Air Sealing Weekend: The Ten Cheapest Fixes

Air sealing is the project that makes the next inch of insulation matter. Warm air leaks into the attic through hatches, wire holes, and the tops of walls, and cold air is pulled in lower down to replace it. The ten fixes below are the ones that usually cost less than a tank of fuel and can be done with a caulk gun, weatherstrip, and a careful afternoon in the attic.

They are not a blower-door test. A blower door would rank the leaks with a number. If you cannot borrow one, you can still close the holes you can see and feel. Start at the ceiling, because that is where the pressure from warm air is trying to leave.

Read the heat-loss guide first if you have not measured anything. If the calculator already says the windows and the air-change line dominate, this weekend is the right project. If the ceiling is bare, plan to seal and then insulate. Do not pile new insulation over an open hatch and call it done.

Difficulty

Level

Beginner

Time

A weekend

Skills

Caulking, cutting weatherstrip, and moving carefully in an attic

Cost estimate

$25–70

One or two tubes of sealant, a can of low-expansion foam, a roll of weatherstrip, and a door sweep cover a small house. A large attic with many penetrations can use the whole budget on foam. Skip outlet gaskets if the money is better spent on the hatch.

Materials

Item	Qty	Notes
Paintable acrylic latex or silicone sealant	2 tubes	Acrylic where you will paint. Silicone where it stays wet. Fire-rated sealant at a rated assembly.
Low-expansion spray foam	1 can	Window-and-door foam. High-expansion foam bows thin frames and is a poor choice around wires in a tight box.
Closed-cell foam tape	One roll	For the attic hatch and, if needed, an exterior door stop.
Door sweep	1	The threshold gap is often larger than the leaks around a window.
Caulk gun, utility knife, gloves, headlamp	1 set	A plank across joists if you must cross the attic.

The ten fixes, in the order that usually matters

1. Weatherstrip the exterior doors. Close the door on a strip of paper and pull. If the paper slides out easily, that spot is open. Apply a self-adhesive strip to the stop, not to the door face, and do not build the strip so thick that the latch no longer catches.

2. Add or replace the door sweep. A gap that shows daylight under a door leaks like an open duct. A sweep should meet the threshold along the whole width without scraping so hard that you stop using the door.

3. Gasket the attic hatch. The hatch is a hole in the best place for warm air to escape. Stick closed-cell tape on the stop so the hatch compresses it. Then screw a piece of rigid foam to the attic side of the hatch, thicker than whatever is convenient, and keep it from catching on the frame.

4. Seal the pull-down stair. A folding stair is a larger hatch. An insulated cover that sits over the opening from above, with a gasket, is the usual fix. Weatherstrip on the stair panel alone rarely survives the hinges.

5. Foam or caulk the wire and pipe holes in the top plates. From the attic, look along the tops of the walls. Every cable that disappears into the wall is a path. A small shot of low-expansion foam, or sealant if the gap is a crack, closes it. Do not fill an electrical box. Do not bury a junction that someone will need to open.

6. Treat recessed lights as a fire question first. If the fixture is not rated for insulation contact, do not cover it and do not foam the inside of the can. An airtight, insulation-contact fixture can have the gap around its housing caulked to the drywall. When you are unsure of the rating, leave the insulation away from the can and seal only what the manufacturer allows. Replacing a leaky can with a rated airtight fixture is sometimes the real fix, and that replacement is electrical work.

7. Seal the rim joist where you can reach it. The rim, or band, joist at the edge of the basement or crawlspace is a long joint between the wood frame and the foundation. Sealant at the gaps you can see is the cheap step. A later step is a piece of rigid foam cut to fit each bay and sealed at the edges. Keep foam away from a flue, a chimney, and any combustible clearance the appliance instructions demand. Foam plastic often needs a thermal barrier, such as drywall, where code says it cannot stay bare. Check that before you line a whole basement.

8. Gasket a few exterior-wall outlets, then stop. Foam gaskets behind outlet and switch covers on outside walls make a small difference and cost little. They are not a substitute for the attic. Turn the circuit off before you remove a cover, and do not do this on a backstabbed, cracked, or warm device. If the box is loose or the wiring looks damaged, leave it for an electrician.

9. Fix the dryer duct damper. A dryer termination that sticks open is a hole, and a duct full of lint is a fire. Clean the duct, confirm the damper flaps shut, and do not vent a dryer into an attic or a crawlspace to "save the heat."

10. Close the fireplace when it is not a fire. An open damper lets heated air go straight up the chimney. Close it when the fireplace is cold. A wood stove or a gas appliance is a different device with its own air requirement. Do not cap a flue that an appliance still uses. Glass doors on a masonry fireplace reduce the room air lost up the throat while a fire is burning, and they do not make a damp chimney safe.

What this weekend will not do

It will not turn a single-pane window into a double-pane window. It will not fix a wall with no cavity insulation. It will not tell you the new air-change rate. Afterward, the heat-loss sketch is worth running again with a lower air-change guess, and the honest follow-up is a blower-door test if you can arrange one.

If a combustion appliance is in the house, do the seals with the carbon monoxide alarms working, and watch for backdrafting at the water heater or furnace after you close the large holes. A house can be too tight for an orphaned atmospheric water heater. That is a reason to stop and get a combustion-safety check, not a reason to skip the hatch gasket and hope.

1. Walk the ceiling and the doors with a tissue

   On a cold, windy day, a tissue or a stick of incense shows moving air at hatches, outlets, and door edges. Mark the hits. Do not use a flame.

2. Buy for the hits you marked

   Weatherstrip and a sweep for doors. Gasket and foam for the hatch. One can of low-expansion foam for the top plates. Fire-rated sealant only where you are sealing a rated penetration you understand.

3. Do the attic in one trip

   Hatch gasket, stair cover if you have one, top-plate holes, and a decision on each recessed light. Wear a mask if the insulation is dusty. Walk on joists.

4. Do the doors and the dryer on the ground

   Weatherstrip, sweep, and a dryer duct that actually closes. Skip any flue you are not sure is abandoned.

5. Recheck combustion appliances

   Run the furnace or water heater and look for a strong draft in the right direction. If you smell flue gas, stop using that appliance and have it checked.

6. Write down what you closed

   The next insulation or solar project should start from a ceiling that is no longer a collection of holes.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

• https://www.builditsolar.com/References/Calculators/InsulUpgrd/InsulUpgrade.htm
• https://www.builditsolar.com/Projects/Conservation/PCWindowCvr/PCStormWindow.htm
• https://www.builditsolar.com/Projects/Conservation/ThermalShades/Shurcliff%20Thermal%20Shades.htm
• https://www.builditsolar.com/Projects/Conservation/bubblewrap.htm
• https://www.builditsolar.com/Projects/Conservation/Recirc/RecircEnergy.htm
• https://www.builditsolar.com/Experimental/GreyWaterHX/
• https://www.builditsolar.com/References/Half/ProjectsConservation.htm