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Will I rot out my plywood roof decking if I condition my attic space?

Will I rot out my plywood roof decking if I condition my attic space? I have a standing-seam metal roof with 15 lb. felt paper as an underlayment. I am in a coastal climate on the Puget sound in WA state (marine zone 4).

I am considering a conditioned attic space in my home that I am remodeling. We want to use the space for storage and a play space for the kids. I am certain the interior space would have sufficient ventilation from the HRV.

What I am concerned about is rotting out the roof decking since I have an insufficient moisture barrier (greater than 1 perm) under the metal roofing. The metal roof is well constructed and should prevent vapor drive (as I understand it my metal roof should not be as susceptible to inward vapor drive as would other roofing materials) but should I be concerned about condensation? The metal roof is not an air barrier and I am pretty certain the tarpaper is not perfectly applied and covering the decking underneath the metal roof.

Asked by mathew BEAN
Posted Tue, 02/11/2014 - 14:04
Edited Tue, 02/11/2014 - 14:14

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3 Answers

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Also I would be air sealing the underside of the roofing assembly and then insulating with rigid foam sealed at the seams with canned foam followed by cellulose held in place with batting.

Answered by mathew BEAN
Posted Tue, 02/11/2014 - 14:11

2.
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Mathew,
It sounds like you want to install insulation in your sloping roof assembly to create an unvented, conditioned attic. This kind of work is done all the time. However, it's easy to get the details wrong and make a mistake. I suggest that you read these two articles:

Creating a Conditioned Attic

How to Build an Insulated Cathedral Ceiling

It sounds like you want to use the "cut-and-cobble" method of insulation. I don't recommend this approach for cathedral ceilings, since future air leaks can lead to rot. To learn more about cut-and-cobble, and why you shouldn't use it for a sloped ceiling, see Cut-and-Cobble Insulation.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 02/11/2014 - 14:20
Edited Tue, 02/11/2014 - 14:23.

3.
Helpful? 0

Thanks Martin that first article answered my question thoroughly. Yes I am insulating a sloping roof 4/12 pitch to create a conditioned attic. From my reading it looks like it will be best to insulate below the trusses leaving the air channel that goes from the eves to the ridge vents intact. Probably the most economical will be to create a sufficiently large cavity below the rafters and fill it with cellulose.

Answered by mathew BEAN
Posted Tue, 02/11/2014 - 15:52

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