thermal bridging conditions but still falls short in some cases. As we continue to increase insulation
levels, thermal bridges assume increasing significance as a proportion of the overall heat loss. They can also create problems due to condensation at local cold spots. Yet, despite all this, thermal bridges do not appear to feature highly in the consideration of those designing external walls. A study by the Joseph Rowntree
Foundation on sustainable housing at Elm Tree Mews (Journal, December 2010, pages 7 and 23), found that the design calculations seriously underestimated the extent of thermal bridging, resulting in heat loss of 50% higher than expected. This appears to have been partly due to the assumptions included in the calculations not having been checked against the actual construction details. It is essential that those taking
responsibility for building performance calculations are fully involved in the design of the building envelope. There is a danger that any disconnection between those who detail the construction and those who understand the thermal performance could lead to serious consequences, possibly even failure of a structure due to condensation damage. We may have accounted for non-
homogeneous construction, but there are still more factors that we must consider in order to completely understand the steady-state heat loss. The calculations traditionally use a single temperature point to represent internal and external conditions, typically the air temperature. Yet in modern low-energy buildings it is not uncommon to find a mixture of radiant and air heating sources. We often choose radiant heating systems
for their ability to transfer energy to surrounding surfaces, without significant effect on the air temperature. Radiant heating therefore could create greater heat loss, due to the increased absorption at the internal surface, than would be indicated by simple conduction of heat from the air. The CIBSE heat loss calculations now include a heat source factor to account for the differential heat loss by fabric and ventilation conduction due to the balance of radiant and air heating. Similarly we should also account for
heat loss or gain by radiation from the external surface of the building. In an urban situation the heat loss from the roof to a cold night sky may be much more significant
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than heat loss from the walls which are surrounded by other buildings also radiating heat. Conversely the absorption of solar energy during the day may in fact be greater than the notional heat loss. There is no simple way of analysing
the radiant contribution to building heat loss but nevertheless it should still be considered when making choices about the materials used in building envelopes. In particular this might start to inform choices about the use of materials with high emissivity or those which are highly transparent to radiation such as glass curtain walls. For the complete picture we should also
consider the dynamic thermal response of a building, not just the steady-state condition. A proportion of the heat flowing through the envelope, in either direction, will heat up the construction materials. It is only when the temperature of the material has been raised that onward transmission takes place. This introduces both attenuation and a time delay, fundamentally changing the envelope’s response to diurnal variations in temperatures and solar radiation. The attenuation is known as ‘decrement’ and the time lag as the ‘decrement delay’. Decrement is used when calculating summertime cooling loads but, as we continue to drive for improved
August 2011 CIBSE Journal 39
This display of corks in a restaurant window indicates the problems that can be created by simply adding more insulation to an envelope construction without considering the vapour permeability. The corks act as an insulant, changing the temperature gradient, but they do not inhibit the passage of water vapour. Condensation occurs outside the insulation where the temperature drops below the dewpoint
Doug King
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