Vented Rain Screens - Mind the Gap

by ‐ Tags: alternative energy, building science, green living

A vented rain screen is a fancy way of describing a gap, or airspace, that's left between the siding and sheathing of a building.  It sounds like a simple thing--a gap--but it can have a significant impact on the durability of the structure in areas that receive significant rainfall.  For buildings with a cladding that can hold water, such as brick, it is especially important.  

One prime example of its importance comes to us from Canada.  In the late 1980's and 1990's, British Columbia was in the midsts of a building boom.  Despite being built to the building code of the time, many of these buildings ran into moisture issues that caused extensive wall and roof rot.  These buildings are usually referred to as "leaky condos," though what they are actually suffering from is premature building envelope failure.  This envelope failure resulted from the walls getting wet and not being able to adequately dry for extended periods of time.  It's estimated that about halfof the 160,000 units built during this building boom in BC are "leaky condos" and it the eventual total cost of repairs will be in the range of $3-4 billion.   

This is not the only place failures like this have occurred.  Seattle and other areas of Washington State have encountered what they call "condo rot."  And a large mid-west builder, Zaring Homes, was ushered into bankruptcy because of rotting, moldy houses.  In areas that receive a good amount of rain, installing a rain screen is an easy way to help prevent serious issues down the road.  It's not a magic bullet, but it's an important part of a "belt and suspenders" approach to keeping buildings dry.  

 

There are several ways to implement a rain screen.  There are products that can be installed or furring strips can be put on the house.  Furring strips can be either wood, plastic, or metal.  The main goal is just to have an adequate gap (usually 1/2" or greater) between the cladding and the sheathing so that air can circulate, moisture can drain, things can dry, and there's an extra barrier between rain and the important parts of the building.  

There are other products that accomplish this same effect.  These are typically either a dimpled mat, a crinkled housewrap, or a textured mesh.  

The choice of using furring strips or a specific product largely depends on budget, design limitations and if any of the products add functionality that is desirable for that project.  Overall, as long as an airspace is provided that is allowed to vent and drain, a good choice has been made.  And while installing a rain screen will add to the initial cost of the project, it is without a doubt less expensive than any remediation that would need to be done down the road as a result of moisture damage.