A Window Is a Hole
in Your Insulation

🏠 The Hole in the Wall

Stand in any room in your house in January and put your hand flat on an outside wall. Then put it on the glass. That difference is the whole of this page.

A framed and insulated wall is somewhere around R-13 to R-30, depending on when the house was built. A single pane of glass is about R-1. A standard double-glazed unit is about R-2. So in the middle of a wall doing its job, there is a rectangle doing between a thirteenth and a thirtieth of it — and in most houses there are a dozen or more of them.

That last figure is the Department of Energy's, and it is the number worth carrying: heat lost and gained through windows accounts for something like a quarter to a third of what a house spends on heating and cooling. Not a rounding error. A quarter of the bill, leaving through the part of the wall you can see through.

And it is losing heat three different ways at once, which matters because the fixes address different ones.

Everything below is aimed at one or more of those three. And the order to do them in is not the order people usually do them in.

🍿 What a Film Kit Actually Is

The boxes are in every hardware store from October, and a lot of people buy one without ever being told what is in it or what it does.

It is a roll of clear plastic sheet and a roll of double-sided tape. That is the entire product. You clean the window trim, run the tape around it, stretch the sheet across the whole opening and press it on, then go over the plastic with a hair dryer, which shrinks it drum-tight and takes the wrinkles out. The result is a sealed transparent membrane a few inches in front of the glass.

Here is what it is doing, because it is widely misunderstood. The plastic is not insulation. It is a few thousandths of an inch thick and its own R-value is essentially nothing. What it gives you is two other things:

So the honest verdict. On an old single-glazed window that rattles in the wind, a film kit is genuinely worth the money and the hour. On a tight modern double-glazed unit it does very little, because both of the things it provides are already there.

Its real drawbacks are not about performance. It is a one-season product — the sheet gets thrown away in spring and the tape frequently takes a strip of paint with it. The window cannot be opened all winter, which matters more than people expect in a house that needs airing. And it never looks like anything but plastic over a window.

🧩 The Board That Goes in the Opening

Now the method this page is really about, because it is better than the film by a distance and almost nobody is selling it to you.

You buy a sheet of rigid foam insulation board — the faced polyisocyanurate kind, foil on both sides, half an inch thick, sold in four-foot by eight-foot sheets in the insulation aisle of any lumber yard. You cut a panel to fit each window opening exactly, and in winter you push it into the opening where it stays by friction alone. In spring you pull them out and stack them in the garage.

An inch-thick sheet is about R-6 if you want more and have the depth in the reveal to take it.

And the reason it beats everything else on cost is that it is bought once. One four-by-eight sheet covers roughly two average windows. After the afternoon you spend cutting them, the panels cost nothing ever again — no tape every autumn, no plastic in the bin every spring, no batteries, nothing to wear out. They will outlast the person who cut them, which on this site is generally the test.

There is one honest trade and it is not small: you cannot see out, and the room goes dark. That is the entire case against it, and it decides which windows get a board and which get something else. More on that at the bottom of this page.

📏 How to Do It

This is an afternoon's work for a whole house, done once, on a table or a pair of sawhorses in the garage.

One refinement worth the extra ten minutes. Run a strip of self-adhesive foam weatherstrip around the edge of the panel rather than relying on the foam itself to seal. It makes a slightly loose panel grip properly, it takes up the out-of-square, and it lets you pull the panel out without scraping the paint off the reveal.

And if a window is in constant use, a panel can be cut for the bottom sash only. Half the glass covered is half the loss stopped, and you keep the daylight and the view out of the top.

⚠️ Four Things to Get Right

None of these stop the method being a good one. All of them are worth knowing before you cut anything.

1. It is a plastic, and plastics burn

Rigid foam insulation is a foam plastic. It burns, and it produces heavy black smoke when it does. That is why the building codes require foam plastic that is permanently installed in a house to be covered by a thermal barrier — normally half-inch gypsum board — before anyone lives behind it.

A removable panel you push into a window in November and take out in March is a different thing from insulation built into a wall, and it is not what that rule is written about. But the material behaves the same way regardless of what the rule calls it, so:

2. Never block an escape window

This is the one that could cost somebody their life, and it takes one minute of thought to get right.

Bedrooms are required to have a window big enough to climb out of in a fire — an emergency escape and rescue opening, in the code's language — and in a good many houses there is exactly one per bedroom. A friction-fit board over it in the dark, in smoke, by somebody who has just woken up, is a barrier at the worst possible moment.

So: work out which window in each bedroom is the escape route, and do not board that one. Board the other one, or fit a panel to the bottom sash only, or use one of the see-through options below. If there is only one window in the room, it does not get a panel. Your own fire department will tell you what counts where you live, and they would far rather have that conversation in October.

3. Condensation behind the board

A sealed panel makes the glass behind it colder than it was, because you have stopped room heat reaching it. Cold glass plus damp household air equals water, and if enough of it runs down onto a wooden sill and sits there all winter you get mould and eventually rot.

It is easily managed and it wants doing deliberately: pull a couple of panels out every few weeks and look. A little misting is normal. Standing water on the sill, black spotting on the frame, or a swollen sill means that window needs more air — take the panel out of that one, or leave a deliberate gap at the top. Rooms that make a lot of moisture — kitchens, bathrooms, and any room where laundry gets dried — are the ones to watch.

4. The room goes dark, and that is not nothing

It is easy to treat daylight as a luxury when you are looking at a heating bill. It is not one, and this site is written for people for whom it matters more than average: light helps you see the edge of a step and the thing on the floor, and winter daylight is doing something for mood at the time of year when that is hardest.

So do not board the room you sit in all day. Board the spare room, the room nobody uses, the north-facing bedroom that has never been warm, the basement windows, a window behind a wardrobe. Use something you can see through on the room where you live. That is the trade, made deliberately, window by window.

☀️ The Other Half of the Year

Here is what makes the board a better buy than anything else on this page, and it is the part most people never think of: the same panels work in July, and they work by a completely different mechanism.

In winter you are stopping heat leaving by conduction. In summer the enemy is sunlight coming through the glass and turning into heat the moment it lands on your floor and furniture. Turn the panel round so the foil faces the glass and you are reflecting a good deal of that sunlight back out before it ever becomes heat in the room. That is the same principle as the reflective screen people put inside a car windscreen, applied to a house.

Where it pays is specific, and it is worth being deliberate rather than boarding everything:

Which means the panels earn their keep twice a year rather than once, and the same afternoon of cutting covers both. That is a rare thing on any list of home improvements.

👁️ The Third Thing It Does — They Cannot See In

This came from a family who had been doing it for years, and it had nothing to do with the heating bill. A boarded window cannot be looked through. Somebody walking past a house, or standing in the yard at dusk having a good look, gets a dark rectangle and nothing else.

It sounds like a small thing until you look at what the research says burglars actually do, and then it stops sounding small.

The most useful study on this is the University of North Carolina at Charlotte survey of 422 convicted burglars in North Carolina, Kentucky and Ohio, which simply asked them how they chose houses and what put them off. The prime deterrent they named was the presence of people — near the house or inside it. Most of them go to real trouble to avoid meeting anybody, and the reasons they give are not moral ones: they are afraid of being hurt, of being caught, and of the much heavier sentence that comes with a confrontation.

And there is a companion to that worth knowing, because it is the same question asked through a different hole: a cheap device exists that lets somebody outside look in through an ordinary front-door peephole. The fix is a piece of tape or a two-dollar cover, and it is set out on the home and property security page.

They do knock, of course — ringing the doorbell and waiting is one of the standard methods. But a window tells them a great deal more than a doorbell does, and it tells them without anyone knowing they asked. Whether a light is on and nobody is moving. Whether there is one cup on the table. Whether there is a walker beside the chair, or a stick by the door, or the television playing to an empty room. Whether the person in there is on their own.

A panel in the opening takes all of that away at once. There is nothing to read. No layout, no routine, no headcount, no walking aid, no idea whether the room beyond is empty or has four people in it.

And sound goes straight through it

Here is the part that makes it better than a curtain or a blind, and it is the family's own observation. Half an inch of foam stops heat and light. It does very little to sound.

So a radio playing in the kitchen, a television, somebody on the telephone, two people talking — all of that still reaches the yard. From outside, the house plainly has somebody in it, and there is no way to check who or how many. The one channel that would settle it has been closed, and the one that suggests an occupied house is still open.

That is a genuinely good trick, and it costs nothing extra, because the panel is already in the window for the heat.

The dog, which the research backs outright

The study puts it in almost exactly these words: alarms and dogs act as substitutes for occupancy. They do the job that a person in the house does, which is to make the place feel like somewhere you will be met.

And a dog is at its best in precisely this situation. It hears somebody at the window long before you do, and it does not need to see them either. A person standing at a boarded window, unable to see in, with a dog going off on the other side of it, has learned two things at once: there is something in there, and he cannot find out what. That is the whole calculation, and it is the point at which an easier house two streets over becomes the sensible option.

We have a page on that, written straight, with the injury figures put properly rather than scarily: a dog after 50.

And a word on the everyday version of this, which has nothing to do with crime. A ground-floor bedroom on a street, a bathroom window facing a neighbour's kitchen, a front room where people walk past three feet from the glass — a lot of older people quietly keep the curtains shut all winter in rooms like that, and lose the daylight anyway. If the curtains are closed regardless, a panel behind them costs you nothing you had not already given up, and pays you back in heat.

Worth knowing about the source: that burglary study was funded by the research foundation of the electronic security industry — people who sell alarms. The findings about occupancy and about dogs match older independent research, and we have said who paid for it so you can weigh it yourself.

🚪 The Door, and the Towel Your Grandmother Used

A door is a window you walk through. Same three leaks — straight through the material, around the edges, and the cold-air current down the face of it — except a door has one opening no window has, which is the gap along the bottom that the whole thing has to swing over.

That gap is usually the single biggest hole in the door, and it is where the oldest fix in the book comes in.

Grandmothers have been doing that for a century and it is still right. The shops now sell a tidy version of the same idea — a fabric tube filled with something heavy, sold as a draught stopper or a door snake, in patterns and colours — and the better ones are double: two tubes joined by a flat strip that slides under the door, so the thing straddles the gap and moves with the door instead of being kicked across the floor every time somebody opens it. That is the one genuine improvement on the towel, and it costs a few dollars.

Either way it belongs against every door you are not using: the front door on a windy night, and above all the doors of the rooms you have shut off, which is the whole one-warm-room plan on the heating page made airtight at the bottom.

But be honest about what it is. A towel is a chore. It has to be put down, kicked aside, and put down again, every day, by somebody who has to bend to the floor to do it — and that is exactly the kind of small repeated bending that quietly stops happening. It is the right answer tonight. It is not the right answer for the next fifteen winters.

So test the seals, which takes five minutes

Weatherstripping is the strip of foam, felt, vinyl or rubber running round the frame that the door squashes against when it shuts. It perishes. It goes hard, it flattens, it tears at the corners, and on a lot of houses it has not been touched since the door was hung.

There are three tests, all free, and the Department of Energy's own building laboratory and the state extension services all give the same ones.

And then replace it, which is genuinely not difficult

This is a job people pay for and should not. It is a tape measure, a utility knife and a screwdriver, and the materials come off a rack at any hardware store for a few dollars a door.

One honest note on the savings, because the numbers get thrown around loosely. Different sources quote different figures for what sealing a house saves, and they disagree enough that we will not print one. What is not in dispute is the Department of Energy's position that caulking and weatherstripping are among the fastest-paying improvements there are — the materials commonly recover their cost inside a year — and that air leaking through gaps is a large share of what a house spends on heating and cooling.

🛠️ Everything Else, Honestly Ranked

Somebody asked whether there is anything better than the cut-to-fit board. The honest answer is: not on insulation per dollar. What beats it is anything you can still see through — and those all cost considerably more. Here is the whole field, cheapest first.

❓ Questions People Ask

💰 Why This One Is Bought Once

Almost everything that saves money on heating is a bill. A better furnace is a bill. A heat pump is a bill. A space heater is a bill every month for as long as you own it, and the cheapest heat on this site is still heat you are buying.

That is the whole argument of this site in one sheet of foam board, and it is why this page exists rather than a page about replacing windows.

The timing argument is the familiar one and it is real. Cutting eight panels is a morning of measuring, kneeling at window reveals, carrying a full sheet in from the car and running a knife along a straightedge. It is not hard work. It is work that is much easier at sixty than it will be at eighty — and the panels do not care how old you are when you put them in each October. That job stays easy: lift, push, done.

Do it in the autumn, before the aisle empties and before the cold arrives. And if the reason you are reading this is that the heating bill has become genuinely frightening rather than merely annoying, do the free things first, and then read the page about help paying your utility bill — because two federal programmes exist for exactly that, one of them insulates houses for nothing, and older households are given priority by federal rule in both.