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?
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.
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
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
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.
remove the "not" from my address to email
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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