Quick answer: PIR thickness affects thermal performance, but a PIR board does not have one universal finished U-value on its own. The U-value is calculated for the complete roof, wall or floor build-up.
Our stocked PIR board specification has a declared thermal conductivity of λ 0.022 W/mK. If you are choosing a board thickness, start with the required construction and thermal target, then select the PIR thickness that allows the complete build-up to achieve it.
U-value measures the whole construction
A U-value measures how easily heat passes through a complete building element such as a wall, roof or floor. It is expressed in W/m²K, and a lower number means less heat is passing through the construction.
The calculation includes more than the insulation. Timber studs or joists, masonry, plasterboard, cavities, roof decks and other layers can all change the result.
PIR thickness and U-value are not the same thing
It is tempting to ask for the “U-value of 100mm PIR”, but that skips an important step. A 100mm PIR board can form part of several different constructions, and those constructions can finish with different U-values.
For example, PIR fitted continuously across a structure does not behave exactly the same as PIR fitted between timber members because the timber creates a separate heat path through the build-up.
PIR thermal conductivity is λ 0.022 W/mK
The PIR board specification we stock has a declared thermal conductivity, or lambda value, of 0.022 W/mK.
Lambda describes how readily heat passes through the insulation material itself. It is useful for thermal calculations, but it is not the same as the finished U-value of a roof, wall or floor.
What thickness of PIR insulation do I need?
There is no single PIR thickness that suits every job. Choose the thickness from the required construction build-up, the available depth and the thermal performance the project needs to achieve.
If the project already has a specification, follow that. If a U-value has to be demonstrated, calculate the complete build-up using the technical data for the board being installed rather than selecting the thickness from a generic table.
Thicker PIR gives more thermal resistance
All else being equal, increasing the PIR thickness increases the insulation layer's thermal resistance. That is why 100mm PIR provides more resistance than 50mm PIR of the same specification.
The finished U-value, however, still depends on the rest of the construction. Doubling the insulation thickness does not mean the complete wall, roof or floor U-value simply halves.
Why we do not publish one generic PIR U-value table
We sell PIR insulation by specification rather than by brand. The supplied brand can vary, while the specification we buy remains the same.
For project calculations or compliance paperwork, use the technical data supplied with the board being installed and calculate the complete construction rather than relying on a generic thickness-to-U-value table.
Shop PIR insulation by thickness
Our PIR insulation range includes rigid PIR board and PIR insulated plasterboard. For standalone rigid insulation, shop 25mm, 50mm, 75mm, 100mm and 150mm PIR boards.
If you are new to the material, read What Is PIR Insulation? first.
