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Rigid Insulation Roof Construction #9874 02/18/06 06:24 PM
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Dave Petrina Offline OP
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I've asked this before but didn't get any response so I'll try again.

Has anyone designed/constructed a roof using rigid polyurethane foam insulation (not SIPs) applied directly over the T&G?

I'm interested in some of the details (ie: ridge, faschia, strapping for roof material).

Re: Rigid Insulation Roof Construction #9875 02/19/06 06:08 AM
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Emmett Greenleaf Offline
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Dave,
Not an experiential answer to your query but your question raises some issues in my mind.
Why would you consider any roof insulation material which is in itself pretty fragile and has limited insulating properties compared to other choices like the new 6" sips which approach R40+ (not including other material above and below)?
Especially in light of the BC climate, not exactly the snow free tropics.
deralte (aka Emmett)

Re: Rigid Insulation Roof Construction #9876 02/19/06 06:20 AM
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chris robinson Offline
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yep,i've paricipated in the construction of one.
it was a two bent great room on a stick frame, with ridge, plate and two purlins each side as i recall. t'n'g from ridge down and overhung plate by a couple of feet.
we ran two by six on edge ridge to plate at either end and across the bottom, perpendicular to the roof pitch. just a big box screwed to the t'n'g using small metal L brackets. inside this went VB and then 5 1/2" rigid foam in large sheets. this material had a shiplap type joint, i think. joints and space around the edges were filled with canned expanding foam. the sheets ran right to the ridge and the gap at the ridge was covered with misc. offcuts and foam.
strapping was 2 x 4 screwed right through everything, shimmed where necessary to meet the stick frame.
lookouts were framed on the front side to allow continuation of the t/g, 2x8 subfascia and support the decorative timber truss at the gable. these were just 2x6 with 2x4 that ran over the 2x6 at the edge ot the timber frame and were screwed right through the foam and in to the t'n'g, just like the roof strapping.
2x6 sub-fascia was attatched at the eaves, again with small metal brackets. i guess this could be either plumb or perpendicular to the roof. perpendicular in this case.
i think steel went directly over all this.

Re: Rigid Insulation Roof Construction #9877 02/19/06 04:00 PM
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Dave Petrina Offline OP
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Thanks for the information Chris. Exactly what I was looking for. Just a bit of clarification. Did the 2x4 strapping bear directly on the polyurethane? How was is secured - panel screws? I presume the strapping ran parallel to the ridge at 2ft O.C.?

Thanks again

Re: Rigid Insulation Roof Construction #9878 02/19/06 04:10 PM
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Dave Petrina Offline OP
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Hi Emmett,

The rationale for using polyurethane sheets versus SIPs on the roof are:

1. It costs a fraction of what SIPs cost (atleast around here)
2. It has the same insulation value as a polyurethane SIP (R6-7 per inch)
3. It is a lot lighter than a SIP and therefore can be handled and installed without a crane

Polyurethane sheets are quite common in commercial construction but I wasn't sure if anyone had tried it for residential. In my view, in some applications a SIP on the roof can be a bit overkill. ie: do you really need the 2 sheets of OSB if you have 2x T&G? Metal roofs (quite common in snow country) work fine on non-continuous support (strapping) if the pitch is steep (10:12, 12:12).

Re: Rigid Insulation Roof Construction #9879 02/21/06 05:14 AM
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chris robinson Offline
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dave
the strapping was secured with long panel screws and bears directly on the foam.

check this out
http://www.htf.ab.ca/george/pdfresources/wall-roof-detail-3d.pdf

i concur with your thoughts on built up versus panels. two people can relatively easily put this all together with ladders and scaffold, with no need for crane, though if it's there for the raising why not keep it for the panels.

and 2x6 sure seems strong enough that you don't need the structural part of the SIP. and i guess you can screw strapping to it pretty much wherever you want rather than trying to hit rafters or putting down horizontal strapping over vertical strapping on a purlin roof.

Re: Rigid Insulation Roof Construction #9880 02/21/06 05:30 AM
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Emmett Greenleaf Offline
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Dave,
Good to see people doing serious cost-benefit analysis to achieve a common goal. Makes me want to rethink my cost model.
The pix on your other reply is great.
Now the question that remains is how does moisture move throught such a build-up ?
deralte

Re: Rigid Insulation Roof Construction #9881 02/21/06 03:04 PM
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Jim Rogers Offline
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Dave:
I tried to send you some info via regular email and it came back, as address unknown.
Can you send me your address again?
Jim Rogers


Whatever you do, have fun doing it!
Re: Rigid Insulation Roof Construction #9882 02/22/06 05:26 PM
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Dave Petrina Offline OP
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Thanks for the picture Chris - most helpful. Glad to see I'm not reinventing the wheel here.

Emmett,

Regarding moisture movement through this assembly, I think it would be similar to a SIP if you sealed the seams with expanding foam. My understanding is that the permeability of polyurethane is low enough that you don't require a seperate vapour barrier (provided you seal the seams with foam). I suspect most building inspectors around here would prefer to see a seperate vapour barrier though (6 mil poly).

Re: Rigid Insulation Roof Construction #9883 02/22/06 09:50 PM
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Emmett Greenleaf Offline
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Dave,
Ok, let's use a poly sheet or similar wrap for a moisture barrier. Where you gonna put it ?
On the inside to keep ambient moisture outside of the structure ? On the outside (like a house wrap) to contain interior moisture and keep any excess exterior from intruding ?
Next sub topic to consider: Metal roofing, what goes between the metal roofing and the sub roofing if anything ? Use the natural air flow of the formed "standing seam" channels topped with a vented ridge ?
deralte

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