Clarification on 2009 IRC code requirement vs what’s stated in article on building a Cathedral Ceiling
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From the article I read: (i capitalized what im trying to get clarification on)
If you want to install a combination of rigid foam ON TOP of your roof sheathing and air-permeable insulation between your rafters, you need to be sure that your rigid foam is thick enough to keep your roof sheathing above the dew point. Guidelines to achieve that goal are included in the 2009 International Residential Code (IRC).
According to section R806.4 of the 2009 IRC, “rigid board or sheet insulation shall be installed DIRECTLY ABOVE the structural roof sheathing as specified in Table R806.4 for condensation control.” The table specifies the minimum R-value for the foam installed on top of the sheathing (not the R-value for the whole roof assembly).
From the code i read:
5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the UNDERSIDE of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.
So my question is, does code allow you to do either of these:
1. Rigid foam on top of roof sheathing to minimum thickness in chart and fiberglass filling the cavity between rafters.
2. Rigid foam on bottom of roof sheathing to minimum thickness in chart and fiberglass filling the remainder of the cavity between rafters.
It seems to me that the code only states #2 as an option, while the article seems to imply that only #1 is option.
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Alan,
Here's a link to an article that answers your question: How to Build an Insulated Cathedral Ceiling.
Either method can be used.
My article states: "There are three ways to build an unvented roof assembly:
1. Install spray foam (either open-cell foam or closed-cell foam) against the underside of the roof sheathing, and no other type of insulation. Be sure that the thickness of spray foam is adequate to meet minimum code requirements. Remember that open-cell spray foam is risky in a cold climate unless the underside of the cured foam is covered with gypsum drywall that has been painted with vapor-retarder paint. Vapor-retarder paint is ineffective if it is sprayed directly on the cured foam.
2. Install rigid foam insulation above the roof sheathing and air-permeable insulation between the rafters. If you choose this method, it's possible to install vent channels between the top of the rigid foam and the top layer of roof sheathing by installing a series of parallel 2x4s — one above each rafter — extending from soffit to ridge.
3. Install a layer of closed-cell spray foam against the underside of the roof sheathing, and fill the rest of the rafter cavity with an air-permeable insulation."
Thanks Martin, I have read the article several times. My question was relative to what was allowed in the code. what i found in the code was:
5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the UNDERSIDE of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.
I couldn't find where the code permitted the rigid foam to be on top of the sheathing as it seems to specifically state "underneath"
Alan,
There are two code sections that discuss different ways to build an acceptable unvented insulated cathedral ceiling, and you quoted both of them. The section that addresses the use of rigid foam insulation above the roof sheathing is, as you pointed out, section R806.4 of the 2009 IRC.
Here is a link to another one of my articles that discusses these issues: Creating a Conditioned Attic.