Heat insulation
Last updated: February 28, 2009.
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).
How does heat escape from your home?
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 cosy warm home is leaking heat to the freezing cold world all around it:
- 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.
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.
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!
How heat insulation works
Suppose you've just poured yourself a hot cup of coffee. 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 no 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, any 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.)
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 developed a more practical way to use this technlogy 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 applies equally
well to your home.
Many homes, for example, 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. 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.
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).
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.
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.
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.
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.
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).
Further Reading
