Air sealing – tape and caulk
I found an older GBA article (What is the best sealant to use for airsealing?) from July 28, 2009 but I’m wondering if the info in this article is still up to date?
https://www.greenbuildingadvisor.com/question/what-is-the-best-sealant-to-use-for-airsealing
I’m building a heated/cooled workshop in zone 4A, using ZIP system (non-insulated) sheets on the exterior, 2×6 studs with fiberglass batt insulation, OSB on the inside and latex paint. Exterior will be vinyl siding. I intend to use the ZIP system tape for the exterior joints between sheets. The building will have a slab foundation and scissor trussed vented attic.
1) For sealing the ZIP panel to the top and bottom plates of the wall, is polyurethane caulk the go-to solution? Any specific brand recommendations?
2) On the interior side, is it better/preferable to use a sealing tape or a type of caulk to seal between the OSB joints? If I’m painting the inside, does paint stick to tape? (Since this is a workshop, it doesn’t have to be perfect but I’d still like it to look nice.)
3) How important is air sealing the interior joints of the OSB if I’m sealing the exterior joints? I understand I need to seal the exterior OSB to the top plate and continue the air sealing on the interior side of the ceiling to fully enclose the space. Does sealing the interior wall joints provide a measurable difference? The easy answer is “yes” but I’m hoping someone can provide some real-world experience.
4) Any thoughts regarding ZIP system tape vs ZIP liquid flashing?
If anyone can recommend specific brands, that’s great. I’d love to get advice from people that have used the product.
Thank you much.
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Replies
1- Polyurethane caulk is probably the most durable of the commonly available sealants, and is what I'd use for any kind of "seal the sheathing to the structure" type of applications. I use products from Loctite's PL line myself.
2- Tape will probably be more reliable over time here, provided you use a good tape that will stay stuck. Soemtimes priming the OSB first will help the tape to stick if it has any problems, so I would do a test-stick first before applying the tape everywhere. Prime over the paint when your prime the wall if you'll be painting it. You'll have a difficult time getting things to stick to polyethylene tape surfaces, and I can't really say I've tried to paint a lot of tape. I know you can paint vinly tapes, but if you don't put a coat of primer over first, all the tape will telegraph through your finish coats.
3- It's belt and suspenders. The general idea is it's difficult to get a perfect air barrier, so if you have some redundancy (like seal the exterior sheathing AND the interior drywall), then you'll have better overall performance. Sealing the interior also helps to keep moisture out of the wall, which is a Good Thing. I personally always air seal both the interior and exterior wall surfaces.
4- I've never really used the liquid system, so can't offer much here. I would expect the liquid flashing would be less likely to have problems adhering to irregular surfaces, which could be a plus.
Bill
For under the bottom plate, a gasket seems like a good idea.
You don't need to seal the sheathing to framing, you need to connect it to the rest of your air barrier.
In case of the foundation, this means either tape rated for concrete or liquid flash to the foundation.
The ceiling is a bit more challenging. What I found is the simplest is to use wide strip of flashing tape (3m8067 is good for this) with only half the backing removed. Adhere the tape to the top of your sheathing and flap it over the top plate to the inside. Set your roof on top of your wall over the tape, once the roof is up install blocking between the rafters/trusses by the exterior wall. Staple the tape to this blocking. Install the ceiling air barrier (ie 6mil poly) and remove the rest of the backing to adhere it to the air barrier.
Of course it doesn't hurt to adhere the sheathing and have a solid sill gasket as backup.
I would not go crazy on air sealing the interior OSB. A bead of caulk behind the panels around the perimeter is good enough. Try to install the panels as stand ups instead of horizontal as now all the edges directly over studs. This is all I did (solid air barrier on the outside, something but not overkill on the inside) for a garage to studio conversion for my wife and the place costs nothing to heat and is very comfortable.
I'll share some ZIP tape vs liquid flash thoughts from my own experience:
You can use both of them on the same project; there is no need to exclusively choose one or the other. You can use any mixture of seam/flashing tape, stretch tape, and liquid flash.
If you can use tape for a panel joint, do it. It's faster, cheaper, and easier to tape a simple joint than to apply/tool liquid flash. Liquid Flash definitely has its place, like sheathing-to-concrete transitions, and on fussy details like window bucks and steps/flanges that would drive you crazy trying to tape. The liquid flash works great on complex stuff, just apply and spread out. Liquid flash also works great to strike nail heads and fill shiner holes, and to touch up incidental damage in a panel field.
If you have to transition from one to the other, the liquid flash is applied over the tape, not the tape over the liquid flash. Some tapes (the ones without a peel-off liner) will need to be wiped with acetone at the transition area (linered tapes don't have a release coating, so there is no need).
If you wanted a slightly more finished look on the interior, would you consider installing thin wood strips (like lath) to bridge the osb joint? You could caulk the strips as you install. Not saying it's superior from air sealing perspective but would finish the panels in a different way.
Not specifically recommending OSI Quad Max but you may want to look into it as an option. I've used it more than polyurethane so can't compare but it seems to stick well and is also a bit messy.
If you want a non hardening sealant like tremco (perhaps for sealing membranes to framing) but not black and with fewer VOC's (i think) you could look into contega hf from 475. it's pricey, so it's probably worth asking of you really need it.