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Forums > Green Building Technologies > Structural Insulated Panels (SIPs) > Subject: Type of Underlayment Over Roof SIPs

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eastsoundUser is Offline
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Posts:15
03/11/2008 12:51 PM  

My project is a timber frame that has 1x8 T&G Doug Fir over the roof timbers.  Since the project needed to go through the winter before being enclosed with polyurethane SIPs, I attached a layer of non-vapor permeable, waterproof synthetic underlayment on top of the T&G.  In addition to waterproofing the exterior of the T&G, I wanted to protect the interior skin of the SIPs from interior (vapor) moisture once the structure is enclosed.

I am about to have the roof PU SIPs attached on top of the existing underlayment.  Aside from roofing felt, which has a vapor permeability rating of 5+, is it apporpriate to use a synthetic underlayment that has no vapor permeability?  If I did so, what is the possibility I would trap vapor between the lower and upper sheets of non-permeable underlayment.

Advice and comments are eagerly welcomed.
lambabbeyUser is Offline
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Posts:71
03/17/2008 11:22 PM  
I'm a novice here, but I've been looking at underlay treatments to go between my roof SIPs and the metal roofing directly above it.   From the digging around that I've done, the two best products I've been able to find on the market for my needs are the following:

Delta-Trela

RoofTopGuard II


These may not be ideal for your particular application, but I urge you to check them out.   I don't know enough about the difference between the problems trying to avoid and the one you're trying to avoid to know whether these products could be used for both.

Personally, I'm leaning toward the Delta-Trela.

Cheers,


John
John A Gasbarre
Lamb Abbey Orchards
PO Box 623
Union, ME 04863
E-mail: orchard@lambabbey.com
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Forums > Green Building Technologies > Structural Insulated Panels (SIPs) > Type of Underlayment Over Roof SIPs


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