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Rigid foam insulation in a cathedral ceiling?

We are building a tiny house currently with cathedral ceilings and 2x6 rafters. I have access to large quantities of rigid foam and would like to make use of it. Can I fill the rafter spaces with layers of rigid foam and spray can foam the seams and cracks of each layer as an air barrier and keep the roof unvented? I would then like to add another layer of rigid under the rafters with taped seams to prevent thermal bridging and air leakage once again. Then I would strap under that so I could still install T&G on the ceiling. Above the rafters I could either go with 2x4 strapping then 30lb felt then metal roof, or half inch sheathing then felt then metal. With sheathing that would mean there would be no venting. I have read on here that I may not need venting if I have the seal of rigid foam with canned spray foam to seal. Any insight into this would be greatly appreciated. Oh and I am in climate zone 2b, pretty much in the rocky mountains of southern alberta. Cold and dry here. Thanks.

Asked by jordan Saunders
Posted Tue, 08/05/2014 - 01:41
Edited Tue, 08/05/2014 - 07:03

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2 Answers

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Jordan,
The method you are proposing is called cut-and-cobble. It falls into a gray area -- not really supported by existing building codes (assuming you want to build an unvented assembly), but not expressly forbidden either. This method is never used by professional installation contractors.

For more information on this method, see Cut-and-Cobble Insulation.

There have been several anecdotal reports of failures when the cut-and-cobble approach is used for unvented cathedral ceilings. You can read about those failures on the page I linked to, and then make your own decision.

For more information on the many acceptable ways to insulate a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 08/05/2014 - 07:02
Edited Tue, 08/05/2014 - 07:21.

3.
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I am not a cut and cobbler.

Vent above your cobbling if you must cobble. Moisture leaks will have an escape route.

We vent rigid foam roofs and do not vent sprayfoam roofs.

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 08/06/2014 - 10:53

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