Assumptions in our U-value calculations


One of the things that you may have noticed when using our online U-Value calculator (and if you haven’t used it yet then take a look), is that there are a number of caveats that are shown when you are working through the calculation.

This small print at the bottom of the screen may be something you have overlooked but it does form a critical part of our U-value calculator and we thought it would be worth explaining in a bit more detail about what these caveats are and why we have included them.

We want to give you as much information about how we have made these calculations so that you can be assured that the number we give you is correct and you have the complete picture of all the detail that went into your calculation. This way you know you can rely on the calculation we provide as we have taken other factors into account. These usually relate to assumptions that we have made when working out the U-value and are different for each build up. And because the calculation is carried out by an expert BBA approved person certified in carrying out U-Value calculations, you can be assured that the calculation follows the correct procedure.

There are several different types of assumption and these are explained in a bit more detail below.

What they are:

Construction Guidance

Some of the caveats given in the calculator give guidance regarding how the insulation is fixed or added to the construction. For example to achieve some of the U-values requires two layers of insulation, and in these circumstances the thicker layer should always be on the outside of the construction.

Another example is that of green roofs, where the ‘green’ part of the roof may have some insulating properties which could result in a lower U-value, however as this is very much dependent on the plants, thickness of soil, how wet the soil is etc., it is standard procedure not to take this into account when calculating the U-value, however we wanted to make sure that you are aware of what we are taking into account when calculated your U-value.

Thermal Bridging & Mechanical Fixings

We said in an earlier post that the U-Value is calculated by adding together the reciprocal of the resistivity of all the elements making up the particular application; however there are often other aspects which need to be taken into account; for example the impact of mortar around blocks or corrections for mechanical fixings to hold insulation in place, all of which need to be considered.

The other aspects which we have taken into account are covered in our caveats.

For example on rendered solid block walls we have a caveat regarding the thermal bridging factor assumed for mortar joints, which means that there is a different heat path through the structure than that taken through the lightweight blocks, and we have allowed for this within our U-value calculation.

For cavity walls we have a caveat regarding the stainless steel flexible wall ties which would connect the two leafs of the cavity. The assumption we are making with this caveat is regarding the amount of heat lost by the fixings. Obviously the wall tie will carry more heat than the insulation and we have taken this into account when calculating the U-Value.

Overall the heat lost through the fixings depends on the number, size and material of the fixings. For each application that this applies to we have made an assumption as to the number of fixings that are used, based on the guidance that we give within our product literature.

Why we made assumptions

These assumptions are the same as those we would make if we were speaking to you on the phone and used in our literature to calculate a U-value. We will always let you know when we are making an assumption or correction, but if you have any questions about them let us know, either in the comments below or by emailing us directly on

Don’t forget to try out our new U-value calculator at To find out more about the u-value calculator and how it works check out the rest of the posts in this series which are linked below.

To keep up to date with all our latest blog posts you can follow the blog by clicking this RSS Feed link or by following us on Twitter @KingspanIns_UK or on Linkedin.

Share this blog post with your friends and colleagues by clicking on the social media icons below.

Print this page

Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

Leave a Reply

Your email address will not be published. Required fields are marked *