On 1/31/2012 6:08 PM, Justin Time wrote:
wrote in message
...
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?

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.



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.



--
Steve Barker
remove the "not" from my address to email