Polyiso and loose-filled cellulose insulation in cathedral ceiling
I have a home built in the 1950s in Tennessee. The PO closed attic to make a large family room, bathroom, and full bath. No insulation has been installed in the knee walls or the slope of the ceiling. The space behind the knee walls is roughly another 800 sq. ft. of space, that I would like to turn into conditioned space.
The attic (also not insulated) has a ridge vent and gable vent. I have actual 2×8 rafters with no soffit vents. I have found 4×8 – 2.5″ PolyIso at a great price. My thought is to run the polyiso on the interior side of the rafters and loose-fill cellulose between the rafter cavity to get a close enough R-38.
Can this been done? Do I need to place XPS against the sheathing? Vapor barrier? Cut in soffits?
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Replies
L.R.,
I don't know what a P.O. is -- for me, it's usually a post office.
If you intend to install cellulose between the rafters, you must first create ventilation channels on the underside of the roof sheathing. This approach only works if (a) you can install soffit vents, (b) you can install a ridge vent, (c) the roof has no hips, valleys, skylights, or dormers, and (d) the ventilation channels can extend in an uninterrupted fashion from the soffit vent to the ridge vent (or vented attic).
For more information on creating a vented roof assembly (as well as a few ideas on unvented roof assemblies), see this article: How to Build an Insulated Cathedral Ceiling.
P.O. = Previous Owner
Norman,
Thanks. My goal on Green Building Advisor is clarity -- so I aim for as few abbreviations as possible. If an abbreviation sends me to Google, communication has failed.
There is no need for a vapor barrier or XPS anywhere in this assembly, but it needs to be vented soffit-to-ridge (as Martin aptly stated), and air tight to the interior.
Reclaimed roofing polyiso is good for R5.5/inch, or ~R14 for 2.5" goods. A 2x8 is 7.25" deep, so with the 1" vent gap you're looking at only ~6" (~R22) for the cellulose), or R36 at center cavity. But with the R14 thermal break of the polyiso over the rafters it will outperform R38 thermally bridged by joists.
Most of TN is in zone 4, where the IRC calls out R49, but the western end of the state is zone 3, where R38 would meet IRC code. See the orange counties in this map:
https://energycode.pnl.gov/EnergyCodeReqs/images/tennessee.png
https://up.codes/viewer/wyoming/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.2
While 2.5" continuous polyiso + 6" of cavity cellulose will beat code on a U-factor basis in zone 3 , it doesn't quite make it for zone 4, but it won't be terrible.
https://up.codes/viewer/wyoming/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.4