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grover
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# Posted: 11 Sep 2014 02:14pm
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My roof is OSB, then felt paper, then metal. The spray foam company said they always do 6 inches of open cell spray foam on the underside of the roof and that makes any attic area you have a "conditioned space". Doesn't sound like enough R value but they say it is air tight and should be no problem heating and cooling. (The way I understand it, it is air tight but water vapor can still penetrate it since it is open cell) Then I ran across an article that described a roof tear off on a house that was spray foamed like I have described. The OSB was basically rotting. Anyway, that picture has scared me and I trying to see what you all think about spray foam. Is it a great solution to drafty homes or is it a problem waiting to happen
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Wilbour
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# Posted: 11 Sep 2014 02:37pm
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It was suggested that I do foam but I am still going to do the styrene form from bottom to top and fiberglass pink. With vapor barrier
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Wilbour
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# Posted: 11 Sep 2014 02:39pm
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Humidity should never reach the insulation but if it does you want it blown away asap especially with osb
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bldginsp
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# Posted: 11 Sep 2014 03:37pm
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I've seen closed cel used in this kind of situation, the idea being that it provides a moisture barrier. Don't know why open cel would be used on a roof- I thought it was used mostly in walls. But I don't have long term experience with any of these spray foams
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razmichael
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# Posted: 11 Sep 2014 05:45pm
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I went with a "hot roof" design using closed cell spray foam ( R36). Lots of information on a hot roof design if you google it. Not everyone supports it and there is a dependency on what climate zone you are in. The basic key is to have the sheathing and roof completely isolated from the interior and with enough insulation to prevent the condensation point from being into the sheathing. With Standard insulation and vapour barrier it is very hard to ensure this as any hole with allow the warm air to get into the insulation. I'm no expert but all the research I found discussed closed cell to act as the vapour barrier and moisture barrier (and it has a higher R value per inch) rather than open cell.
From one site:
"There are 2 Types of Spray Foam insulation, Open and Closed Cell. Open Cell is mainly used as an air barrier but closed cell is an Air, Moisture and Vapor barrier."
Keep in mind, as I stated at the start, not everyone is comfortable with a hot roof. Some links:
Hot Roof
Building Science - note that this site may have other references as well.
R value - will obviously vary by the actual manufacturer but this shows the large difference between open and closed.
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MtnDon
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# Posted: 11 Sep 2014 07:53pm - Edited by: MtnDon
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article on the topic. I had issues with the doc displaying in my browser, but it saved to the hard drive okay.
Longish, but probaly a worthwhile time investment.
I didn't check on what the r-value of 6" of open cell foam would be. Check what minimum the energy code calls for where you are located.
FWIW, I like the idea of a closed cell foam installation when someone wants a cathedral ceiling. Other than that I'd go for a conditioned duct work space in a home that used an air distribution system for heating and/or cooling. All the insulation is applied on top of that space and there would be an attic above. A 'regular' attic with good ventilation. But that won't work over cathedral ceilings.
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Tarmetto
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# Posted: 11 Sep 2014 08:14pm
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I haven't researched this idea, but my plan was to sheath the roof in 1/2" OSB (standard batt insulation underneath), then horizontal 2 x 4 battens 24" OC to screw the metal roof panels to...but 1.5" foam board between the battens (on top of the OSB). Not sure at all if this would be a good plan...but that's what I plan to research. I figured to use some sort of wide seal tape to cover the battens and lap over the flush foam board.
Does anybody see a problem with this plan? Maybe tar paper on top of all of that?
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Don_P
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# Posted: 11 Sep 2014 10:36pm
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Yes, the fiberglass insulation needs to have a ventilated air space above it. The foam above the sheathing would be on the outside of the envelope, basically just hanging out outdoors.
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grover
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# Posted: 13 Sep 2014 12:00am
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This is the article that I was referring to.
http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp- roof-sheathing?page=1
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Em Ty
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# Posted: 13 Sep 2014 10:04am
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I took building science in school and wanted to stay in that field after graduation, though I've taken a round-about path. I now do mould remediation and I see a lot of attics and a lack of sufficient ventilation is the biggest issue I see with conventional attics, followed by incomplete vapour barriers.
Our prof was, and is, well known for his research in building science and he was a firm believer in 'hot roof' construction. It's a better option than conventional construction and is used in just about every commercial and industrial building with a conditioned space. The reason it wasn't used on residential construction when I was in school was because it wasn't accepted unless it was approved by an engineer, and residential homes generally didn't warrant insulation re-designs by engineers. I know it's gained recognition over the years, but you may still have to have it approved by an engineer for your particular building department's approval. Civil engineers make glaciers seem hasty at times.
Having studied it and having seen hundreds of attics a year, it's the only roof insulation approach that I'd use.
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morock
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# Posted: 13 Sep 2014 10:34am
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I agree, "hot roof" (closed cell foam) is the best way to go. You don't really need 6 inches of foam either. The major benefit is the 100% air barrier it provides. I have worked jobs in old houses with real 2x4 roof constuction in livable attic space, it worked amazingly well.....in Canada. The major downfall is you really need to inspect and keep an eye on your roof as the foam will never alert you to a leak and the structure will rot out before you know it.
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Em Ty
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# Posted: 13 Sep 2014 12:09pm
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You won't need 6 inches of foam, but you do need to make sure that the dew point occurs in the closed-cell foam layer at your design worst-case scenario. Usually 2" is the accepted minimum, but I'd feel better with 3" in many places in Canada. You can also look into using a mixture of sprayfoam and the XPS foam sheet insulation and sealing the voids with cans of expanding foam.
If you're building to code you'll need R50 in the roof, which would require 8" of sprayfoam or 10-12" of other insulation. The main paths of heat loss are due to air leaks and windows. The best approach is to try to totally seal the structure and that will give you the best return on your money. Windows are like thermal holes in the wall and spending lots of money on slightly higher R-values isn't going to yield much better results in your heat loss. If your wall is R20, even a high R-value window with R7 is going to lose a lot more heat than the surrounding wall, so smaller is better if your goal is to reduce heat-loss.
If you have a very tight structure, you can tolerate more window loss and even get by with less insulation, especially for a small cabin.
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SonnyB
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# Posted: 26 Sep 2014 11:17am
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I just finished, last Oct., my main home and spray foamed it completely. I have been very happy with the results! My home is roughly 3800 sq ft on two floors with a 4 ton and 3 ton hvac units and my highest electrical bill has been $139! I keep the home at 73 degrees. I am going to spray foam my cabin also. I love the stuff!!
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jameswhitsett
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# Posted: 30 Sep 2014 08:29am
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Insulation is done to save substantial amount of energy while heating and cooling. We have a 30'x50' steel building denver and we were completely irritated due to growing molds then our building professional tell us to use spray foam insulation. We had some roof leaks due to drilling of screws which are sorted after using this insulation scheme.
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