Tackling the impact of thermal bridging

With the drive towards improved energy efficiency, thermal bridging has become a ‘hot topic’ for housebuilders. insulation manufacturer marmox explains why, and describes some solutions to the problem.

It has been calculated by The Energy Saving Trust that in a typical detached property, an unaddressed thermal bridge can account for 30 per cent of a building’s heat loss.

One of the worst areas of heat loss through a thermal bridge is where the floor meets the wall, allowing heat to be transmitted to the outside. In this area, up to 50 per cent of heat in an otherwise wellinsulated room can be lost.

The BRE defines a thermal bridge as follows: ‘A thermal bridge, also called a cold bridge, is an area of a building construction which has a significantly higher heat transfer than the surrounding materials. This is typically where there is either a break in the insulation, less insulation, or the insulation is penetrated by an element with a higher thermal conductivity. Where the building is situated in a cold climate (such as the UK) this can result in additional heat loss at these points.’

All Building Regulations in the UK and Ireland now say: “The building fabric should be continuous over the whole building envelope and constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps in the various elements.”

Building regulations

The non-dwelling and dwelling versions of the Building Regulations and Energy Conservation Part L (England, Wales, R.o.l), Section 6, Part D (Northern Ireland) require continuity of insulation at the wall-floor junction. Building regulations now require that heat loss due to thermal bridging should be taken into account in SAP calculations (for dwellings) and SBEM calculations (for buildings other than dwellings) at the design stage.

The heat loss through the wall/floor junction is expressed as a ψ value, which is the linear thermal transmittance, and the cumulative heat loss as a Y-value. The design of the junctions should be based on Accredited Construction Details (ACDs) or Enhanced Construction Details (ECDs). SAP, SBEM or other approved software then allocate a specific default Y-value for each type of junction.

Stopping heat loss at the thermal bridge

One of the latest solutions to addressing and stopping heat loss at the wall-floor junction is the use of high insulating, load-bearing building blocks, designed to replace the course of brick or block at the bottom of a wall.

The heat loss for a thermal bridge is determined by its linear thermal transmittance. This is measured in Watts for every metre of the thermal bridge – for every degree difference in the temperature between the inside and the outside of a room. The UK maximum allowable heat loss at a thermal bridge (wall/floor junction) is 0.16 W/m.K. Using Accredited Construction Details (ACD) can reduce this figure to 0.08 W/m.K. An Enhanced Construction Design (ECD) with additional interleaf insulation and edge insulation will be 0.04 W/m.K.

Y-values (heat loss across the length of the cold bridge) are calculated from the ψ values, and are used by the designers in the SAP/SBEM calculation of the building. In the absence of knowing the true Y-values, Building Regulations require a ‘default Y-value’ to be used which makes it harder to achieve a good energy rating, and in turn could result in non-compliance with Part L/Section 6.

These default Y- values add an approximated additional heat loss to the whole property, which is less accurate than calculating it using the approved data provided by a product manufacturer.

Moving towards the 2019 zero carbon home standard

English Building Regulations for dwellings (Part L1A: 2013) now include another measurement of heat loss: Fabric Energy Efficiency. This standard is based on the Ene2 points calculation used in the Code for Sustainable Homes. This requirement is met by ensuring continuity of the insulation on the fabric of the building. It is measured in terms of kilowatt-hours of energy lost for every square metre.

The 2019 regulations for zero carbon buildings (non-domestic) will demand a 100 per cent improvement in the overall heat loss (the Building Emissions/Energy Rate) compared with the previous standard.

Offsetting the cost

Using high insulating, load-bearing building blocks can even allow the level of insulation required elsewhere to be reduced. Incorporating such products into a design will reduce the Y-value used in SAP or SBEM. Because SAP/ SBEM are concerned with the overall heat loss, if the Y-value is reduced (meaning the heat loss at the thermal bridge is reduced), the U-value of the wall or floor insulation could possibly be slightly compromised, yet still the overall heat loss or DER (Dwelling Efficiency Rate) would be the same.

With the need to improve energy efficiency and move closer towards the target of zero carbon homes, housebuilders and developers will continue to face the challenge of controlling heat losses due to thermal bridging. Using high insulating building blocks to reduce heat loss at the wall-floor junction is estimated be able to reduce the stated research levels of 30 per cent to approximately two per cent, and reduce the daily CO2 emissions associated with this wasted heat from 1.5 kg to 0.1 kg.

Article from Marmox Specialist Building Products