Thermal bypass

The significance of thermal bypass has only recently come to light due to the efforts of building performance practitioners. In one study, ventilated party walls between terraced homes and flats meant that measured heat loss from the homes was far higher than anticipated.

Other examples include ‘wind-washing’, where fibrous insulation materials do not perform as well as expected due to air movement on their outer edge drawing warmth from them. This has led to the proposed use of a ‘wind barrier’ to the outer face of insulation, as shown in the loft insulation described here.

Thermal bypass is a less-known cousin of air leakage, discussed below. With thermal bypass, air gets in or out of the building fabric but only part of the way. Because buildings tend to have lots of cavities, this air can cause havoc once inside the building fabric, cooling the building fabric from within or depositing warm, moist air in places that can’t be seen or reached.

Corners form their own thermal bridge, often colder than adjacent wall areas. The mould here is also probably related to being normally behind a sofa with little air movement. The image in the top corner is a thermal or thermographic image (infrared), which shows the surface temperatures of the surroundings. There are several examples of thermal images in this document.

Because thermal bypass, like air leakage, exploits gaps in construction as much as cavities, it is also relevant to issues of quality and workmanship on site. Many construction anomalies picked up by thermographic cameras are assumed to show thermal bridging, whereas they often show cold air movement indicating a gap somewhere there shouldn’t be, i.e. thermal bypass.