U-Values: What Are They and How Do I Calculate Mine?

If you have been working on converting a garage or loft you may have started shopping around for insulation. If so, you will have almost certainly come across the term ‘U-Value.’ It is commonly used as a selling point, but not very often is it explained exactly what they are or why they are so important. This is despite the fact that this know-how is key in not only finding the right insulation for you but is also necessary to meet certain aspects of the building regulations and to get planning permission sorted for your project.

Below, we’ll explain, in simple terms, exactly what U-Values are, why they matter and how to achieve the required value for the different parts of your home.

Table of Contents:

What are U-Values & Why do They Matter?

Short Answer

In very simple terms, the U-Value is a number assigned to a section of your building (roofs, walls, floors etc.) to determine how effective they are preventing the transmittance of heat. The lower the U-Value the better the insulation. This means your house will stay warm in the winter and cool in the summer.

Long Answer

The U-value is a total sum of all the thermal resistances of every layer that makes up an element of your building. Measured in W/m²K or Watts per Meter Square Kelvin, it tells you how quickly heat can enter or escape from any given section of a structure and also includes adjustments for any fixings or air gaps. So, a low U-Value such as 0.022 means that a very small amount of heat is being transmitted per square meter, thus making it a high performing layer of insulation.

Beyond making your home, extension and any conversion more comfortable, meeting a certain standard of U-Value when carrying out building work is a requirement of Part L of the Building Regulations (England and Wales) and Section Six of the Scottish Standards.

You can think of the U-value as the final grade of your insulation, taking into account every component that has been incorporated into a given element. It is calculated using the following formula:

In this formula R refers to the total of every R-Value of that particular section of building (roof, wall, floor etc.) including the fixed internal resistance or Rsi and fixed external resistance or Rso.

R-Value: Measure of an individual product’s thermal resistance. The higher the R-Value the better it is at resisting heat transference. It is calculated by dividing the thickness of the material by its lambda value (K-Value) which itself is a measure of thermal conductivity.

This makes your final calculation look something like this, with R1 and R2 being separate additional components you have added such as PIR boards:

Using this calculation can provide you with a generalised idea of what your U-Value will be. However, it does not take into account any heat loss that may occur from thermal bridging or other influencing factors outside of your insulating material.

Thermal Bridging: A result of two materials with low insulating properties coming into contact with each other to form a bridge between the inside and outside of your building. It causes the loss of heat and is therefore something to be avoided. This can and should be done when building your structure or by the insulation of existing thermal bridges.

What U-Value Do I Need?

If you are not qualified, navigating your way through the technical documents can be a daunting task. To make it a little easier we’ve listed out the basic requirements that you should be looking to meet for new builds, extensions and refurbishments.

Residential New Builds (Best Starting Point)

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

England

0.16

0.11

0.11

0.11/0.11

Wales

0.16

0.11

0.11

0.11/0.11

Scotland

0.15

0.13

0.10

0.10/.10

Non-Residential New Builds (Best Starting Point)

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

England

0.22

0.18

0.14

0.14/0.14

Wales

0.22

0.18

0.14

0.14/0.14

Scotland

0.18

0.15

0.14

0.14/0.14

Residential Extensions

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

England

0.28

0.22

0.18

0.16/0.18

Wales

0.21

0.18

0.15

0.15/0.15

Scotland*

1.     0.17

2.     0.22

1.     0.15

2.     0.18

1.     0.13

2.     0.18

1.     0.11/0.15

2.     0.13/0.18

*value 1 is for when the existing walls and roof values are worse than 0.70 and the floors worse than 0.25, otherwise use value 2.

Non-Residential Extensions

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

England

0.28

0.22

0.18

0.16/0.18

Wales (Domestic in Character)

0.21

0.18

0.15

0.15/0.15

Wales (Other)

0.26

0.22

0.18

0.15/0.18

Scotland*

0.25

0.20

0.15

0.15/0.15

* Refurbishments/extensions/conversions of unheated buildings

Residential Refurbishments

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

England**

0.30¹/0.55²

0.25

0.18

0.16/0.18

Wales**

0.30¹/0.55²

0.25

0.18

0.16/0.18

Scotland*

1.     0.17

2.     0.22

1.     0.15

2.     0.18

1.     0.13

2.     0.18

1.     0.11/0.15

2.     0.13/0.18

*value 1 is for when the existing walls and roof values are worse than 0.70 and the floors worse than 0.25, otherwise use value 2.

** ¹Used for internal or external wall insulation ²Used for cavity insulation

Non-Residential Refurbishments

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

England**

0.30¹/0.55²

0.25

0.18

0.16/0.18

Wales**

0.30¹/0.55²

0.25

0.18

0.16/0.18

Scotland*

0.25

0.20

0.15

0.15/0.15

*Refurbishments/extensions/conversions of unheated buildings

** ¹Used for internal or external wall insulation ²Used for cavity insulation

Conversion of Heated Buildings (Scotland Only)

 

Wall

Floor

Flat Roof

Pitched Roof (Ceiling/Rafter)

Residential

0.30

0.25

0.25

0.25/0.25

Non-Residential

0.30

0.25

0.25

0.25/0.25

How to Achieve U-Values

As the above may have you thinking, manually calculating your U-Value is anything but easy, which makes the task of ensuring you’re up to code a bit of a head scratcher. Fortunately, there are simple ways around this.

Other than consulting a professional, a great way to determine your U-Value, along with what you may need to get it up to the recommended number is by using a ‘U-Value’ calculator. Kingspan, a leading manufacture of insulation, for example have a fantastic online version which can get you to your value in a matter of minutes.

If you already know what you need to meet your U-Value goals, we have an extensive range of insulation options from high end brands such as SuperFoil, Actis and Celotex ready to be delivered to your doorstep. Simply order online, get in touch via our chat or contact us directly at 01295 565 565. We’ll be happy to help you find the best products at the best prices, so that you can complete your project with confidence.

PLEASE NOTE: THIS GUIDE ON U-VALUES HAS BEEN WRITTEN FOR INFORMATION PURPOSES AND WE THEREFORE TAKE NO RESPONSIBILITY FOR ANY PURCHASING DECISIONS YOU MAKE OR INSTALLATION PROCESSES YOU FOLLOW AS A RESULT OF READING THIS ARTICLE. WHILST WE ACT AS A RETAILER, WE ARE NOT EXPERTS NOR QUALIFIED IN INSULATION OR ANY OTHER ROOFING MATERIAL. WE THEREFORE RECOMMEND THAT YOU REFER TO YOUR MANUFACTURER'S GUIDE AND, IF APPROPRIATE, CONSULT A QUALIFIED PROFESSIONAL.