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Is it okay to put an unvented hot roof under a wood shingle roof?

I have a wood shingle roof directly over tongue and groove sheathing. Can I put an unvented hot roof under the shingles? My plan is to put five inches of ISO directly on the underside of the sheathing. I cannot find anything on the Internet that discusses this issue. I live on Cape Cod. Thank you for your help.

Asked by Randy Aubrecht
Posted Wed, 06/18/2014 - 21:41

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5 Answers

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1.
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Randy,
For the most long-lasting installation of cedar roof shingles, the shingles should be installed over skip sheathing, so that there is some air behind the shingles. This allows the shingles to dry from the back side as well as the top side.

If the shingles are installed over continuous board sheathing, you don't want any insulation on the interior side of the sheathing. You want the sheathing boards to be able to dry towards the attic.

So I don't think your plan is a good one.

The method you are proposing is called "cut and cobble." The cut-and-cobble method is not recommended for unvented cathedral roof assemblies. For more information, see Cut-and-Cobble Insulation.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 06/19/2014 - 07:01

2.
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Thank you Martin for your advice. I have two other questions. Would it make a difference if I put three inches of ISO and then spray two inches of closed cell foam over the top? Second would this technique work if I replaced the wood roof with fiberglass?
Thanks

Answered by Randy Aubrecht
Posted Thu, 06/19/2014 - 21:35

3.
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Randy,
If you seal everything up with at least 2 inches of closed-cell spray foam, your roof assembly is less risky. But you still need a vent channel under the sheathing if you have cedar shingles.

You have several possible choices here. One method is to include a vent channel under the roof sheathing, connected to soffit vents and ridge vents. Another possibility, as you noted, is to use a different kind of roofing.

No matter what you do, you want to strive to install at least the minimum code required R-value, which is R-38 in your climate zone.

For more information on all of your options, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 06/20/2014 - 04:08

4.
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Thank you for your help.

Answered by Randy Aubrecht
Posted Fri, 06/20/2014 - 05:33

5.
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Massachusetts state building codes are based on IRC2009 & IECC2012, the latter of which specifies R49 as the code min for attics.

http://www.iccsafe.org/gr/Pages/MA.aspx

http://www.mass.gov/eopss/consumer-prot-and-bus-lic/license-type/buildin...

You have another 2 weeks before that takes effect.

Answered by Dana Dorsett
Posted Fri, 06/20/2014 - 18:22
Edited Fri, 06/20/2014 - 18:25.

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