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How to insulate a cathedral ceiling?

I am planning a deep energy retrofit of my home. I am located in Calgary Alberta. Climate zone 3a. Average winter temperature in Dec and Jan is -14c (7 deg Fahrenheit). But it can get down to minus -50f, and -20f is not unusual for extend periods of time during the winter.

My living room has a cathedral ceiling over it. The existing roof/ceiling construction is: paint, GWB, poly V.B., 2x10 joists, 8”batt insulation, 1” roof decking, 1/4” ply, SBS roof membranes. The roof is presently vented with eve vents down low and the roof dumps into a vented attic on one side of the house. I will be adding a second floor where the attic is presently located so the top venting will be interrupted.

I do not want to loose any height in the living room so my intention is to remove the membrane and add insulation above the roof sheathing. I would prefer not to use foam for environmental reasons(persistent chemicals). I plan to add minimum 6” of Roxul (R-24) over the sheathing.
I have several concerns with this approach:

One, this roof assembly becomes a semi (half) vented roof assembly. I am concerned that I maybe paying a significant energy penalty by maintaining the venting in the joist space? If cold air is flushing this space what good is the insulation above?
If I seal the joist space I may have condensation problems.
Has anyone had experience with this type of retrofit condition? Any suggestions would be greatly appreciated.

Asked by David Richardson
Posted Fri, 01/03/2014 - 17:40
Edited Fri, 01/03/2014 - 17:51


3 Answers

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If you want to add insulation above your roof sheathing, you have to seal your vent openings. Even better: you should gain access to the rafter bays and fill the ventilation channels with insulation (in addition to sealing the vent openings, of course).

In your climate, I strongly advise you to use some type of foam insulation -- either spray foam or rigid foam -- if you aren't willing to lose any ceiling height.

For more information, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/03/2014 - 17:49

Helpful? 0

Thank you for your comments Martin
After some consideration I am now considering insulating this cathedral ceiling from below. I plan to add 2" of poly iso under the joists giving a thermal break and 5" of poly iso between the rafters with 3.5" batt over that. This will leave a 1" air space at the top of the joist space for venting. My concern is: the dew point will likely fall within the poly iso layer, is this a problem ? Has anyone had experience with this sort of application. Should I just be using batt above the 2" of poly iso to encourage venting? Obviously the poly iso provides greater thermal performance hence my desire to use it.

Answered by David Richardson
Posted Mon, 03/17/2014 - 17:42

Helpful? 0

You wrote, "I plan to add 2 inches of polyiso under the joists [rafters?] giving a thermal break and 5 inches of polyiso between the rafters with 3.5 inch batt over that."

Sometimes people misuse the terms "under" and "over" when describing ceiling assemblies. I'm going to read your comment literally, however, and assume that, from the top down, you are describing the following assembly:

A vented air space
A 3 1/2 inch fiberglass batt [with no air barrier above it?]
5 inches of polyiso, installed according to the "cut-and-cobble" method
A continuous 2-inch-thick layer of polyiso under the rafters

Is that your plan?

I'm not a fan of cut-and-cobble for cathedral ceilings, but cut-and-cobble can work if the ceiling assembly is vented, as you propose. For more information, see Cut-and-Cobble Insulation.

Your assembly is unusual. For one thing, it's a good idea to have an air barrier above any fiberglass batts.

A better assembly would be, from the top down:

A vented air space
A baffle to create the air space -- for example, 1-inch EPS
A layer of fluffy insulation to fill the rafter bays
A continuous layer of 2 to 4 inches of polyiso under the rafters

Answered by Martin Holladay, GBA Advisor
Posted Tue, 03/18/2014 - 04:44

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