Spray Foam Conditioned Unvented Attic in Severe Cold Climate

12 years ago

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!!!

Comments (5)

renovator8
12 years ago
What is the specification for the foam? Is it open or closed cell? What is the density and is it closed or open cell?
energy_rater_la
12 years ago
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.
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Open cell polyurethane spray on the underside of your roof will be way below the minimum recommended R Value for new wood framed homes, according to the US Department of Energy. If you have typical 2x6" rafters, you will only be getting R20 vs. the minimum recommended R30. actually 2x6 rafters are 5.5" filling the rafter bay with 6" of open cell foam meets the catherdalized insulation standard. foam covers faces of 2x's to minimize thermal bridging. see pictures linked in my previous post. Plus, unless the job includes sealing all the openings into the attic from the exterior, you will not be getting the full benefits of the insulation. (See linked document.) document linked is promoting rigid foam sheathing to exterior of roof. OP is applying foam to interior of attic at roofline. when roofline is foamed the air and thermal barrier is now at the roofline not the attic floor. leakage sites that were problematic with insulation at attic floor are no longer sucking (negative pressure) from the conditioned space below. Only if you have those leaks sealed and use closed cell foam does spraying the underside of the roof make economic sense. open cell is used in hot humid climates. not closed cell. Unless you have HVAC in your attic, you will likely be getting a much better return by insulating the attic floor as suggested above. Unless being the operative word... 90% of homes in the south have hvac in attic. and ductwork in the attic. putting equipment and ductwork under the insulation (foam on roofline) allows equipment and ducts to perform. putting foam on attic floor still puts equipment and ducts in 130 degree attic. Spraying R20 under your roof on the Georgia shores and calling that fine and dandy? I wouldn't have anything to do with either of these contractors. If you want an impartial assessment, read the linked materials or contact an energy consultant in your area. Resnet is a good source for an independent energy rater. best of luck op
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It seems to me that you should answer two questions. Do you want conditioned space in your attic? How much money are you willing to spend? Our story: In our 1/2 story remodel we have conditioned living space. It had always been living space (at least for 70 or more years of our home's 97 years). During the depression there had been vermiculite in the space above the ceiling. In the 1950's parts of it got fiberglass bats in the space above the ceiling. (It had always been finished like an after thought, pressboard walls, and even cardboard over 1xs during the depression!) When we gutted the poorly finished space, we decided to have cathedral or loft type ceilings. Our roof does not have any vented soffits, nor a ridge vent either, so we had to have spray foam insulation in order to do the cathedral ceilings and have some kind of insulation. It did cost a small fortune (in our estimation), but it is highly effective. Between that and foam backed roof sheathing we have a cold deck roof. Unless the snow load is wet/heavy, we don't have to shovel it; the entire roof melts evenly with virtually zero icicles and no ice dams. The 1/2 story space needs very little heat, there is more sound proofing to the outside than before, and this will sound odd, but the foam literally glues your house together. Kind of hard to describe. Our space has been inspected by structural engineers, don't worry! But before the foam was sprayed you'd arrive at the top of the stairs and walking in certain spots created "creaking". After the foam was installed, no more creaking. We are very satisfied with spray foam. YMMV Others who will post here clearly prefer blown in cellulose. Since I have no experience with that type of insulation, I hope they will post about it. Hope you get the information you need and end up with a space that meets your needs.
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A couple fo things: Open cell foam. Note the word "open". It will absorb moisture vapor and hold hold that vapor. It can eventually saturate and hold liquid moisture, just like a saturated sponge. Open-cell foam is indeed classified as an air infiltration barrier. But it still needs a vapor barrier to prevent the open-cell structure from holding moisture. I use both open- and closed-cell foams. But I'd never use open-cell in a roof. Never. In my opinion, and it is my opinion, closed is the better option. If you are going to do an unvented roof, then the foam does need to be applied directly to the underside of the sheathing. To the house a a whole... When the thermal envelope of a house is tinkered with, you need to take a holistic approach. You sealed the lid. If you didn't already do so, you need to address the basement as well as the living spaces below the lid. For most houses, moisture is generated in the basement as moisture wicks through the slab and/or foundation. That moisture can work its way into the living spaces through stairways, doors, and utility penetrations in the first floor platform. That moisture, plus moisture generated in the living spaces, can work its way into the attic. In older leaky houses, or houses with vented attics, the moisture can be whisked away through air movement. That same air movement gets rid of moisture vapor, but it can kill the thermal efficiency of your envelope. You sealed the lid of your house, eliminating an escape route for that moisture vapor. Your open-cell foam may now be acting as a holding medium for that moisture. Thus the high humidity levels in your attic, as well as liquid moisture draining out of your soffits. Address moisture sources in your basement as you can. Seal penetrations in your first floor platform. Address moisture generated in the living spaces; cooktops, showers, etc. Seal penetrations in your attic floor. After a dry spell when your open-cell foam has been able to go through a drying cycle and it's no longer holding water, get a vapor barrier on that foam. An easy way might be to use a topical coating to "paint" right on the surface of the foam. The coating can act as a vapor barrier as well as an intumescent barrier.
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"But what happens when you close off the roof and soffit venting to make it unvented space, then tighten the ceiling and ducts?" you don't tighten the ceiling plane. the whole reason foam sealed attics are so efficient is because you are moving the air & thermal barrier from the attic floor to the roofline. when that happens, the many holes in the ceilings are no longer sucking air from the attic into the living space. thermal bypasses( like fireplaces that you can see into from the attic) no longer pull attic temps into the bypass. ductwork & equipment in the attic is no longer in the hottest part of the house. the average duct system has 30% leakage. why would you need to add a duct to the attic...there is plenty of leakage. 80% of homes already have oversized hvac systems...why open the door to having the units "sized" to include the attic? many hvac companies size units by rules of thumb...these aren't the companies who will actually learn to size for unvented attics...they'll just add another few tons to the house. Joe has really gone after ASHRAE's 62.2 ventilation strategy. and good for him! he also realizes that what works for cold climates (on which codes are based) does not work in hot humid climates. see for yourself: http://www.buildingscience.com/documents/bareports/ba-1309-ventilation-system-effectiveness-and-indoor-air-quality-impacts http://www.buildingscience.com/documents/bareports/ba-1310-supplemental-dehumidification-warm-humid-climates I think these are the correct links...but I can rest easy Worthy will correct me if they aren't ! LOL! "conditioned space on the cheap" just ...wow.
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live_wire_oak
12 years ago
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.
renovator8
12 years ago
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.
worthy
12 years ago
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

Spray Foam Conditioned Unvented Attic in Severe Cold Climate

Comments (5)

renovator8

energy_rater_la

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