How to Stop Window Drafts Without Plastic Film

Shrink film works. That is the problem: it works just well enough that people keep buying it every October instead of fixing the window once.

Every fall we get calls from Boston homeowners who have been taping plastic over their windows for five, ten, sometimes twenty years. The film is not a scam. It stops air movement, and stopping air movement is most of the battle. But it is a seasonal patch on a permanent problem, and it hides the thing that actually needs attention. Here is what film does, what it does not do, and what to do instead.

First, find out where the air is coming from

Most people seal the wrong place. Before you buy anything, spend ten minutes finding the leak. Pick a windy day, close the window, and hold a lit incense stick or a thin strip of tissue near each of these spots in turn:

• The meeting rail. The horizontal joint in the middle where the upper and lower sash overlap. On an old double hung this is usually the worst leak in the window, and it is the one nobody checks.

• The side channels. The vertical gap the sash slides in. Second worst, and it gets worse as the wood dries and shrinks.

• The bottom rail against the sill. Should seal when the window is locked. Often does not, because the lock no longer pulls the sashes together.

• The top of the upper sash. Frequently painted shut, which accidentally seals it. If it is not painted shut, it leaks.

• Around the trim, where the casing meets the wall. This is the only one caulk fixes.

If the smoke moves at the meeting rail and you caulk the trim, you have accomplished nothing and you will still be cold. We see this constantly. Diagnose first.

What plastic film actually does

Shrink film creates a sealed air gap between the plastic and the glass. That gap does two useful things: it stops the air leaking through the window from reaching the room, and the trapped air adds a small amount of insulation value, roughly comparable to adding a second pane. On a single pane window in a cold room, the difference is real and you can feel it.

Where it falls short:

• It seals the window shut for the entire heating season. No ventilation, no opening the window on a warm February day, and no egress in a bedroom that needs it.

• It has to be redone every year. Three hours of work per floor, every October, forever.

• The tape takes finish with it. On painted trim it usually comes off clean. On varnished or historic woodwork it often does not, and we have repaired more than a few sashes where twenty seasons of tape stripped the finish down to bare wood.

• It traps moisture. Warm indoor air still reaches the cold glass through gaps in the film. Condensation collects on the sill behind the plastic where you cannot see it or wipe it. That is how a sound sill turns into a rotted one.

• It hides the real damage. A window that needs film every year is telling you something: the weatherstripping is gone, the sash no longer fits, or the glazing putty has failed. Covering it up for six months a year is why small problems become sash replacements.

The cost math

A film kit runs about $10 to $15 per window and lasts one season. Ten windows is roughly $120 a year plus an afternoon of your time. Over ten years that is $1,200 and ten afternoons, and at the end of it you still have a drafty window.

Bronze weatherstripping on the same window is $275, installed once, and lasts thirty years or more. It pays back against the film alone in about two decades, but that is the wrong way to look at it: it pays back against your heating bill in two to three winters, and unlike film it does not have to come off in the spring. The window still opens.

What to do instead, in order of what actually helps

1. Fix the lock

On a double hung, the sash lock is not a security device. It is a clamp that pulls the two sashes tight against each other at the meeting rail. When it is loose, worn, or mounted slightly off after a paint job, the sashes never close tight and the biggest leak in the window stays open. Adjusting or replacing a sash lock is the cheapest meaningful thing you can do, and sometimes it is the whole fix.

2. Weatherstrip the sash

Spring bronze in the jambs and at the meeting rail seals the window where it actually leaks. It is thin, it disappears when the window is closed, it survives repainting, and it is reversible, which matters on a historic house. This is the permanent version of what the film is doing. We cover the full system, including the tested performance of each material, in our guide to weatherstripping historic wood windows.

3. Re-glaze failed putty

If the putty holding the glass has cracked, shrunk, or fallen out in places, air comes straight through the sash itself, and no amount of perimeter sealing will stop it. Re-glazing also keeps water out of the sash, which is what prevents the rot that ends in a rebuild.

4. Deal with the ropes while you are at it

If the sashes are painted shut or the cords are broken, the sashes have to come out for weatherstripping anyway. Doing sash cord repair and weatherstripping in one visit costs much less than two separate jobs, and you get a window that both opens easily and seals shut.

5. Heavy curtains, honestly

Lined curtains or cellular shades cut radiant heat loss and slow air movement at the window. They do not seal anything, so they are not a substitute for the work above. But they help, they cost nothing to operate, and they are the one item on this list you can do this afternoon.

Do you have to replace the window?

Almost never. The wood in a pre-war Boston sash is old growth, tighter grained and more rot resistant than anything sold today, and a drafty window is a sealing and hardware problem, not a dead window. A properly weatherstripped double hung with a storm panel performs close to a modern double glazed unit, at a fraction of the cost, and it keeps the original glass and proportions. Replacement makes sense when the frame is structurally gone, and even then we usually rebuild the rotted section instead of tearing the window out.

Timing

The right time to do this is September and October. The work is easier in mild weather, the schedule is open, and you get the savings across the whole heating season instead of the second half of it. By the time the first real cold snap hits, everyone calls at once.

If you are about to buy another box of film, get an assessment first. We will tell you which windows are worth doing, which ones are already sealing fine, and what it costs per window. Call (617) 982-1211 or see our weatherstripping and insulation service. We work across Boston, Newton, Brookline, Wellesley, and the Greater Boston area.