How to calculate a U-Value the old-fashioned way. Here’s what you need…


We know there are computer programmes and our very own lovely U-value Calculator, but behind the simplicity is a lot of supporting guidance, documentation, protocols, rules and regulations. So here’s a quick run-through of what you may need if you’re calculating a U-value by hand.


BR 443 – Conventions for u-value calculations.
A general document outlining conventions for undertaking u-value calculations for various common types of construction, with references to additional supporting documents, and explanations of why some common conventions are adopted, e.g. why a 15% timber bridge is used for timber frame walls. The document also contains a number of methods for adopting adjustments for potential bridges in insulation layers, such as loft hatches, recessed light fittings etc.


BS EN ISO 6946: 2007 – Building components & building elements – Thermal resistance & thermal transmittance – Calculation methods.
This is the main calculation protocol, and outlines the method (i.e. formulas) for calculating thermal transmittance (u-value) for constructions, including those with thermally bridged layers. The annexes to the document include a number of additional calculation procedures, to calculate the r-value of components, or adjustments due to various factors (e.g. such as surface or cavity r-values, or adjustments due to mechanical fixings, or rainwater in inverted roofs).


BS EN ISO 13370: 2007 – Thermal performance of buildings – Heat transfer via the ground – calculation methods.
This document covers calculation methods for ground floors (solid and suspended) and basement constructions where all or part of the heat loss from the construction is through the ground beneath or adjacent to the construction.


BS EN ISO 10456: 2007 – Building materials & products – Hygrothermal properties – Tabulated design values & procedures for declared & design thermal conductivity.
There are two main items in this document, the first is a number of tables which contain generic material properties, which are one of the approved data sources for taking product properties from. The second covers thermal conductivity used in u-values, ISO 6946 & ISO 13770 are written around using design thermal conductivity values, a declared conductivity can be converted to a design value using various adjustments from this document, covering potential differences in temperature, moisture and ageing (as ageing is already covered by our product standards this does not apply to our products).


BRE Digest 465 – U-values for light steel frame construction.
Due to the high thermal conductivity of steel frames, constructions containing these elements can often not be modelled accurately via the standard process from ISO 6946, due to the large difference which occurs between the upper and lower limits of resistance. This document covers an adjusted simplified method which allows these elements to be modelled without the requirement for numerical analysis to be undertaken.


Steel Construction Institute Paper 312.
This document is similar to the previous item, but covers an alternative steel construction type, covering a simplified process to calculate u-values for built up metal cladding systems which utilise rail and bracket spacer systems.


BRE Foundation for Built Environment 42 – U-value conventions in practice, worked examples using BR 443.
This document is intended to illustrate how the principals outlined in BR 443 would be implemented in practice. It illustrates the method though which a u-value is calculated for a number of common constructions. It should be noted that this document currently contains quite a few errors, so I would suggest its not that important to study in depth currently, but you should be aware of what it covers, and it may be of some use in indicating how the formulas from the various other documents are used.


BBA information Bulletin 3 – Reflective foil insulation – Conventions for u-values.
This document contains additional guidance on the implementation of multi-foil insulations within u-value calculations, relating to how items such as localised compression, bridging by adjacent timbers, and r-values for core and adjacent cavities should be dealt with.


BBA information Bulletin 4 – Inverted Roofs – Drainage and u-values.
This document contains additional guidance on how the information contained within annex D of ISO 6946, and ISO 10456 should be implemented with regards to corrections for rainwater cooling effects, and changes in insulation performance due to in application moisture content.


BBA information Bulletin 5 – Reflective breather membranes in timber frame walls – Thermal performance claims.
This document contains some guidance relating to reflective breathable membranes, but can also be applied to reflective VCL’s or in some cases multi-foils, with regards to how performance is quoted for these materials, and issues that need to be considered with regards to certain types of data, such as directly measured r-values when incorporating these products into u-value calculations.

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