I’m working on an addition at my bosses house. We are putting cedar lap siding over tyvek. I have heard that water reacts with the tannins in cedar allowing it to pass through tyvek, this is a concern because of the lacation of his house, it’s on a mountainside in alaska where they regularly get 100 mph winds and occasionaly up to 125mph, so no matter what you do your going to get some water behind the siding. When I told my boss about the tyvek-cedar thing, his fix was to put a layer of 15# felt paper over the tyvek as a barrier between that and the cedar. I was a little sceptical of this plan but did’nt have any good reasons to dispute it, which is my reason for writing this, Does any have any advise on this issue?
Thanks Josh.
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I think there's something about this on the Cedar Bureau site, and I even think they recommend felt over tyvek if you feel you have to use tyvek in the first place. Whether or not tyvek has any value with felt over it is up to debate. Anyway, the felt's a good idea under cedar, but those 125 mph winds sound like a drag.
Was the cedar backprimed?
If cedar is properly backprimed...not a washcoat...you can apply it directly over a spun-poly housewrap.
Otherwise, you're correct, you don't want cedar in direct contact with housewraps.
If no backpriming, plain felt would be a better choice than tyvek.
In your climate, a rain screen would have been a nice detail as well. With that you could have used either felt or spun-poly.
Cedar, and some other woods have water soluble extractants that, when they come in contact with spun-poly housewraps, they destroy the micropore structure of the housewrap. Instead of having a housewrap the sheds water droplets but allows moisture vapor to pass through, you end up with something similar to a paper towel. It turns to mush when wet and holds water Not the properties you want in a housewrap.
Guys,
There is a spun bond Poly Prop highly permeable building wrap called Roofshield/wallshield that can have Cedar shingles installed direct onto the membrane. Roofshield has a perm rating of over 200 and the nature of the membrane being spun bond can cope with the hundreds of fixings applied through the membrane. Roofshield has also under gone testing and can cope with the cedar oils leaching out onto the membrane.
Hope that helps
Dave, thanks, I hadn't heard of that product before.
What you are thinking of has an universall name of Vaporshield. http://www.vaporshield.com. 1-866-731-7663.
I asked the company rep the differnce between the wall and roof versions and he said they are the same thing, I guess it just depends on what app you use. Its perm rating or breathability is 212 perms. For comparrison #15 felt is 8 perms,Tyvek 28 perms, Ic& water shield .05 perms.
I asked the rep if this stuff was like a gortex jacket for your house, and he said that was a very good analogy.
Beacone sales is going to start carrying the stuff any day now(the container is en route) and they told me it will be abought .47c a sq ft. Rolls are 59"W x 164' L. There also flashing kits available.
If you used this stuff in conjunction with home slicker, aka ceader breather, and dipped sidding you woud have a top of the line installation. IMHO