How do you assess heat and moisture transfer through a building element?
PIR insulation
A reliable result starts with a correct description of the wall or roof. You need to know all the layers, their thicknesses and properties, as well as the conditions on both sides of the building envelope. The low lambda of PIR helps to reduce the insulation thickness. However, it is not enough to assess the entire system. The thermal calculation must be supplemented by checking moisture, thermal bridges and the airtightness of joints. Record the layer build-up from the interior to the exterior. Include plaster, masonry, existing insulation, air cavity and finish. Do not omit old insulation just because it will not be replaced. During renovation, its condition and actual thickness may require an inspection opening. A photo of the façade does not provide this data.
What data should you start with?
Record the layer build-up from the interior to the exterior. Include plaster, masonry, existing insulation, air cavity and finish. Do not omit old insulation just because it will not be replaced. During renovation, its condition and actual thickness may require opening up. A photo of the façade does not provide this data.
For each layer, the parameters needed for the calculation are required. For the insulation, check the lambda and thermal resistance from the documentation. For the moisture analysis, properties relating to vapour transmission are added. Values for the foam alone do not automatically describe a board with a gas-tight facing. Internal conditions also matter. A dry warehouse and a humid laundry should not receive an identical set of assumptions.
What does lambda tell you, and what does the U-value tell you?
Lambda describes heat conduction through a material. The lower it is, the greater the resistance a layer of a given thickness can provide. The resistance of a homogeneous layer is calculated as R = d/λ. The thickness d must be given in metres. For example, 0.10 m of insulation with λ = 0.022 W/(m·K) gives approximately 4.55 m²K/W.
The U-value relates to the entire building element. In a simple build-up it results from the reciprocal of the sum of the resistances of the layers and surfaces. Appropriate corrections must then be taken into account. The result of 4.55 is therefore not a U-value. Nor can it be assumed that every PIR board has an identical lambda. It varies with the type of product, the facing and sometimes also the thickness.
Why is U alone not enough?
Two walls with a similar U-value may behave differently at a window, ring beam or floor slab. In these places the heat flow path deviates from the simple layer build-up. A thermal bridge occurs. A calculation for a flat section of wall will not show the full extent of this phenomenon.
The design must therefore resolve the junctions between building elements. With internal insulation, the reveals and the junction of the wall with the floor and ceiling are particularly important. On the roof, fasteners and the substructure are added. A continuous PIR layer can reduce the influence of the rafters, but it does not make all thermal bridges disappear. Details and an assessment of the temperature at the most exposed points are needed.
How do you check the risk of moisture?
The moisture analysis answers a different question from the calculation of heat losses. It checks whether water vapour can lead to unfavourable conditions at the surface or within the building element. It requires data on temperature, humidity and the sequence of layers. Finding a single dew point on a graph is not enough.
The simple calculation method has a defined scope. It does not fully describe driving rain, capillary action or air leakage. With old masonry or internal insulation, a more detailed time-dependent analysis may be needed. The result is only meaningful once it is established where moisture enters the wall and how it can leave.
How do I check the calculator result?
A good report allows the calculation to be reproduced. It should include layer thicknesses, the parameters adopted, temperatures, humidity and a description of the method used. If the tool shows only a green message, ask for the full schedule. Without it, it is difficult to assess whether the right material was selected.
- Check the units and the order of layers.
- Compare the parameters with the current product data sheet.
- Establish whether fixings and joints have been taken into account.
- Check the scope of the moisture analysis.
Also record the less favourable variant. A small change in moisture or in the thickness of the old masonry can show whether the solution has an adequate margin.
When analysing variants, keep the same geometry and service conditions. Changing the material, temperature and surface area at the same time will make comparison difficult. The report should separate confirmed data from assumptions. If it is not known what is inside the cavity of the old wall, describe this uncertainty. It is then possible to calculate more than one variant and indicate which inspection will decide the choice. This is far more useful than giving a result with many decimal places.
What should I pass on to the contractor?
The contractor needs more than the board thickness. The documentation should include the fixing method, sealing of joints, details at openings and the required finishing layers. These determine whether the actual building element will match the calculation model.
Before covering the insulation, it is worth checking the continuity of the layers and taking photos of the details. A product must not be replaced with another board on the basis of the same thickness alone. Changing the facing can change the moisture behaviour, and a different lambda can worsen the thermal result. A calculation is practical when it clearly indicates the material, the conditions of its use and the elements requiring acceptance.
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