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Tim Dittmar
Junior Member
Username: Tim_dittmar

Post Number: 12
Registered: 05-2008
Posted on Thursday, July 10, 2008 - 11:32 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Marilyn- there are ideas going around about indoor air pollution being heightened by houses being too tight- you may be heading that way from what you've described.. do you plan this attic to be heated-finished footage? In many regards, it may be wise to provide for introduced eave or soffet air to move on the underside of the roof deck and be vented(gable-wise or by ridgeventing) to the outside- windbeams above your headroom in that space would provide the framing for a ceiling ventspace for power exhaust or gablevents. Slate retains heat quite dramatically and passive venting might be quite wise- also, were the roof to leak sometime, the above would help to reduce damage from water that was slow to leave these insulated areas. Your query sounds like a multi-variable choice scenario- I'm listed in the consultant section- weekends or evenings are great..
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Marilyn Cusick-Jones
New member
Username: Marilyn

Post Number: 1
Registered: 07-2008
Posted on Wednesday, July 09, 2008 - 10:41 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

QUESTION: Closed cell or open cell insulation
affixed to underside of roof deck under SBS peel
& stick with 3/8" slate roof on top?

I need advice as to whether I
should install open cell foam insulation or closed
cell foam insulation applied to the underside of
a roof deck upon which I have installed a slate
roof.

Facts: I am building on Kiawah Island, South
Carolina (approximately 20 miles southeast of
Charleston, SC) a two story house approximately
1200 feet from the ocean. Most of the roof is
Buckingham Virginia slate with a thickness range
of 3/8" to 1/2" with slate widths of 9", 10" , 11" or
12". Each slate is 18" long with a 7" reveal. The
roof pitch is 12 in 12. The main portion covers
flat floor area of 40 feet by 32 feet. Another portion
coovers flat floor area of 28 feet by 16 feet.

The roof deck consists of #1 grade southern
yellow pine 2 X 8s milled to be tongue
and groove. IN other words the roof deck is 1 1/2"
thick by approximately 7" wide tongue and groove.

Prior to installing the slate the decking was covered
with an SBS single ply peel and stick roof with a 12
year warranty of its own.The slate was then installed
on top of the SBS material. Thus if a hurricane flipped
off shingles, the stickdown would hopefully keep the
house dry.

I plan to have a 2 ton air conditioner for the attic
only which will dehumidify and keep the attic
temperature at approximately 76 to 78 degrees.

Considerations: The closed cell foam insulation is somewhat
appealing because it adheres to the roof decking and rafters
thus providing extra strength in a hurricane. However
from reading The Slate Roof Bible and discussions with framers
in the Charleston area I am concerned that encapsulating the
wood between the airtight SBS stickdown on top and a fairly
airtight closed cell foam underneath could lead to dryrot. With
the attic being airconditioned and dehumidified I don't think
moisture caused rot would be a problem.

Open cell foam insulation has been recommended by
independent thinking persons (as opposed to closed cell
advocates) in that it would allow the wood to "breathe" and
if any moisture were available, having drier air in the attic
would hopefully take care of any moisture problem.

What is your recommendation?

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