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Aerogel thermal insulation acting as a barrier between a gas flame and some wax crayons, by NASA.

Heat insulation

by Chris Woodford. Last updated: October 4, 2016.

If you're out and about in winter and you're feeling cold, chances are you'll put on a hat or another layer of clothing. If you're sitting at home watching television and the same thought strikes you, you're more likely to turn on your heating. Now what if we switched the logic around? What if you ate more food whenever you felt cold and stuck a woolly hat on top of your house each winter? The first wouldn't make much difference: food supplies the energy your body needs, but it doesn't necessarily make you any warmer right there and then. But putting "clothes" on your house—by insulating it—is actually a very good idea: the more heat insulation you have, the less energy escapes, the lower your fuel bills, and the more you help the planet in the fight against global warming. Let's take a closer look!

Photo: Aerogel is one of the world's newest and most exciting insulating materials. Put a slab of aerogel between a gas flame and some wax crayons and the crayons won't melt: the aerogel stops virtually any heat flowing through. One day, we could make all our windows out of aerogel—but scientists have to figure out how to make it transparent first! Photo by courtesy of NASA Jet Propulsion Laboratory (JPL).

Why do we need insulation?

Compared to using age-old technology such as an open-coal fire, most modern heating appliances are actually pretty efficient; look at the chart below and you'll see that, for every joule (the standard modern unit of measuring energy) of fuel you feed into them, you typically get about 70 percent back again as heat (in practical terms, that's what the fuel efficiency percentage means). The real problem with home heating is retaining the heat you produce: in winter, the air surrounding your home and the soil or rock on which it stands are always at a much lower temperature than the building so, no matter how efficient your heating is, your home will still lose heat sooner or later. The answer is, of course, to create a kind of buffer zone in between your warm house and the cold outdoors. This is the basic idea behind heat insulation, which is something most of us think about far too little: according to the US Department of Energy, only a fifth of homes built before 1980 are properly insulated.

Bar chart (histogram) showing the cost and efficiency of nine common fuels used for domestic heating

Chart: Comparing the cost and efficiency of different fuels. How efficiently you can heat your home (and how much it will cost) depends to a large extent on the fuel you use—which is not always something you can change easily. As this chart shows, home heating fuels vary dramatically in cost (with electricity the most expensive and coal the cheapest), though most are about 70 percent efficient or better. Wood is the least efficient fuel, but given its low cost, availability, and sustainability, that doesn't always bother people. Even though coal is the cheapest fuel, it's dirtiness and other environmental drawbacks have made it much less popular in recent decades. Natural gas owes its popularity to its relativity low cost and high efficiency. The blue bars in this chart show the cost in dollars per million btu of nine common residential fuels (read the vertical axis on the left). The red bars alongside show the efficiency of each fuel as a percentage (read the vertical axis on the right). Drawn using data for 2014 courtesy of US Energy Administration.

How does heat escape from your home?

Where does heat escape from a home? Shows rough percentage heat losses from the walls, roof, floor, doors, and windows.

Artwork: Where does the heat escape in a typical home? It varies from building to building, but these are some rough, typical estimates. The walls give the biggest heat loss, followed by the doors and windows, the roof, and the floor.

Why does heat escape from your home in the first place? To understand that, it helps to know a little bit about the science of heat. As you probably know, heat travels in three different ways by processes called conduction, convection, and radiation. (If you're not sure of the difference, take a look at our main article on heat for a quick recap.) Knowing about these three types of heat flow, it's easy to see lots of ways in which your cozy warm home is leaking heat to the freezing cold world all around it:

  1. Your house is standing on cold soil or rock, so heat flows down directly into the Earth by conduction.
  2. Heat travels by conduction through the solid walls and roof of your home. On the outside, the outer walls and the roof tiles are hotter than the atmosphere around them, so the cold air near to them heats up and flows away by convection.
  3. Your house may seem like a big complex space with lots going on inside in but, from the point of view of physics, it's exactly the same as a camp fire in the middle of vast, cold surroundings: it constantly radiates heat into the atmosphere.

The more heat escapes from your home, the colder it gets inside, so the more you have to use your heating and the more it costs you. The more you use your heating, the more fuel has to be burned somewhere (either in your own home or in a power plant up-state), the more carbon dioxide gas is produced, and the worse global warming becomes. It's far better to insulate your home and reduce the heat losses. That way, you'll need to use your heating much less. The great thing about home insulation is that it usually pays for itself quite quickly in lower fuel bills. Before long, it's even making you money! And it's helping the planet too.

How heat insulation works

Suppose you've just poured yourself a hot cup of coffee. A fundamental rule of physics called the second law of thermodynamics says it's never going to stay that way: pretty soon, it's going to be a cold cup of coffee instead. What can you do to postpone the inevitable? Somehow you need to stop heat escaping by conduction, convection, and radiation.

The first thing you could do is put a lid on. By stopping hot air rising and falling above the cup, you'll be cutting down heat losses by convection. Some heat is also going to be disappearing through the bottom of the hot cup into the cold table it's standing on. What if you could surround the cup with a layer of air? Then very little conduction could take place. So maybe have a second cup outside the first one with an air gap (or, better still, a vacuum) in between. That's convection and conduction just about licked, but what about radiation? If you were to wrap aluminum foil round the outer cup, most of the infrared radiation the hot coffee gives off will be reflected back inside it, so that should solve that problem too. Apply all three of those solutions—a lid, an air gap, and a metallic coating—and what you have is effectively a vacuum flask: a really effective way of keeping hot drinks hot. (It's also good at keeping cold drinks cold, because it stops heat flowing in just as effectively as it stops heat flowing out.) It's worth noting, just in passing, that most takeaway stores give you hot drinks in nasty-tasting polystyrene containers. Ever wondered why? The answer is simply that polystyrene (and especially expanded polystyrene, filled with air—the crumbly kind you get in packaging materials) is a superb heat insulator (check out the table below and you'll see it rates better than double- and triple-glazing).

A sample of compact vacuum insulation CVI material developed by the US DOE NREL

Photo: Vacuums coated with metal are among the best insulators, but they're not always suitable for everyday uses. In the late 1980s, two scientists working at the National Renewable Energy Laboratory, David Benson and Thomas Potter, developed a more practical way to use this technology called compact vacuum insulation (CVI). The outer metal plates, held apart by ceramic spacers, seal an insulating vacuum inside. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

The best way to insulate your home

Now, unfortunately, we can't build our houses exactly like a vacuum flask. We have to have air to breathe, so a vacuum's out of the question. Most people like windows too, so living in a sealed box lined with metallic foil isn't that practical either. But the basic principle of cutting down heat losses from conduction, convection, and radiation still applies nevertheless.

If you want to improve your insulation, you need to take a very systematic approach, considering every possible way in which cold air can enter your home and heat can escape. You need to work your way around the entire building looking at every door, wall, window, roof, and other potential source of heat loss in turn. How much loft insulation do you have and could you do with some more? Is your home suitable for cavity-wall insulation and have you worked out the likely savings and payback period? How much energy are you losing through those drafty old sash windows? Have you thought about investing in caulking, secondary glazing, heavy curtains, magnetically attached plastic sheets, or some other means of keeping out the cold?


An insulated wall in an eco home

Photo: Reduce the energy losses from your home by filling the walls full of foam insulation. This eco-home is being insulated with Icynene, a plastic insulation material similar to that used in pillows and mattresses. Photo by Paul Norton courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Many homes have what are called cavity walls with two layers of brick or blocks between the inner rooms and the world outside and an air gap between the walls. The air gap reduces heat losses from the walls by both conduction and convection: conduction, because heat can't conduct through gases; convection, because there's relatively little air between the walls and it's sealed in, so convection currents can't really circulate.

By itself, air isn't the best insulating material to have between your walls. It's actually far more effective to have the cavities in your walls filled with expanding foam or another really good insulating material that stops heat escaping. Cavity-wall insulation, as this is known, takes only hours to install and costs relatively little. Cavity walls are often filled with loosely packed, air-filled materials such as vermiculite, shredded recycled paper, or glass fibers (specially treated to make them fireproof). These materials work in exactly the same way that your clothes work: extra layers of clothing make you warmer by trapping air—and it's the air, as much as (or more than) the clothes themselves, that stops heat escaping.

Which are the best home insulation materials?

Floor insulation materials with R-30 insulating value

Some forms of insulation are better than others, but how can you compare them? The best way is to look out for a measurement called R-value. The R-value of a material is its thermal resistance: how effectively it resists heat flowing through it. The bigger the value, the greater the resistance, and the more effective the material is as a heat insulator.

Photo: You can reduce heat losses through your floor by building your home on a thick insulating material like this, which has an R value of 30. Photo by Paul Norton courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).


Since warm air rises, plenty of heat escapes through the roof of your home (just as lots of heat escapes from your body through your head, if you don't wear a hat). Most people also have insulation inside the roof (loft area) of their homes, but there's really no such thing as too much insulation. Loft insulation is generally made from the same materials as cavity-wall fillings—such things as rock wool and fiberglass.

Radiation losses

Wall and roof insulation cuts down on heat losses by convection and conduction, but what about radiation? In a vacuum flask, that problem's solved by having a reflective metallic lining—and the same idea can be used in homes too. Some homeowners install thin sheets of reflective metallic aluminum in the walls, floors, or ceilings to cut down on radiation losses. Good products of this kind can reduce radiation losses by as much as 97 percent. You can find out more by searching on "reflective insulation" or "radiant barrier" in one of the Google boxes on this page.

Double glazing air gap reduces heat loss.

R-values for single, double, and triple glazing with and without low-e glass.

That still leaves the windows as a major source of heat loss, but there are ways to tackle that problem too. Double-glazed windows have two panes of glass separated by a sealed air gap. The air stops heat losses by conduction and convection, while the extra pane of glass reflects more light and heat radiation back into your home and reduces heat losses that way too. You can have your windows treated with a very thin reflective metallic coating or made from special thermal glazing (such as Pilkington-K, which traps heat a bit like a greenhouse) that reduces heat losses even further. (Read more in our main article on heat-reflective windows.)

Generally, the more insulation you have, the warmer you'll be. But the amount you need varies depending on where you live and how cold it gets.

Photo: Double glazing: the air gap between the two panes of glass provides heat insulation—and soundproofing too. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

Chart: Switching from single- to double- or even triple glazing can make a big difference (darker blue), especially if you use low-e, heat-reflective glass (lighter blue). The numbers shown are R-values, with a 0.5 inch air gap.

Curtains and blinds

If you can't insulate your windows, for any reason, curtains and blinds can make a difference. Remember that the purpose of curtains is not simply to give you privacy: good curtains should trap a significant volume of air between the fabric and the window and stop it from moving; it's the air that gives you the insulation and not (generally) the fabric of the curtains themselves. So you need curtains that seal at the sides and reach snugly to the floor (or touch the windowsill). The more air you trap between the fabric and the window the better your curtains will be as heat insulators. You might prefer the convenience of blinds, but they are almost never as effective as curtains, partly because most blinds have air gaps in them (so they don't create any kind of an air seal) and also because blinds tend to be fitted closer to the glass so the volume of air they trap is greatly reduced.

Insulate yourself

If your heating bills are really starting to get to you, or your home is so old and drafty that you simply can't keep heat in it for any length of time, why not shift your focus away from heating the building to keeping your own body warm? Use a moderate amount of heating each day to keep your home in good condition and avoid problems such as damp and condensation, but don't have your heating on as long or as high as you would normally. Instead, buy yourself some thermal underwear (merino wool is particularly good—and often sold as "base layer" garments in outdoor activity shops) and put on more layers of clothing on top. Another option is to keep one or two rooms in your home comfortably warm and only heat the others occasionally, in rotation, when you feel they're becoming too cold.

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Text copyright © Chris Woodford 2008, 2016. All rights reserved. Full copyright notice and terms of use.

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