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).

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)

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.

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

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).

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.

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

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

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).

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

the poly vapour barrier went between the foam and tongue and groove. not sure why, i've just been the poor fellow on the roof in the sleet doing the work. i also figured that closed cell foam with the seams well filled with expanding foam should be a suitable VB, maybe it's just for code and or engineers.

rock on

Typically a vapor barrier is used to prevent migration of water vapor from an area of warmer tempeature to an area of cold where it would condense to liquid. The rule for insullation is "vapor barrier to the warm side". (Think of how we install fiberglass batts.) Water vapor seeks to go from warm to cold. While this doesn't specifically address whether or not a vapor barrier is needed with a rigid foam application, it does justify locating it between the first layer of sheathing and the foam.
A house wrap is intended to keep water and wind infiltration to a minimum. There are some who believe that a properly isnstalled skin (sididng, masonry, etc.) should keep water out and a little bit of air movement behind the skin is not a bad thing, thus they don't believe in using house wraps. The real enemy of an enclosure system is the water vapor that infiltrates it and condenses on the cooler surfaces which then suffer rot and mold. This is akin to having a leak which lets rain in.
I hope this helps clarify.

D&D,
Hi Dan, havent seen u since Ferry Farm. Maybe see u on the VMI proj.
Your guidance on moisture barrier on the warm side of the structure is in sync with my prior experience and training.
back to air relief for the metal roof. The Muellar Co in TX is a proponent of using some mylar backed closed cell foam bout 3/8" thick underneath it's 36" wide steel roofing. Shiny side up. this puts a smooth surface under the integral channels in this standing seam look alike and when topped with vented ridge the air wash helps in solar reflection and moisture reduction. The foam also serves to seal the stainless screws coming thru the panels to hold them down (in concert with the live rubber compound sealing washers under the screw heads).
So we end up with a cool, dry, solar reflecting and very light weight roofing system at reduced cost. hmmmmmmmmmmmm

does anyone have first hand experience with P2000 insulation system. I am not connected with company, just interested. one inch thick extruded polystyrene sandwiched between layers of reflective sheets tests at R-27. air tight so no vapor barrier of house wrap needed. seens ideal for timber frame roof and wall strcuture. sounds too good, but company has been around for 10 years.

Great sketch by Chris.

I have a similar configuration on a cabin. I have 1 1/2" thick T&G running perpendicular to the ridge, (no rafter support- just post and beam).

I hope to add 2" thick rigid insulation directly to the top of a shingled roof and then add metal roofing. (there is no attic)

I thought I could glue the insulation down to the shingles and then screw the metal roof over that using screws long enough to drive into the T&G about 1". However, the concern is that the insualtion will compress slightly and prevent a good fastening situation.

Instead, if I screw down 1x4" boards on top of the insulation to hold it down, I can use the 1x4"s and screw the metal roofing directly to them. This gives me 3/4" gap between the metal roofing and the insulation. The insulation acts as the vapor barrier.

Are there any reasons that this will not work? Thanks.

MJC,

The cold roof detail is a good one.

It's been explained to me that a vapor barier is not near the issue when the dew point falls within something solid.

As for gluing the foam, I would be very careful. Most glues eat foam alive, so check compatability.

Best,

Zach

I have used foil face insulation, and taped every seam with aluminum tape to create the vapor retarding barrier.

I think the strapping idea is a good one. I would allow for ventilation under the metal roofing so that it won't be acting as a second vapor barrier.

Pete

Zach,

Thanks for the tip on the potential glue problem.

When you say "something solid", I assume you mean the rigid insulation vs. a soft fiberglass batt or something. Thanks.

Pete,

Thanks for the reply.
What foil faced material did you use? The blue board is R=5 per inch. Thanks.

This thread is a bit old but have a follow up question mostly to Dave if he is still around. How was the roof vented or was it not vented. Seems to be a position that you don't have to vent if you use foam or ridgid insulation. Any ones thoughts.

MJC,

Sorry, the thread is indeed old but if you are still out there, the foil face is rigid 'isocyanurate' type insyulation.

In recent years it has been getting much less expensive than styrofoam (here in NH), and gives better R-values. Especially if the reflective faces are used with a small air-gap.

Pete

I have done just what you described. Our layers were as follows (top to bottom):

-Shingles.
-#15 felt paper.
-1/2" plywood sheathing.
-2x4 strapping (flat) 16" OC.
-3 layers of 1-1/2" Firestone PolyISO polyurethane foam (staggered joints).
-1 layer of Ice & Water Shield directly on top of the pine t&g for our vapour barrier.

The 2x4 strapping was secured to the timbers with 10" screws (to sandwich the foam in place).

This proved to be an easy (and inexpensive) way to close in and insulate the roof system.

I'm interested in the technique,, but dont' have anything to add,,

I think they covered it well. I use the same system with some slight variances. I'm very pleased with how it works and performs. Tim

Do you need ventilation between the foam and the sheeting as you would with fiberglass insulation, or not?

Water vapor is far less likely to reach the dew point and condense at some point during its migration through foam, than it might be as it migrates through glass...

And IMO it is unwise to put a convection current (the ventilation) next to such a potentially volatile fuel supply as foam, as hot cinders my very well be drawn into a cold roof during a potentially beatable fire -

I typically sheath tight to foam and provide a secondary cold roof above it

Jim, I be building a similar roof on my home. If you have any installation information, drawings, photos etc... I'd appreciate receiving them as well. You can email me at: mauricep1@xplornet.con