I have a question I'd like to throw out there.

I'm looking at enclosure systems for my timber frame cabin/house. I was originally thinking of SIPs, but am now re-thinking the idea, mostly because of budget issues. I have another idea that I've been thinking about...

My roof system is a common purlin system with purlins at 48" OC. I was thinking about adding 2X6 rafters on top of the purlins (with a spacer in there for slipping in drywall), putting fiberglas batt insulation between these rafters. Adding one or two layers of rigid foam insulation on top of the rafters, and OSB, and then standing seam roof.

I was thinking about using Atlas Roofing's ACFoam, cross-vent panels (rigid foam with OSB sheathing and a built-in air space).

I was thionking of a similar system for the walls.

Any thoughts on this idea? or any experience with the Atlas Roofing products? Any idea where one would put a vapor barier in such a configuration?

Any thoughts are welcome.

Jon

Jon:

I would put the vapor barrier to the interior surface because you don't want water vapor condensing in the fibreglass insulation (and thus causing rot). The standing seam roof will provide some reflective value for external radiant heat (and I think Englert offers coatings that claim to improve on that as well - depends on whose manufacturing the raw coil for the roof). I am not familiar with the Atlas product but am wondering if it might not make more sense just to stick with rigid foam for your whole roof system. For example: If you use 2 layers of 2" Thermax rigid foam with foil you will get an R value of about R22-30 (I honestly can't remember the final R value but you get around R7/inch plus some bonus for the foil). If you have the time this may save you some bucks. You then are left with some air space beneath the deck for ventilation.

hope this helps,
Andy in NH

Andy,

Thanks for the input. I agree that the vapor barrier should go towards the inside of the wall system. I wonder how this might be accomplished, however. Ideally (I think) the plastic would go between the studs and the timber frame, but when do you attach it? It would seem to be impossible to have a continuous layer if you wait and attach it from the inside once all the wiring and insulation goes in.

Also, how about wall systems? Is it possible to use a similar system (i.e. studs and then a couple layers of rigid foam)? If so, does the rigid foam need to be supported from below somehow?

So many questions...

Jon

Jon,

I to am considering the Atlas Nail base for my roof. I'll be applying it as follows: 1x base sheathing over my common rafer, major purlin roof system, then vapor barrier, then Atlas Vent R, then metal roofing. I'm going to have a catherdral ceiling, as i'm guessing you are, and my local building inspector passed my plan for 5.5" Atlas Vent-R for an R value of 28.6.

I'll be studding my frame with 2x4's from the inside (between the timbers). This only leaves me with 3" reveal on my 7" timbers and completely buries many of my braces, but makes wiring easy and allows me to enclose the frame quickly by being able to nail up my rough sawn 1x base sheathing directly to the outside of the frame (like you would a barn). I'll be insulating my walls with HP R-13 fiberglass bats vs. the standard r11 to make up for the "low" R in the roof.

Personally, I don't like the super air tightness you get from using SIP's all around. It requires the need for an active venilation system to keep clean air in the house. Not to mention, bugs love to live in the stuff. I'd prefer my envelope to be a bit less efficient, spend a little more on heat ( I burn my own wood anyway) in the winter and not have to persuade clean air to circulate thru my house.