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 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).
Put simply: we need insulation because fuel is expensive and
fuels that burn damage the environment, one way or another.
Some fuels are more expensive than others; some are more harmful than others;
some are more efficient than others.
But even efficient fuels cost money—so the less of them you burn, the better.
Compared to using age-old technology such as an open-coal fire, most modern heating appliances are
actually pretty efficient; look at the red bars in 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).
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 and natural gas 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 one of the cheapest fuels, it's dirtiness and other environmental drawbacks have made it much less popular in recent decades. Natural gas owes its popularity to its low cost and high efficiency.
Chart: Comparing the cost and efficiency of different fuels. 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 2020 data from various different market sources,
including the US Energy Administration.
The efficiency data doesn't really change from year to year.)
Over the last few decades, there's been a major shift in energy use: modern homeowners spend a much higher percentage of their utility bills powering appliances and air conditioning than they did back in the late 20th century. Take a look at the donut chart below. In 1978 (inner ring), about 80 percent of home energy use went to heating homes (66 percent) and hot water (14 percent). Today, those figures have changed quite a bit and much less of our energy (62 percent) is now used this way (heating comes in at 43 percent and hot water at 19 percent).
That's still almost two thirds of your bills, however—and a powerful incentive to insulate and improve efficiency!
Charts: US home energy use from 1978 (inner ring), 2009 (middle ring), and 2015 (outer ring).
Blue = home heating, Orange = appliances and electronics, Yellow = water heating, Green = air conditioning. Data from Residential Energy Consumption Surveys (RECS), US Energy Information Administration (EIA).
Holding onto your heat
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;
so, as you can see from the chart below, most of us believe our properties are better insulated than they actually are.
(The good news is that standards are rising. Over a quarter of new homes now meet ENERGY STAR® specifications,
according to the US Energy Information Administration, which means they use 15 percent less energy than those built to 2009 building codes.)
Chart: Over 95 percent of homes built in the 1990s and after are either well- or adequately insulated,
according to the perceptions of their owners, compared to just 68 percent built before 1950.
(In reality, many homes are much more poorly insulated than their owners believe.)
Drawn using data from [PDF] Householder's Perceptions of Insulation Adequacy and Drafts in the Home in 2001 by Behjat Hojjati, US Energy Information Administration, 2004.
How does heat escape from your home?
Your house is
standing on cold soil or rock, so heat flows down directly into the
Earth by conduction.
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.
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.
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:
Well-insulated homes that hold onto their heat in winter tend to be better at keeping the heat out in summer, so any
improvements you make to your insulation should also help to keep your
air conditioning bills down.
That matters because "air con" is now the fastest growing use of energy in buildings
(both homes and commercial buildings), according to the US Energy Information Administration.
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).
Photo: Above: 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).
Photo: Below: A similar idea is at work in products like Superfoil, an affordable insulating material that (if you peel it apart) is much like bubble-wrap, only it's sandwiched between thin layers of aluminum foil instead of paper.
According to the manufacturers, the basic version has an R-value of around 0.97–2.33 (depending on where you use it), though the thicker versions manage somewhat better.
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?
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?
Some forms of insulation are better than others, but how can you compare them? The
best way is to look out for measurements called R-values and U-values.
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
Single glass: 0.9.
Air: 1 (0.5-4 inch air gap).
Double-glazing: 2.0 (with 0.5 inch air gap).
Vermiculite: 2.5 per inch.
Fiberglass: 3 per inch.
Triple-glazing: 3.2 (with 0.5 inch air gap).
Expanded polystyrene: 4 per inch.
Polyurethane: 6-7 per inch
Polyisocyanurate (foil-faced): 7 per inch.
Aerogel: Space-age insulating material: 10
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).
Another common measurement you'll see is called the U-value, which is the total amount of heat lost through a certain thickness of insulating material. The lower the U-value, the lower the heat flow and
the better the material does it job as an insulator (so that's the opposite of R-value, where higher values are better). U-values and R-values are obviously related concepts, but U-values are more accurate. Where R-values only consider conduction losses, U-values take account of losses due to conduction, radiation, and convection. The conduction loss is the reciprocal of the R-value (that's one divided by the R-value), then you add radiation and convection losses onto that to get the total U-value.
Generally, we're only interested in comparing different materials, so all you
really need to remember is that high R-values and low U-values are good things.
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.
But it's also made from air. If you use your loft for storage and pile things on top of insulation,
so squashing it down, you remove some of the air and make it less effective.
A study by Britain's National Physical Laboratory found that squashed insulation loses almost half of its performance.
Photo: Double glazing: the air gap between the two panes of glass provides heat insulation—and soundproofing too.
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 search boxes on
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.
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
Lining your curtains is also a good idea—and a heat-reflective lining works in at least three different ways.
First, it reflects heat back into the room, so cutting losses by radiation. Second, the extra layer of cloth
traps another insulating layer of air between itself and the main curtain. Third, it makes the entire curtain heavier,
less likely to blow in the wind, and more likely to trap that all-important air curtain in front of the window.
Don't forget that curtains work both ways: they can keep heat out as well as in. Drawing your curtains in summer
is a great way to keep rooms cool and reduce the need for air conditioning.
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.
Insulation versus ventilation
The better insulated your home, the less well ventilated it's likely to be. Although that doesn't sound like a problem,
it certainly can be: the air in a home needs to be changed reasonably often to avoid problems like condensation and damp,
and potentially dangerous indoor pollution (from things like cooking and heating). Exactly how often the air needs to be refreshed depends on how big the space is, how many people are inside it, and what sorts of things they're doing (a bathroom or kitchen generally needs more ventilation than a living space, for example). Insulation and ventilation don't have to be enemies, however; there are technical solutions to the problem, notably heat-recovery ventilation (HRV) systems that
use heat exchangers to catch the warm, stale air flowing out of a building and reheat the cool, fresh air flowing in the other way.
Find out more
On this website
Heat: The science of heat energy explored in more detail.
Energy Saver: Insulation: A great collection of material from energy.gov, including comparisons of different insulation materials, where and how to insulate, and the differences between insulating old and new buildings.
The Energy-Efficient Home by Patrick Waterfield. The Crowood Press, 2012. A short primer written in clear, non-technical prose.
Green from the Ground Up by David Johnston and Scott Gibson. The Taunton Press, 2008. Chapter 2 "The House as a System" offers a good introduction to the principles of "building science" that you'll need to understand to make an effectively insulated home.
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