What Is A U-Value?
In relation to insulating the home, a U-Value is the flow and loss of heat through the materials that make up the building itself, including bricks, flooring, roofing and windows. The U-Value is measured in watts at the rate of heat transfer. The important point to note is that the lower the U-Value is, the better the material will be at insulating the home. The higher the U-Value, the more heat is lost.
A U-Value essentially measures the amount of heat lost through a square metre of the relevant material for every degree difference in temperature between the inside of the building and the outside.
Why Do I Need To Know About U-Values?
If you are insulating your home or working as a building contractor, you need to try and ensure that the thermal performance of the building is at an optimal standard. The R-Value is used to calculate the U-Value of the insulation materials that you’re using. By knowing the Values, you can learn how effective the building will be at preventing heat from transmitting between the inside and outside of a building.
The lower the U-Value of part of the building’s fabric is, the slower that any heat can transmit through it. Thus, it will perform much better as an insulator. Practically, this can mean that if you ensure the U-Values of your building materials are as low as they can be, the better it will be for your living conditions. A properly insulated building with low U-Values will also mean that less energy will be required to ensure the building is at an optimal temperature, particularly in the winter. Low U-Values can save you money on heating bills, as you’re likely to need to rely on in-house radiators to keep heat in the home.
As referred to earlier, if you are a building constructor, you will need to know the U-Values of all building components such as walls, roofs or windows, so that you can be aware of how well (or badly) the required component will transmit heat from the inside of the building to the outside (and thus how much heat will be lost).
Photo by Greg Rosenke
What Is An R-Value?
An R-Value refers to a material’s ability to resist heat flowing through it at a certain thickness. The higher the R-Value, the better it can resist heat transfer. Thus, when you are searching for the best materials to insulate a home, it is best to look for materials with a high R-Value.
Why Do I Need To Know About R-Values?
R-Values are important when constructing a building as there are several different factors that come into play here. The best insulation material will have a high R-Value with a low thickness, showing that it is as good at reducing the amount of heat lost as other thicker materials.
The R-Value will vary with the thickness of the material used, and is calculated by dividing the thickness of the material (in metres) by its thermal conductivity (or lambda value, explained below). An R-Value is measured in metres squared Kelvin per Watt (m2K/W).
An important aspect of knowing the R-Values is so that you can compare two insulating materials by their thermal conductivity, while also allowing you to see the impact of adding thicker layers of the same material.
Depending on which part of a building you are looking to insulate, there will be different recommendations. As an example, the recommendation for a loft insulation is between 6.1 and 7.
What Is A Lambda Value?
The lambda value, sometimes referred to as the K-Value, measures a material’s thermal conductivity (or how well the material conducts heat) in W/mK (Watts per metre Kelvin). The lower the lambda value, the lower the heat loss, and thus, the better the insulation.
Why Do I Need To Know About Lambda Values?
It’s crucial to know the lambda values of different materials in order to know what you are working with. For example:
- Wood fibre insulation has Thermal Conductivity of 0.038 W/mK
- Glass fibre insulation has Thermal Conductivity of 0.044 W/mK
- Dense concrete has Thermal Conductivity of around 1.5 W/mK
Remember that the lower the lambda value the better, as the material will conduct less heat energy.
What U-Value Do I Need?
The U-Values you will need will depend on which part of a building you are insulating, as well as if it is a new build, you’re adding an extension to an existing building or it’s for a refurbishment. Suggested U-Values also vary depending on which country of the UK you are in. Here are the suggested U-Values building regulations for each country for both domestic and non-domestic buildings.
England
Domestic
| New Build – Starting Point | Existing Building - Extension | Existing Building - Refurbishment | |
| Wall | 0.16 | 0.28 | 0.30 (internal or external wall) / 0.55 (cavity insulation) |
| Floor | 0.11 | 0.22 | 0.25 |
| Pitched Roof – Ceiling Level | 0.11 | 0.16 | 0.16 |
| Pitched Roof – Rafter Level | 0.11 | 0.18 | 0.18 |
| Flat Roof | 0.11 | 0.18 | 0.18 |
Non-Domestic
| New Build – Starting Point | Existing Building - Extension | Existing Building - Refurbishment | |
| Wall | 0.22 | 0.28 | 0.30 (internal or external wall) / 0.55 (cavity insulation) |
| Floor | 0.18 | 0.22 | 0.25 |
| Pitched Roof – Ceiling Level | 0.14 | 0.16 | 0.16 |
| Pitched Roof – Rafter Level | 0.14 | 0.18 | 0.18 |
| Flat Roof | 0.14 | 0.18 | 0.18 |
Wales
Domestic
| New Build – Starting Point | Existing Building - Extension | Existing Building - Refurbishment | |
| Wall | 0.16 | 0.21 | 0.30 (internal or external wall) / 0.55 (cavity insulation) |
| Floor | 0.11 | 0.18 | 0.25 |
| Pitched Roof – Ceiling Level | 0.11 | 0.15 | 0.16 |
| Pitched Roof – Rafter Level | 0.11 | 0.15 | 0.18 |
| Flat Roof | 0.11 | 0.15 | 0.18 |
Non-Domestic
| New Build – Starting Point | Existing Building – Extension (Domestic in Character) | Existing Building- Extension (Other Buildings) | Existing Building - Refurbishment | |
| Wall | 0.22 | 0.21 | 0.26 | 0.30 (internal or external wall) / 0.55 (cavity insulation) |
| Floor | 0.18 | 0.18 | 0.22 | 0.25 |
| Pitched Roof – Ceiling Level | 0.14 | 0.15 | 0.15 | 0.25 |
| Pitched Rood – Rafter Level | 0.14 | 0.15 | 0.18 | 0.25 |
| Flat Roof | 0.14 | 0.15 | 0.18 | 0.25 |
Scotland
Domestic
| New Build – Starting Point | Existing Building – Extension and Refurb 1 * | Existing Building – Extension and Refurb 2 * | Conversion of Heated Buildings | |
| Wall | 0.15 | 0.17 | 0.22 | 0.30 |
| Floor | 0.13 | 0.15 | 0.18 | 0.25 |
| Pitched Roof – Ceiling Level | 0.10 | 0.11 | 0.15 | 0.25 |
| Pitched Rood – Rafter Level | 0.10 | 0.13 | 0.18 | 0.25 |
| Flat Roof | 0.10 | 0.13 | 0.18 | 0.25 |
1 * This column relates to extensions where the existing building’s wall and roof U-Values are worse than 0.70 W/ m²K (walls) and 0.25 W/ m²K (ceiling).
2 * This column is for any other extensions, non-exempt conservatories and the conversion of unheated buildings.
Non-Domestic
| New Build – Starting Point | Existing Buildings – Refurb, Extension and Conversion of Unheated Buildings | Existing Buildings – Refurb, Extension and Conversion of Unheated Buildings | |
| Wall | 0.18 | 0.25 | 0.30 |
| Floor | 0.15 | 0.20 | 0.25 |
| Pitched Roof – Ceiling Level | 0.14 | 0.15 | 0.25 |
| Pitched Rood – Rafter Level | 0.14 | 0.15 | 0.25 |
| Flat Roof | 0.14 | 0.15 | 0.25 |
How Do I Calculate My U-Value?
Calculating the U-Value of a material can be a complex process. It is essentially the sum of the thermal resistances (R-Values) of all the layers that make up a building element, including any adjustments for fixings or air gaps.
You must first know the R-Values in order to calculate the U-Value. If you buy the insulation materials from a store, then the R-Values will likely have already been worked out for you.
The Total Resistance (Rt) is then the sum of all R-Values combined. Once you have the Rt value, the U-Value can be calculated using the following formula:
U = 1 divided by Rt
Online calculators can also be used to help you work out the U-Value easily.
Don’t forget to read through our guide to insulation types before getting started with any insulation work in your home.
