There are 46 rim joist bays in our basement. I know because I numbered every one of them with a carpenter's pencil on Saturday morning, starting at the sump pit and working clockwise, while the first load of laundry ran behind me.
Why the rim joists
In a basement like ours, the floor joists sit on a wood sill plate on top of the concrete block walls, and the rim joist is the board that closes off the ends of the joists all the way around the house. On our two long walls, that makes a row of little boxes between the joist ends. Ours were bare wood, with a few gaps at the sill you could slide a pencil into, the cold block below and the outside air an inch and a half of lumber away.
The energy assessment two weeks ago flagged them, but I already knew. Last winter, standing at the washer in January, you could feel a breeze on your ankles from that corner. Rim joists were also the easiest item on the assessor's list to do myself. Nothing higher than the second step of a stepladder, no attic, just a lot of cutting and a lot of foam.
Materials, and a call to the city
Most foam board isn't meant to be left exposed inside a house, because of fire. Before buying anything, I called the city's building inspection office and asked about leaving foil-faced foam exposed in an unfinished basement. The answer I got was to use a board that's listed for exposed installation, or else cover it. So I bought fire-rated, foil-faced board rated to be left exposed. It cost more than the regular stuff. Another city might answer differently, which is the whole reason to call.
What we spent, $310 total:
- Three 4-by-8 sheets of 2-inch fire-rated foil-faced foam board: $171
- A foam gun: $45
- Four cans of gun foam: $38
- Foil tape: $15
- Respirator cartridges and gloves: $29
- A long serrated knife for cutting foam: $12
The foam gun was the best $45 of the weekend. The cans with the little straw are fine for a gap or two. For a whole basement, the gun lays a bead as thin as a pencil, stops without dripping, and a can goes a lot further.
The weekend
Saturday was all measuring and cutting. The bays are close to the same size, but not the same, because nothing in a 1952 house is. I measured each one, wrote the size on the joist next to its number and cut every piece on the garage workbench, a quarter inch undersized so it would slip in. Then I labeled each piece with its bay number and stacked them in order in a laundry basket. At work, we'd call that kitting: every part for every job in one place before anybody starts assembling. It was the most satisfying hour of the weekend.
Sunday was install. Push a piece into its bay, foam the gap around all four edges, tape the seams where the board meets the sill. Around wires, I cut notches and foamed them in gently. Around the frost-free sillcock I put in back in May, I notched the board for the pipe and foamed carefully. I kept everything clear of the dryer vent.
My first mistake was doing the first can without gloves, because I was "only going to do one bay." Foam doesn't wash off. It wears off, over about three days. Tessa looked at my hands at dinner and said, "You look like you lost a fight with a marshmallow."
The second thing isn't a mistake, just a choice. The bay on the back wall behind the dishwasher sits right where the kitchen water line comes up from the basement, the PEX we ran in July. It's crowded: the water line, the drain, a cable and an old abandoned pipe hole. I couldn't get a board in without wrestling the pipes, it was late Sunday and I was tired, so I skipped it. I wrote "COME BACK" on the joist next to it, with a plan to return with a can of foam and a smaller knife. The other 45 bays got done.
Did it work?
Thursday morning it was 39 degrees outside, the coldest morning of the fall so far. I went down to switch the laundry and stood at the washer in my socks. No breeze on my ankles. That corner used to feel like standing near an open window, and now it just feels like a basement.
The real number comes later this month, when the insulation crew finishes and the blower door goes back in the front door for the retest. I'm curious how much of the change will come from the basement boxes and how much from the attic. There's no way to separate the two, which bothers me more than it probably should.
Trip count: 2. The second one was for a fourth can of foam.
