[Greenbuilding] Polyiso strength on roof

[RT]

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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