Replies

1. GBA Editor
   Martin Holladay | Sep 08, 2016 03:44pm | #1

   Douglas,
   You wrote that you were "caught off guard that there might circumstances where a rain screen in not considered necessary."

   I guess this discussion hinges around the definition of "necessary."

   In most areas of the U.S., building codes do not require rainscreen gaps. If "necessary" means "code-required," then rainscreen gaps are unnecessary.

   I've heard the "20 inches of rain annually" rule of thumb before. It's just a rule of thumb.

   Ultimately, whether or not to include a rainscreen gap is a judgment call. For more information, see All About Rainscreens.

2. 
   Doug_Epperly | Sep 08, 2016 04:13pm | #2

   Martin,

   Thanks for the response. I said "caught of guard" mostly because of my own ignorance. Planning this house over the last year, I had come to think of a rain screen as a fairly common sense and valuable addition to the wall assembly. I was only surprised because I had not really thought about different rainfall levels and how those might factor into need for a rain screen. I'm still very much learning about building science.

   I have previously read All About Rainscreens, and appreciate the insights it provided.

   So, if you were building a home in an area of Montana that has less than 14 inches of annual rain fall, what do you think you would do?

   Best wishes, Doug

3. GBA Editor
   Martin Holladay | Sep 08, 2016 04:30pm | #3

   Douglas,
   I'm a fan of rainscreen gaps, so if I could possibly afford the gap, I would provide it.

4. Expert Member
   MALCOLM TAYLOR | Sep 08, 2016 04:36pm | #4

   Douglas,
   As Dana has pointed out in some past threads, annual rainfall can be a bit of a misleading measure by which to assess the demands a climate may make of building assemblies. There are a lot of places with higher rates that the Pacific North-West, but it's the way the rain comes down, as a constant drizzle, rather than a one-in-a-while deluge, that makes rain-screen assemblies here so useful.

   In a lot of regions which experience low rainfall there are probably sites with micro-climates where rain screens are very beneficial. North facing walls with overhanging trees, houses near streams, or more generally houses with wall assemblies that may be susceptible to damp sheathing due to levels of insulation or the drying path chosen.

   "Necessary" I would take to mean houses where omitting it would lead to pre-mature failure of the building assemblies. But (beyond perhaps in wildfire country) I can't imagine a house where including a rain screen wouldn't be beneficial to both the underlying wall and the cladding. Considering how little additional effort or expense is involved, it seems like a good idea everywhere.

5. Expert Member
   Dana Dorsett | Sep 08, 2016 05:16pm | #5

   Most of MT is pretty high & dry, and mostly US zones 6B. Independently of the exterior moisture drive issues, in a zone 6 location rainscreen adds a lot of resilience from wintertime moisture drives from the interior (from vapor diffusion, or incidental air leaks), for wood sheathing frame construction, by given the sheathing a good drying path toward the exterior. With an air tight moderately vapor retardent interior you don't absolutely NEED a rainscreen, but it's pretty cheap insurance.

   Things that mitigate exterior moisture drives are deep roof overhangs and modest average wind speeds, both of which limit the amount of direct wetting the wall would see in a year. Siding type also makes a difference- cedar shakes are fairly self-venting and can dry reasonably quickly without a rainscreen, stucco or masonry absorb rain and dew, and really need a rainscreen in almost any climate. (The cavity between the brick and sheathing in brick veneer construction is a rainscreen.) Vinyl or aluminum siding is inherently back ventilated & moisture tolerant, and building in a rainscreen would not provide much additional benefit.

   Even with a rainscreen the type of sheathing also makes a difference. OSB and plywood are fairly vapor tight at about 1 perm when dry, and can still be under 10 perms at moisture levels that would support mold. Exterior grade gypsum board and asphalted fiberboard are both fairly moisture tolerant, and more vapor open.

   The stack up also matters. If you have foil faced foam on the exterior of the structural sheathing, the rainscreen really only affects the drying of the siding, and if the siding is moisture tolerant it can be somewhat moot, though the capillary break provided by an air gap is still useful for houses where there is direct wetting from wind-driven rain, or even chronic heavy dew wetting, to keep that moisture from wicking inward through seams in the foam, etc.

   So...

   ... what other features were you considering including in your wall design?

6. 
   Doug_Epperly | Sep 08, 2016 05:52pm | #6

   Martin and Malcolm,

   Thank you for sharing your thoughts. They are very helpful.

   Dana,

   Thanks for your thoughts as well. Our current wall assembly is (from outside in): LP Smartside, rainscreen (specific type not yet selected), WRB (leaning towards Blueskin), plywood, dense pack double stud wall w/ a total 12 inch cavity, and drywall. Intent is to eliminate as much air leakage as possible in the double stud wall.

   Doug

7. Expert Member
   Dana Dorsett | Sep 09, 2016 06:13pm | #7

   In zone 5 you could technically get away with just the rainscreen, but in zone 6 you'll need a class-II vapor retarder on the interior side (or a "smart" vapor retarder), even if it's air tight. The cheapest solution is usually half-perm paint aka "vapor barrier latex primer", which reduces the vapor permeance of the paint layer to something less than 1 perm, but that also limits drying toward the interior.

   A more resilient solution is to use a smart vapor retarder, such as 2-mil nylon (Certainteed MemBrain) under the drywall. The nylon vapor retarder stays under 1-perm whenever the air in the cavity next to the sheet has a low relative humidity, say, 35% or less, which it will all winter, since it's dew point (a measure of the absolute humidity) will track the temperature of the sheathing, whereas the temperature of the cavity air near the nylon will be nearly room temperature. So when it's say +15F outside, the sheathing will be less than 20F, lowering the dew point of the cavity air down to 20F. In a 68F room the sheet nylon and the air next to it will still be above 65F. At a dew point of 20F the relative humidity of 65F air is 16-17%, WELL below where the permeance of 2 mil nylon is below 1 perm. See:

   http://www.naturalspacesdomes.com/dome_store/dome_insulation_systems/images/Membrain3.jpg

   But if there is ever enough moisture in the cavity to support mold growth, the RH is much higher, and the limiting factor of drying toward the interior becomes the vapor retardency of the paint- about 3-5 perms for a couple coats of interior latex on top of a latex primer, which is an order of magnitude faster than drying through half-perm paint.

   There are other smart vapor retarders out there as well, with somewhat different curves, but the principle is the same. MemBrain just happens to be easier to find (you can even order it online through Lowes or Home Depot now), and cheaper than some others. The material cost of 2-mil would be between 10-15 cents per square foot, depending on local pricing, compared to ~5 cents per square foot for VB latex primer. Whether it's worth the upcharge from vapor barrier latex is really your call. Either would meet code, but the broadsheet nylon can enhance air tightness (if detailed as an air barrier), without reducing drying rates the way 4-mil polyethylene would.

   The rainscreen isn't a product with a "type", it is defined by the assembly. A vented air gap between the siding and the next layer meets that definition.