Replies

1. 
   Lucy Foxworth|Sep 07, 2014 10:36pm|#1

   Pierre,

   I'm fairly sure Martin Holladay, GBA editor will answer your question in the morning. He gets up crazy early.

   But there have been discussions about this topic before. Drilling holes in your sheathing is contrary to most of the concepts you'll read about in this website. One basic building concept is to make sure air does not flow through your building assembly because air carries moisture with it as well as reducing the effectiveness of your insulation.

   Here is a link to a very good, but fairly lengthy discussion about it on GBA in 2012.//m.etiketa4.com/community/forum/general-questions/23219/am-thinking-following-wall-and-roof-assembly-bc-cost-effecti

2. Expert Member
   Malcolm Taylor|Sep 08, 2014 12:25am|#2

   I've become a lot more appreciative of Martin's point of view on this since then, but at the risk of opening this whole can of worms, there is still something that troubles me.
   When we discuss the problem of wet sheathing, one solution suggested is to switch from panel type materials to wood boards (the idea being that they have a better potential of drying to the exterior due to the gaps between them), or to omit the sheathing altogether and rely on a membrane type WRB.
   But in all these options they aren't relying on the Sheathing to deal with wind wash and air-sealing. What is the difference between them?

3. 
   Kevin Dickson, MSME|Sep 08, 2014 01:54am|#3

   A self adhesive vapor open weather and air barrier should do the trick:

   http://www.cosella-dorken.com/bvf-ca-en/products/wall_ext/wrb/products/vent_sa.php
   http://us.henry.com/fileadmin/pdf/literature/BlueskinVP_Brochure_25Jun09.pdf

4. GBA Editor
   Martin Holladay|Sep 08, 2014 06:09am|#4

   Pierre,
   Every wall needs at least one air barrier. If you are installing fibrous insulation like fiberglass or mineral wool, it's a good idea to have two air barriers -- one on the exterior side of the insulation, and one on the interior side of the insulation. In no cases is it a good idea to encourage air movement through your wall assembly.

   If you use old-fashioned board sheathing, the sheathing layer is clearly not an air barrier. One can create an exterior air barrier in that case with high quality housewrap, carefully taped -- an imperfect approach in my opinion, but still an option -- or with one or two layers of taped rigid foam on the exterior side of the board sheathing.

   If you use plywood or OSB sheathing, the easiest way to proceed is to tape the seams of the sheathing to create an exterior air barrier. For more information on this approach, seeAirtight Wall and Roof Sheathing.

   It really doesn't make any sense to install a panel which is a fairly decent air barrier (OSB) and then to drill holes in it. If you drill holes in your OSB, you are almost certainly degrading the thermal performance of the wall assembly.

   Worries about the "cold OSB" problem are legitimate. (For more information on this problem, seeHow Risky Is Cold OSB Wall Sheathing?) But the solution to these problems is not to drill holes in your wall. Possible solutions include (1) protecting the OSB with a layer of exterior rigid foam to raise its temperature, (2) including a ventilated rainscreen gap on the exterior side of the OSB to help it dry faster, or (3) switching to plywood.

   Finally, the title of your question -- "Humidity being trapped between an inside vapor barrier and OSB?" -- leads me to believe that you are installing polyethylene on the interior side of the wall. (This is especially likely if you are building in Canada.) In general, interior polyethylene can make moisture problems in walls worse -- especially if the house has air conditioning. If you are worried about these issues, and it sounds like you are, you should switch from interior polyethylene to a "smart" vapor retarder like MemBrain.

5. GBA Editor
   Martin Holladay|Sep 08, 2014 07:44am|#5

   Pierre,
   I never directly addressed the question of your architect's advice. In case my previous answer wasn't clear, it's worth mentioning that your architect is wrong.

   You might want to ask your architect this question: "Why do you think that the 1/2-inch holes will help? Do you think that they will help because they will increase air flow, or because they will increase the rate of drying by diffusion?"

   如果架构师回答:“因为孔increase air flow," you can explain: "But it makes no sense to encourage air flow through a wall assembly. That's the easiest way to undermine the performance of the wall insulation."

   如果架构师回答:“因为孔increase the rate of drying by diffusion," you can explain: "But diffusion drying is proportional to the area of the material through which the diffusion is occurring. If the wall consists of 98% OSB and 2% holes, then the diffusion through the holes represents only a very tiny improvement in the rate of drying by diffusion. For all intents and purposes, the holes won't help diffusion drying -- unless you want to make the wall 50% holes and 50% OSB."

6. 
   Pierre Gingras|Sep 08, 2014 09:20am|#6

   Thanks Martin for your answer. It does make sense.
   The whole issue has been raised because I was planning to use a (BP) R4 sheating instead of the OSB on this new construction in an effort to try to avoid this humidly "trap". Unfortunately the R4 has been rejected by the engineer on the job because he says it does not have enough strength to act as a good wall bracing for this house structure unless we do all kind of complicated therefore expensive compensation scenario. So we are back with OSB and with the humidy concern.
   I wish someone would come up with an OSB replacement that would have the strength without being a vapor barrier.
   Regards

7. 
   Aj Builder, Upstate NY Zone 6a|Sep 08, 2014 09:58am|#7

   Pierre, install top quality plywood. Have Owens Corning Energy Complete sealing system if using there products. Then use Membrain instead of poly.http://www.ocenergycomplete.com/

   There are walls with much more insulation and many other materials... read up here and at buildingscience,com.... some ramblings follow;

   Once you spend for a premium build, Icynene water blown products become competitive, double stud superinsulated walls and more,,, SIPs

   We have added rigid foam on the inside under the drywall around these parts for years. OSB under vinyl siding to me holds up OK since the siding acts like it has a built in rain gap.... if sheathing can dry that's good, to me applying rigid foam over OSB (exterior rigid foam) should never happen. If water gets in the tight gap it will rot the wall because it won't get out before rot starts.

8. Expert Member
   Malcolm Taylor|Sep 08, 2014 11:09am|#8

   "If the architect answers, "Because the holes will increase the rate of drying by diffusion," you can explain: "But diffusion drying is proportional to the area of the material through which the diffusion is occurring. If the wall consists of 98% OSB and 2% holes, then the diffusion through the holes represents only a very tiny improvement in the rate of drying by diffusion."

   There is the very obvious answer that has been eluding me.