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Rainscreen Drainage & Ventilation Mats

Dan Johnson

Rainscreen Drainage & Ventilation Mats

Posted: August 21, 2020 | Project Management

When it comes to designing high-performance buildings, moisture control becomes a predominant issue. Rainscreen drainage and ventilation mats provide your wall systems with countless performance advantages:

1. Drainage: Rainscreen mats installed between your building’s WRB and exterior cladding create a capillary break and space for water drainage, allowing it to mitigate moisture penetration. These products feature a filter fabric bonded to a polypropylene filament core.

2. Ventilation: Rainscreen mats provide continuous ventilation by convection or stack effect. As the wall heats up, warm moist air in the cavity rises and escapes out the top, while cool dry air is introduced at the bottom. This cyclic ventilation dissipates moisture, keeping your building dry.

3. Open drainage path: Chambered configurations with strapping or furring strips can only provide drainage and ventilation within a confined section. Rainscreen mats with a minimum 90 percent open area ensure a clear space for multi-directional drainage and drying.

4. Moisture protection: Filter fabrics with some rainscreen mats are enhanced to function like an extra layer of house wrap. They are fully tested to prevent water penetration from the outside while keeping your building dry on the inside.

5. Back-side drying: The perm rating of some filter fabrics exceeds 200. This means they promote backside drying of your exterior materials. Moisture in the façade evaporates through the fabric, allowing the wall to dry from the inside out.

6. Flat walls: Not all rainscreen products are able to resist point loads from siding attachment, resulting in a wavy façade. Most rainscreen mats offer a robust heavy-duty fabric that better distributes compression loads, ensuring a flat uniform surface results.

7. Retain space: All rainscreen materials look like they’re providing a consistent drainage path behind claddings. The truth is that many cannot hold up to the normal stresses from cladding installations. Once nails, screws or other forces are applied, they compress substantially, compromising the drainage space. New advancements in geometric configurations on these polypropylene cores better resist compression, resulting in the industry’s most rigid materials. These improvements ensure the drainage space is maintained across your wall.

8. Uniform thickness: Rainscreen mats contribute to uniform thickness in your scratch and brown coats, unlike furred or strapping systems. Variations in stucco thickness can often result in thin areas on top of the furring, which in turn are prone to vertical cracking. This is never the case with rainscreen mats.

9. Cushion: On the topic of stucco, rainscreen mat’s flexible cores also function as a cushion between your building and exterior façade. This reduces stress due to thermal movement or settlement, and has proven to reduce cracking in stucco and stone veneer systems.

10. Validation: Rainscreen mats have been a component within cladding assemblies across the U.S. for 25 years. From Boston to Seattle, San Francisco to Miami, rainscreen mats have found acceptance with some of the most recognized and distinguished building enclosure specialists in North America. Many believe that rainscreens offer such amazing redundancy and are so forgiving that every envelope consultant should make it their number-one recommendation, and that rainscreens should be the standard of care.

Rainscreen mats are endorsed by numerous industry associations, including the Stucco Resource Guide published by the Northwest Wall & Ceiling Bureau ( In both new construction and cases of cladding remediation, rainscreen drainage and ventilation mats accommodate essential criteria in today’s construction environment. Forensic evaluations have long noted compromise from moisture penetration increases significantly if water is unable to evaporate or drain, causing accelerated consequential damage.

As early as 1997, polypropylene mats were incorporated within EIFS assemblies to enhance water management and drainage. Installing the mats over a WRB followed by mechanically attached insulation resulted in the first rainscreen exterior insulation and finish systems. These initial projects have exceeded expectations during forensic evaluations — confirming the ability to manage water penetration and help eliminate subsequent considerations.

By 2001, the first designed rainscreen stucco systems were available. Simply incorporating a rainscreen mat over the weather resistive barrier(s) allowed code compliant stucco to be created that had the ability to address moisture-related issues. This was an interesting development: as rainscreen stuccos found an immediate acceptance with many enclosure specialists, the problem was subcontractor familiarity. To complement construction documents, specifications and details, “how to” videos were produced to better illustrate the configuration and design so that accurate proposals could be developed reflecting the scope of work.

Then came discussions regarding materials compatibility with surfactant-laden sidings and potential effects on some WRBs. Many design professionals found rainscreen mats to be just the right solution for this consideration. They presented a low-cost drainage and ventilation medium that also created separation. This allowed optimum performance of each individual product within the configuration. These early siding projects continue to perform admirably for all involved today.

Manufactured stone veneer is another cladding that benefits from rainscreen mats. Consider a location like the Pacific Northwest: you may have more water going in than evaporating out of a wall for six months a year. A typical scratch coat, installed over a grade “D” paper, can be like holding a wet sponge on a 60-minute-rated material.

Rainscreens creating a separation have proven to be an effective defense against premature compromise. Saturated scratch coats never even touch the weather resistive barriers.

Rainscreen mats performed so admirably over the last 20 years they’ve truly become standard practice for building scientists, envelope engineers, consultants, architectural design firms, and many of America’s largest developers, both residential and commercial.

These rainscreen mats are available in a variety of profiles thicknesses, from a nominal 1/8" to 1", depending on desired cavity space and cladding selection. Most agree a ¼" (nominal 7mm) is the minimum thickness to fully realize the advantages of drainage, capillary break, and ventilation.

These mats are equally applicable for stucco, manufactured and natural stone, fiber-cement, brick, standard lap and cedar sidings, EIFS, metal, and other wall system materials.

The rainscreen technique achieves desired results with limited additional expense. The incorporation of rainscreen can run as little as $.49 per square foot for materials. With the simplicity of installation, these mats are very affordable; they literally roll out like housewrap and hammer-staple in place. With over 200 million feet of “Waterway” currently in service and an exponential increase in demand, you can feel confident in this effective rainscreen concept.

About the Author: Dan Johnson is the general manager for STUC-O-FLEX International, Inc. They introduced “WATERWAY” Rainscreen & Ventilation Mats back in 1997. He can be reached at 800.305.1045 or