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Spray Foam Conditioned Unvented Attic in Severe Cold Climate
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Posted by northernmama (My Page) on Sat, Feb 4, 12 at 11:34
| Someone please help me! Plan to finish & condition our 1930s attic space into bedrooms, bath, and hobby room. We live in north central Montana, where winters are severe. I have people telling me it is fine to use spray foam to completely seal off the space from the outside (we are not re-doing the roof, which is shingled). My gut tells me that the house needs to breathe and I'm very scared about letting someone seal it off. Can anyone tell me if this has proven to be okay to do? All the research that's out there is very confusing, and I can't find anything related to our particular climate. We're short on vertical space up there, so the spray foam holds advantages from that point of view. House is wood frame/brick exterior. Thanks so much for any input!!! |
Follow-Up Postings:
RE: Spray Foam Conditioned Unvented Attic in Severe Cold Climate
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| What is the specification for the foam? Is it open or closed cell? What is the density and is it closed or open cell? |
RE: Spray Foam Conditioned Unvented Attic in Severe Cold Climate
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| http://www.buildingscience.com/documents/guides-and-manuals/gm-spray-foam-guide/view one article that is definately worth the time to read. best of luck. |
RE: Spray Foam Conditioned Unvented Attic in Severe Cold Climate
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| If it's tight up there already, you probably do not really have enough room to achieve the R values needed in your climate, even with foam. You'll need about 10" of closed cell foam (EXPENSIVE) to achieve the recommended R60 for your climate. And you'll need to have 7' head height after it's all done. Plus, are the ceiling joists of your existing home of sufficient size for a live load? Many times they will need to be larger/taller in order to provide the correct support for living space instead of storage space. That also cuts into your required headroom. You might be better off design wise and insulation wise to just yank the roof and add a second floor. And that's pretty expensive too, so at that point, an addition may make more sense as a less expensive choice. You need the input of a trusted contractor in your area to fully explore the building requirements vs. costs vs. achieved benefits of any of these projects. |
RE: Spray Foam Conditioned Unvented Attic in Severe Cold Climate
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| The concept of allowing a house to "breathe" is often misunderstood. The concept is really about the movement of moisture out of cavities. The reason to ventilate a roof rafter cavity is to allow moisture to escape if it gets into that space. When the rafter cavity is filled with closed-cell foam it is difficult for water to get into it but to be safe I like to see the roof sheathing covered with Grace Ice & Water Shield and new roofing installed. |
RE: Spray Foam Conditioned Unvented Attic in Severe Cold Climate
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| Since you're planning to use a height-challenged attic as living space and you don't want to touch the roof, you've limited your choices. The ceiling becomes a cathedral ceiling and the best way to reach Code R levels is high- density closed-cell foam followed by one-two inches of extruded polystyrene foam board. But with typical 2"x6" roof rafters (the sloped boards), you'll still be below recommended insulation levels. Not to mention problems pointed out by other posters: limited headroom, possible leakage from the roof, inadequate ceiling joists that will become floor joists. In addition to the excellent article linked by energy rater, you might want to look at further items at the Building Science Corp. site: Unvented Roof Assemblies for All Climates, Building Profile: Very Cold Climate. According to the Canada Housing and Mortgage Corporation, recent research indicates there's not much difference between conditions in vented and unvented attics. |
Here is a link that might be useful: How Much Insulation is Needed
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