What is the most practical air sealing technique for new construction?
I am building a home in climate zone C4. My wall assembly will consist of 2×6’s on 24″ centers, OSB sheathing, then a layer of 1″ XPS insulation, with my wrb on the outside of the insulation. I will then have rough cut reverse board and batten siding.
What is the most practical method of air sealing my house? I have read articles where people caulk around every stud and framing member on the inside of their wall cavity. Is this best? Wouldn’t it be easier to caulk around the outside edges of my sheathing? What kind of caulking do you recommend? Or should I tape it? and if so, is there some kind of tape that will adhere to OSB permanently and is readily available (big box store)?
I was not planning on taping my seams on the xps insulation. Partly because it will be behind my wrb and partly because I haven’t heard of a tape that works really well. Is this foolish?
Thank you for any advice. It is very much appreciated.
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Replies
Kail,
First of all, you can buy tapes to seal the seams of XPS or OSB. For information on what tapes to buy, see Return to the Backyard Tape Test.
If you live in the U.S., you shouldn't limit yourself to tapes that you find at Home Depot. It's easy to shop for tapes on the web. These tapes can be delivered to your door in a few days. It would be nuts to try to tape the seams of your OSB or XPS with the wrong tape, just because you couldn't plan ahead enough to order the tape a few days before you need it. (After all, you are building a house. So plan ahead.)
If I were you, I would tape the seams of the OSB sheathing with Siga Wigluv or 3M All Weather Flashing tape. Remember to come up with a plan to seal air leakage where your ceiling air barrier (or roof air barrier) meets your wall air barrier. You also need a plan to seal penetrations through your wall.
Thanks for the great advise Martin. Do you think a 2" wide tape will be sufficient?
Kail,
Yes, 2-inch wide tape should work for OSB seams or XPS seams.
Sills are notoriously leaky; use a rubber building gasket under the sill and under your exterior walls. Tape all of your sheathing joints. Caulk the top/sides/bottom/edges of the wall sheathing. Use a good, taped membrane under your trusses, or sheath and tape the bottom of the trusses with plywood, and do this before you stand up the inside walls. Tape your windows. The new tapes are revolutionary and work.
Do you guys recommend a particular type of caulking that would have a good quality to cost ratio?
I have wondered the same thing, Kail. I'm building a new home and have decided to cover the entire exterior of my OSB sheathing with a peel and stick membrane. I will be covering about 1900 square feet at a cost of around $1100. I know it's expensive compared to taping seams or caulking, but I figure it's the best solution. It will easily cover the sills and joints that I've heard notoriously leak air. In addition, the membrane has some self-healing properties that will help seal around screw penetrations through the membrane. Plus, if any area does get punctured, the rest of the membrane is still stuck to the OSB and incoming/escaping air won't be able to easily transfer to other areas. This membrane will function as my air and vapor barrier and WRB. Unless I'm missing something, this seems like the best (albeit more expensive) solution.
Adam,
It really depends on your drying strategy and the make-up of your wall. Covering the entire surface with a membrane precludes drying to the exterior and can create a wrong side vapour barrier. It only works in some situations.
Adam,
The only time when it is safe to install a vapor barrier on the exterior side of your wall sheathing is when you plan to put 100% of your wall insulation on the exterior side of the peel-and-stick. This approach is called PERSIST; here is a link to an article that explains the system: Getting Insulation Out of Your Walls and Ceilings.
If you have insulation between your studs, your plan will lead to moisture accumulation and rot.
Adam and Kail, if you're looking for the BEST solution, consider a liquid-applied weather barrier (LAWB). Unlike impermeable peel-and-sticks, these products are permeable to allow drying to the outside, but completely block liquid water and, of course, air. Typically they're used in commercial construction, but I did my house. I have no data to support their superiority, but I'm just not convinced of the longevity of any exterior tape. LAWB probably costs more in labor and materials than peel-and-sticks, and certainly much more than sheet housewrap. But I'm hoping it's a more effective and durable barrier.
For more information on liquid-applied WRBs, see Housewrap in a Can: Liquid-Applied WRBs.
It's also worth pointing out that a few manufacturers make vapor-permeable peel-and-stick products. One such product is WrapShield SA Self-Adhered.
Martin and all,
I am doing a PERSIST/REMOTE wall system, but using a traditional vented roof with blown cellulose. My empty stud wall will be sheathed with OSB, covered with peel-stick, and then covered with 6-8" of EPS. Vertical furring strips and Smartside siding will cover that, creating 3/4" rainscreen gap. So yes, any drying of the wall will be to the interior of the membrane. I live in zone 7a, eastern North Dakota. Thanks for the info about the vapor-permeable membrane, Martin. I will contact a rep and ask about pricing. One question though, does it really matter if the membrane is vapor-permeable since I have multiple thick layers of foam on the exterior, which is virtually impermeable?
Adam,
If you are building a PERSIST wall, with all of the insulation on the exterior side of your peel-and-stick, it's perfectly OK to use a vapor-impermeable peel-and-stick product.
Paying for a more expensive vapor-permeable product would be a waste of money.
Martin, I have been using the 3M All Weather tape you recommended and it is really great stuff. How would you seal the ceiling air barrier to my wall air barrier? On my gable ends do I need to seal the outside of my sheathing above my ceiling line? Or is this unnecessary? I am planning on installing 1/2" polyiso on the lid of my ceiling and sealing it before sheetrock.
Kail,
You need to install a membrane (for example, polyethylene or peel-and-stick) or rigid material (for example, OSB) on top of your wall plates, and this membrane or material should cantilever or extend into the interior of the house so that you can tape or seal your gypsum drywall to the extension. On the exterior, the membrane or material should be sealed or taped to the air barrier of your wall.
Here are two links to help you:
REMOTE: A Manual
A Practical Air Sealing Sequence
The first image below is from the REMOTE manual; the second is from the referenced FHB article.
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