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What is Thermal Bridging?

You might have heard about thermal bridging, perhaps mentioned by your builder or architect. But what exactly is it?

Thermal bridging occurs in building envelopes when gaps or breaks in the insulation envelope create pathways for heat loss to bypass thermal insulation. Thermal bridging also occurs in building envelopes when materials with higher thermal conductivity value are used – like steel, timber and concrete. When these materials are used, they can create pathways for heat to bypass the thermal insulation, as the heat always looks for the path of least resistance.

Thermal image (inset) of a residential building showing thermal bridging around openings and junctions.

Thermal image (inset) of a residential building showing thermal bridging around openings and junctions.

For any project, it is the responsibility of all workers on site to pay close attention to detailing at junctions and service penetrations of the external fabric of the building, to avoid thermal bridging.

 

What this means for heat loss and comfort

The thermal envelope is the connection of your building’s walls, roof, foundation, windows and doors. If this connection is unbroken, the rate of heat flow from the building is minimised. As all heat loss cannot be avoided, its effects are minimised with a constant slow rate of heat flow through the entire fabric. Thermal bridging causes increased heat flow in different parts of the building fabric, meaning that it is harder to predict how much energy will be needed to maintain a comfortable temperature within.

Cold bridging is basically cold areas on the internal surface of the envelope and, as we know, heat energy flows from areas of higher temperature to areas of lower temperature. The heat within the building is already trying to flow to the lower temperatures outside but this flow rate is dramatically increased through the cold spots. When there are fluctuations or changes in temperature caused by hot air inside trying to move outside, this creates air currents.

These air currents within the building, which are increased especially around cold areas, are experienced as drafts by the occupants and will negatively affect your comfort level. This usually results means you turn up or boost the heating simply because of the discomfort caused by the draft from the cold bridging! Eliminating the cold bridging as much as possible could reduce the drafts and increase the level of comfort for the building’s occupants, and with less or no energy wastage.

 

Surface & interstitial condensation and air quality considerations

Ok, here comes some more science…

The colder the air, the less water vapour it can hold. For example, if the room temperature is 20 C° and the humidity level is measured at 50%, then the air is holding half of its maximum amount of moisture. As the temperature is lowered in the house the humidity level will rise. If the humidity level reaches 100% and the temperature is still falling, condensation begins. This condensation can form where thermal bridging has created cold areas on the internal surface of the building. Warm moisture-laden air condenses on cold surfaces thereby increasing dampness.

Water vapour in the air is absorbed by surrounding materials, including the materials that make up the building fabric. However as vapour passes through the fabric of the building and where the temperature is dropping inside of the fabric the vapour will reach Dew Point and start to condense at a location within the wall fabric.

Section through external wall illustrating the Dew Point. The drops show where mould might grow within your walls.

Section through external wall illustrating the Dew Point. The drops show where mould might grow within your walls.

 

What does this mean to you?

Warm damp air leaking through the building structure can lead to condensation within the fabric (Interstitial Condensation), which can reduce insulation performance and cause fabric deterioration. If the relative humidity levels in the building exceed 70% for prolonged periods, there is a high probability that the condensation occurring on cold surfaces will lead to mould growth.

This can seriously affect the quality of the air for the occupants and mould spores can have a detrimental effect on human respiratory system. Therefore, thermal bridging can lead to condensation and possibly mould growth.

Visible condensation and mould growth may be avoided by ensuring adequate insulation detailing and controlled ventilation in all parts of a building. However, with interstitial condensation, mould growth can take place within the building fabric structure where it is invisible; hence, cannot be cleaned off, creating an unhealthy living environment for the occupants. Mould spores have a detrimental effect on human respiratory system and with over 470,000 people in Ireland suffering from asthma, this emphasises the importance of managing condensation in building fabric.

Black mould, known as Stachybotrys chartarum, at a window reveal

Black mould, known as Stachybotrys chartarum, at a window reveal

 

Make sure your building is designed right and built right for a healthy and comfortable home.

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