All About Heat Loss – U Values Explained

It’s estimated that the average UK home loses about 20% of its heat through the windows. If your average heating bill comes to, say, £50 a month, chances are that about £10 is drifting out your windows! Winters can be a chilly business in this country, and with energy costs rising every year, it goes without saying that the better-insulated your home is the warmer (and richer) you’ll be.

While most of us discuss the temperature of our homes in words like ‘warm,’ ‘cold,’ ‘draughty,’ or ‘snug,’ experts rely on the term ‘u value’ to determine how much heat your home retains in colder seasons.

Although there’s a fair bit of science behind the concept of u values, they are not that difficult to understand. Read on for a short primer on u values and what to consider when installing double glazing in your home.

The basic science of heat

A cup of hot coffee can be used to demonstrate heat transference

A ‘u value’ is simply a way to measure how well a building material such as glass or wood conducts heat. U values are also called u factors, though the former term is more common in the UK.

The letter ‘u’ represents a scientific concept called ‘overall heat transfer coefficient,’ but that’s not too important to remember.

Every material on earth, from a plank of wood to a pint of lager, contains molecules. These molecules move at different speeds, depending on a number of factors.

When molecules in a material move quickly, they transfer heat. When a fast-moving molecule bumps into a neighbouring molecule, that molecule begins to move more quickly as well. In this way, heat spreads from molecule to molecule, starting a chain reaction that conducts heat through the material.

When you hold a cup of hot coffee, for example, the quick-moving molecules in the coffee bump into the molecules in your coffee cup, prompting them to move faster as well. Soon, the heat of the coffee transfers to, and warms, the entire cup.

When you hold the cup in your hands, the fast-moving coffee cup molecules bump into the molecules in your skin, spreading their heat energy to your body and forcing you to put the hot cup down.

You can imagine the same process in reverse when you hold a pint of cold lager: the molecules in your body move faster and are hotter than the molecules in your beer glass, so they encourage the pint glass molecules to move faster.

When the glass warms up, heat transfers to the beer itself.

Heat conduction

U values are important because different materials conduct heat at different rates. If you wrap a towel around your coffee cup, for example, you’re able to hold the hot cup for longer than you can hold it with your bare heads.

This is because materials with molecules that are closer together, like the porcelain of a coffee cup or the glass of a lager mug, conduct heat more efficiently than materials in which molecules are spread further apart, like the fibres and trapped air of a towel.

In scientific terms, the former (porcelain and glass) have higher ‘thermal conductivity’ than the latter (air and towel fibers).

In order to assign a standard measurement to a material’s thermal conductivity or ability to conduct heat, scientists test the material under standard conditions, then assign it a numerical u value based on its ‘performance’ in the test.

Everything on earth, from coffee to beer to your own body, has a u value. The lower the number, the worse the material is at conducting heat.

U values and double glazing

When it comes to windows, the lower the u value the better. Since, as mentioned above, about 20% of the heat in an average UK home escapes through the windows, the worse those windows are at conducting heat the lower that percentage of lost or wasted heat becomes.

Almost all double glazing manufacturers now assign u values to their windows and doors to clearly indicate their products’ ability to keep heat inside a home.

While double glazed windows always have lower u values than single glazed windows, certain types of double glazing insulate better than others.

The type of double glazing with the lowest u value is one in which the pane of glass that faces the outside world is slightly tinted and the pane that faces into your home is slightly reflective.

This type of double glazed window usually has a u value of 1.8. Compared to double glazing with untreated panes of glass, which have u values of around 2.7, these treated windows are significantly more efficient.

However, considering that single glazed windows carry u values of around 5.3,  either type of double glazing is clearly better at keeping heat in your home.

Special considerations

Double glazing with treated glass is, as you might expect, often more expensive than double glazing with untreated glass. If you are considering installing double glazing in your home and are not sure which type of glass to choose, keep in mind that you can install different types of double glazing in different windows in your home.

The u value of your double glazing becomes more important in winter!

A north-facing window, for example, is likely to conduct more heat out of your home than a south-facing window. This is simply because, due to the sun’s positioning the winter sky, the north-facing side of a building tends to be cooler.

The greater temperature difference between the inside and outside of a north-facing wall means that more heat is likely to escape from a north-facing window.

Therefore, if you are not able to install double glazing with treated gas in every window in your home, it makes the most sense to install treated windows only in north-facing walls.

If you are considering installing double glazing in your home, all you really need remember about u values is to compare the listed u values of each manufacturer’s windows.

As far as your budget allows, choose the windows with the lowest u value and in order to reduce as much as possible the amount of money you spend on lost heat.