Spray foam against plywood roof decking questions
I currently have the following roofing system and am located in Northern WV in climate zone 5. Fabral standing seams metal roofing, tar paper and then plywood on 2′ center bonus room trusses. I am getting quotes on insulating the underside of the roof decking with spray foam. The HVAC units and ductwork are located in the attic space beside the bonus room. Here are my questions and concerns.
1) I have a vented soffit and vented ridge vent at the moment and should I put up something to keep the air flow before spraying the foam or create a unvented attic space instead with the foam?
2) Should I go with open or closed cell against the roof decking due to vapor barriers and moisture buildup and concerns.
3) What R value/Inches of foam should be on the attic. I have had to people out here to give me a quote and they both seem to want to skimp on the foam and only do around 20 R value but both are using products from LaPolla.
I can’t go back and put insulation under the standing seam so this all needs to be done correctly from the inside and any suggestions would be great. Please let me know if there is any other information I should provide or take a look at. I have read many posts but wanted to make sure since there is a lot of mis-information out there.
Thanks
Brian Mallow
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Brian,
Q. "I have a vented soffit and vented ridge vent at the moment and should I put up something to keep the air flow before spraying the foam or create a unvented attic space instead with the foam?"
A. Either approach can work, as long as you get the details right. If you include a vent channel, it will be easier to replace any rotten roof sheathing in the future. For more information on the pluses and minuses of each approach, see How to Build an Insulated Cathedral Ceiling.
Q. "Should I go with open or closed-cell against the roof decking due to vapor barriers and moisture buildup and concerns?"
A. Again, either product can work, as long as you get the details right; but there is a fair amount of evidence than closed-cell spray foam is less risky than open-cell spray foam. For more information on this issue, see:
Open-Cell Spray Foam and Damp Roof Sheathing
Does Open-Cell Spray Foam Really Rot Roofs?
Q. "I have had two people out here to give me a quote and they both seem to want to skimp on the foam and only do around R-20."
A. You are right to be suspicious of any contractor who urges you to install less than the minimum code requirement for R-value in your climate zone. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.
Martin, Thanks for the quick responses. It seems I have two choices from the looks of it. Either vented or non vented with closed cell and an R value of at least 38. I try to build/do everything by myself but sometimes I have to rely on contractors and like to double check the information they are trying to feed me.
Thanks Again