Thermal Bridging

Thermal bridge (cold bridge) – a junction where thermal insulation is not continuously and cause heat loss (air leakages). It usually occurs when structural element passes through insulation layer or the insulation is not thick enough where two construction elements meet.

Repeating thermal bridges

Repeating thermal bridges occurs where the structural elements with the low thermal conductivity repetitive cross the higher thermal conductivity layers. For example, timber studs bridge the layer of insulation, steel wall ties in the masonry cavity external wall, mortar joints in lightweight concrete blockwork (because the mortar has higher thermal conductivity compared with the blocks). These should be included when calculating within the main building element U-values.

Non-repeating thermal bridges

This mostly appears around loft hatches, around openings (doors, windows), where internal walls or floors penetrate the thermal envelope, etc. These bridges should be considered separately from main building element U-values.

Geometric thermal bridges

Geometric thermal bridges are result of complex building shape. They can be 2-dimentional or 3-dimentional, depending on where they occur. Mostly geometric thermal bridges appears at the junction of wall/roof, at the corner of external walls, at the wall/floor junction.

Building Regulations

At the part L1A of Approved Documents it is said that ‘The building fabric should be constructed to a reasonable standard so that:
a. the insulation is reasonably continuous over the whole building envelope; and
b. the air permeability is within reasonable limits.’

Ways to avoid thermal bridging

• Keep the house design as simple as possible. The smaller external area and amount of junctions the less air leakage routes in the house.

• Where possible, do not interrupt the thermal envelope (Do pen on section test to check if there is no passes through the insulation).

• If the thermal envelope is interrupted by water pipes, vents, windows, doors, etc, thermal resistance in the insulation should be as high as possible.
• At the junctions of building elements there should be no gaps
• For regular thermal bridges such as wall ties or mortar joints, the fabric been used should be high thermal resistance.

To conclude thermal bridging has a significant impact on the thermal and energy performance. New build houses (sustainable/zero carbon) are highly insulated, so, any heat loss is essential when trying to achieve the highest U-values.

Case study


For example I will use the same Greenwatt way development as for my previous posts.





The houses were design with very carefully detailing to avoid air leaking. For this they used special tapes and seals, done airtightness testing through the construction process, etc. Nevertheless, they had to deal with few challenges, such as north facing roof light/natural ventilation, which penetrates the ceiling cassette and cause heat loss. It needed to be very well insulated. The most difficult to insulate was 1 bedroom flat above the bin and bicycle store as the dwelling has 4 external walls, an exposed floor and an exposed roof. Also, in order to achieve high air tightness levels, post boxes had to be taken out from the design, external mailboxes are used instead.

Insulated roof light/natural ventilation

The balconies are supported on an independent structure to avoid thermal bridging

As a result, all these houses has a very limited heating demand 80% less than homes built to 2006 Building Regulation standards and 90% less than a typical existing home.

References:

'Greenwatt way' [Online] Availabe at: http://www.thisisconcrete.co.uk/home_page/case_studies/greenwatt_way.aspx [Accessed at 15th of November, 2013]

‘Greenwatt Way. A zero carbon homes newbuild case study’, 2011. Energy saving trust.

Historic Scotland Alba Aosmhor. ‘Fabric improvements for energy efficiency in traditional buildings’ [Online] Availabe at: http://www.historic-scotland.gov.uk/fabric_improvements.pdf [Accessed at 15th of November, 2013]

‘Greenwatt way’ [Online] Available at: http://www.house-builder.co.uk/documents/WILFORD-Chris.pdf [Accessed at 15th of November, 2013]

‘Fabric first’, October edition, 2010. Energy saving trust

Richards Partington Architects, 2012. ‘Understanding overheating – where to start’ NHBC Foundation

‘Thermal bridging’ [Online] Available at: http://www.leedsmet.ac.uk/teaching/vsite/low_carbon_housing/thermal_bridging/introduction/index.htm [Accessed at 15th of November, 2013]

Row, M., 2012, ‘Thermal bridge- what is it and how to avoid? [Online] Available at: http://www.insulationshop.co/Thermal_bridge_-_What_is_it_and_how_to_avoid%20_it [Accessed at 15th of November, 2013]