U-value for dummies

May 3, 2018

If you’ve been browsing doors for a while, you’ve definitely encountered the term ‘U-value’. But what does it mean?

 A U-value is a measure of thermal transmittance – sometimes confused with a K-value, which measures thermal conductivity.

So, a U-value refers to the rate of heat transferred per second, per square meter, per degree of temperature difference across a wall surface. Got all that?

U-value is expressed as W/m²K.

That translates as Watts per Square Meter Kelvin. In practical terms, it tells us how effective a wall is as an insulator. The lower the U-value, the better the insulation and vice-versa. Therefore, it’s an important measurement for certifying construction materials.

The average U-value of an insulated sectional door (5 x 5 meters) is 1.0 W/m²K, for example. A value close to 1 is good; below 1 is great; and the higher it goes, the less effective the material is.

A door isn’t a wall, of course, but when it’s closed it still functions as one. That means it should be able to maintain the indoor temperature as much as possible – for comfort as well as saving energy.

DIY calculation

If you want to do your own calculations, you can use the following formula:

Qw = (Ta – Tb) x A / Rw

  • Qw is your U-value (or W/m²K)
  • Ta is the temperature on the inside of the door
  • Tb is the temperature on the outside of the door
  • A is the surface area of the door in m²
  • Rw is the thermal resistance expressed as (m²K)/W

Why do I need to worry about it?

Mostly, you don’t need to bother with the calculations yourself. For one thing, it’s difficult. But also, it should be included on the information sheet delivered with your door.

Taking note of the U-values of your entrances, however, is becoming more important as regulations to reduce building emissions are becoming more and more common.

Regulations that come into force at the end of 2020 dictate that all new buildings should be nearly zero energy.  And under this, doors, among other components, have their own U-value requirements. Industrial doors for example cannot exceed a U-value of 1.4 W/m²K.

So, it’s worth keeping an eye on your U-values not only to make sure you’re getting the most out of your doors, but also to make sure that you’re meeting regulations.


Want more information? Find your local contact at assaabloyentrance.com