Basement Weatherization

We’ve jumped into winter with a dump of snow and a drop in temperatures, and this is our first winter since we moved down to the basement. The radiant heated floors are a big plus, and having warm feet and 72° indoor temperatures is a welcome change from the upstairs, where our temporary radiators were not up to the task of heating our drafty house, even when supplemented by electric heaters. We had a bit of a problem earlier in the season, mostly because I wasn’t paying attention to a critical detail of our new heating system. In addition to the thermostat and the boiler temperature, there is a dial that controls how warm the basement floor loops can get. Lester, our radiant installer, had left it at a reasonable 80°, but since our brick exterior walls aren’t insulated (yet) and there’s nothing between our heads and the unheated upstairs but a layer of OSB, that wasn’t cutting it for keeping our air temperature where we wanted it. Once I discovered the dial (and Sarah pointed out that she had told me about it after Lester told her), we cranked it up and now the basement is staying warm, mostly.

Window caulking and foam

I say mostly because there was a pretty big exception. Even before the dramatic temperature drop of this week (-7° last night), the kids’ bedroom was cold and our bedroom wasn’t much better. I tried feeling with my hand, using a laser thermometer, and blowing out a candle to watch the smoke, but I couldn’t isolate where the cold air was coming in. I found a few spots around one of the windows in the bay and I caulked it, but it didn’t make much of a difference. We finally broke down and bought a tiny thermal camera that plugs into a smart phone. It’s a lot cheaper than a full size thermal camera, and it actually does a pretty decent job. In the image below, you can see the spectrum temperature range on the left side, from black to white.

Bay window thermal image

I took it into the kids’ room and started finding cold spots (and thus leaks) right away. Most of this was centered on the bay windows, so I spent a fair amount of time caulking and spray-foaming all around, switching back to the camera to get new readings on the heat in various spots. I found a lot of the cold air was coming from the top of the brick walls along the front, and I filled those cavities with “big gap” Great Stuff.

Rob installs rigid foam

After this work, we were still not satisfied with how cold it was, and the camera started pointing us to the sides of the house where the floor joists above rest on the top of the brick wall, notched into a 6×8 rim board. We plan to fill all of these floor joist cavities at the outside wall with proper closed-cell spray foam, but not until all the mechanicals are in and we can do the rest of the exterior walls upstairs at the same time. Given that, we didn’t want to try and fill these all with Great Stuff, and Roxul mineral wool batts wouldn’t do much to stop the air flow. Instead, Rob, Mike, and David came down to help and we cut pieces of leftover 2″ rigid foam (originally for under the basement slab) and fit them between each joist for the length of the kids’ bedroom. The effect was dramatic: the room went from 10° colder than the living room to 3°. The next evening I followed up with our bedroom, but I ran out of extra foam before I could fill all of the joist bays. Even so, it was a noticeable improvement. I went back and found some more cold spots with the camera that I added some spray foam to and all-in-all it’s a lot more comfortable. Of course, now Sarah is saying the kitchen is cold, so I may still have some more work in front of me, but the camera is proving invaluable. Plus, I can lend it to friends so they can use it on their own houses. Thanks to Mike, Rob, and David for their assistance!