Seal the rim joist and your basement stops fighting you

Quick answer: Cut rigid foam board to fit each joist bay, seal every edge with fire-block canned foam, and don't stuff fiberglass in there. Check the wood for rot first. Check your local code for a fire covering. Most houses take one weekend.

Start at the ugliest spot in the basement

Walk down with a flashlight and look up at the top of the foundation wall. That band of lumber sitting on the concrete, running the whole perimeter, is the rim joist. The sill plate is the board under it. Between them and the concrete are seams you can often slide a business card into.

Cold air comes through there in winter. Humid air comes through in summer. Both are why your floors are cold and your basement smells like a wet dog.

If you see pink or yellow fiberglass shoved into those bays, look closer. It's usually gray along the edges, a little dark, maybe damp. That's the problem, not the fix. Fiberglass stops almost no air. Moist air slips through, hits cold wood, and condenses. The batt holds that water against your house like a sponge.

The job is air sealing first and insulation second. Foam board does both.

Inspect before you buy anything

Take a screwdriver and probe the sill plate and rim joist every foot or so. Solid wood resists. Rot gives way like cake.

Look for these:

• Water stains or white crusty residue on the concrete
• Dark, fuzzy growth on the wood
• Sawdust piles or mud tubes, which mean insects
• Daylight, or a draft you can feel with the back of your hand

Soft wood, live insects or a wall that's actively leaking means stop. Insulating over a problem hides it and feeds it. Fix the water first, which usually means gutters, grading and downspouts, then insulate. A termite or rot problem is a pest-control or contractor call. Knowing when to stop is half of home repair, and a free estimate costs nothing.

Also note what passes through the rim: the dryer vent, cables, pipes, a gas line, maybe an old wire that doesn't look like modern wiring. You'll work around all of it. If you find cloth-wrapped wiring that looks ancient, leave it alone and call an electrician.

What you need

• Rigid foam board, 1.5 to 2 inches thick. Polyiso, XPS and EPS all work. Polyiso gives you the most R-value per inch, and EPS is the cheapest and most forgiving.
• Cans of fire-block foam sealant, the kind labeled for fire blocking. Buy more than you think. A bay eats a surprising amount.
• A caulk gun and a tube of sealant rated for gaps in wood and concrete
• A utility knife with a fresh blade, a tape measure and a straightedge
• Safety glasses, gloves and a respirator rated for the foam you're using
• A marker, because you're going to label things

You already own most of this if you've read the ten tools that cover household repairs.

Cut and fit

Every bay is a different size. Houses settle and framers have bad days. Don't measure one and cut twenty.

Measure each bay at the top, middle and bottom. Use the smallest number. Cut the board about a quarter inch smaller than the opening in both directions. Score it with the knife along a straightedge, snap it, and cut the back skin. Write the bay number on the board and on the joist.

Before you install anything, sweep the ledge and vacuum the dust. Foam sticks to clean surfaces and holds on nothing else.

Then seal. Run a bead of caulk or foam along the seam between the concrete and the sill, and the seam between the sill and the rim joist. Do the same around pipes and wires that pass through. This step matters more than the foam board. It's also the dull one most people skip.

Press the board into the bay. It should sit snug with a thin gap around all four edges. Fill that gap with fire-block foam, shaking the can first and going slowly. Foam expands, so fill about half the gap and let it grow. Overdo it and it pushes the board out and leaves you scraping cured foam off your joists.

For a thicker result, glue a second layer over the first with the seams offset. Seal those edges too.

Tape the board seams if you like. It's cheap and makes the finished wall look tidy.

The fire rule people miss

Foam plastic burns. Many building codes require it to be covered in a basement by a thermal barrier, usually half-inch drywall, or by a product tested and approved for exposed use. Some places carve out exceptions for rim joists. Rules vary by city and by year.

Call your local building department before you start. Ask what's required for foam on a rim joist in an unfinished basement. The call takes five minutes. It can save you from tearing out finished work, or from a surprise at resale.

If you hang drywall over the foam, leave the gap to the framing sealed and finish it the way you would any wall.

Your furnace and water heater deserve a look

Tighten a basement and you change how air moves in it. If you have a furnace or water heater that vents up a flue using the house's air, sealing leaks can affect how well it draws. Carbon monoxide is odorless.

Before you start, put a working CO detector on every floor. After you finish, run the furnace and water heater and make sure they vent normally. If you're unsure, have an HVAC or gas technician check the draft. Sealing the rim doesn't automatically cause a problem, but you don't want to find out the hard way.

If you live in an area with radon, test after you seal. Test kits are cheap at hardware stores.

Foam board or a pro with a spray rig?

Closed-cell spray foam gives a tighter seal in odd spots, and it needs training, a respirator and a rig. Skip DIY spray kits for a whole basement. Hire a licensed installer, get the quote in writing, and ask what covers the foam to meet fire code. Our piece on what your home inspector won't check is a good list of what to ask about while someone's in the basement.

Foam board and cans cost less and let you do the work at your own pace. You'll pay for that in an afternoon of cutting, and the quarter-inch gaps won't be perfect. Fine. They'll be sealed.

What you'll notice

The floor above feels different the first cold morning. The musty smell in a corner fades over a few weeks. The furnace cycles a little less, and the old gray fiberglass is in a trash bag instead of holding a puddle against your house.

Put "check the rim joist" on your fall list in the home maintenance calendar. Look for new gaps, damp spots and pest signs once a year. Sweep up your foam scraps when you finish. The next person to work down there, maybe you in ten years, will notice.