[Greenbuilding] Polyiso strength on roof

Steve Houlihan sho11 at comcast.net
Sun Dec 7 16:16:38 CST 2014


What about the sealed attic with spray foam from under the rafters that 
is being promoted so much lately.  How do you protect that? I've heard 
of some spray-on intumescent paint but I don't see it being used.

Also I see spray foam applied to floor joists in basements and 
crawlspaces.  What is the situation there?

Steve Houlihan

On 12/7/14 12:38 PM, RT wrote:
> On Sun, 07 Dec 2014 10:52:20 -0500, Bill Allen 
> <bill.allen at verizon.net> wrote:
>
>> Hi all
>> I am about to reroof my 1926 colonial and am looking to also insulate 
>> per the BSC "deep energy retrofit" plans consisting of plywood 
>> sheathing, 2 layers of 2" polyiso, another layer of plywood, then 
>> roofing (in my case, standing seam metal roofing).
>
> Bill;
>
> Since you are planning to expose the existing rafters anyway, I'd be 
> inclined to turn the existing rafters into spaced rafters (aka 
> "parallel chord trusses") by suspending a 2x4 above the existing 
> rafter to create a member that is at least 24" deep allowing you to 
> install at least 18-20 inches of non-plastechhhh! insulation and leave 
> a ventilation space of at least 4" over. I would be inclined to use 
> Roxul mineral wool as the insulation material.
>
> Not only does the creating of the deeper roof framing cavity allow for 
> a substantial amount of thermal insulation, it substantially improves 
> the structural strength of the rafters -- always a Good Thing in this 
> age of Climate Change when weather can severely misbehave.
>
> Adding the new top chord is as simple as installing a few pairs of 
> 3/8-1/2" ply or OSB gussets (say at 1/3 pts but confirm with your 
> engineer for your specific situation) along the length of the existing 
> rafters. One small compromise would be to install blocks of 1.5" EPS 
> in the space between the two gussets to ensure that the space gets 
> insulated, creating a mini-SIPS panel at each gusset area.





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