Amber
Registered User
Hello,
I have a project that will be under IECC and IRC 2021 codes. I want to do an unvented roof assembly per IRC R806.5. On previous jobs, I have done unvented attics and have installed a combination of spray foam to meet the air impermeable R values in Table R806.5 and then batts or blown cellulose to get the roof to the required R-value for energy code requirements. I am now working on a job where we want to do a SIPS roof with blown cellulose below that, so in this case I would need to meet the requirement under R806.5.1.4 which says "Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45 degrees F. For calculation purposes, an interior air temperature of 68 degrees F is assumed and the exterior air temperature is assumed to be the monthly average outside temperature of the three coldest months" What the heck am I supposed to do with that? This seems in stark contrast to the section I had been using where the impermeable insulation is below the sheathing like when I did spray foam...That section 5.1.3 states "Where both air-impermeable and air-permeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R values in Table R806.5 for condensation control. The air permeable insulation shall be installed directly under the air impermeable insulation". This makes perfect sense to me. Why on earth do they make it so convoluted to try to figure out how thick the air impermeable insulation is if you want to put it above the sheathing like with a SIPS panel? Any insight is appreciated.
I have a project that will be under IECC and IRC 2021 codes. I want to do an unvented roof assembly per IRC R806.5. On previous jobs, I have done unvented attics and have installed a combination of spray foam to meet the air impermeable R values in Table R806.5 and then batts or blown cellulose to get the roof to the required R-value for energy code requirements. I am now working on a job where we want to do a SIPS roof with blown cellulose below that, so in this case I would need to meet the requirement under R806.5.1.4 which says "Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45 degrees F. For calculation purposes, an interior air temperature of 68 degrees F is assumed and the exterior air temperature is assumed to be the monthly average outside temperature of the three coldest months" What the heck am I supposed to do with that? This seems in stark contrast to the section I had been using where the impermeable insulation is below the sheathing like when I did spray foam...That section 5.1.3 states "Where both air-impermeable and air-permeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R values in Table R806.5 for condensation control. The air permeable insulation shall be installed directly under the air impermeable insulation". This makes perfect sense to me. Why on earth do they make it so convoluted to try to figure out how thick the air impermeable insulation is if you want to put it above the sheathing like with a SIPS panel? Any insight is appreciated.