Getting to the Bottom of Moisture Management

Getting to the Bottom of Moisture Managment

Published By: Construction Canada | Date: February 2015 | Author: John Koester

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Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

There are numerous factors/phenomena that create misunderstandings that result in improper or inadequate moisture management design for the exterior building envelope. Let’s take a look at two of them, gravity and temperature.

It’s the lowest point that’s the wettest. Why, because that’s where the water is! (see figure 1) That’s because of the influence of gravity. Now the low point of an exterior wall system of a building doesn’t necessarily have to be the top of the footing (see figure 2) or the top of the stem wall. (see figure 3)

A low point in an exterior building envelope is a location that stops or slows moisture in liquid form from proceeding in a gravity-induced downward direction.  Sometimes this is intentionally done with a designed flashing system and sometimes it is just the result of other components of the exterior building envelope system coming together to stop or seriously slow the downward path of liquid water.  (see figure 4 and figure 5)

It’s the highest point that’s the wettest.  Why, because that’s where the water is!  (see figure 6) That’s because of the influence of temperature.  Everyone, or at least I hope every construction professional, can understand why a low point of a construction detail would be the wettest, but why would a high point/the top of a wall system be the wettest?  The answer is found in the fact that H2O can be in three forms:  a solid (ice), a liquid (water) and a gas (water vapor).

The most obvious conclusion is that there is a leak, a source of liquid water coming down and in from a higher point, and in most cases, that is exactly what is happening.  But there may be a number of other scenarios that could explain water and water patterns at a high point of an exterior wall detail.

One of them is that H2O in vapor form will move upward with warm air to a higher point of a construction detail and come in contact with air or a surface that is cool enough to be a dew point and condense into water droplets and liquid water and wet a construction detail. (see figure 7)

Figure 7 Figure 8 Figure 9 Figure 10

This can happen on exterior surfaces of a construction detail or on the interior surfaces of a construction detail (inside a wall).  (see figure 8)  The other part of the answer has to do with why a high point of a wall would be cool enough to be a dew point when hot air rises and the top of the wall and ceilings should be warmer.  This phenomenon is usually the result of a thermo bridge from the cold exterior of an exterior building envelope to a warm interior of an exterior building envelope with enough intensity to overcome the ability of the interior ambient air temperature to warm the construction detail. (see figure 7 and  figure 8)

The ingredients required to allow this to happen are an opening to allow a sufficient amount of cool air to pass into or through the construction detail at this point or a material that can transfer this difference in temperature, “good conductors” such as steel, glass, solid concrete, or water (water transmits temperature 25 times better than open air). Wet construction materials are good conductors of temperature and poor insulators. The best insulations that are wet are very poor insulators. So when a wet pattern in a high point of a construction detail is contributing to condensation, there very well may be an associated water source or wet material that has promoted cold temperature transfer (a water source that causes a temperature transfer and a dew point and a condensation wetting pattern but is not actually leaking or absorbing into a construction detail to wet it). (See figure 7 and figure 8)

Another mechanism that transports moisture in liquid form into and through construction details is absorption.  The general rule/law of physics says moisture will usually move from a high concentration of water into and through materials to a lesser concentration of water (drier materials) in an attempt to equalize the concentration of water.  The distances that this moisture can travel into and through construction materials is truly surprising!  (see figure 9)

If certain conditions exist, continuous source of liquid moisture, absence of driving mechanism and construction materials that funnel and or encapsulate (two or more) vapor retarders in the same exterior building envelope; the travel distance can be great. (see figure 10)

These distances will be expanded proportionally with the volume of the liquid moisture source. A large volume of liquid water has greater weight (pull of gravity) propelling the liquid moisture into and through construction materials.

It should be apparent that designing an exterior building envelope moisture management system brings a number of factors into consideration.

  • The potential amounts of moisture to be managed
  • How often the construction detail is exposed to this amount of moisture
  • The duration of the exposure
  • The physical form of the moisture (liquid, solid, gas)

The first, and most important, moisture management design requirement for your building’s details is “Do not let them get wet!”   They get wet quickly but they dry out slowly.  The second moisture management requirement is “Get the moisture away from, off of and out of your construction detail as quickly as possible.”  It’s about “Time.” The amount of time moisture is in, on or near your construction detail is really what you are managing.

Drainage is “the movement of moisture from one point where it is not desired to a preferred location.”  In most cases this is from a high point in a construction detail or surface on a construction detail to a lower point out of, or off of that construction detail.  In the interior of a wall of an exterior building envelope, this is a code compliant requirement.  Section 1403.2, Weather Protection, of the 2012 IBC states, “Exterior walls shall provide the building with a weather-resistant exterior wall envelope.  The exterior wall envelope shall include flashing, as described in Section 1405.4.  The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer, as described in Section 1404.2, and a means for draining water that enters the assembly to the exterior.  Protection against condensation in the exterior wall assembly shall be provided in accordance with Section 1405.3.”

Figure 11

The moisture that may enter the exterior building envelope must be provided with a designed passageway to move it from a high point of entry to a lower point and allow it to exit the exterior building envelope to the exterior of the building.  The technology that allows liquid moisture to move downward in the interior of an exterior building envelope is a core, cavity or a rainscreen drainage plane material.  The technology that stops the downward movement of liquid moisture is a waterproof flashing material.  The technology that gives liquid moisture a reason to go in one direction or another is slope-to-drain/elevation variation – “high to low.”  The technology that creates the opening from the interior of the wall to the exterior of the veneer/rainscreen material (brick, stone, stucco, etc.) is the weep.  (See figure 11)

All of these four components are absolutely critical for an exterior building moisture management system to work.  Weeps do not work until the liquid moisture gets to them.  If the moisture that is attempting to move downward in a core, cavity or rainscreen is obstructed or slowed, it may just absorb into adjoining construction material and/or deeper into the exterior building envelope.  If and when the liquid moisture gets to a flashing/water stop, if there isn’t any slope-to-drain to the exterior of the exterior building envelope, it may just accumulate and absorb into the adjoining construction material or find its way deeper into the exterior building envelope through a void in imperfect flashing or out through veneer material through an undesigned pathway.  If the weep holes are not at the lowest point of the core, cavity or rainscreen drainage plane, liquid water may accumulate and find its way deeper into the exterior building envelope or through veneer materials through an undesigned pathway.

All of these consequences of an improperly designed moisture management system are potentially very serious and may result in exterior building envelope component failure or total system failure, up to and including structural failure.  An exterior building envelope moisture management system’s reliance on all components of the system to function in concert with each other cannot be stressed enough!

Figure 11

Figure 13 Figure 14

The last and lowest (but certainly not “lowliest”) components are in many cases the least understood and prioritized – these are the weeps and weep screeds. (See figure 12 and figure 13)

In the case of weeps, materials used, the number used and their location is completely without reason.  Materials that are often used are thought to wick water out of a core or cavity.  Here is some really good advice – Do not get into a wicking contest with masonry materials!  A weep of any kind, good or bad, every 48 inches is of little value.  A weep of any kind that is not at the lowest point of a core or cavity which is at the top surface of the flashing or water stop, is of little value.  A weep that creates a hole in a veneer from the interior of the core or cavity to the exterior of the veneer is just another hole in the veneer that may let liquid moisture into the wall as well as let it out of the wall if there is no slope-to-drain that keeps it out and drains it out of the wall.

Weep screeds have very similar moisture management responsibilities, but in many cases they do not function appropriately.  The notion that a shrinkage crack between the metal and cementitious material (scratch coat, brown coat and bedding and grouting mortar) is a dependable moisture management detail is ludicrous. (see figure 14)

But for many popular/commonly installed weep screeds, that’s exactly what the literature says.  An example taken from a manufacturer’s weep screed literature states, “The ‘V’ stop is punched with holes primarily intended as keying mechanisms.  These also offer minor moisture weeping capabilities.  As stucco cures it shrinks slightly away from the ‘V’ stop allowing moisture to flow down the building paper and exit down the sloped surface.”

A weep and weep screed must have compatible moisture moving capacity with the core, cavity or rainscreen drainage plane above.  If not, the slowing of moisture movement will cause an accumulative effect with all the negative consequences.

Figure 15

What makes a good weep?  There are specific criteria to follow when choosing weep technology.  Masonry Design and Detailing (Bell, 1987) defines weep holes as “Openings placed in mortar joints of facing material at the level of the flashing to permit the escape of moisture.”  The voids/tunnels and channels that a weep creates through the bed joint of mortar, scratch coat, adhering mortar, etc. must be at the lowest point of a core, cavity or rainscreen drainage plane – where the water is – and must be no further than 12 inches apart. (see figure 15)

There is no need to consider modular configuration of masonry units because the voids/tunnels and channels are in the bottom side of the bed joint of mortar and do not affect layout or coursing.

What makes a good weep screed?  There are specific criteria to follow when choosing weep technology.   According to, “A weep screed is a type of building material used along the base of an exterior stucco wall.  The screed serves as a vent so that the moisture can escape the stucco wall finish just above the foundation.”  It is a device used to terminate the bottom of a cementitious-based thin veneer rainscreen.  This device should allow liquid moisture that drains down the back side of a thin veneer rainscreen and on the surface of the weather resistant barrier in the rainscreen drainage plane to freely exit the thin veneer 1.5” to 2” below the bottom of the framed wall system and the top of the stem wall.

Figure 16

Section 2512.1.2, Weep Screeds, of the 2012 International Building Code states, “A minimum 0.019-inch (0.5 mm) (No. 26 galvanized sheet gage), corrosion-resistant weep screed or plastic weep screed, with a minimum vertical attachment flange of 3.5 inches (89 mm) shall be provided at or below the foundation plate line on exterior stud walls in accordance with ASTM C 926.  The weep screed shall be placed a minimum of 4 inches (102 mm) above the earth or 2 inches (51 mm) above paved areas and shall be of a type that will allow trapped water to drain to the exterior of the building.  The weather-resistant barrier shall lap the attachment flange.  The exterior lath shall cover and terminate on the attachment flange of the weep screed.”  (see figure 16)

Figure 17-18

It is critical that the drainage holes/weep holes be numerous and not more than 1 inch (or so) apart and located at the bottom of the weep screed and against the 3.5” back flange to allow for direct contact with the bottom of the rainscreen drainage plane.  (see figure 17, figure 17A, and figure 17B) This is critical for two reasons:  First, the liquid moisture that drains down the rainscreen drainage plane will have an immediate exit point.  Second, the cementitious materials used to create the thin veneer will not have access to block them.  (see figure 18, figure 18A, and figure–– 18B)


There is little doubt that moisture will enter the building envelope and that it needs to be drained.  This has become more and more the accepted practice, and in many cases, code mandated.  Unfortunately, the liquid moisture gets drained to the bottom of the wall with little thought given as to how it will get out.  This article has presented pragmatic solutions to getting the moisture out at the bottom.  It is up to the reader to make prudent choices in the materials and methods they choose to employ.

Drainable is Sustainable

Drainable is Sustainable

Published By: Masonry Magazine | Date: December 2011 | Author: John Koester

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Most exterior surfaces of a vertical wall system do allow some moisture to penetrate deeper into the exterior building envelope. Why do building professionals allow this; don’t we have adequate waterproofing systems? Part of the answer is “Of course we know and of course we do, but that isn’t the hole/whole story!” The hole/whole story, the more complete story, is that some exterior building surfaces of the exterior building envelope are responsible for more than just moisture management. These responsibilities include aesthetics, structural support, signage support, mechanical support and protection (veneers acting as shields). In addition to all these responsibilities, they also have to manage moisture! So the hole/whole story is a little more complicated! As part of the process of moving forward, let’s take a look at an example of a typical exterior wall (exterior building envelope). Obviously, there are many types of wall systems, but let’s use this one (Figure 2.) to simplify the discussion as much as possible. Figure 2

Rainscreen Components

The Rainscreen is the most exterior surface of an exterior building envelope. It is the first surface that has the potential to change the exterior environment to satisfy a desired interior environment: wet to dry, hot to cool, cold to warm. It can be constructed of various materials including brick, stone, stucco, steel, wood, glass, plastics, and fabrics. The next section, the Rainscreen Drainage Plane, is the space from the backside of the rainscreen to the front surface of the moisture resistant material. It can be either a wall design feature (such as a space) or a product. The third section, the Water Resistive Barrier, can be a waterproofing product (usually a sprayed or rolled on chemical compound) or a weather-resistant material (two layers of construction paper or a building wrap). It can also be both. The final section, the Structural Wall, can be composed of a variety of materials including wood or steel studs, cmu or poured concrete. Figure 3In most cases there is a requirement for insulation. It comes in many forms and can be placed in various locations in this detail. It is an extremely important and extremely complicated character in the hole/whole story and must be included in the moisture management equation. (Figure 3.) All forms of insulation involve unique moisture management issues that are beyond the scope of this article; however, they must be seriously examined when constructing a moisture management plan for the rainscreen building envelope.

Sustainable Building and Moisture Management

Sustainable building isn’t possible without moisture management. For those forced to live or work in a building without it, life becomes a health and safety nightmare. Bold statements, but totally supportable. According to the HUD’s Path Project, “Moisture, in all of its physical forms, is commonly regarded as the single greatest threat to durability and the long-term performance of the housing stock. Excessive exposure to moisture is not only a common cause of significant damage to many types of building components and materials, it also can lead to unhealthy indoor living environments. A long list of serious adverse effects can result from moisture problems in houses. There is wide agreement that successful management of moisture in its various forms is essential for houses (buildings) to be durable, safe and energy efficient.” In its 2004 report, the Path Projected listed the following outcomes of uncontrolled moisture in the building envelope:

  • Decay of wood and corrosion of metals
  • Infestation by termites and other destructive insects
  • Negative impacts on indoor air quality
  • Growth of mold, mildew and other biological contaminants
  • Reduced building material strength
  • Expansion/contraction damage to materials
  • Reduced thermal resistance of wet insulation
  • Premature failures of paints and coatings
  • Damage to building contents
  • Negative effects on building aesthetics

Enter the key phrase sustainable rainscreen building envelope into Google and you will get more than 18,000 results, including MTI’s “Drainable is Sustainable” presentation delivered at last fall’s technical meeting of the Sealant Waterproo ng and Restoration Institute. A key point of the presentation is that a moisture management solution for the rainscreen building envelope requires a systemic/holistic approach. There is no single magic bullet; it takes a well-thought-out, coordinated system of products and processes designed and implemented by a team of professionals working collaboratively at every stage of the project to reach a successful outcome.

Motorcycles, Rainsuits and Moisture Management

Motorcyclist rainTo illustrate the importance of a coordinated system in moisture management, I’ll use the analogy of a motorcyclist riding towards an approaching storm. It’s a nice sunny day, so I decide to go for a motorcycle ride. Being an experienced motorcyclist, I always have raingear in my saddlebags because it’s summer and anything is possible! As I move through the countryside, I notice that the sky is darkening and a storm is imminent so I pull over and put on my rain suit. In a matter of minutes, the rain starts. It’s light at first but soon becomes heavy, and it’s coupled with a driving wind. Rain is forced around my windshield and into my eyes greatly limiting my ability to see the road. Water cascades off my helmet and runs down the back of my neck soaking my shirt. The water on the highway flies upward leaking into my boots through the seams and around the tongue. To make matters even worse, it’s a hot, humid day so beads of condensation start to form on the inside of my rain suit making for an increasingly miserable rideObviously evenen though I thought I was prepared for rain, I hadn’t looked at all possibilities.” Even though I had a collection of items designed to keep me dry, I hadn’t fully thought through the outcome, and I hadn’t properly combined the items into a functioning system. If I had used goggles or a helmet with a visor, I could have seen the road better. If I had used the hood on my jacket and worn it under the helmet, I wouldn’t have gotten rain down my back. If I had used a rain jacket with vents, air could have moved around inside the system and reduced the condensation. Finally, had I worn rain boots with my rain pants lapped over the top of the boots and fastened snugly, I wouldn’t have gotten wet from the water spraying up from the road. So what can be learned from this analogy about the importance of a system in solving the building envelope moisture management problem? Hopefully, it’s that simple solutions don’t always work. As much as the motorcycle rider would like to just throw on a waterproof jacket and waterproof pants and be off again, it just doesn’t work. It takes many products, put on in the right order and at the right time, to create a positive result. We need to look at how many factors are in play and then employ several moisture management solutions as part of a system to solve the problem.

The Importance of Holistic-Systemic Building

This same idea of a coordinated, multi-component solution can be applied to the people designing, specifying and constructing a building. Gone are the days when designers, specifiers, and contractors could successfully do their jobs in a vacuum. There are just too many new products, processes and complex codes for the “Lone Ranger” approach to work. Everyone must collaborate and communicate if a sustainable, healthy building is the goal. This brings us back to our hole/whole story concept. Merriam-Websters Dictionary defines holistic as: “relating to or concerned with wholes or with complete systems rather than with the analysis of, treatment of, or dissection into parts.” Knowing how each product used in the construction of the building reacts with the rest of the products is critical! Is it compatible with the rest of the system, or does it create unintended consequences? Have the parties designing, specifying and installing the products looked at all the possible outcomes of the each procedure and each product with the rest of the system? That’s the “whole” part of the concept. The “hole” part of the concept refers to the importance of drainage in an effective moisture management solu- tion in the rainscreen building envelope. The normal interpretation of a hole in a building envelope is a nega- tive one. However, all building envelopes leak. Moisture can enter from the outside in multiple ways including wind-driven rain and cracks in the veneer. Moisture can enter from the inside in the form of condensation. So if we know water is going to get in, how is it going to get out? WeepsIn many places, drainage of the exterior building envelope is a code requirement. Section 1403 Performance Requirements (International Building Code 2009, pp. 277-278) states: “1403.2 Weather Protection. Exterior walls shall provide the building with a weather-resistant exterior wall envelope. The exterior wall envelope shall include ashing, as described in Section 1405.4. The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior veneer, as described in Section 1404.2, and a means for draining water that enters the assembly to the exterior.” Subsection - 1b of the National Building Code of Canada 2005, Volume 1 states: “The second plane (the drainage plane) of protection shall be designed and constructed to i. intercept all rain and snow that gets past the first plane (the veneer) of protection and, ii. effectively dissipate any rain or snow to the exterior”

Real Waterproofing Involves Real Drainage

Moisture DamageIn his August 2010 article Putting in Holes to Stop Leaks, Brett Newkirk wrote, “A wise Canadian once said, ‘It is amazing how many leaks you can stop by putting holes in a building.’” The holes referenced are weeps, but not just any weeps. Gone are the days of the “wicking” rope weep or the weep tube. If these old-style weeps worked at all, it was because some water leaked out of the moisture voids next to them and not through the weep itself. Many times, these antiquated weep devices weren’t even placed at the lowest point in the wall. An effective weep must be placed at the lowest point in the wall (the bed-joint-of-mortar). Weeps also need to be place frequently to be effective. The channeled weeps in Figure 6. above are placed 10” on center. Over the last 20-30 years we have experienced a myriad of moisture-related problems in the building envelope. The increased use of rainscreen technology by designers has seen more than its share. There are many reasons for moisture problems in the rainscreen building envelope. Some of the most frequent issues include:

  • Tighter building envelopes increased mold and rot problem
  • Many modern materials created unintended moisture barriers
  • Increased insulation led to condensation problems

The Three Key Components

Figure 10Figure 11Figure 11Figure 12

Rainscreen walls with a wide variety of veneers are sustainable when combined with the right moisture management system. This effective moisture management system includes three key components. First, there must be a space from the backside of the rainscreen (veneer) to the face (exterior surface) of the WRB. (The minimum depth is still being debated, but should be at least 1/8”. In Canada 10mm is required.) This space must be uninterrupted, predictable, constant and stable, and the materials used to create it should have very low absorbency. Second, we need to dump the water (weep holes) out of the rainscreen drainage plane at every possible opportunity. Water should be dumped at transition details (e.g. brick to stucco, siding to brick or stucco, etc.), projection details (e.g. windows at top, windows at bottom) and wall terminations (e.g. bottom of wall, top of wall). The final component is air pressure control. A well-designed rainscreen drainage plane system helps maintain pressure equalization. It lessens the chance that high-pressure air, contaminated with moisture, will move from high pressure to low pressure and deeper into the building envelope. And with venting at the top and bottom of the wall, air ow may assist in drying the building envelope. (Figure 12.) Lu Bu Wei, a Chinese philosopher wrote, “It follows, then, that all things work together...The Sage examines them in order to observe their similarities.” In other words looking for one simple solution to preventing mois- ture problems in something as complex as a modern building is unrealistic. There are too many things that come into play: materials used, the laws of physics, the people involved, location/climate, the many systems brought together in the building. To reach a true “waterproofing” result we must work together as a team and we must create the right moisture management system. And that’s the hole/whole story! Weeps draining brick wall

Rainscreen Drainage Planes vs Furring Strips

Rainscreen Drainage Planes Vs Furring Strips

Published By: Construction Canada | Date: December 2011 | Author: John Koester & Mark A. Johnson

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The construction industry and more specifically, the building envelope sector of the construction industry, seems to embrace the status quo with exceptional tenacity. There are obvious reasons for this unwillingness to change. Manufacturers of products currently utilized in building envelopes don't want to see change, and many tradespeople don't want to learn new techniques.

“For time and the world do not stand still.  Change is the law of life.  And those who look only to the past or the present are certain to miss the future.” John Fitzgerald Kennedy The idea that “Change” is a law of life has been proven true again and again since the dawn of time.  In our own lifetime think how much technology has changed the way we live in just a few short years.  And yet we seem so slow to change.  Many of us actually dread the idea!  Our present habits are like the comfortable old jeans we refuse to throw out despite the stains and holes.  Resistance to change is as predictable as the knowledge that change is inevitable. The construction industry and more specifically, the building envelope sector of the construction industry, seems to embrace the status quo with exceptional tenacity.   There are obvious reasons for this unwillingness to change.  Clearly, manufacturers of products currently utilized in building envelopes don’t want to see change unless they have another product that fits with the new way of doing things.  Also, many tradespeople don’t want to learn new techniques when current practice has served them well for so long.  There is a long list of players who could be negatively impacted by the implementation of new technologies, new materials and new techniques including the shipping and storage industry, unions, contractors, raw materials producers, architects, specifiers and codes officials.  Building is a complex process, and there are countless individuals and organizations involved.

Forces of Change

It is unfortunate that “change” is often accompanied by the words “forced to.”  While force may bring about superficial change, it rarely engenders quality of change.  One of the more common forms of force used to change the construction industry is through codes.  The dictionary defines the word codes as “any set of standards set forth and enforced by a local government agency for the protection of public safety, health, etc., as in the structural safety of buildings.”  Codes require the building industry to conform to a “minimum standard.”   It is the very least you can do without breaking the law and being subjected to fines and/or imprisonment.  There’s probably little realization among the general public that buildings are being constructed with the goal of meeting a “minimum standard.” Economics is the other factor that brings about change.  If a product is cheaper, smarter and sexier, and if consumers become aware of it, they’ll demand the new technology.  Industry will also shift to new technologies if they see an economic and/or marketing benefit.  For example, how many products have suddenly become “green?” The focus of this article is the adoption of modern rainscreen drainage technologies by the construction industry as a solution to the entrapped moisture problem.  The issue was first addressed in Canada in the last part of the 20th Century because widespread failures of the exterior building envelope were occurring.  Building scientists discovered that a space between the backside of the veneer and the front of the backup wall could alleviate the problem, and code was written to force this change. In 2005 the National Building Code of Canada was modified to include Section, Minimum Protection from Precipitation Ingress.  This section states, “an open drainage material, not less than 10mm thick and with a cross-sectional area that is not less than 80% open, is installed between cladding and the backing, over the full height and width of the wall.”

Furring Strip Drainage Systems

Example AContractors began using wood furring strips to create this space to allow moisture to drain down and out of the building envelope and to equalize air pressure between the front and the back of the rainscreen building envelope.  If installed correctly, they provided this required space behind the rainscreen for moisture to move out, and they allowed for air pressure equalization.  As time passed, new types of materials (plastic, metal, etc.) began to replace wood furring strips. In most furring strip applications, additional materials are required to complete and support the construction of this rainscreen drainage plane detail.  These supporting materials include some type of semi-rigid moisture-resistant panel or roll stock material used in combination with construction paper as a slip-sheet. There are several inherent problems with this type of rainscreen drainage system.  The two that are probably the most negative are:

  • Too many components (cost issue)
  • Too labor-intensive (cost, time, and complexity issues)

Another factor that negatively impacts this system is its violation of the most basic principle of moisture management; it fails to provide a means for moisture to effectively exit the building envelope.  (Example B)   The International Building Code, Section 1403.2 states, “The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by providing…a means for draining water that enters the assembly to the exterior.”  Section 703.1.1 of the International Residential Building Code affirms this point. The three required components of a well-designed moisture management system to stop moisture penetration include:

  • Moisture-Resistant Materials (flashings, coatings, WRBs, etc.)
  • Slope-to-Drain (to direct moisture out of the construction detail)
  • Predictable Open Channels (pathway to the exterior of the building/building details that allow moisture an unobstructed pathway out of the construction detail)

Example BThe furring strip rainscreen drainage system pictured in Examples A and B interrupts moisture at the building paper slip-sheet surface installed in front of the semi-rigid asphalt board but doesn’t provide a designed detail for this obstructed moisture to drain to the exterior (See Example B – Top Down View).

  1. Sheathing
  2. Fastener
  3. Intermediate Furring
  4. Building Paper
  5. 3-Ply Semi-Rigid Asphalt Board
  6. Building Paper - Slip Sheet
  7. Stucco Lath
  8. Three-Course Stucco
  9. Stucco Cracking
  10. Moisture

The Waterproofing Issue

The required drainage that must be provided in front of a moisture “water stop” has a long history of being misunderstood and is often improperly detailed or even omitted in the design and construction of the exterior building envelope.  The root cause of this problem may be the construction industry’s unrealistic comfort level with the idea of “waterproofing.”  The reason for waterproofing’s preeminence can be at least partially explained by the petro-chemical industry’s rapid growth resulting in large volumes of byproducts in need of disposal (e.g. coal tar pitch from the coking of coal, asphalt from the distillation of petroleum, etc.).  When these byproducts were found to be excellent waterproofing materials, a market was needed; enter the construction industry! Prior to this period, about 125 years ago, various types of metals such as copper and lead, and some tree pitches and animal fat impregnated fabrics, were the only moisture-resistant materials available.  The construction industry’s only real moisture control was slope-to-drain.  Slabs and boards of semi moisture-resistant material were installed shingle-fashion on roofs with high pitches; they were also  installed vertically on walls.  Coal tar pitch and asphalt changed all that and led to developments like “low slope” or “flat roofs.”  The petro-chemical industry modified and improved on these original materials and also added many high-quality synthetic waterproofing materials.  These abundant, high-quality waterproofing materials distracted the construction industry’s attention away from the need for drainage. Example C

The Need for Drainage

With the avalanche of building envelope failures over the last two or three decades, the need for the inclusion of drainage in the building envelope moisture control equation has become increasingly clear.  Furring strips were an early answer, and were, at least for a time, an imperfect solution.  However, the placement of the furring strips created a new problem.  The early use of furring strips to create a drainage plane looked something like Example C. The furring strips, attached at each stud location, pushed against the scratch coat of mortar bellying it out and allowing the furring strip to come in close proximity to the stucco lath.  This created a much thinner layer of stucco at each furring strip and often led to stress cracks in the three-course stucco. (See #8 Example C) Example DIn an attempt to improve this application, an additional furring strip was installed midway between the 16” on- center furring strips. (See #3 Example D)  This alleviated some of the bellying/cracking problem, but it also created a new issue.  The labor forces would occasionally fasten the expanded metal lath to this nonstructural termination (the intermediary furring strip); this resulted in more extreme cracking. Excessive, uncontrolled cracking in the rainscreen of the exterior building envelope is unsightly, and it creates an opportunity for excessive moisture to egress into the rainscreen and potentially deeper into the exterior building envelope.  To make matters worse, the cracks were occurring at fastening points.  This fact potentially allowed egressing moisture to have a direct pathway along fastener shanks and into the stud cavity (See #5 & #6 Example E).  All because the labor force incorrectly used non-structural details as attachment points. Example EIncorrect placement of fasteners that penetrate to the backside of the backup wall sheathing and on through to the backside of roof sheathing does cause real “leak” issues.   However, the moisture patterns seen on the backside of the sheathing may not be from egressing moisture!  Instead, they could be a water pattern caused by the melting of frost balls or the run off of condensation.  What is really happening is that the shanks of the misplaced fasteners may not be pathways for liquid moisture; rather, they are pathways for cold temperature.  The result is the points of the misplaced fasteners are cold enough to create dew points that in turn allow for condensation in the form of frost balls or water droplets (See #4, #5 & #6 Example F).  So the old technology seems to have quite a bit of room for improvement.Example F

The New Technology

The new technology that is attempting to improve the rainscreen drainage detail is the use of “mat” materials.  Most of these mats come in the form of roll stock, and they are created with various manufacturing procedures and materials.  The primary manufacturing procedures and materials are as follows:

  • Continuous vacuum molding of various types of plastic sheet roll stock material creating a variety of shapes or raised patterns and dimples in the material.  A second manufacturing process involves adhering a synthetic fabric to the tops of the raised pattern.  (Example G)
  • The extruding of various types of plastics into filaments into a variety dimensions then compressing and organizing them into continuous mats of various thickness and widths. A second manufacturing process involves adhering a synthetic fabric to one or both sides of this entangled filament mat.  (Example H)
  • The extruding of various types of plastics into filaments of different dimensions and compressing them into a sheet. This sheet is then formed into a variety of corrugated patterns.  (Example I)
  • The forming and perforation of a plastic roll stock material into a variety of corrugated patterns and thicknesses. A second manufacturing process adheres a synthetic fabric to one side.  (Example J)

Examples G - JThese four manufacturing procedures are used to create six of the major product lines that represent the new “mat” rainscreen drainage plane technology in North America.  They differ from one another in drainage efficiency, compressive strength, and ease of installation.  What they all bring to the construction industry, and specifically to the exterior building envelope segment of the construction industry, is ease of installation and in most cases, a more predictable cost-effective rainscreen drainage plane.  When choosing which one of these new rainscreen drainage plane technologies to use in your next project, select the one that best suits your veneer/rainscreen type and one that has a CCMC evaluation number.

Window Rough Opening Moisture Management

Moisture Management in Window Rough Openings

Published By: Masonry Magazine | Date: February 2011 | Author: John Koester

The phrase “leaky, drafty windows” is common in the construction industry and, unfortunately, it seems too often true. However, the perception may not be the reality. There may be leaks and drafts in close proximity to the window, but the window units themselves may not have anything to do with the problem. In many cases it is the wall system that the window unit is installed in that may be the actual source of the leaks and drafts. We need to focus our attention on the rough opening to conquer some of the air and moisture problems blamed on windows.

Window DamageThe areas where two or more different construction products, details and systems intersect are always places of potential risk. Incompatibility of products or designs and poor communication between the various people involved (architects, contractors, tradespeople) are some of the more common scenarios that result in failures. In many cases the solution to the “leaky, drafty window” problem is to focus on the materials and methods used around the window rough opening.

Exterior building envelope construction systems (roofs and walls) often fail in the detailing of openings, projections and transitions. The detailing process is complicated because it involves more than a single individual or discipline. The challenge is managing the various disciplines using a wide range of materials into a cohesive unit. The need for holistic building is imperative. Each party involved needs to know how their task, and materials used to complete that task, impact the final result. This may sound impossible, but it’s not. It may be difficult, but it can be accomplished, and if the parties involved in the construction process are truly committed to sustainable building, it must be accomplished!

Start by understanding that perfection is the ultimate goal. The reality of imperfection is the risk of some type of failure. The real task is to modify risk in order to minimize failure. The first and most important task of moisture management, as it relates to construction products and details, is keeping moisture off of them. If water can’t get to the detail, it can’t damage it. The second task is to isolate as many details as possible. Identify risk zones and design details and concentrate on their intrinsic weaknesses. Once identified create a moisture solution. When a potential problem isn’t addressed in one area, it often leads to failure in another. A poorly prepared rough opening develops leaks that then get blamed on the window. In reality had the window detail been properly addressed in the first place, a failure in an adjacent detail may not have happened. Many wall failures, both structural and veneer-related, are a result of a moisture management failure on a window installation (specifically, problems with preparation of the rough opening).

Risk Zones Of a Window Rough Opening

Moisture risk at the top of a window detail can come from a number of sources.

  • Condensation or frost accumulation from warm, moist air coming in contact with dew point temperature in the air or on surfaces of the interior of the window rough opening and on the surfaces of the window unit
  • Openings or voids in the waterproofing material at the top of the window (installation flange and flashings)
  • Moisture that may have entered the wall system above the window and moved from the high point of entrance down to the top of the window detail with an open waterproofing system.

When there are voids or chases in construction details of the exterior building envelope, the air that is in these voids, or that can move into them, must be controlled or conditioned. The control mechanism is usually some kind of vapor retarder membrane placed on the warm side of the detail and made as airtight as possible. When warm moist air can’t meet a dew point temperature, there is no condensation; therefore, no frosting. These voids also need to be filled with some sort of insulation to interrupt temperature transfer – no dew point temperature, no condensation problem.

To a lesser degree, the voids at the sides of the window rough opening have the same condensation concerns as the top and bottom because the surfaces are vertical rather than horizontal. However, they are dependent on the top of the window being properly treated to manage moisture correctly so that it doesn’t allow moisture to move down into the voids at the side. The bottom of the window area is of greater concern. It can be compared to the bottom of a bucket; everything runs downward.

Historical Moisture Diversion
Historical Window Moisture Diversion

Any and all liquid that gets into the window’s rough opening will accumulate at the bottom of the window rough opening. The accumulation of moisture at a low point in a window rough opening leads to absorption and migration into the surrounding details resulting in one of the most common failure scenarios in the construction industry. Cover the construction details that you don’t want to get wet with moisture-proof or moisture-resistant materials. Moisture should also be diverted away from construction details with drainage products as quickly as possible to minimize risk. The combination of a well-designed drainage system and a moisture-resistant or waterproofing system is the ultimate detail to manage moisture!

Historical Basis for Moisture Diversion

Diverting water away from window details is not a new idea nor is it a new technology. Designing a pattern in the veneer immediately above a window has a long and successful history. What is not commonly known or understood in today’s construction and design industry is that most of the patterns in older historical building veneers were there to manage moisture and to move it off of, and away from, sensitive details such as windows and doors.

This historical idea of diverting moisture away from sensitive details with architectural details can be applied today, but with a twist. A thin veneer with a predictable rain screen drainage plane affords an opportunity to apply this moisture diverter technology on the inside of the void of the rain screen drainage plane rather than by adding architectural details on the outside.

Here are two examples of this moisture diversion practice of creating a detail above the window to move moisture away from the top and out and around the sides, one from the past and MTI’s “inside the envelope” solution.

Moisture Diversion Technology

The bottoms of chases in framed construction window rough openings should be addressed with the following details and materials:

Figure A. Detail

  • Installation DrawingsThe construction materials that make up the bottom of this detail must be covered with a waterproofing material that turns up the sides of the window rough opening a minimum of eight inches
  • The top surface of the bottom of the window rough opening must be sloped to drain to the exterior of the building
  • The back edge of the bottom of the window rough opening must have an elevation change that is higher and creates a back dam.

Figure B. Detail

  • A pathway must be provided for moisture to move out of and off this detail. This needs to be done for moisture that may enter at the sides as well as at the bottom
  • A pathway must be provided for moisture to exit the wall detail once it has drained out of the window rough opening. The Window Drainage Plane™ material will move water from the rough opening into the wall drainage plane (such as Gravity Cavity™ or Sure Cavity™).

Figure C. Detail

The next step involves moving water away from the top. Remember that moisture moves downward. If we follow the examples from history that created external details away from the top of windows and doors, much of the moisture problem can be eliminated from the window rough opening. The only difference is that the moisture diverting mechanism is inside the building envelope.

  • A moisture diverter (such as MTI’s Moisture Diverter™ DS2858) is placed above the window rough opening. It should slope 1/4-inch per one foot and should extend at least four inches passed the side of the rough opening
  • Apply Flashing Tape to the top edge of the Moisture Diverter. The layers of construction paper should overlap the top of the moisture diverter and extend down into the trough.

Window Top Down View

Incorporating these practices into rough opening design gets moisture away from, off of and out of the window construction detail as quickly as possible. These practices constitute a well-designed rough opening. The voids at the top, sides, and bottom need to have predictable pathways to drain moisture that may enter these voids or that may condense in them. The voids also need to be insulated because they can allow air infiltration with negative results. The idea that moisture can’t get in, or that if the voids are filled with insulation there isn’t enough room for moisture, is just wishful thinking!

Moisture Diverter provides drainage at the top of the window (see Figure C.). To provide drainage on the side, install Gravity Cavity strips on the sides of the rough opening (see Figure C.) and on the sides of the window frame. It is critical that these strips are centered on the sides of the rough opening and the sides of the window frame, and that the edges of the Gravity Cavity strips are at least 3/4-inch in from each edge of the rough opening (see Figure D.).

Photo 1Insulate the top, sides and bottomof the rough opening with low expasion foam. The Gravity Cavity will provide the drainage required for the side voids of the rough opening; the Moisture Diverter will provide drainage for the the top. The foam insulation will seal off the interior  of the detail.

Use Window Drainage Plane for drainage at the bottom of the rough opening (see Figure E.). The Window Drainage Plane will also provide a pathway for moisture to enter the rainscreen drainage plane system.


Figure EGood construction design is identifying and prioritizing goals. (Do I want it strong? Do I want it lightweight? etc.) Sometimes the critical requirements of a construction detail or system infringe on long-held beliefs, and sometimes they seem to infringe on each other. Sometimes, they just seem like too much work. However, if our goal really is perfection, and if we really believe in sustainability and “going green,” we will seek what truly works.

Materials, technology and methods are completely different than they were just a decade or two ago. Society’s needs have also changed and the push is on to be responsible, to limit our use of energy and to conserve other resources. We have the knowledge and the materials to conquer the “leaky, drafty window” problem. We just need to employ the will to get the job done!