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What is a U-Value?

Where you have a wall… or a floor… or a roof… or any large element, and where you have differing temperatures on either side of that element, heat will transfer from one side to the other. This is why our homes lose heat! In the winter there can be a temperature difference of 20 degrees between the inside and outside. Heat from your home escapes through the fabric, and is replaced by colder air.

Every material has a ‘thermal conductivity’ – measured in watts per metres Kelvin (w/mK) and often written as lambda (?). Insulation materials have a very good, or low, conductivity (around 0.02 – 0.04). Masonry is around 0.1 to 0.8, right through to metals at the top (aluminium at 160 and copper at 380).

By bringing together thicknesses of materials, their thermal conductivity and air resistances, you get a U-Value.

A U-Value is shown in watts per metres squared Kelvin (W/m2K). A modern external wall of a house would achieve a U-Value around 0.25 (the lower the better)… a single pane window would be around 4.8. As you may expect, materials with lower conductivity (insulation's) will play a much bigger role in the overall U-Value than materials with higher conductivity. This means increasing your insulation layer by 50mm will noticeably reduce your U-Value. Increasing your block-work by 50mm is unlikely to make any difference at all. In construction, it’s common to have areas of ‘repeating thermal bridges’. This is where your insulation layer is affected by the structure of a building (such as timber joists in the loft space, or cavity ties in the walls). The U-Value should include these repeating bridges within the calculation. These are treated very differently to ‘non-repeating thermal bridges’. Look out for our blogs on what is a ? (PSI)-value and what is a Y-value.

Jon Ponting

Author: Jon Ponting

This article was published by Jon Ponting on 02.10.2014.