What is a U Value and Why is it Important for Windows and Skylights?
The purpose of the U value is to measure the amount of heat passing through various building materials, i.e. thermal conductivity. In other words, it shows the thermal performance of a window or another building element. It is essential to know this value because it indicates how much heat enters your home in the summer and how much hot air remains inside during the winter. Good windows and doors can often contribute to less need for air conditioning and make the home more energy efficient.
What is a U-value? Why is it Important?
Mathematically speaking, the u value is calculated by using a reciprocal of the sum of the thermal resistance of each material making up the specific building element. Apart from material resistances, the internal and external faces have fixed values of resistances as well, which also need to be added. The U value for windows can be worked out with the equation W/m²K. It describes the heat transfer in watts through a square metre of glass divided by the temperature difference across the structure.
Sometimes you can also find the ug value and uw value that show the energy efficiency of the pane and the window, respectively.
Essentially, the lower the u values are, the less heat loss takes place and the higher the window energy rating is. Low u values are very important for external wall windows and roof windows as well. The most thermally efficient windows have the lowest u values on the market.
All of that is important because you want your home to have fewer hot and cold spots, as well as to be more energy efficient. If you are looking to reduce your energy bills, you should start with the windows.
Double glazing u values
Double glazing is the most widespread glazing in the UK. It consists of two panels and a spacer bar between them. This gap between the two panels of glass is what traps the hot air and it makes it possible to maintain the desired degree difference between the outside and inside. Double glazing glass has a low u value within the set parameters in the building regulations. With average u values of 1.6 W/m2K, it is suitable for all properties. It achieves good energy efficiency at a reasonable price.
Triple glazing u values
Triple glazing is more advanced than double glazing. It features an additional, third pane of glass with gaps in between filled with argon gas. This creates a second gap that reduces solar heat gain and has minimal u value. Triple glazing is the pinnacle in window solutions and glazed doors, with typical u values of approximately 0.8 W/m2K. It is more than enough to satisfy building regulations.
Triple glazing is a huge investment, as it is much more expensive than double glazing. Lower u values lead to better energy efficiency and lower energy bills. However, it is important to consult with an expert on whether the investment is worth it, considering it may not be necessary to equip every square metre of glass in your home with triple glazing if you live in a warmer area. Check out our post on what is triple glazed glass for more information.
You may be interested in our article going into detail and comparing double glazing vs triple glazing.
U-Values and Building Regulations
In the UK there are building regulations that set the maximum u value for buildings, which essentially outlines the least energy-efficient windows that builders can install in a home. This also applies to various renovation projects and building extensions. And while it is not only the u value for windows that contributes to how energy-efficient new builds are, it is extremely important to follow the set standards.
Current regulations state that U values for new windows and existing dwellings must be no higher than 1.6W/(m²K). As a reference, single-glazed windows often have a U value that exceeds 5, making them illegal to install due to poor thermal properties.
How are rooflight U-values calculated?
Manufacturers of both flat and pitched roof windows and skylights should be able to calculate any window u value. One way for them to do this is by testing their product in a hot box. This is a special kind of machine that tests how good of an insulator the product is. During this test, the product is exposed to hot and cold elements, just like it would be in the real world.
By exposing either side of the window to hot and cold elements, experts can then measure the thermal performance and thermal resistances. The results indicate what amount of heat transfer occurs.
There are u value calculators that show the amount of heat transferred through a window. A low u value means less heat loss and a higher u value means more thermal transmission will occur.
There are also a few other key factors to consider when measuring window u values:
- The UF value - measures the thermal performance of the window frame
- The UG value - measures the thermal performance of the glass panel
- PSIG - measures the thermal performance of the glass edge
The ultimate U value can vary depending on these aspects, and a few extra measurements, like the opening mechanism of the window and its overall design.
U values are important to know because they indicate how energy efficient the skylights, windows and doors in your home are. They show the amount of heat lost through these surfaces in the winter and the amount entering in the summer. Double-glazed and triple-glazed windows have low u value, which means they don't transfer heat and there is minimal loss occurring. Knowing the exact value is also important because there are strict regulations about it that must be followed.
Frequently Asked Questions
What is a good U-value for windows?
Good u value is anything below the current regulations of 1.4 - 1.6 W/(m²K). This is the minimum u value that new buildings should have, which means single-glazed windows will probably not cut it. Owners and builders need to consider double-glazed windows or even triple glazing in order to meet the u values and g values requirements.
What is the best U-value for double glazing?
Double-glazed windows have a U value between 3.5 and 1.2. The lower end of that falls within the requirements of building regulations in the UK. A lower U value means less transference of heat from the inside and outside.
Is 1.2 a good U-value for windows?
In accordance with the current regulations in the UK, domestic windows should have u values no higher than 1.4 W/(m²K). Double-glazed windows typically show values between 1.2 and 3.7, whereas for triple-glazed windows it is below 1 W/m²K.
What is 1.4 U-value glazing?
For a window with a U value of 1.4, it indicates that 1.4 watts are transmitted through the material for each degree difference between the outside and inside environments.
Difference Between U-Value and Window Energy Rating
U values show how much heat is transferred from one side of the window or door to the other. Energy rating is a broader term, which encompasses the energy efficiency of a window or door. The latter takes into account several different factors and measurements, including U values, G values, R-value, air permeability and more.
What does a high U-value mean?
It indicates that a lot of heat is lost through the window surface. Any windows with a U value higher than 1.4 should be upgraded to ensure adequate energy efficiency.
What is G value?
G value represents the solar transmittance through the glass and other transparent materials. G value helps determine the solar heat gain from the sun. Using the energy of the sun in the winter is good, but it can be more troublesome in the winter.