Taping Exterior OSB Joints

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Old 08-04-15, 06:51 AM
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Question Taping Exterior OSB Joints

I have been reading GBA and there is discussion on the benefits of taping the OSB Sheathing joints in terms of energy gain from the air sealing. Have any of your done this? I am wondering if it is worth the hassle.
 
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Old 08-04-15, 07:07 AM
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The only joints I have ever taped are the joints on Zip System sheathing. That is the standard practice with that sheathing primarily because the sheathing does not need a WRB when the seams are taped. The Zip system sheathing is smoother and has a painted surface that makes it a better surface for butyl tape to stick to.

As for taping the joints on OSB, the method usually preferred is to tape the WRB instead (Tyvek or similar) which in theory would accomplish the same thing, making it your primary air barrier. Tapes stick better to WRB than to OSB.

One insulation subcontractor I was around would caulk the seams in the framing before insulating. (double studs, header edges, bottom plates, etc.) He also foamed behind outlets.
 
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Old 08-04-15, 07:26 AM
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I stopped building before all of the emphasis on air sealing hit, and have only prescribed to these methods from my energy auditing work. But the problem I see is that each step is part of the whole project. Where do you want your air barrier? Inside or outside it should be continuous from foundation to above the top plate. Are you installing a vapor barrier on the inside? A plastic vapor barrier becomes another air barrier and although double air barriers have not been identified as a problem (yet) one has to wonder just how tight is enough. Beyond some level you have to introduce more fresh air to make up for what used to leak in. I know, controlling that air is better, but we have some very old homes that have been rather leaky that have survived for generations.

If you are including all of the other energy efficiency measures and shooting for a very tight home, then sealing the osb is probably a good part of the package. As for tape on osb, O have had poor results, it just didn't last, different application. Mastic with fiber mesh or as X stated, a couple of cases of caulk (i used construction adhesive) and catch all of the seams. Just my opinion, no building science corp articles behind it.

Bud
 
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Old 08-04-15, 05:33 PM
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According to this Tape Test from GBA, one tape that did hold up in the 8 month test was 3M All Weather Flashing Tape. I purchased this coincidentally a few months ago when looking for a good window flashing tape.

I was planning on using this tape for the OSB Joints if I were to tape them. I realize that house wrap is install over the OSB, and the house wrap can/should be taped. I still assume air can permeate through the wrap and even draft at the bottom or top edge of the house wrap. How would air/vapor not travel from the bottom of the house wrap to the interior wall?

I do not want to spend extra time and money on something that does not benefit me of course -- Sounds like this is not the best idea.
 
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Old 08-04-15, 06:01 PM
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When the WRB is installed as an air barrier, ALL edges are taped. Top, bottom, and seams. And IMO you are right, it's not the best idea. You could do it but whats the point. Are you taping the top and bottom edges of the OSB too? If not, then why do the seams.

Almost no one installs the WRB as an air barrier, but it "can be". Generally its main purpose is to shed WATER should any water get past the exterior siding/cladding. I would never install the WRB as an air barrier because its my belief that you have to allow for drainage even behind the WRB, just in case. Some people skip tape the WRB at the bottom to allow for drainage if needed. This is the sort of thing happens when people overthink each step. Not saying you are overthinking it, it's good to think things through when making decisions.

I've seen articles where the building science "extremists" will lay the membrane down on the top of the foundation or glued/stapled to the bottom of the sill plate (before the sill plate is installed.) Then once the floor is on, the walls are built and it is sheathed, that membrane can be wrapped up onto the sheathing completely air sealing that entire area. Like I said, its a bit extreme.
 
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Old 08-04-15, 06:41 PM
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So is there any air barrier in a common built house now-a-days if you do not use rigid foam on the exterior? Because using any plastic or vapor barrier on the inside would be a recipe for disaster.
 
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Old 08-04-15, 08:17 PM
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Its pretty common in our area to put up a 4 mil poly vapor barrier on the interior. About half still do it, the other half don't. Kind of depends what sort of insulation you plan to use in the walls. You could look at the WRB a half-assed air barrier the way most of us do it. (open at the top and bottom, tape the seams) Once the siding is on, its pressed back pretty tight.
 
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Old 08-04-15, 10:08 PM
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I really want nothing half-assed on my house build... So back to taping the OSB joints I shall go. I hear a lot of bad results from interior vapor barriers/poly film. Tons of articles on why or why not to use it. Most of them only say it's okay in extreme cold climates.
 
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Old 08-05-15, 04:23 AM
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XSLeeper is giving you the best information, localized that you can get, IMO. I am in the South, so we do things a little differently sometimes. Yeah, we talk slow, too . You want the exterior of the house to breathe, thus the WRB. Keeps water out, lets air move. If you want the "best", consider Roxul insulation in the walls. Requires no vapor barrier, is waterproof, fire retardant, mold resistant and vermin proof. Beats Fiberglas any day. I've never heard of taping OSB joints, but it could have its virtues. Expensive, you bet.
 
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Old 08-05-15, 06:51 AM
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Guess I shouldn't have used the word half-assed. I agree about the poly, I'm not one of its proponents. I've seen mold behind it in our climate. But like I said, some still do it, some don't. But we have the extremes... where dewpoints can be 80+ in the summer and can be -20F in the winter. I'm more of the opinion that a vapor retarder is sufficient. Your paint. Drywall is the interior air barrier, housewrap can be a partial air barrier to allow a drainage plane. Take extra measures to insulate and seal outlets and penetrations.
 
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