How to seal windows for winter from the inside

There is a right order for this job. It is not the order the shops suggest, because the shops sell film. Film is excellent and it comes last, after the things that are free or permanent.

Step 1: tighten the lock. It costs nothing.

On a double-hung window, the cam lock is a clamping device: turning it pulls the upper and lower sash tightly together at the meeting rail. If it is loose, misaligned, or painted so it only half-turns, that joint stays open along the entire width of the window regardless of what you seal it with.

1. 1Turn the lock and watch the sashes. They should visibly pull together.
2. 2If the cam does not engage fully, loosen its two screws, close the lock, and retighten them in the engaged position.
3. 3If the lock is painted, free it with a blade rather than forcing it.
4. 4On casement windows the equivalent check is the handle and the hinge stays: a casement that no longer pulls tight onto its seal needs the keeps adjusting, not more weatherstrip.

Step 2: find the leaks before you seal anything

1. 1Close all the interior doors in the room and switch on a bathroom or kitchen extractor fan. That slightly depressurizes the house and pulls outside air in through every gap.
2. 2Light an incense stick and hold it a couple of inches from the window's edges, moving slowly.
3. 3Watch the smoke, not the stick. Where it pulls sideways or breaks up, air is coming in.
4. 4Mark each spot with masking tape. Work the meeting rail, both sash channels, the sill, and the joint where the frame meets the wall.

Step 3: seal the sash

• Sash channels get a V-strip. Tape will not survive being slid across.
• Meeting rail gets a V-strip or a thin D-section — thin enough that the lock still closes.
• Under the lower sash gets a D-section on the sill, or rope caulk if the window is staying shut.
• Measure first. A seal thicker than the gap stops the window closing, and a stiff window gets its weatherstrip torn off by March.

Step 4: caulk the perimeter

Remove any cracked old caulk, push backer rod into anything deeper than about a quarter inch, and run a fresh bead where the frame meets the wall. This is permanent, it is the one step you never have to repeat, and on an old house it is often the largest single improvement of the day.

Step 5: film the window

1. 1Clean the frame with alcohol and let it dry. Skipping this is why film kits fail.
2. 2Run the double-sided tape in one continuous path around the frame, with no breaks at the corners.
3. 3Press the film on loosely, top first, then sides, then bottom.
4. 4Shrink it with a hair dryer on high, moving constantly, from the middle outward.
5. 5Trim the excess with a fresh blade flat against the tape.
6. 6Re-run the incense test around the tape line. One unsealed corner undoes the whole window.

What to leave alone

• Weep holes in a vinyl or aluminum frame. Those small slots drain rainwater out of the frame. Sealed, they hold water inside it.
• Trickle vents, if your windows have them. They exist to ventilate the room; blocking them moves the moisture problem to a wall you cannot see.
• Your egress window. Every bedroom needs one window that opens. Do not film it, do not rope-caulk it.

Once the windows are done, the same evening's work applies to the doors: best weather stripping for doors for the frames and best door sweeps for the gaps underneath.