Rainscreens: 5 Things You Need to Know

  1. A rainscreen is like a shield. The shield protects you from most attacks. If something penetrates the shield you'll be fine if there's a gap between you and the shield. If there isn't a gap, you're in trouble.
  2. No wall is waterproof. Sealers, waterproofers and other materials break down over time, errors take place, the ground settles, cracks form, and eventually water will get in.
  3. A little water can do a lot of damage. A small amount of moisture trapped in a wall leads to mold, rot, and negative effects on the sustainability of the building & health of its occupants. Moisture remediation is a billion dollar industry. 
  4. Rainscreen walls have a minimum 1/8" drainage gap. A wall without a drainage gap is not a rainscreen, it's just a veneer.  A 1/8" gap is the minimum space to create a capillary break for drainage.
  5. Rainscreen walls are robust & cost effective. Imperfections in the wall are less likely to lead to problems with a rainscreen system. Including a rainscreen system typically adds $1-2 per square foot to the construction cost. Remediation can cost over $60 per square foot. 

Extend Your Learning: What is a rainscreen? Why do I need a rainscreen? 

A rainscreen system is the embodiment of building scientist Dr. Joseph Lstibrurek’s two rules of moisture management:

“Keep water out; Let water out if it gets in.”

The rainscreen is the first line of defense against the effects of moisture on the wall detail. A rainscreen is the weather-facing surface of an exterior wall detail that stands off at least 1/8” from the moisture-resistant surface of the structural back up wall. In many cases this would be a brick veneer, stone veneer or other types of masonry veneers. A very important, though often ignored, point about rainscreens: A veneer that does not stand off from the moisture-resistant surface of the structural backup wall to create a cavity is NOT a true rainscreen; it is just a veneer.

No building envelope is 100% waterproof. The addition of a drainage plane mitigates the risk of moisture intrusion from unavoidable imperfections in the wall materials and construction.

Sure Cavity Rainscreen Drainage Plane System

The Rainscreen Drainage Gap

A variety of techniques and products can be used to create the air gap needed in a rainscreen system. Sure Cavity and Gravity Cavity are made of a rigid corrugated plastic with a mortar blocking fabric that covers the exterior wall surface and provides a predictable drainage gap and an unobstructed path for moisture to exit the wall system. This system, combined with appropriate weeps and ventilation, creates a ventilated cavity which encourages drying and mitigates the risks of moisture in both liquid and vapor form.

In addition to protecting against moisture, a rainscreen also helps protect the interior of the wall detail from a variation in air pressure (wind), ultraviolet rays of the sun and other severe weather conditions. View MTI's Rainscreen Drainage Plane Products

Ready to Rainscreen, or Have Questions? We Can Help With Both.

 1-800-879-3348 or  Contact Us 

Moisture Testing: Rainscreen Vs. No Rainscreen

Don’t Two Layers of WRB Provide Drainage?

Building codes call for two layers of WRB to create a “drainage plane” in the wall system, with the outer layer of WRB acting as a “sacrificial layer.” MTI conducted Independently verified testing to measure the moisture levels of the layers in a system with the Sure Cavity rainscreen system compared to a system without a rainscreen. The system without the rainscreen allowed moisture levels to exceed 16% for nearly 5 days and over 20% for nearly 4 days from a single wetting event (Wood Moisture Equivalent levels of 16-20% promote fungal growth, and 20% or higher promote rot). In contrast, the testing verified that Sure Cavity rainscreen system prevented the moisture-sensitive layers from getting wet. Read the full testing report.

Does “Drainable Housewrap” Create a Rainscreen?

No. Cementitious materials such as stucco have a long history of bonding to plastic building materials like housewrap. Many housewraps claim to be “drainable” by incorporating wrinkles or raised patterns. However, these products do not create a 1/8" rainscreen gap. Using a rainscreen with a three dimensional drainage plane with rigid channels guarantees that the rainscreen will drain water than gets past the exterior veneer.

Does a rainscreen diminish R-Value?

Rigid insulation with taped seams is being included in many building envelopes in order to meet new energy code requirements. Drainage is essential in these wall systems, and is typically included exterior of the rigid insulation, between the insulation and veneer which has no effect on the R-Value. In systems where the rigid boardstock insulation creates the exterior veneer (such as EIFS) research has shown that a drainage gap of 1/8” behind rigid insulation allows drainage and maintains over 90% of R-Value.

Which Geographic Regions Require a Rainscreen?

Precipitation Map

The Whole Building Design Guide recommends that rainscreens be incorporated in areas that receive 40” (1000 mm) of rain per year or more, but rainscreens benefit buildings in all geographic areas by mitigating the risk of moisture intrusion. Wall failure can occur anywhere, including areas that typically receive low rainfall - there have even been failures attributed to lawn irrigation systems in arid regions. A very small amount of entrapped moisture can create major problems. Creating a predictable drainage gap in a rainscreen system eliminates those concerns.

Sure Cavity Keeps Walls Dry

Sure Cavity's rigid channels are proven to promote rapid drainage and ventilaiton.

“True Channel” Technology
• Unobstructed Drainage & Ventilation
• Resists Compression
• Reduces Waviness

Performance Backed by Independent Testing
• Exceeds ASTM 2273
• Prevents moisture related damage 

Mortar Blocking Fabric
• Easy Installation
• Labor and Cost Saving

• Not a vapor retarder
• Promotes Cross Ventilation

96% Recycled Materials, Made in USA
Details on Sure Cavity

Got Questions? We Have Answers.

 1-800-879-3348 or  Contact Us 

Cost-Risk Analysis: Rainscreen Vs Failure

Using a rainscreen system adds minimal cost and complexity to a building envelope, and significantly reduces the risk of moisture related wall failure. Rainscreens are like cheap insurance for architects, contractors and owners. At a cost of $1-2 per square foot installed rainscreens make economic sense when compared to the risk of over $60 per square foot to remediate failed walls.

Getting to the Bottom: Weeps and Weep Screeds

A rainscreen drainage gap requires that the moisture has a pathway out of the wall, and gravity necessitates this should be at the lowest point in the wall system. The two most common pathways for water to exit at the bottom of the wall are weeps and weep screeds.

Weeps are used in cavity masonry construction, such as brick or full stone walls. The most common types of weeps are plastic tubes and ropes, but neither of these devices provides a clear path for liquid water at the lowest point in the system. The best weeps put the drainage pathway directly on top of the flashing. Learn about MTI’s Weeps 

A weep screed is a common termination for the bottom thin veneer walls such as stucco, adhered stone & brick, and fiber cement siding. The weep screed does not support the veneer but provides protection along the bottom edge. Many weep screeds feature small holes that are designed for mortar attachment and instead rely on hairline cracks for weeping. A quality weep screed will feature large weep holes or slots directly below the drainage gap. Learn About MTI’s Weep Screeds