Let’s talk about insulation Values!

At Celtic Sustainables we get a lot of calls regarding insulation, a lot of people are unsure about thermal values and how much insulation they need to install to hit the right U values etc. Thermal conductivity, R-Values and U-values can all seem a bit confusing, however we hope this explanation will help to make things a little clearer.

Thermal Conductivity

written as W/mK (Watts per Metre Kelvin)

This figure is the basis of all insulation and heat loss calculations. It should be available for every type of insulation. If it’s not printed on the packaging it will be listed on the manufacturers website or on the product datasheet.

Thermal conductivity indicates how easily heat will flow through a material, whether it is a brick or a layer of insulation. This measurement doesn’t relate to the thickness of the material; the number is the same whatever the thickness.

When comparing Thermal conductivity the lower the number the better

The Thermal conductivity of the material is used to calculate R values.

R-Value

written as m2K/W (metres squared Kelvin per Watt)

The R-Value is the measure of a material’s resistance to heat flow at a specific thickness. The more resistance a material has to heat flow, the higher the number.
To calculate a materials R-value, you need to divide the thickness of the material (in metres) by the Thermal conductivity (in W/mK).

So 150 mm of insulation with a Thermal conductivity of 0.039, has an R Value of 3.85
The equation for this is 0.15 (m) / 0.039 = 3.85 m2K/W (metres squared Kelvin per Watt)

When you are comparing the R Values of materials, the higher the number the better.

U-Value

written as W/m2K (Watts per metre squared Kelvin)

So, we’ve established that the R-Value of a material is its resistance to heat loss. Conversely, the U value of a material is the amount of heat that is lost through conduction. So in its simplest form, the U-value of a single material is the inverse of the R-Value. To calculate the U-Value of a single material we divide 1 by the R-Value.

If the R-value of a material is 3.85 the U-Value would be 1/3.85 = 0.26.

When comparing U Values, the lower the number the better.

However, the U-Value equation is usually used to calculate how much heat is lost through a section of construction e.g a wall, floor or roof. To do this we need to add a few more numbers into the calculation. We need to add the heat lost through conduction by the additional materials in the construction as well as heat lost through the construction through convection and radiation.

The equation for calculating the full U-Value is: U = 1/ [ Rsi + R1 + R2 +R3… + Rso]

Rsi is the surface resistance of the internal face
Typical internal surface resistance: Roof / Ceiling 0.1, Wall 0.12, Floor 0.14.

Rso is the surface resistance of the external face.
Typical external surface resistance: Roof / Ceiling 0.04, Wall 0.06, Floor 0.04.

R1, R2, R3 etc is the resistance (R-value) of each individual material in the construction.

As you can see it’s quite an involved calculation. There are a couple of u value calculators available on-line which can make things a little easier.

Take a look at https://www.ubakus.de/u-wert-rechner/? Who offer a free demo version of their commercial calculator.

How much Insulation do I need?

The answer to that will depend on:

• what type of building you are insulating (domestic or non-domestic, new build or refurbishment), 
• where you need to insulate (walls, roof, floor etc), 
• how the building work is constructed (materials used etc) and,
• where in the UK the project is located (the rules are can differ for England, Scotland, Wales and NI).

To find out what minimum U-Value you need to achieve for your project to comply with building regulations, please go to:

• The Planning Portal for projects in England or Wales (don't forget to change the top drop-down menu to "Wales" if the project is located there)
• Gov.scot for projects in Scotland
• Building Control Northern Ireland for projects located in Northern Ireland

So there we are, hopefully this will help a little in your understanding of Insulation values. If you do have any questions or would like to discuss your insulation requirements, please feel free to give us a call on 01239 777009 or email  sales@celticsustainables.co.uk

Celtic Sustainables Ltd does not accept liability for loss or damage arising from the use of this information.