Before diving into the regulations concerning cavity wall insulation on new builds, you probably should familiarise yourself with the ‘U’ value. Also known as thermal transmittance. The U-value measures the rate of health loss of a fabric. This may be your walls, roof, floors or even windows. Each fabric has its own U-value or rate at which it transmits thermal heat.
The graphic represents this equation.
Where ‘R’ represents represents thermal resistance of an element (internal wall OR cavity wall insulation etc ).
You may be wondering, what relevance does this ‘U-value’ have? Put shortly, it’s a number that buidling regulations use to check that your cavity walls are compliant.
The important thing to note is that a lower U-value it preferable from an insulation perspective. For example, metals typically have a high U-value and are good conductors of heat. Hence why your frying pan has a copper base. On the flip side a well-insulated wall should have a U-value of approximately 0.25.
U-value for external walls which property developers must meet. What is the target setting?
In simple terms, the U-value of your external walls should be less than 0.30 (L1A 2013). If the U-value of your new dwelling is above 0.30, then your home will not comply to building regulations.
As of June 2022 new build homes in England must cut an additional 30% in carbon emissions.
Although this statement is not directly aimed at cavity wall insulation, its important to understand its significance. As its compulsory to meet this target set by the DLUHC, one of the ways in which we can meet this is to minimise the amount of heat lost to your cavity walls.
In this example, you can pretend ther’s 4 layers. Each layer has its own thermal resistance, thus having its own independent U-value. However, to calculate the U-value of the external wall we must calculate the 4 layers as one item. Hopefully the equation mentioned earlier in this post will make a little more sense now. Lets brake it down.
Thermal resistance of gypsum skim, R1 = 0.17
Thermal resistance of internal leaf, R2 = 0.44 (4-inch face brick)
Thermal resistance of cavity wall insulation, R3 = 6.81 (Xtratherm PIR Insulation 150mm)
Thermal resistance of external leaf, R4 = 0.44 (4-inch face brick)
Note
Thermal resistance = Thickness / Thermal Conductivity
Xtratherm PIR thickness = 150mm or 0.15m
Xtratherm PIR thermal conductivity = 0.022
Hence, Xtratherm PIR thermal resistance, R4= 6.81
