I live in climate 3 (texas) and I am thinking about a roof with only rigid foam outside the roof sheathing. I want to avoid fiberglass bats if possible for chemical reasons.

I plan on putting furring strips/planks on the foam to hold it down and lift the sheet metal roof up a little to allow air to flow under it.

However, Climate 3 requires a roof with R30 and 2" of rigid foam is only about R10. I'm not sure how many inches I can put before the roof becomes unstable though. Is 6" ok if I put OSB on top of it to sandwich it?

https://energycode.pnl.gov/EnergyCodeReqs/?state=Texas

You can use 12" if you want. If you are concerned about chemicals, I'd avoid OSB and use solid wood battens. Or cellulose or rock wool for insulation.

I'd put most of my efforts into high ventilation rates (with an ERV) and monitoring interior pressure (never maintain a negative pressure). Also use an interior side air barrier film. And check for radon (even at lower than "acceptable" levels).

Posted By xeoncross on 25 Nov 2014 10:10 PM
I live in climate 3 (texas) and I am thinking about a roof with only rigid foam outside the roof sheathing. I want to avoid fiberglass bats if possible for chemical reasons.

I plan on putting furring strips/planks on the foam to hold it down and lift the sheet metal roof up a little to allow air to flow under it.

However, Climate 3 requires a roof with R30 and 2" of rigid foam is only about R10. I'm not sure how many inches I can put before the roof becomes unstable though. Is 6" ok if I put OSB on top of it to sandwich it?

https://energycode.pnl.gov/EnergyCodeReqs/?state=Texas

This build-up sounds like a disaster. None of the above will pass sound construction codes. You can't just put rigid foam on top of a roof and then throw pieces of OSB on top to try and "sandwich it".

As far as "chemicals" go, they are everywhere and even wood has chemicals in it.

We've installed 7" of foam on top of old plank sheathing (so the sheathing is visible in the attic), then 2x4's to hold down the foam (with 12 or 13" screws) then either a layer of plywood for asphalt or metal roofing, or cross strapping for wood shakes. I have no idea whether this meets code in tornado areas, but those screws will not release easily. This is being done across the country with good results.

How the hell do you hit the rafters consistently with foot-long screws?!!

I find anything over 7" unwieldy, but then again, I'm not practicing very often.

Using 4-4.5" polyisocyanurate nailbase clad panels (R20-22-ish) on top of the roof deck and 5.5" rock wool or cellulose (R20-23-ish) under the roof deck has pretty good performance with few chemical issues (unless you react to newsprint ink, in which case stick with rock wool.)

Posted By Dana1 on 13 Jan 2015 04:32 PM
How the hell do you hit the rafters consistently with foot-long screws?!!

Measure twice, screw once, Grasshopper.