Air sealing exterior walls in a benign climate
Hello Martin,
I am removing and replacing the siding on an old Victorian located in Berkeley, CA which I believe is climate zone 3. After stripping off the old layers of siding, we will insulate, install plywood sheathing, (nailed at 4 at eh edges and 6 in the field for seismic stiffness), and then a drainable house wrap and fiber cement siding.
My architect says our climate is mild enough that the R-15 we can get from well-installed batt insulation should adequate in the walls without any additional foam on the exterior, if we do a really good job of sealing the exterior. Do you agree?
I understand the use of sealing tapes that would seal all the joints and seams in the plywood and also the laps in the house-wrap. My other question stems from reading a number of articles in GBA which suggest sealing the sheathing to the studs. I have see several recommendations: one is construction adhesive (which dries hard and brittle); the other is polyurethane which stays somewhat soft ( but I don’t know for how long); and the 3rd is acoustic caulk which is supposed to stay soft forever. What do you think of this technique to prevent air movement between stud bays and which caulk or sealant would you recommend?
Thanks,
Craig in Berkeley, CA
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Replies
Craig,
Q. "My architect says our climate is mild enough that the R-15 we can get from well-installed batt insulation should adequate in the walls without any additional foam on the exterior, if we do a really good job of sealing the exterior. Do you agree?"
A. There is no single answer to this question -- other than the obvious answer that you are obliged, at a minimum, to meet local code requirements. If you were building a new house in Climate Zone 3, the 2012 IRC would require (according the the prescriptive requirements in Table R402.1.1) that you install at least R-20 insulation between your studs, or R-13 between your studs plus R-5 insulation on the exterior. So it's not crazy to consider installing a layer of exterior rigid foam or exterior mineral wool on a siding replacement job in your climate zone. Remember, you won't have another chance like this for about 60 years -- the next time your replace your siding.
Q. "My other question stems from reading a number of articles in GBA which suggest sealing the sheathing to the studs. ... What do you think of this technique to prevent air movement between stud bays?"
A. I don't know whether your house has no sheathing -- a fairly common situation in Berkeley -- or existing board sheathing. In either case, installing new plywood sheathing gives you an opportunity to add a fairly airtight barrier on the exterior side of your stud walls. If you have full access to the stud bays from the exterior -- in other words, if there is no existing sheathing -- I suppose that you could prevent air movement between stud bays by installing spray polyurethane foam between the studs. That isn't necessary, though. In most cases, simply establishing an exterior air barrier by taping the plywood seams is enough of an improvement to make a huge difference. Either Zip System tape or Siga Wigluv tape would be an excellent tape for this purpose.
Air sealing existing homes is tricky. Your walls might have air movement near the bottoms of the stud bays (leaks allowing crawl space air to enter the stud bays) or air movement near the top plates (into the attic). These leaks must be tracked down and addressed if possible.
There's no threshold where all of a sudden the insulation becomes enough whereas with less it wasn't. It's just a tradeoff between cost and energy use. So there's no one answer--it depends on things like how much you value reducing your fossil fuel consumption, how conservative you want to be about protecting yourself from future energy cost increases and whether you'd be paying cash or getting a loan to pay for the work, as well what heat source you are using and how skilled your contractor is at installing exterior foam efficiently and inexpensively. But as Martin points out, the code minimum is a good starting point to get an idea of what a good tradeoff is, and you can decide whether you want to go beyond that, or, for a retrofit, go below it.
You asked about sealants. Martin suggested tape instead. If you do use sealant, be aware that standard acoustical sealant is very smelly and continues to be smelly for weeks to months after it's installed. The vapors are probably bad for you, although it's hard to tell since they are largely from crude oil that contains a mix of many different VOCs. My new favorite alternative is Contega HF imported to the US by 475 building products. It stops smelling pretty quickly, and the solvent is ethanol, so it's not any worse for you than breathing the air in a no-smoking bar.
If you go with building tapes there are OK tapes and then really good tapes. Some tests done show that SIGA building tapes hold really well. I would price them out and see what you come up with. You only get one chance to do it right before you seal up the wall.
Followup questions: I would prefer to go with polyiso for the exterior inch of insulation over the plywood sheathing because it seems to have a lower environmental impact. 1. Does the foil facing create an unwanted vapor barrier? 2. If I tape the joints well and provide a rainscreen gap with 1/4" strapping, can I skip the housewrap and expect that the foil facing is an adequate WRB?
Craig,
Walls with exterior rigid foam are designed to dry to the interior. The fact that the polyiso is a vapor barrier will not cause problems in your climate zone.
If you want, you can detail the exterior foam as a water-resistive barrier (WRB), and skip the housewrap. However, if you decide to go that route, you should know that the details are somewhat tricky. Here is a link to an article that explains what you need to know: Using Rigid Foam As a Water-Resistive Barrier.