Thermal Isolation 101

8th Apr 2021

A simple guide to U-value measurement in construction


What is it?

A U-value is a sum of the thermal resistances of the layers that make up a complete building element such as a wall, a floor, or a roof. It also includes adjustments for fixings or air gaps.

A U-value value is shown in units of W/m²·K and shows the ability of an element to transmit heat from a warm space to a cool space inside or outside a building. The lower the U-value, the better insulated the building element.

When is it used and why?

U-values are used in the construction of all buildings. A building element’s U-value is vital in understanding if a building’s thermal performance will comply with any required building regulations and/or standards.

These standards are in place for a number of reasons. Firstly, they ensure that the building is a safe and comfortable place to inhabit, whether that’s as a home or to spend an amount of time in for work or leisure. Ensuring a comfortable temperature varies in approach depending on where you are in the world.

For example, Mecca in Saudi Arabia is the warmest inhabited place on earth where its average annual temperature is 30 degrees Celsius. Cooling the inside of a building in Mecca to ensure a comfortable environment would be essential, whereas in Ottawa in Canada, warming the building up from the lowest average winter temperate of –14 degrees Celsius would be key.

Secondly, standards are in place to reduce a building’s impact on the environment. A better-insulated building will mean less wasted energy as heat or cold air is lost.

How does a U-value work?

Here’s an example of how a U-value works:

The U-value of a single sheet of glass as found in a traditional pane of glass (as opposed to the more modern double or triple glazed panes more commonly used now) is 6.0W/m2K.

That means that for every degree of temperature of difference between the outside and inside, every square metre of the glass would lose 6 watts. So, if we were enjoying a chilly day in London and the temperature difference on a typical cold day was 17 degrees, then the amount of heat loss would be 17 x 6 = 102 watts per metre square.

The U-value of a pane of triple-glazing – more often used for its higher level of insulation, can be as low as 0.7W/m2K. The amount of heat loss would be substantially less. 17 x 0.7 = 11.9 W/m2K

What are the benefits?

The benefit of using U-values to calculate thermal performance when constructing a building is that you end up with a compliant and habitable building. If you use U-values to go above and beyond when it comes to insulating a building, you also end up with a key selling point.

Energy efficiency and sustainability are massive buzzwords when it comes to construction and investment. Getting investment for a development will be much easier if you can prove that your building design is set to be as energy efficient as possible.

The reason for this is simple – people will pay more for a space that will cost them less to run, as well as being more environmentally friendly.

Based on average property prices in England, there is a strong correlation between a stronger energy efficiency rating and a higher house price. The graph below highlights the price increase as a result of raising a home’s Energy Performance Certificate (EPC) from a G rating through to the higher A ratings. Property value can be as much as 14% higher where the EPC is higher.

u values house prices

Did you find this Farrat 101 Article useful?

For more information on U-values and to explore how Farrat can support architects, engineers, and developers with optimal thermal insulation, please visit our structural thermal breaks page.