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Doubled-studded walls & cold sheathing in WA

I live in western Washington state and I am designing a house and would like to incorporate some energy efficiencies in the design. I am mostly sold on double-studded walls however I do have reservations about cold sheathing. My builder is new to double-studded walls but willing to give it a go, as a result I am learning as much as can to help ask and inform as we move forward.

Q: Is this approach sufficient to lower cold sheathing in my environment: 1.) have a rain-screen 2.) install a interior vapor barrier and make sure all the seams are taped well 3.) mechanical ventilation system and 4.) lots of caulking (electrical boxes, windows, top plate/bottom plate)

Q. How important is it to use plywood over OSB (trimming costs and I trying to decide if using OSB makes sense )

Any suggestions greatly appreciated. Thanks in advance

Asked by Michael Larrabee
Posted Tue, 12/17/2013 - 00:49

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Michael,
I assume that you are aware of cold sheathing risks and want to lower those risks. You may have already seen the articles on GBA that address this issue; but in case you haven't, here are links to two articles you might want to read:

How Risky Is Cold OSB Wall Sheathing?

Monitoring Moisture Levels in Double-Stud Walls

Some of the questions you raise have no certain answer. Researchers are now studying these issues and are still collecting data.

One thing's for sure: your plan to "install a interior vapor barrier" is misguided and is based on old ideas that have been discredited. Interior vapor barriers are not required by code and are not recommended. Building codes sometimes require an interior vapor retarder, a less stringent layer than a vapor barrier. (This requirement can be satisfied by a layer of vapor-retarder paint.) For more information on this issue, see:

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

Do I Need a Vapor Retarder?

When you ask about "lots of caulking," you're on the right track, but your description is simplistic. You don't really need "lots of caulking." What you need instead is an air sealing plan -- which means that you need to know the location of your air barrier, and you need to have a plan to make it continuous -- and an air sealing goal (usually expressed in terms of air changes per hour at 50 pascals). You also need to verify that you have met this goal by performing a blower door test. For more information on these issues, see:

Questions and Answers About Air Barriers

Blower Door Basics

Finally, you asked, "How important is it to use plywood over OSB?"

Here's my answer: If you are worried about the cold sheathing problem -- and it seems as if you are -- then it's very important. Choosing plywood will help you sleep better at night.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 12/17/2013 - 06:52

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