A Quick Guide to U Value

What are they? How can you achieve your desired U-Value?

These are recurring questions we’re often asked and there’s no one-size-fits-all approach. 

In short, a U-Value is a measure of how well a building element (floor, wall, roof) retains heat. (U-Value unit is W/m²K)

To achieve a desired U-Value, a list of components are required and on average, the insulation does about 90% of the work to achieve said U-Value. 

U-Values can look overwhelming at first and this is mainly down to the number of factors that need to be considered in a calculation.

To give you an idea of what’s considered, we’ve provided some factors below.

  • Area of the building element (floor, wall, roof)
  • What membranes are being used (breather membranes, VCLs)
  • Are there fixings being used
  • Are there air cavities in the build-up 
  • What part of the country are you located 
  • Perimeter / area ratio (Floors only)
  • Internal & external surface resistance

Here’s an example of what a manufacturer's U-Value calculation may look like. (Floor calculation)

 U-Value Guide

We understand that all this jargon may look somewhat daunting but we’re going to break it down as easily as possible for you. 

The two main values needed in calculating U-Values are Thermal Conductivity and R-Value.

Thermal conductivity (W/mK)

This is the thermal performance of the insulation and can vary depending on what type of insulation is being used.

The lower the thermal conductivity of the insulation, the better. 

You’ll often see 0.022W/mK, this is the thermal conductivity of standard PIR such as Celotex.

There’s Kingspans K103 which is 0.019W/mK and Optim-R which is 0.007W/mK. 

Thermal conductivity is used to calculate the R-Value of a material which we’ll touch on next.

R-Value (m²K/W)

This is the thermal resistance of an insulation. 

The higher the R-Value, the better the material is at retaining heat.

R-Values are calculated by dividing the insulations thickness (in metres) by its thermal conductivity.

An example of this is 100mm PIR - 0.1 ÷ 0.022 = 4.54 

This is typically rounded down to the second decimal place so it would be shown as 4.50 on the manufacturers data sheet an example of which is below from Celotex.

U Value

U-Values (W/m2K)

In its simplest form, U-Values can be calculated by adding together the R-Values of all elements within a building component and dividing them into 1. 

Lets take 100mm PIR on its own as a simple example. 

The R-Value of 100mm PIR is 4.50, 1 ÷ 4.50 = 0.22 W/m2K. This is the U-Value achieved by the insulation on its own with nothing else taken in to consideration. 

This is the most basic form of calculating a U-Value but the same principle applies when adding together all of the elements.

You could have a combination of 100mm PIR + 100mm Loft Roll for example. 

The R-Value of 100mm PIR being 4.50 + 100mm Loft Roll which is 2.25 equates to a combined R-Value of 6.75. (1 ÷ 6.75 = 0.148, this is rounded up to 0.15 W/m2K)

U-Value Guide

We’ve outlined some of the more popular U-Values below and how they can be achieved.

It’s important to note that these are guidelines only and while U-Values can be achieved in most circumstances using these materials, we can’t guarantee them. 

Pitched roof 0.18 or better - 150mm deep rafters

Celotex/PIR 100mm between rafters, 50mm below.

Kingspan K107 100mm between rafters, K118 37.5mm below.

Pitched roof 0.15 or better  - 150mm deep rafters

Celotex/PIR 100mm between rafters, 70mm below.

Kingspan K107 100mm between rafters, K118 57.5mm below.

Cavity wall 0.20 or better - Full fill

Celotex Thermaclass 90mm

Kingspan K106 90mm

Isover CWS32 150mm

Cavity wall 0.20 or better - Partial fill

Celotex CW4000 100mm

Kingspan TW50 100mm

Timber frame wall 0.20 or better - Masonry outer leaf 

Celotex/PIR 80mm between studs, 30mm inside.

Kingspan K112 70mm between studs, K118 42.5mm inside. 

Solid ground floor 0.18 or better - P/A ratio 0.4

Celotex/PIR 90mm

Kingspan K103 75mm

Solid ground floor 0.16 or better - P/A ratio 0.4

Celotex/PIR 100mm

Kingspan K103 90mm

Please note this is for guidance purposes only and results are not guaranteed as calculations are based on assumptions of standard building practices. We recommend contacting a manufacturer for confirmed U-Value calculations. 

  • technical@kingspaninsulation.co.uk 
  • technicalsupport@saint-gobain.com
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