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.