We are re-doing a hipped roof cathedral ceiling that had chronic moisture problems due to lack of venting & that it was sheathed with 1×6 T&G planks.

 We are removing the T&G, firing down the 2×6 rafters to a full 12” using plywood gussets & 2×4’s as the bottom cord and re-insulate with R-38 Fiberglass including rafter vents

 The T&G planking will be replaced with ½ GWB

 We are adding soffit vents (None existed before) and adding Cobra hip-ridge vents 1/3 of the way down the hips. (It is not recommended to vent hips more than 1/3 of the way down). Below the 1/3 point we will drill the rafters to allow air flow up to the Cobra hip-ridge vents. I also think there will be some cross ventilation between bays due to our plywood gusset/2×4 build down.

Because the existing rafters are irregularly spaced between 20” and 25” on center we are planning to use un-faced fiberglass press fit in to place.

After the fiberglass is installed, we plan to strap the ceiling across the rafters with 5/4×4 – 16” OC

There won’t be any recessed lights or other perforations in the new sheetrock so it should be pretty air tight.

My questions is before the strapping goes on should I install some kind of vapor retarder?

Considering Aqua Bar B asphalt Kraft paper, a class 2 vapor barrier or CertainTeeds Membrain, both products are readily available at my local supplier or HD and relatively low cost.

From what I can see on the web the perm rating of Kraft paper like the Aqua Bar B is 0.4 to 4.2 and the Membrain is 1 to 10

Your thoughts on this?

Thanks – Jud

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