exterior sheathing - how to caulk?

Hello folks, I did some searching but was unable to find anything very cut-and-dried regarding exterior sheathing and the best way to seal it (if at all). I'm doing double wall construction, and my plans call for two layers of 3/4" OSB on the outside of my "outer" set of walls, with lapped hardie-plank siding outside of that. I've seen a couple references to caulking the perimeter joints of the sheathing with acoustic sealant (i.e. at the edges of the walls): And how about between the joints: With gypboard, this is accomplished with tape - what would be the equivalent with exterior sheathing? I'm also concerned about making the exterior sheathing too airtight - wouldn't that make it too difficult for the humidity in the cavity between the double walls to equalize with the outside humidity, thus trapping moisture and possibly contributing to a mold problem (which is definitly an issue here in Portland)? Let me know what you guys think. ---Devin

It's good to see you back in action. We've missed you around here. It looks like you posed this question on your last thread but it didn't get any action... :roll: Is the OSB tongue and groove? Usually 3/4" is. If it is, I would think some acoustic sealant would do the trick, but it's probably not too critical. As long as you position the vapor retarder in the correct location as Rod stated in the last thread you started, I would think you'd be OK. Got pictures of your progress? We'd sure like to see them! --Keith :mrgreen:

Thanks Keith - right now I don't have much to show photo-wise, because we still haven't broken ground, but we're supposed to start this week! Long story short: ---We were on the verge of hiring a contractor twice, but thankfully didn't hire either one of them ---We now have a building permit, and didn't have to hire an architect or designer to do the drawings - the downside of this is that it took me this long to learn what I needed to learn to get it right ---We are now the proud parents of a beautiful baby girl (9 weeks this Thursday) :D So, lots has been going on. We think that we've found a great contractor, and we're finally to the point where we feel like we can resume this project. 8) I'll definitely be taking pictures (& posting them!) Back to the subject at hand... So, I'd want to have them put a bit of caulk in the grooves, then let it kind of get squished in there as the OSB panels are put together? That seems like plenty of seal to me - then I could have them caulk around the perimeter from the inside. I'd be interested to hear from someone who didn't do any caulking of their exterior sheathing/siding and how their studio performs isolation-wise... ---D

Congratulations on the new arrival!! 8) Congratulations on getting the permit! I recall that wasn't a sure bet in your case, given the expansion of the building and all. When you have time it would be great to read more details about that experience. You're to be commended and will be rewarded for taking your time on the design. Let's see those drawings that scored you your permit when you can! 8) The idea of putting sealant in the "groove" seems like a winner to me. (But, then again, what the heck do I know? ;-)) --Keith :mrgreen:

Actually for your outer leaf if it IS T&G I would stagger all joints and NOT caulk at all; varying moisture content of outer walls will usually cause some movement of materials, and the T&G allows this. By offsetting joints between inner and outer layers, leakage will be minimised enough that I don't think it would make a difference. You should be fine just caulking along studs between frame and innermost layer of the outer leaf. Following proper nailing schedule for each layer and you will be fine. I'm assuming you'll have a Tyvek wrap under the Hardy; this will keep wind-blown moisture from causing too much penetration, and will also allow breathing since it's NOT a vapor barrier. (your only vapor barrier in this climate should be at the inner leaf) HTH, and congrats on the new baby... Steve