Spray Foam Insulation

When I watch some home construction and repair shows, I often see them use open and closed cell spray foam insulation. Often the spray the foam in the attic between the rafters and in direct contact with the roof. Does the foam preclude the need for the roof to breathe? When the attic is insulated this way, is the air flow through the eaves intentionally blocked?
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I wonder, I think the shingles would get hotter and fail quicker, I saw it happen with an undervented ceiling to roof design. And when you get a leak will you ever be able to notice it before rott starts, the foam might allow the water to be hidden and run to the outside so its not noticed. Its just another idea that people will pay a fortune to fix in 50 years, but who cares seems to be the attitude everywhere.
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On 1/30/2012 9:57 AM, mcp6453 wrote:

In the last year, I've worked on two house renovations where this was used. They used open cell on the walls and closed cell on the basement ceilings and attic rafters. This stuff will really make any new wiring or plumbing a real chore. It also effectively makes the attic space part of the heated envelope. Because it seals the house so tightly, they had to install a whole house ventilation system so the house could "breath". I'm not sure what the energy savings will be, but my initial thinking is that it can't be good for indoor air pollution
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Depends on the foam. There are various types of "Green" friendly foam which have no harmful effects.
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wrote:

Except sealing the house too well. Even with no offgassing from the foam, a too-tight house has air quality problems.
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The theory is to seal the entire attic to make it the same temperature as the house. Apparently, this "Hot Roof" system is predominantly being used in the warmer climate states. Tests have indicated when outside temps reach above 100, the attic is still 70 to 80. The theory claims by blocking the ventilation, it will not be subject to drastic ambient temperature changes as it remains mostly constant with the inside temp of the home. Therefore, no moisture will develop to cause mold. Also, the foam adheres to the wood and anything it makes contact with, becoming one whole object, so to speak and also serves as a vapor barrier. Thus, moisture cannot penetrate it.
Close sell foam is mostly used in application which may encounter water buildup, such as flood zones and lower grading along with tighter areas. Close cell has a very low permeance rate (vapor perm rating) of about 1 for 2" (think of the 1 as a drop of water the size of a pin head). Open cell has a higher permeance rate which is about 10 for 5". The confusion is what to use under the roof deck. Many use closed cell due to it's higher R-value (about 6 to 6.5 per inch) and will divert water if there is a roof leak. Open cell has a lower R-value (about 3.5 to 4 per inch) and due to it's permeance, will allow water to pass, or in other words, will open when wet, but reseal when it fully dries. This can be a good option to find a roof leak.
As for shingle life, it's indicated not to have any bearing on the shingle life but I haven't read anywhere of a long term study.
Open cell foam is sprayed using water while closed cell is used with a spraying agent, though, can be also used with water but often isn't. Open cell foam is more Green friendly than closed.
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On 1/31/2012 6:08 PM, Justin Time wrote:

BUT, having said all that, OPEN cell foam should NEVER be used on an exterior wall, so therefore is basically only good for sound deadening applications on interior walls and floors. Always used closed cell foam on exterior walls. And it doesn't affect the shingle life at all. That's a wives tale.
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mcp6453 wrote:

I agree that it makes no sense to apply spray-foam insulation to the underside of the roof decking above an attic space that is not part of the household environment or habbitable envelope.
The attic is open space that's directly connected (relatively speaking) to the outside with an unobstructed air pathway (roof vents, ridge vents, soffet vents, etc) -> or at least it should be. But because of the need to keep out small animals, insects like bees, etc, the attic space is not afforded a large open aperture to allow for free air movement - so it will get hotter in summer (and generally will also be warmer in winter) compared to ambient outdoor temperature.
Hence the application of spray foam to the underside of the deck is a waste, and very likely does lead to hotter shingle temperature and with that will be a decreased shingle life span.
Justin Time wrote:

I think you mean "by insulating the underside of the deck" - which prevents deck ventilation,

That doesn't make sense. Any moisture in the attic will surely "boil" away when the attic air temp is 100f more readily than when it's 70 or 80f. I don't think that keeping the attic air temp at a more mild 70 or 80f in the summer (and probably 30 to 40f in winter) will be good at combatting mold. Quite the opposite infact.
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