Saturday, 3 October 2015

What is a Thermal Bridge

The Context

Standards of energy performance in UK buildings have been increasing gradually over the last 50 years, particularly since the update to Part L in 2006, which brought the UK in line with EU legislation (the Energy Performance of Buildings Directive). One of the main features designers have used to reduce heat loss is insulation. As levels of insulation, and the performance of other aspects is increased, features that previously had a small impact on the overall performance of a building increase in importance. To stretch an analogy, when the low hanging fruit has been taken, we have to reach higher for further improvements. Thermal Bridging is the fruit that’s a few branches up.


A thermal bridge could simply be described as a discontinuity in insulation. In places, by design or by issues on site, there are gaps in insulation, filled by materials (including air) that conduct heat better, so that more heat flows through that point than the surrounding insulation. Often, but not always, this is due to the building’s structure, for example the studs of a timber frame, where there is insulation placed between the studs.


There are several types of thermal bridge, which are accounted for in different ways in thermal models such as SAP.


Repeating thermal bridging is generally spread across the area of a wall, roof or floor in a regular way. It is usually accounted for in the U-value calculation. Examples would include wall ties in a cavity masonry wall, and the studs of a timber frame wall. In the first example, the ties conduct heat better than the insulation material so heat passes along them more easily than it does through the insulation, so the presence of the wall ties increases the rate at which heat passes through the wall. U-value calculations done in accordance with the relevant Standards should take such bridging into account using the methods described in the standards.


Non-repeating thermal bridging covers everything else, and can be broken down into sub-categories. The main two categories are linear thermal bridges, and point thermal bridges. Linear thermal bridges are more common and the bulk of the work of Advanced Details.


Linear thermal bridges typically occur where one element of a building joins a different one, for example at the eaves where a wall meets a roof. They can also happen in the middle of an element, for example a steel post in the insulation zone of a wall. The bridge occurs in two dimensions, visualised via an architectural detail drawing.
Point thermal bridges occur where insulation is ‘punctured’, usually by a structural element. A steel beam passing perpendicular through the insulation would be an example. N.B. if the beam was parallel to the insulation this would probably be a linear thermal bridge.

What is a psi value

A psi-value is similar to a U-value: it is a number that represents the rate of heatloss through a linear thermal bridge. ‘Psi’ is the english written version of the greek letter Ψ(uppercase) or ψ (lowercase), pronounced “sigh”. It has the units W/m.K, which is similar to the units for a U-value, W/m2.K. Note that for a U-value, the unit contains m2, a measure of area because a U-value relate to a planar element, whereas a psi-value contains m, a measure of length because it is linear. Both units contain W for Watts, which is the rate of heat transfer, and K, which is Kelvin, representing the difference in temperature between inside and outside.
Much like a U-value, a high psi-value is generally a bad thing, because it means more heat loss, and a low psi value is good because it means less heat loss. A Ψ-value should not be confused with the Y-value, (the characters can sometimes look similar) which is a number that SAP uses to represent all of the heatloss due to thermal bridges in one particular building – more on that later.
The other role of psi values is to account for the difference between our simplified thermal model consisting of simple planar representations (Euclidian planes) and real building elements that have a thickness. In the middle of a wall, assuming the wall is much longer and taller than it is thick, it is an acceptable simplification to assume it is a Euclidian plane with a given U-value, because the heat flow can be represented in one dimension, perpendicular to the plane. However at the junctions with other walls, the roof, floor and windows, the heat flow is more complex and not perpendicular to the wall or whatever it is joined to; the heat flow becomes two-dimensional. Instead of a one dimensional calculation (a U-value) it is necessary to undertake a two-dimensional calculation to evaluate the psi-value. As a result, these calculations are much more laborious than a U-value calculation.

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