To lower the risk of wall rot, it often makes sense to provide a ventilated air gap between your siding and your sheathing
Posted on Mar 1, 2013 by Martin Holladay, GBA Advisor
UPDATED June 17, 2013 with new product information
Twenty years ago, very few residential builders knew what a rainscreen was. These days, however, it’s no longer unusual to see siding being installed on vertical furring strips or a plastic drainage mat. As rainscreens become more common, mainstream builders are beginning to ask, “What’s a rainscreen? How do I know if I need one?”
This article will pull together information to answer the most common questions about rainscreen gaps between siding and sheathing.
What’s a rainscreen?
You can’t really point to a rainscreen, because it isn’t a thing — it’s a system. That’s why I prefer to talk about a “rainscreen installation,” a “rainscreen approach,” or a “rainscreen gap.”
For most residential builders, a rainscreen siding installation is one that includes an air gap between the siding and the water-resistive barrier (the asphalt felt or housewrap).
Some purists insist that you can’t call it a rainscreen gap unless you have ventilation gaps at the top of the wall as well as the bottom of the wall. If it only has drainage gaps at the bottom, but no openings at the top, the purists insist that the system is more accurately called a “cavity wall.”
However, the distinction between a cavity wall and a rainscreen wall isn’t particularly useful — in part because the distinction is based on a faulty understanding of “pressure equalization” principles — so I side with those who use the word “rainscreen” for either type of wall.
Every well-designed rainscreen wall needs:
- a water-resistive barrier (WRB);
- an air gap between the WRB and the back of the siding;
- flashings at all penetrations and vulnerable areas; and (usually)
- weep holes at the bottom of the wall.
Ventilation openings at the top of the wall are optional.
Why would you want a rainscreen?
Rainscreen gaps help walls manage moisture. A rainscreen gap helps to dry the sheathing, which may accumulate moisture during cold weather. It also helps to dry the siding when it is soaked by rain.
These benefits are due to four functions of a rainscreen gap:
- The gap provides a capillary break between the back of the siding and the WRB. This limits wicking.
- The gap allows moisture held in the siding and sheathing to be redistributed to adjacent materials, by evaporation and diffusion, thereby limiting damage due to water that concentrates at leak points.
- The gap provides a path for liquid water to drain down to the weep holes at the bottom of the wall.
- If there are ventilation openings at the top of the gap, the rainscreen provides a path for moving ventilation air. This ventilation air rises due to the stack effect, which is strongest when sun shines on the wall. Research shows that this type of ventilation is a powerful drying mechanism.
Read the rest of the article at: http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-rainscreens