In Part 1 of this article, we introduced the two main types of products you’ll use for air sealing—and more importantly, outlined a framework for prioritizing air sealing projects in your own home. In Part 2, let’s walk through that framework and introduce some of the specific projects you might encounter along the way, including the products you’ll likely use for each. Future articles will provide detailed, step-by-step instructions for many of these projects.
As an illustration, let’s pretend this house drawn by the Historic American Buildings Survey is the one you’re paying to heat and cool:
For the sake of simplicity, let’s assume this house is using a modern HVAC system housed in the basement, and that the fireplaces you see have become ornamental. The orange outline indicates some ductwork running from a furnace or fan coil unit (controlled by a thermostat mounted below the steps, as shown in blue). Just like in the article on your thermal envelope, I’ve highlighted the area you’re trying to heat and cool in green. The dashed red line surrounding it is your thermal envelope, and it’s where you’ll be focusing your air sealing efforts (in three dimensions, of course). Notice how this house’s three (!) attics and one basement are not included. Unless someone is living in one of those spaces, there’s no reason to be wasting energy conditioning them—and no reason to waste your time and money making sure they’re perfectly air-tight.
OK, let’s go through the framework, step by step. (A quick note: there are definitely projects where it makes sense to think about air sealing and insulating at the same time while you’re doing the work, but for simplicity this article only focuses on air sealing.)
1. Big Holes in Critical Locations
In Part 1 of this article, I mentioned some locations that I would consider critical in a typical house. Due to stack effect, those include the top of your thermal envelope, and the bottom of your thermal envelope when above a basement. I also included ductwork running through unconditioned spaces, and the area near your thermostat. So what projects will address the biggest holes in the most critical locations in the example house above?
- Plug those chimneys. Notice how those two tall stacks of brick shoot right up through the thermal envelope? Don’t forget that chimneys are literally designed to draw warm air out of your house. To top it off, this particular house is old enough that there’s a chance it has no dampers at all—mine doesn’t, and it was built a century later. No dampers means a hole in your thermal envelope of something like 1 to 3 square feet, per chimney—hopefully your largest openings by far! Even if your chimneys do have dampers, there’s a very good chance they don’t make a tight seal. You can make and install your own removable airtight plug for somewhere between cheap and free, and it’s one of the quickest projects you can check off your list.
- Seal electrical boxes, attic hatches, chimney perimeters, and other penetrations in the top of the envelope. Remember, all those little gaps add up to one big hole. This house has a lot of attic space, and therefore a lot of ceiling underneath. At probably the most important location in your thermal envelope, it’s good to be a little fussy, which means this is a project—really, a bunch of little projects—that should take you some time. Working from above and below (that is, from the attic and from under the ceiling), you’ll probably want to:
- Caulk all the ceiling junction boxes.
- Apply weatherstripping to the attic hatch(es) and/or door(s) and caulk their trim.
- Caulk the perimeter of the chimneys where they penetrate the ceilings (you may have to use fire-rated materials).
- Caulk or spray foam other joints, gaps, or cracks in the ceiling plane visible from the attic side.
- Seal the perimeter of the basement ceiling, as well as wiring, plumbing, and other penetrations. It’s probably OK to be a little less fussy here—for example, if you don’t have a subfloor, I wouldn’t recommend trying to air seal the joints in the floorboards over your head. However, a typical basement ceiling in an old home will still take you a while to air seal. Working primarily from below (that is, from the basement), you’ll probably want to:
- Caulk or spray foam wiring and plumbing penetrations.
- Caulk the perimeter of the chimney where it penetrates the ceiling (you may have to use fire-rated materials).
- Caulk or spray foam joints and gaps at the perimeter of the basement where the walls and floors come together—forget about what’s a rim joist vs. a sill plate (or whether your house even has those), just focus on the idea of keeping outside air outside, and keeping outside and basement air separate from the first floor living area.
- Working from the first floor, caulk the joint where the baseboards meet the floor. If your baseboards have a natural finish and brown caulk is a bad match, at least buy some felt and stuff it in the largest gaps.
- Seal the basement ductwork. You’re paying to heat and cool the air in your ductwork (orange outline), so let’s make sure that air doesn’t leak out into an unconditioned space before it gets to you. Depending on how much ductwork is running laterally in the basement, this could be a relatively short or a pretty time-consuming job—but of course the bigger the job, the bigger the potential savings. (I sealed something like 750 linear feet of joints in my basement ductwork!) You’ll want to use duct mastic, not foil tape (and absolutely not duct tape). Otherwise you’ll be doing the job again in a few years.
- Apply weatherstripping to the basement door near the thermostat. OK, I’m making this up a little bit based on my own experience—let’s pretend that the door shown next to the thermostat (blue outline) leads down to the unconditioned basement. Cold air leaking through that door could trigger the thermostat unnecessarily, resulting in the HVAC system running too frequently. Sealing the door perimeter will involve weatherstripping, a door sweep, and possibly a new threshold.
2. Big Holes in Other Locations
I’ll repeat what I said in the first half of the article: at a certain point, you’re going to have to start relying on your senses. Seal the holes you can see or feel in your daily life. Don’t just rely on a list put together by Green Old Home, the Department of Energy, or anybody else. You should know your old home better than anyone—giving you an advantage over any professional you could hire, as long as you’re paying attention.
Obviously we don’t know what it feels like to live in this example house (though you apparently you can tour it), but let’s use our imagination to identify some more projects that will air seal some other big holes:
- Apply weatherstripping to the exterior doors. Since we already addressed any attic doors and the basement door near the thermostat, it looks like the only remaining doors at the edge of our thermal envelope are leading to the exterior. If you can see daylight around the edges of a door, or if you feel drafts in the winter, spend some time adding weatherstripping, a door sweep, and maybe a new threshold.
- Tighten the windows. You may have noticed that windows weren’t included on the list of “critical locations.” That’s because weatherization professionals have come to deemphasize them compared to the other parts of the envelope that we’ve been discussing. However, they’re still important to address once you’ve got the top and bottom of our envelope under control—especially if you can see daylight around the edges or feel cold air pouring in! There are a few approaches here that you can take separately, or combined:
- Apply weatherstripping to any windows without it.
- Apply plastic window film to all your windows in the winter, when you’re not using them. (Think about whether you ever use your basement or crawlspace windows, if they’re part of your thermal envelope—you might be able to leave the film on all year!) This is a bit of a workaround, because the goal is to create a secondary airtight barrier rather than worry about getting your existing window super tight. (Though if you can do both, there is an even greater benefit.)
- Better yet, install an operable interior or exterior storm window. Same principle, and you don’t have to remove and reinstall every year like film (using tape that can damage your finishes).
- If you’re feeling more ambitious, reglaze your old windows. This means partially dismantling your trim, removing the sash, and the putty or stops that are holding your glass in place. When you reinstall the putty and reassemble your window, you can make adjustments and add sealant to make the whole assembly more airtight. Combine this with a storm window, and you’ve got two tight planes with an air gap—similar to the concept behind modern insulated glass units (but repairable!).
- Caulk electrical boxes in exterior walls. Remember, all those little holes add up to one big one.
- Seal the intersection of the porch roof and the second floor. The porch roof is probably supported by joists that continue straight through the wall to support the second floor. The result is that there is a big hole in your envelope—the space(s) between the joists. You may encounter a similar situation with continuous joists above other unconditioned spaces like garages or enclosed porches. The tricky part is accessing the joist cavity above the ceiling. If you can access it, you’ll probably want to seal the big hole(s) with rigid foam board installed with spray foam at the perimeter. If you can’t access the area and don’t want to demo your porch ceiling, you may want to air seal the porch ceiling instead—essentially redefining your thermal envelope—using the techniques for other ceilings to try to minimize air flow to and from that joist cavity.
3. Small Holes in Critical Locations
Let’s keep this one short, since we’ve already covered critical locations. Just remind yourself that once you’ve taken care of the biggest holes in your thermal envelope, it’s time to make sure you’ve really done an A+ job tightening up those critical areas. Is there a project on your uppermost ceilings that you put off because it was too much of a pain? Are there parts of your basement ceiling you didn’t seal because you noticed other, bigger issues elsewhere? Did you really apply mastic to 100% of the seams in your ductwork? Double-check.
The point is to make sure those critical locations are as tight as can be before you move on. Now that you’ve been air sealing for awhile, there’s a good chance you’ll notice things gaps and cracks that you didn’t before. Plus, it’s never a bad idea to double-check your work.
4. Small Holes in Other Locations
OK, it’s finally time to get really picky. We’ve already covered the major types of air sealing projects—now you’re just going to apply those same techniques along the entire perimeter of your thermal envelope. Remembering to think in three dimensions, the goal is for you to be able to walk around your home following the continuous boundary where air is not allowed to pass. Your caulk gun will be your best friend at this stage, because caulk is uniquely qualified to handle those remaining 1/16″ gaps. And thanks to your earlier work in the attic and basement, you’ll have enough practice to be confident making seals that are small, subtle, and don’t look terrible.
The other thing you might want to think about at this point is those walls and ceilings of your house that are outside your thermal envelope. You know, the exterior walls and ceilings of your vestibule, sun porch, garage, etc. They’re not a good place to start weatherizing your home, but if you tighten them up you’ll create a nice buffer of semi-conditioned spaces between your conditioned spaces and the unconditioned outdoors.
But I’ll be honest: you’re almost certainly in the land of diminishing returns at this point. By the time you’re caulking the tiny gap below the baseboards of your sun porch, you’ve probably already achieved the majority of your energy and money savings from air sealing. However, I encourage you to finish the job for two reasons:
- Imagine the sense of pride you’ll feel when you know you did literally everything you could, short of remodeling your whole house, to air seal it.
- The better your house is sealed, the better your insulation will perform.
Yes—when you’re done air sealing, it will finally be time to shift your attention to insulating your old home. Whether, where, and how much to insulate are subjects for another day. But while you’ve still got your caulk gun out, remind yourself that you’re working on part of a system. And the better you optimize the first part of the system, the better the second part will function.
But let’s snap back to the present. Your old home still needs a lot of air sealing. It’s time to talk about caulk.