What are Thermal Values?

All new buildings must meet an overall thermal level so walls floors and ceilings need to be designed to meet the minimum requirements laid down by construction rules. By adding insulations and barriers, the minimum levels can be improved. Normally, the thicker the component is the better – but this is not the case with insulation. With modern materials a 40mm thick insulation can contribute to a much lower U-value than a 200mm thick wool product could! Urban Marque Ltd spend a great deal of time identifying and testing new products so we are always at the cutting edge and able to provide thermal calculations to our clients to prove that our choice of materials and components will meet their specified demand.

So what is a U-Value?

U-value is a measure of how much heat will pass through a thermal object, like a wall or window. A low U-value means little heat will pass through it. U-value is measured in W/m2K – that is, Watts per square metre per degree Kelvin (1 degree Kelvin is the same as 1 degree Centigrade, but with a different zero).

So if a wall has a U-value of 1.0, then 1 m2 of wall will let 1 Watt pass through it when there is a temperature difference of 1 degree between the inside and outside.

Typical limits of U-value in buildings vary from a maximum of about 5.0 W/m2K for a single-glazed window to a
minimum of about 0.15 W/m2K for a modern roof with 300mm of loft insulation – so the window lets about 30 times as much heat pass through 1m2 of it, compared to the roof.

K-Value

The wall is called a ‘thermal object’ above because its U-value can be calculated from each of the components it is made of.

Each component, say the outer skin of brickwork in a cavity wall, will let heat pass through it based on two factors:
1. Its thermal conductivity
2. How thick it is

Thickness matters because in general the thicker something is, the slower heat will pass through it – a thick jumper keeps heat in better than a thin jumper.

The thermal conductivity is a measure of how easily heat passes through the material and is called the k-value (little ‘k’ which helps stop confusing it with big ‘K’ degrees Kelvin!). k-value is measured in W/mK. Thermal conductivity is an inherent property of a material, like stiffness or density.

The U-value of a single component is given by the formula U = k / l, conductivity divided by thickness (in thermal calcs the thickness always seems to be represented by ‘l’, not ‘t’ as you might expect).

So that outer skin of brickwork, on its own, might have a thermal conductivity, k of 0.84 W/mK and a thickness of 100mm (or 0.10m), so U = 0.84/0.10 = 8.4 W/m2K – yes, worse than a single-glazed window!

Typical limits of k-value in building materials are a maximum of around 2.0 W/mK for concrete and sandstone to a minimum of about 0.025 W/mK for modern insulation materials.

R-Value

– You will come across this in UK evaluations, so it’s worth mentioning.
America uses a different way of expressing resistance to heat flow, using the R-value, thermal resistance. This is just the inverse of the U-value, so 1/ R = U. Of course they don’t use metric units, so you can’t compare their R-values directly with ours.

So big R-values are more resistant to heat flow. In (European) SI units a U-value of 0.2 will have an R-value of 5.0 m2K/W and this would be equivalent to an American R-value of 28 (units are h.ft.ºF/Btu) – American R-values are 5.67 times SI R-values.

We have to use R-values in thermal calculations because we can’t add up the U-values of each component in a thermal object – but we can add up each component’s r-value to get the total R-value (little ‘r’ is used for the components and big ‘R’ is used for the whole thermal object).
So R = r1 + r2 + r3 + r4 and so on. Each individual r is the inverse of the u-value, so r = l / k, thickness divided by conductivity.