How to select the drainable WRB with the right amount of vapor transfer?
In another thread of mine, Ben Obdyke’s Hydrogap and Tyvek Commerical Wrap D have been mentioned. I had originally focused on using Hydrogap as my WRB and drainage enhancer but now I am curious about how it compares to Commercial Wrap D, especially with vapor transfer.
I compared the specs from each product as published by the manufacturers and found it interesting that according to ASTM E2273, Commerical D drains more water than the Hydrogap (98% vs. 96% respectively). It was also interesting that according to ASTM E96, Commerical D has a much higher moisture vapor transmission rate than Hydrogap (30 perms vs 16 perms respectively).
I’m building in zone 4a and plan to use drywall, 2×6, fiberglass insulation, OSB or Zip + drainable WRB for my walls…should I look for a higher or lower vapor transfer rate with regard to the WRB?
Thanks!
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BrunoF,
I think the answer in most cases is it doesn't matter much. Other materials in the wall assembly will be the limiting factors in how vapour-open it will be. The one situation where a lower perm WRB is useful is if you have inward vapour drive - usually caused by reservoir cladding. Then being less vapour-open can limit the amount of moisture that might get driven into the wall.
I prefer to consider the WRB I’ll be using on a particular wall assembly depending on the cladding I’ll be using, and I prefer to choose WRB with the largest gap to create a capillary break. The wetter your CZ is, the wider the gap, so I tend to select 3D WRBs vs. sheet/roll drain materials, as you plan to select.
Researchers at BSC and RDH, among others, show that a minimum drainage gap should be greater than 3/8”. Also, where you live, the rainfall amount per year, or if you are in a coastal climate may dictate even wider gaps. Attached is a good article from BSC/VC Test Hut with really good information.
Thx folks. In my case I am going to use lapped fiber cement and perhaps some board and baton looking fiber cement for one area.
You have to remember that if you do a swell job on the sheathing, you've essentially got a shell of building that has a permeability of between 1 and 5 at most. OSB's permeability is pretty low, and it's been shown that the diffusion of vapor into them carries much less water than air leaks into the cavity. So while the tyvek may allow more vapor in, it also allows more vapor out. The OSB will stop it from entering your house, but it's nice to have a way for it to escape.
My train of thought has developed into letting the sheathing dry to the outside as much as possible, because it probably wont dry that much to the interior down the road. Do a good job on the taping and fenestrations, and do a good job detailing the air/wrb (tyvek) and you wont have problems.
The paint and caulking on your fiber cement siding is going to do 98% of the leg work here as it is. The flashing will divert the rest. The Tyvek will practically be a secondary air barrier after the siding goes up, and the sheathing the primary.
Knowing you are going to use fiber cement board, I'm pretty sure most of us here on the GBA, when it comes to best installation instructions, we all recommend using of 1x4 rainscreen.
If I can make a suggestion, use 1" R5 taped rigid foam on top of the WRB (like the ones you are planning on), and if you used the approved tape with the rigid foam, it becomes a 2nd WRB. Then I would install the 1x4 rainscreens. This is my preferred installation method. See the attachment I gave you before, its all there.
I have read everything I can get my hands on which has led me to believe that air sealing is the most important thing to tackle behind simple bulk water management. This works well since I can put in some sweat equity taping seams, ensuring gaskets are used, etc. I don't have an unlimited budget so I am also approaching some elements with a "pretty good house" / "good enough beyond code" concept. For that reason I need to stick to materials and techniques that the average crew can handle properly and it seems so far that the concept of a true rain screen is going to cause all kinds of headaches and added cost and isn't a part of the project that I can reliably do myself. I hope that using a drainable housewrap will be good enough to manage bulk water and let the wall dry out in certain times of the year and dry in in other times (central NC, zone 4a) hence my focus on the vapor transfer rates of the drainable WRB.
Hope that helps clairfy some of the additional details and thanks for the document and feedback so far!