Is one room always colder than the rest of the house, no matter what you do with the thermostat?

Walk around the edges of that room on a windy day and hold your hand near the baseboard. If you feel cool air moving, you have found the cheapest insulation upgrade in the house.

Why baseboards leak

Behind the baseboard is a gap between the drywall and the subfloor. In older houses it can be half an inch wide, and it is open to the air space inside the wall.

Cold air comes up through that gap and runs along the floor. Every room in the house has one, and sealing them takes a tube of caulk and a free afternoon.

What you need

• A tube of paintable acrylic latex caulk, the kind marked for interior trim.
• A caulk gun.
• A damp rag.
• Optional but worth it: a caulk smoothing tool.

Skip silicone for this job. It does not take paint and it is a nightmare to clean up.

The technique

Cut the tip at a 45 degree angle, small. Run a thin bead along the seam where the baseboard meets the floor.

Thin bead, not a fat one. You can always add more. A fat bead looks sloppy and wastes half the tube.

Run your finger or the tool along the bead right away to press it in and flatten it. Wipe your finger on the damp rag every few feet so you are not smearing caulk down the wall.

The baseboard you should not caulk

Do not caulk the bottom of a baseboard on a floor that gets wet, such as a bathroom, if the floor is above a crawlspace with poor drainage. You want water to escape, not pool.

Also skip the caulk on the top edge where the baseboard meets the wall if the walls are plaster. Movement will crack the bead within a season.

Let it cure

Give it a full day before painting or moving furniture back. Touch it too soon and it pulls away.

I did an entire bedroom in one afternoon last October. That room now holds heat as well as any other room in the house.