Posted By Bigdaddygb on 14 Mar 2020 05:43 PM
Ok so I am building a new home in Macon , NC starting this year. I
am planning to build as follows. Basement= CMU with water barrier and
drain outside, inside 2 " closed cell spray foam on walls and into floor
joist area and r15 rockwool insulation in studded out wall. Slab to
have 10" of stone under it to provide and area for water to go and not
reach the slab then ( I believe ) its r16 or r25 ips foam underneath in 2
layers covered and sealed with 7mil vapor barrier then 4" of concrete.
Vapor barrier will be carried approx 18" up wall and sealed behind spray
foam on basement wall. The house is a modest rancher with approx. 3000
sq ft and exterior walls to be 2x6 construction with flash and bat of 2"
closed cell foam and R15 rockwool inside. Outside to have zip system
1-1/2" r6.6 sheathing with liquid flashing applied to all nail holes and
joints and seams. Next it will have board and batten siding on it and
Shaker shingles above trim line. Roof is going to have steel roofing
over 7/16" or 5/8" zip roofing ( haven't come to a decision yet) on a
12/12 and 6/12 pitch roofs. My question is this (and some people have
had similar situations to mine but not really exactly like mine) So I
want to enclose the envelope, my HVAC will be in my basement as most of
my ceilings are cathedral or 10 ft ceilings, So I was wondering with
either 2x8 or 2x10 rafters ( still waiting on the manufacturers to get
back about size and options from my architect) can I do a closed cell
spray foam in the rafters without a vented roof? Or do I have to have a
vented roof and block off the eves and run baffles up to ridge vent? I
planned to use an open grate hidden into the attic from a room somewhere
and use a return vent in the attic back to the HVAC from attic to
condition the space. This way it would draw either cooled or warm air in
to the attic and draw it back to the HVAC to process and run through
the house.
Any Suggestions and direction would be great.
Thanks in advance.
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I've been pondering this same issue while considering how I'm going to construct the roof.
The minimum size of the rafter is dictated by the building code, see the span chart in the code book.
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In your area (high wind/hurricane prone, low snow load, low earthquake, not extremely cold winter, hot humid summer) I think I would go at least one size larger rafter than minimum.(this is my opinion, not required) I think that is a 2 x 10 @ 16" o.c.
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There are some very specific code requirements about how to construct a vented or non-vented attic. Read here> https://codes.iccsafe.org/content/NCRC2018/chapter-8-roof-ceiling-construction#NCRC2018_Pt03_Ch08_SecR806 …...
I'm still thinking about it but this is what I'm leaning toward:>
(1.) Standing seam roof
(2.) Grace Ice Water barrier (because of high wind driven rain)
(3.) 5/8" OSB sheathing
(4a.) Air vent space between rafter, above insulation
(4b.) 6" rockwool batt between rafter
(4c.) 4" rockwool batt beneath rafter, diagonal to rafter
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The reason I want that air space is to remove moisture at the roof peak. If the rafter space is unvented and done like building code requires, the wood sheathing is enclosed between water and air barrier so it can't breathe if it gets a leak. That is a sure way to encourage mold/rot.
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I do not like foam insulation unless it is on the outside/exterior of the building envelope. How will you inspect for termites if the insulation on the interior of your basement is covered with spray foam (there is a code requirement)
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I am a retired general contractor in central NC, 45+ years of building experience. IMO, based on your local conditions, 1-Whatever insulation you choose, lean toward materials that will not absorb and hold moisture. 2-Consider a whole house dehumidifier in addition to the AC