U/R-Values – The Difference Between Calculated and In-situ Measurements

U/R Values overview

The U-Value is probably one of the terms which is most used when talking about the energy efficiency of buildings. U/R-values are used to describe the thermal performance of building elements including floors, walls, windows, doors and roofs. This data is then used to calculate the energy performance of the whole building which in turn can impact on the size and cost of the specified insulation, heating system and overall cost of the development.

U/R-values are calculated using a range of computerised systems and in some cases manually. Many U-Value calculators also assume that insulation layers are airtight meaning the use of the airtight layer is necessary to achieve specified U-values.

The installation factors that affect U-Values are airtightness, windtightness, thermal bypass, damp construction, condensation, and cold bridging.

Airtightness and the installation of a vapour control layer (VCL) is possibly the most important part of an installation as it can protect the structure of the roof from condensation, mould and structural damage. It contributes towards noise reduction and allows and insulation to function correctly.

Breathable Construction
The correct installation of the appropriate VCL will ensure that a breathable construction is achieved which means that condensation of roof felt is avoided and that the roof can effectively ‘breath’ correctly. The quality of the installation is of utmost importance as weak points may be formed, which in certain weather conditions may generate condensation in the roof space. As a result, this may lead to mould and/or timber damage. Partel’s extensive range of VCL solutions from IZOPERM (fixed SD of 5), VARA PLUS (variable SD 0.4 to >60 m), to ECHOFOIL IZO (fixed SD 100m), to name a few, ensures that the correct product for each construction type is available. Read more on SD values here.

Thermal Bypass
Thermal bypass can occur when buildings are in fact airtight. When airtight, air can still move around or through insulation. In the roof, this can occur when the felt has not been taped or sealed at the eaves. It can also happen in the walls where external insulation has been poorly fitted or where cavity walls have been partially filled.

Where a roof or window is not windtight, the performance of insulation can be dramatically affected, in that the wind will cool down the insulation. The location of the project, the exposure of the roof and the quality of the workmanship will determine the effect that the wind will have on the calculated U-Value. It can be assumed that the calculated u-value could adjust by 1.5 times where windtightness is ignored. This may not sound like a lot but consider if you had hoped to achieve a U-Value of 0.16 and you actually achieve 0.24, this could result in a significant effect on heating demand and possibly compliance.

Building Regulation

In many countries U-Value calculators assume perfect airtightness and do not allow for on site issues. The issue is that installed or completed U Values may not match designed u-values.

Some countries, as Sweden allow for adjustments to a U -Value based on the installed system essentially to allow for workmanship issues. That together with the usual adjustments related to dimensions, services, material properties, wall ties, noggings and wall plates. Such adjustments to the U-Value are known as ‘’Delta U’’ and the terms are based upon the difference between calculated and actual (measured) u-values. Contractors may endeavour to reduce the figure through controlled/supervised manufacture and detailed planning. The typical adjustment figure in Sweden is between 5% and 20%.


The lower the U-value, the higher the R-value, and the better the thermal performance of the assessed element.

Our U-Value measurement-approach uses heat flux meters (HFM) and temperature sensors to achieve a certified U-value over 72 hours. This system is standardized in ISO 9869 and it’s the only method that delivers accurate quantitative in-situ information about a building envelope. The Measurement System has the potential for use in a number of areas, including refurbishment planning, building inspection, mold detection, quality control and R&D, to ensure insulation has been installed correctly.

The availability of higher quality materials and better knowledge should lead to higher quality construction – something which we at Partel are striving to promote. At Partel we supply a complete range of windtight products, all of which are available with integrated windtight tapes for ease of installation.

Our technical expert advice, WUFI analysis and the smart U-Value measurement approach can give security on the system specification.

References Sean Doran BRE East Kilbride Field investigations of the thermal performance of construction elements as built






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