A Quick and Easy Guide to U-Values

These days it’s all about the U-Values, so you need to get clued up! Everyone loves a nice low U-Value, or is it a high one….? Here are the quick tips you need to make sure you are bang on with your U-Values!

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.

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.

U-Value for Building Regulations

Currently the Building Regs Part L1A (which refers to new dwellings) for limiting fabric parameters U-Values are as follows:

Roof: 0.20 W/m2 K

Wall: 0.30 W/m2 K

Floor: 0.25 W/m2 K

Party Wall: 0.20 W/m2 K

Windows, roof windows, curtain walls, pedestrian doors: 2.00 W/m2 K

These are minimum acceptable values, and it is always advisable to strive to achieve u-values that exceed these minimums by seeking out high standard products that are at the forefront of sustainable design.

If you choose to take your standards a step higher, aim for the concurrent notional dwelling  specification standards set out in Approved Document L1A of:

U-Value of Roof: 0.13 W/m2 K

U-Value of Wall: 0.18 W/m2 K

U-Value of Floor: 0.13 W/m2 K

U-Value of Party Wall: 0.0 W/m2 K

Windows, roof windows, glazed roof lights and glazed doors: 1.40 W/m2 K

Or you could look at the Passiv Haus where in Sweden, to achieve passive house standards, the insulation thickness for a wall would be 335 mm (0.10 W/(m².K) and the roof 500 mm  (U-value 0.066 W/(m² K)!

Generally, my view on this is make it the best it can be.

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.

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 

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).



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:

Rsi Inside SurfaceRso Outside Surface



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



McMullan, R. 2007. Environmental Science in Building

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.