I have been going with a relatively unique (and time-consuming) approach to address two similar situations: vented cathedral ceilings and semi-conditioned attics with HVAC equipment.

I have been combining 3/4" Dow Tuff-R polyiso and R-21 high-density FGB insulation to achieve both an air barrier, a partial radiant barrier, and roughly an R-30 in 2X8 roof rafter assemblies (in older homes in Northern VA).

The 3/4" Dow Tuff-R is set 1" from the sheathing (with a ridge vent retro'd in to the roof, and natural ventilation at the roof-fascia intersection). The edges of the Tuff-R are sealed with closed-cell foam, and the joints taped with Nashua Extreme Weather foil tape. The remainder of the cavity is then filled with R-21 HD batts. Ducts in semi-conditioned attics are often sealed.

With recessed lights in cathedral ceilings, I try to get as much polyiso above the can housing (and swap out bulbs to LEDs), while maintaining an air space beneath the sheathing.

I have come to really appreciate the approach, but I wanted to bounce this off the pros before I continue my love affair with this approach.

Thanks in advance!

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Sounds good at first blush & used similar approach for others (minus leaving can lights) - As an FYI it is R26 technically & I am assuming by radiant barrier this one has the foil facing & it is facing the sheathing?

For the tape, make sure you check with DOW to make sure they approve but that sounds good

How are you getting the 1" gap?

Now the problem I see - even with LEDS, those cans are not rated for foam to be touching them, only items like FG, rock wool, & cellulose should be touching (assuming IC Rating)

Thanks for the reply Sean.

I have been using 1" polyiso sleepers to achieve the 1" air space between the roof sheathing and the polyiso board.

I have indeed been using the foil-faced product.

I will look into the tape and whether Dow approves. Any advice on finding that info out without scouring Dow's entire site?

I will have to look further into the can lights and the issue you raise with foam coming into contact with them.

Look at there spec / installation directions or call there technical support department or regional rep if not listed - generally pretty good at answering questions

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