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Advantech exterior sheathing permeability and wall drying

Hi. I am worried that my plan to use Advantech 1/2" (WRB over it) as my exterior sheathing will not allow my wall to dry to the outside. I have heard that the perm rating of this product is 2-3. Is there way to install the sheathing with very small gaps to improve the chance of moisture to escape? Should I even be worried about it? Thanks.

Asked by Matthew Michaud
Posted Wed, 05/14/2014 - 22:14

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Matthew,
Studies show that damp OSB dries fairly readily. Many OSB-sheathed homes develop damp sheathing in February. The sheathing is usually dry by April.

I don't recommend that you deliberately install OSB with air leaks. Airtight construction methods are always preferable to methods that deliberately include air leaks.

If you want to encourage fast drying of your OSB wall sheathing, I urge you to include a ventilated rainscreen gap between your siding and your sheathing. For more information, see All About Rainscreens.

Your question touches on an ongoing controversy: the "cold OSB" question. For more information on this issue, see How Risky Is Cold OSB Wall Sheathing?

Most experts are less worried about the ability of OSB to dry out (your worry, apparently) than the question of whether it's a good idea for OSB to get damp every winter, and what the results of decades of moisture cycling will do to the OSB.

For those who worry about damp OSB, the standard solution is to install an adequate thickness of insulation (rigid foam or mineral wool) on the exterior side of the OSB to keep the OSB warm and dry.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 05/15/2014 - 05:13

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