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Insulation Values

When designing your extension, you will hear lots about the insulation performance of the new structure. So what’s that all about?

Building regulations specify the minimum performance criteria for the insulation of building elements. Implemented to assist with the overall objective of reducing the carbon footprint, and at the same time make buildings more comfortable to live in. They cover energy efficiency of heating appliances and ventilation too.

Simple to remember – the LOWER the U value the BETTER the insulation. If you have a U value of 0, that means no heat is getting out.

I promise I won’t get too technical, so here goes.


Insulation is a function of the ability of a material to resist the flow of heat from one side (the “hot” side) to the other (the “cold” side) together with the thickness of the material. So different materials have different insulation resistance values referred to as the R-Value and are a function of density and conductivity of the material. The unit is m2K/W.

  • W = Watts is a measure of energy – I’m sure you will be familiar with a 60W bulb or a 1500W steam iron.
  • M2 =Square metre is the area of the surface.
  • K = degree Kelvin – it’s graduated the same as centigrade and a degree change K is the same as a degree change C.

A more useful guide is the thermal transmittance value – or ‘U” Value as it is commonly referred to. This takes the energy flow across a material and expresses it in units of Watts per Square metre per degree Kelvin of heat loss. W/m2K.

That means energy is lost at 1W per 1 °K (C) over a 1m2 area.

Take a typical single glazed glass unit in a traditional pane. It has a U value of perhaps 6.0 W/m2K.

If it’s a chilly day outside – say 3° C – That means it would take 17 X 6 = 102 W to maintain 20° centigrade inside.

For a building regulations compliant window double glazed with a U value of 1.6 W/m2K it would take 17 X 1.6 = 27.2 W
And for a high efficiency glazed unit with a U value of 1.0 W/m2K it would take 17 x 1 = 17 W. That’s 83% less than single glazed and 63.2 % less than building regulations compliant glass. If I said I could reduce your energy bill by 80% you would think I am doing a great job I’m sure.

A bit more detail…

Ok here’s the technical bit.

Materials have a thermal conductivity value – l – it’s called Lambda. It’s measured in W/mK.

For instance, typical l values are:

  • Dense Concrete is around 1.5 W/mK
  • Standard Glass is around 1.0 W/mK
  • Earthwool building Slab insulation around 0.035 W/mK
  • Aluminium is 250 W/mK

The thicker the material, the more insulating it will be. So the R value takes into account the thickness of the material. The value is calculated by R= l/l where l is the thickness of material (in metres) Its useful as by working on the R value a multi layered construction with different materials and thicknesses can be taken into account eg insulated timber wall.

And to get to a U Value it’s the reciprocal of the R value(s)

The U value is 1/R and is in W/m2K.

Also the HIGHER the R value the BETTER the insulation of the material.

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