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Cathedral celling insulation technique in a northern climate

I have seen a few discussions about insulating cathedral ceilings but none that relate to the method that I had in mind for my 24' cathedral here in Vermont.
* Penetrating water sealer on all surfaces that have the potential to rot.
* One inch strapping to create an air gap for ventilation.
* Multiple overlapping layers of ridged foam(open or closed cell?)
* All seams taped because expansion and contraction will break a DIY spray foam seal.
* One or two inches of spray foam insulation applied professionally to hopefully create a secure air seal. Total ridged foam is enough to prevent condensation from temperature variations of the outside and indoor air. What happens when the air seal fails despite your best efforts?
* R 30 batt insulation
* Vapor barrier, vapor resistant or nothing?
* A layer of ridged foam over the celling joists or would it be diminishing returns?
* If a wood celling, Sheetrock first?
* No recessed lighting.

Asked by Gary Spurling
Posted Fri, 04/11/2014 - 08:50

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5 Answers

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1.
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Gary,
Have you seen this article? How to Build an Insulated Cathedral Ceiling.

At first, I thought that you were describing the peanut brittle variation on cut-and-cobble ("Multiple overlapping layers of rigid foam ... One or two inches of spray foam insulation applied professionally to hopefully create a secure air seal"), but they right out of left field you threw in "R-30 batt insulation," and at that point you lost me.

For more information on the peanut brittle method, see Cut-and-Cobble Insulation.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 04/11/2014 - 09:28

2.
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The rigid foam is used to lower the cost while gaining the R value needed. The batt is inline with the flash and batt system. What comes next in my discussion is up for debate. As you know, there are issues with spray foam, closed cell or open cell. I think I would rather deal with a minimum of spray foam when, not if, it will someday be removed.

Answered by Gary Spurling
Posted Fri, 04/11/2014 - 10:40

3.
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Gary,
OK, you are talking about a flash-and-batt ceiling.

1. Properly detailed, a flash-and-batt assembly doesn't have to include ventilation, although the ventilation will do no harm.

2. Make sure that the "flash" layer of your flash-and-batt approach meets the minimum R-value requirements explained in my article, How to Build an Insulated Cathedral Ceiling. The building code requires that this layer (in your case, rigid foam plus spray foam) must have a minimum R-value of R-25 in Vermont.

3. You ask whether rigid foam is open-cell or closed-cell. All three types of rigid foam are closed-cell foams, although polyiso is more likely than the other two types (XPS and EPS) to absorb moisture.

4. If you are building a flash-and-batt assembly, you should not include an interior vapor barrier, because this type of assembly is designed to dry to the interior.

5. If you want to use a flash-and-batt approach, it isn't a good idea to also install a layer of interior rigid foam. That would create the dreaded "foam sandwich." It's better if the assembly can dry to the interior. That said, your choice of flash-and-batt doesn't address thermal bridging through the rafters.

6. If you want to install a board ceiling, then you definitely need an interior air barrier; usually, taped drywall is the best air barrier to choose -- followed by boards if you like the look of boards.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 04/11/2014 - 10:57

4.
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Ok, I understand what you are saying about the foam sandwich.
Is the use of a penetrating wood sealer recommended for the underside of the roof sheathing and roof joist area? In boat construction wood sealers are often used due to the persistent issue of wood rot.
Thanks for the recommendation not to use recessed lighting in a cathedral celling. I would have made that mistake.

Answered by Gary Spurling
Posted Fri, 04/11/2014 - 14:56

5.
Helpful? 0

Gary,
Q. "Is the use of a penetrating wood sealer recommended for the underside of the roof sheathing and roof joist area?"

A. No. My recommendations can be found here: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 04/11/2014 - 17:57

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