[Greenbuilding] Polyiso strength on roof
RT
archilogic at yahoo.ca
Sun Dec 7 14:38:20 CST 2014
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.
--
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c at Y a h o o dot C a >
(winnow the chaff from my edress in your reply)
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