New cathedral ceiling with unfaced f/g batts, no air barrier or ceiling yet – planning t&g pine under…xps? below joists
Hello, all – first time post. I read all of the q&a posts I could find on cathedral ceiling insulation, and the article on how to insulate a cathedral ceiling, along with every response. So I’m informed, but still have questions about my particular situation.
House is new construction, zone 4A that I bought as a shell – framing done, roof sheathing and asphalt shingles done. Most of upper story is a simple gable roof, made with 2×4 trusses, not joists. Ceiling profile does not match roof profile – ceiling is a shallower pitch. The trusses are about 7″ deep at the eaves over the top plate, and 3′ deep at the ridge. Roof had eave vents, and I had a roofer install a ridge vent.
I hired a large, well known local general contractor to finish the house, who turned out to be nearly totally ignorant of any building science principles. They in turn hired a large local insulation contractor who was even more ignorant, to the point of negligence, and I fought both of them every step of the way.
2/3 of the upstairs has been completed – from the inside of the ceiling out: 1/2″ painted drywall, unfaced fiberglass batts between lower section of trusses, flimsy vent chutes stapled haphazardly on lower section of roof deck, osb deck, asphalt felt (presumably), asphalt shingles. Oh, also recessed cans (sealed or IC for what it’s worth) probably without insulation on top of them.
I scrambled around the day the drywall hangers were working to install XPS air dams between the wall top plates and the underside of the flimsy foam vent chutes, sealing with canned foam. Insulation contractor insisted it was OK to just jam fiberglass into the gap, which obviously left the insulation open to wind wash and air infiltration, and also pretty much crushed the vent chutes at the eaves. Pathetic.
GC would not hold off drywallers so I had to rush, but I did at least a 95% job sealing air dams.
There is no air barrier above the fiberglass, except “sort of” where the f/g fills the space between the drywall and the vent chutes/roof deck. I say “sort of” because these are the cheap corrugated chutes that allow like 2 air flow cavities per joist/truss bay. No sealing over the insulation, just stapled up there. As the trusses open up towards the ridge, there is a 1′ + air space, no air barrier, no vent chutes. If you stuck your head up there you could see from one end of the ridge to the other, over the batts.
SO HERE are the QUESTIONS:
I left the 1/3 end of the floor unfinished – 1 bedroom and bath. It’s roughed in, and has the fiberglass insulation in place. I want to insulate this ceiling properly.
1. Martin stresses the importance of an air barrier above and below the fiberglass batts in an assembly with joists. Is the upper barrier as crucial in my truss assembly, which is in effect more like an uninsulated vented attic, with insulated floor?
2. We want to use tongue and groove pine boards for the ceiling surface – obviously not an air barrier. I propose using rigid XPS board under the joists, over the t&g, as a thermal break, extra r-value, and an air barrier. Taped seams obviously. Sound good? Looks like code approves the t&g as a “thermal barrier” for fire safety. Want to avoid the weight and hassle of installing drywall on the ceiling, as it is pretty tall.
3. Must I install some sort of air barrier over the existing fiberglass batts, where there is a void between insulation and roof deck?
4. Must I eliminate IC can lights, even if I can add insulation on top of them?
5. Should I add an air barrier “wall” up in the trusses, laterally separating this soon to be better insulated space from the existing mess of a ceiling?
FWIW I plan to install rigid foam on the top of the roof deck, and seal soffit and ridge vents whenever we have to re-roof, but that is definitely not in the cards now.
Thanks in advance for the help!
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