Roof Ventilation For Foam Insulation?

mattp

I will describe this the best I can until I can get some pics. We just finished a 3 story hip roof on 4 sides that come to a point. There is a dormer on each side of the house that takes up about 80% of each side. There is currently no intake on the entire house. We removed 5 layers, resheathed over plankboards, ice barrier eaves and valleys, perma felt synthethic with black 3 tabs. My plan was to put 750 box vents in the front of each dormer (they are hips too). The homeowner told me he was going to have spray foam installed to the under side of the roof deck and there would be no insulation on the ceiling joists just the rafters. There is no real attic in this house, just a huge loft in the center of the attic with a little attic space in all four corners (will probably be converted to closets). So my question is do I need any ventilation the way this thing is going to be insulated. Thanks guys and I hope I explained this good enough for now.

SLSTech

Check out the building sciences web site for a more in depth answer - short answer --- closed cell foam installed properly works great & no ventilation is required

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genecarp

You do not need to use ventilation if you use iceynene (foam). It is a system that operates without vents. G

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SLSTech

Unless they finally got some testing done - it still not accepted by code as it is an open cell formula. If they wish to use Icenyne they have to spray a 2" lift of close cell foam in first or have foam board above the roof sheathing, then the Icenyne is sprayed in & then their moisture barrier (Granted I have seen it done many times - but they still don't have the approvals & a BI that knows his stuff can cause lots of problems)

I found the Demilic CC system the best

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mggrew

According to the 2009 IRC you can use air-impermeable (closed cell) spray foam insulation as long as you spray the correct thickness for your climate zone to eliminate the possibility of condensation. The issue you may have is covering the insulation properly for fire protection. If the attic can be used for storaage or other purposes you must cover it with a "thermal barrier" as defined in the code. If the attic can only be accessed for servicing utiltiies you are allowed to cover with a lesser "ignition barrier" as defined in the code. So you should check with you local code official to verify how thick the closed cell insulation must be in order to eliminate vents and what kind of protection you will be required to have.

Chris Johnson

Demilac has a product AG which is (As I understand) a combination of Closed and Open cell foam, again as I understand it has an r rating of 4.5 per inch as opposed to 3.8 with selection 500 product.

Should a fire rating be required as I have had on gables, there is a fire stop spray that is applied on the surface of the spray foam after it is completed, it is applied with an airless, it is as thick as molasses.

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loneframer

Without ventilation below the roof sheathing, the roofing product will fail prematurely from what I've been told.

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SLSTech

That was the original thinking, but tests have been done & manufacturers allow it now. I think shingles may get to be a degree or two warmer but that was about it. Sorry I can't find that study at the moment - it may have been linked to other posts on this subject here

FYI - the article from Building Sciences explaining everything I & the others stated above
http://www.buildingscience.com/docum...es?full_view=1

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loneframer

Would that apply to all conditions? My parents live in a Cape Cod style home with a finished attic. There is about a 5' section on the back of the roof where the insulation touches the roof deck and it is noticably more deteriorated than the rest of the roof which is ventilated. The roof is overdue for replacement, but that area definitely shows more wear than the rest.

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SLSTech

No, only CC foam is allowed for this. If this is fiberglass directly against the roof, if actually is trapping & holding more heat against the roof area. The orientation, installation, and many other factors could be contributing to this also. When you redo this section I would remove the sheathing, fill the cavity with some higher R Value insulation while allowing for some room to install some baffles, resheathe & roof

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