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I am preparing my home for a new metal roof. Since my HVAC is located in my attic, I feel it makes more sense to have an unvented attic assembly, especially since my attic easily reaches 130F in our long Dallas, TX summers. I considered spray foam but understand it's not a good option since it can be very toxic and has a high risk of error. Closed cell spray foam paired with a vapor impermeable HT underlayment doesn't seem like a good idea. I am considering using mineral wool insulation to create an unvented attic assembly as dictated by 2018 IRC R806.5 which requires a few extras (some of which I've listed below):
Vapor Diffusion PortVapor Permeable Membrane covering the Diffusion Port
*Supply Fan connected to same HVAC ductwork that supplies the AC for the occupiable space
1) Is this a good ventilation option for my metal roof? I have confirmed with the Insulation Institute that I can use a metal roof/HT underlayment with this new unvented attic assembly method, but I would like to hear from some metal roofing experts.
2) Based on your experience, is there a different installation method for cathedral ceilings with this type of attic assembly? I recall seeing in the Edco installation guide a slightly different method for cathedral ceilings, but I can no longer find that part of guide.
3) Should I install Roxul 30 with Roxul Comfortboard as explained below?
Here are some details about my house.
Location: Dallas, TX northern suburb Climate Zone: 3A - just south of the Warm/Humid line
Roof Pitch: 10/12Roof Decking: OSB that is currently in good condition
Current Roofing: Asphalt ShinglesAttic Space: Due to steep pitch, some areas have 10'+ height. I can stand up nearly everywhere except along perimeter.
Levels: 1 Story with 10'-16' ceilings throughout houseCurrent Soffits: Builder grade OSB w/wood grain on exterior paired with 6"x14" metal slotted soffit vents
Replacement Soffits: Unvented HardieSoffit (concrete board) to help deter rodentsMetal Shingles: Edco Arrowline Slate Shingles
Underlayment: Titanium UDL 50 HT paired with PSU30Mineral Wool Insulation: Roxul 30 (perhaps paired w Comfortboard 80) for total of either R30 or R36
Insulation Installation: Roxul 30 in bw rafters (2x8s) and perhaps Comfortboard 80 installed perpendicularly to rafters/Roxul. IECC Insulation Recommendation for Cathedral Ceilings in Zone 3: R22-R38
*HVAC: 2 separate split systems w/Furnace, evaporator coil, and all ductwork located in attic (16 SEER Condenser, 95% Gas Furnace/Blower, Evaporator Coil) Large system = 4Ton, Small = 2Ton
I wouldn't put any batt or fibrous insulation as the only insulation in any assembly when we are talking about a roof without covering it with a finite and complete air barrier or without some insulation above the roof deck.
I would look at Nailbase or other pre-made type of insulation with plywood or look at some above deck venting and some above deck insulation.
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