A Quick and Easy Guide to U-Values

Loft Conversions

Understanding and measuring U-values has become increasingly important as we aim to improve the sustainability and performance of our buildings.

What is a U-Value?

U-Value – is the measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction.

In other words, a U-value is used to measure how well or how badly a component transmits heat from the inside to the outside. The slower or more difficult it is for heat to transfer through the component, the lower the U-value. This means that we are looking for a lower U-value.

The lower the U-value the better.

When we talk about a component, we can mean a pane of glass, a timber door, or a complete building assembly such as a cavity wall. We can work out how much heat passes through each element of the building assembly and determine the U-value based on the amount of energy lost through a square metre of material.

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What are the units for U-Values?


The U-Value is measured in W/m² K

This is broken down as: The rate of heat flow (in Watts) through 1m² of a structure when there is a temperature difference across the structure of 1 degree (K or ˚C)


Wall 1 with U-Value of 0.3 W/m2 K will lose heat at half the rate of Wall 2 which has a U-Value of 0.6 W/m2 K


So, the LOWER the U-Value, the BETTER.

The lower the u-value the more efficient the construction is at keeping heat flow through the structure to a minimum.


Are U-values important in the Building Regulations Approved Document Part L?


U-values are considered in the Building Regulations Approved Document Part L. For the following references to the Building Regulations we will assume we are always referring to a new build dwelling.


Achieving a specific U-value isn’t a tick box exercise. It is important the building is considered as whole. This is reflected in the way the Approved Document Part L is set out. In order to comply with the building regulations there are a couple of areas to consider:


The building must be designed to show that the carbon dioxide emission rate for the whole of the building  (dwelling CO2 emission rate or DER) does not exceed a maximum or target CO2 emission rate (TER).


Energy loss through the building fabric for the whole building (dwelling fabric energy efficiency DFEE) must not exceed a maximum or target allowance (dwelling target fabric energy efficiency TFEE).


Much of these calculations can be carried out using a SAP software.


The area that relates to U-values is the DFEE and the TFEE.


The DFEE (dwelling fabric energy efficiency) must be no worse than the TFEE (target fabric energy efficiency). The TFEE summarised in ADL1A Section 5 and provides concurrent notional dwelling specification recommendations.


What U-Values do you need for Building Regulations?

Currently the Building Regulations Part L1A (which refers to new dwellings are as follows:
  • U-value requirement External Walls 0.18 W/m²K
  • U-value requirement Party Walls 0.0 W/m²K
  • U-value requirement Floor 0.13 W/m²K
  • U-value requirement Roof 0.13 W/m²K
  • U-value requirement Windows (whole window U-value) 1.4 W/m²K
  • U-value requirement Opaque doors 1.0 W/m²K
  • U-value requirement Semi glazed doors 1.2 W/m²K


Recommended U-Values Domestic

Recommended U-Values non domestic

U-Value Calculator:


There are a few good u-value calculators online, that if you are short on time are worth checking out. Some are for calculating any build up, where as others have been developed by insulation manufacturers that specifically deal with their own products.

Scroll down to view our recommended Online U-value calculators.


How to calculate a u-value


Although online u-value calculators are really useful, and some more intelligent modelling software will calculate the u-value for you, it is worth learning how to do it yourself, just in case it pops up in an exam. After all, it is pretty simple when you know how.

To calculate the u-value of a particular part of the building construction you need to know a little about each element of the construction.

Thermal Resistance (R)

U-values are calculated from the thermal resistances of the parts making up a particular part of the structure. Transmission of heat is opposed in varying amounts dependent on material and surface. Thermal Resistance is defined as a measure of the opposition to heat transfer offered by a particular component in a building element.

In order to calculate thermal resistance, you must know the thickness of the material and the Thermal Conductivity (K) value. These values can be found in the Metric Handbook, or the Architects Pocket Book (the Architects Pocket Book is extremely useful, I would recommend every architecture student has a copy of this book).


Thermal Conductivity of materials

Thermal conductivity of materials (w/mK)



R= Thermal Resistance (m2K/W)

d= Thickness of material (in Metres – very important)

k= thermal conductivity of the material (W/m K)

You must know the thermal resistance (R) in order to calculate the u-value. If you are specifying standard products it is often easy to find the resistance values for these elements. Sometimes it is worth having a look at specific manufacturers websites for these details.

Total Resistance (Rt)


Ra is airspace cavity and values for this can also be found in Architects Pocket Book.

How do you know the Rso and Rsi Value?

Rso is the outside surface resistance, and Rsi is the inside surface resistance. These values are specified in the Architects Pocket Book as:



Now you have your Rt value, the calculation is simple, one divided by the Rt. There you have your U-Value.

Other helpful bits:

Architecture.com U-Value guide BRE Guide to U-Values

One of our readers, Brian, has very kindly provided access to the U-Value calculations of a live project so that you can get a feel for what is involved. Click on the link below to view.

Calculations of area weighted U-values

Brian also provided us with access to the very useful “Part L1B & What you need to know to get your building to pass

Online U-value calculators:

Thermal Calc Online

Vesma Calculator (This one seems the best)

Rockwool U-Value Calculator – This is for Rockwool products only

Kingspan U-Value Calculator – Kingspan products only

British Gypsum Calculator 


McMullan, R. 2007. Environmental Science in Building


Don’t forget you can download our handy guide by clicking on the button below:


My favourite Tools and Resources

I have curated a list of some of the tools and resources I would strongly recommend for anyone studying or working in Architecture.