Permeability of ASTM D 3909 cap sheet; needing roof to dry to outside
mikkelsen
| Posted in General Questions on
4″ of 2 lb closed cell spray foam will go on underside of roof sheathing….so roof needs to dry to outside; hence why I am using #30 felt/tar paper instead of a non-permeable ice & water shield.
However, to get my Class A fire rated system, I need to use an ASTM D 3909 cap sheet over the tar paper. How well is this system going to be able to dry to the outside and will there be any issues with this system?
The roofing material is a synthetic shake; Bellaforte by DaVinci.
Thanks-Eric
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What makes you think the roof will dry to the outside by using tar paper versus ice and water? Research is proving this is not happening. The roofs are rotting out.
There are many articles here and else where describing open cell and closed cell foam sprayed to undersides of roofing rotting out. These assembly's are built as you described. I read a recent post where a professional roofer is recommending to maintain the 1 inch air gap to avoid failure since the previous rule provided by spfi manufactures is proving to back fire on roofers and builders who swear by foam.
The comments here are valuable;
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/75042/Will-Open-Cell-Spray-Foam-Insulation-Really-Rot-Your-Roof
Eric,
Most roofs aren't able to dry to the outside. Usually, that's not a problem.
Richard's recommended approach -- including a ventilation channel under the roof sheathing -- is the most conservative approach, but it isn't the only approach. Most unvented roof assemblies with closed-cell spray foam are doing just fine.
The reports on the performance of unvented roof assemblies with open-cell spray foam are more mixed; that's where most of the damp sheathing problems are occurring.
Thanks for the responses. Richard, the link you referred me to seemed to advocate that this was an acceptable roofing system more than not. I guess my question is still the same...is there a ASTM D 3909 cap sheet that would work better than others in this application? Most builders are using ice & water shield over the sheathing with foam underneath. Should I just add the Ice & water Shield if I'm going to use a 3909 cap sheet for the fire rating? Would the cap sheet negate what I'm trying to accomplish using the #30 tar paper?
At 2" most closed cell foam is still more vapor permeable than the kraft-facers on batts, which is a sufficient amount of drying capacity in most climates.
At 4" it's still 2x more vapor permeable than a layup of #30 felt + asphalt shingles- the tight end of class-II vapor retardency, but still on a true vapor barrier.
I guess I need to decide if I'm going to get any benefit from using #30 felt or if I might as well use ice and water shield and have it be more thoroughly dried in?
also...I'm not required to get a class A fire rated roof but I do have some large ponderosa pines within 40' or so. and so it seemed a highly rated fire roof would be a good idea. Is this rating overkill?
so some options I'm thinking about are:
1. go for the class A fire rating using the 3909 cap sheet...with #30 felt or Ice & water shield?
2. forget the class a fire rating and just use the #30 felt under the synthetic shake.
Thoughts?
Eric,
Only you can decide whether a more fire-resistant roof is worth the additional investment.
Are there any answers to this question: Am I going to get any benefit from using #30 felt when there is a 3909 cap sheet over it or should l just use ice and water shield and have it be more thoroughly dried in? Would the cap sheet negate what I'm trying to accomplish using the #30 tar paper?
Is there a ASTM D 3909 cap sheet that would work better than others in this application?
Can anyone answer these questions?
Eric,
It's unusual to use an ASTM D 3909 cap sheet on a sloped roof assembly with synthetic shake roofing. Usually this type of cap sheet is specificed for low-slope commercial roofs.
With most low-slope commercial roofs, there is no drying to the exterior, so the permeance of the cap sheet is irrelevant.
If you want to install closed-cell spray polyurethane foam on the underside of your roof sheathing, and you also want to install a cap sheet on top of the roof sheathing, your roof sheathing isn't going to dry in either direction, unless some manufacturer makes a high-permeance cap sheet -- and I don't know of one.
So if you want your roof sheathing to dry in at least one direction, you're going to either need to switch to open-cell spray foam on the underside of your roof sheathing, or you are going to have to omit the cap sheet.
Thanks. I just talked to the synthetic shake manufacturer...the material just missed the class A rating by itself without the cap sheet. Since it is not required here and I have a steep pitch roof to help kick embers off and since it comes close to the class A rating anyways, I'm going to skip the cap sheet.
Cheers to those who contributed to these questions. thank you
I would go with the ice & water shield directly onto the roof sheathing and place the finished roofing material over that. It's almost pointless putting tar/building paper underneath a roofing material like a standing seam metal roof because it can't dry to the outside anyway so might as well make it waterproof with ice & water peel & stick membrane.
Closed cell is better than open cell on the underside.