How thick should PIR insulation be? Thermal conductivity and resistance explained

There are also variants with other facings. Polyurethane boards should therefore be compared on the basis of the documents for a specific delivery. Belonging to the PIR family does not by itself guarantee an identical lambda. In an energy comparison the difference should remain visible, even when both products look similar.

The calculation assumes a continuous and homogeneous layer. It does not cover the additional heat flow path through a structural element penetrating the insulation. Nor does it describe gaps created by an installation error. The result should therefore be treated as a starting point for checking the complete building element.

The resistance calculation shows the benefit of a layer of a given thickness. It does not yet say what proportion of the house’s annual losses flows through it. A good result on a small area may matter less than repairing a large air leak. It is therefore worth establishing the area of the building element being upgraded and the other loss paths. Only then does the figure expressed in m²K/W become a useful component in assessing the whole building.

For a simple homogeneous system, U is obtained as the reciprocal of the sum of the thermal resistances of the layers and the heat transfer resistances. The project may nevertheless require additional considerations and corrections. The U-value of the roof should not be calculated solely as the reciprocal of the slab’s R and then presented as a complete assessment.

When comparing variants, keep the same assumptions for the other layers. Change only the element whose benefit you want to check. If you are also improving airtightness and windows, do not attribute the whole effect to the PIR boards alone. This simple order prevents savings from being overstated. It also makes it possible to explain why two houses with the same insulation thickness can have completely different bills.

Let us consider only a calculation example with lambdas of 0.022 and 0.035 W/(m·K). Ten centimetres of the first material provide a resistance similar to that of approximately 15.9 cm of the second material. This is a comparison of layers with equal R. It is not a universal conversion factor for every PIR, every wool or XPS board.

Another centimetre increases the resistance of the insulation, but the effect on the U-value depends on the total sum of resistances. A constant fall in U should not be expected with every equal increase in thickness. It is worth calculating several available variants and comparing them with the cost of the details. Sometimes removing a weak junction will matter more than choosing the thickest board. Polyurethane insulation should be assessed in a real section, taking account of the perimeter and fixings.

When comparing costs, keep the same area and the same price unit. Take account of the real cutting layout and the accessories required by the system. The price of the board alone will not show the difference in the length of fixings or flashings. Only the complete scope makes it possible to assess whether a thinner material offers an economic benefit.

The thermal conductivity stated in the documents is not a guarantee of a single room temperature during a heatwave. A house also gains energy through glazing and from use. Insulation limits the flow but does not by itself remove excess heat. If summer comfort is the aim, include shading and the possibility of night ventilation in the assessment. Polyurethane foam has a defined role in this system, not the task of replacing all other measures.

A continuous layer of PIR outside the structure can reduce repeating thermal bridges, yet this does not mean that thermal bridges disappear completely. Edges, fixing points and connections remain, and their influence must be considered in the project. The profiled edge of the board facilitates installation but does not replace this assessment.

The record of the decision should show the values adopted, the thicknesses and the space constraints. If the supplier changes, the calculation can then be checked again. A note about material equivalence without data is not enough. Polyurethane insulation makes it possible to achieve high resistance in a small thickness, but its economic sense depends on the project. A clear table of variants helps to assess both the thermal benefit and the additional expenditure needed to complete the whole.

Assume initially a target R = 5 m²K/W. The calculation shows 110 mm for a lambda of 0.022. If the selected product line has a different thickness gradation, the nearest suitable variant must be checked. Do not round down without reassessing the required result.

Before buying, it is worth establishing what problem the new insulation is meant to solve. It may be insufficient resistance of the whole building element or a lack of space in a particular detail. These are not identical tasks. The calculation should indicate the thickness that matches the need, and the cost estimate should cover its correct execution. Only then does a low lambda have practical significance. It becomes a verifiable component of the choice, instead of a slogan intended by itself to justify any price or promised bill.