Overheating in Low Energy Buildings

It was disappointing to read the article in the Sunday Times about overheating in buildings, specifically Passivhaus buildings. In our experience, overheating is as much of a problem in ‘eco’ buildings rather than true Passivhaus buildings. Indeed it has been reported that some (non Passivhaus) apartment developments with high proportions of glazed façade, poorly designed and installed communal heating systems and insufficient ventilation in communal areas, temperatures are reaching close to 50oC.
Part of the problem is the traditional approach to Passivhaus design which has been to favour high levels of south facing glazing which if not correctly specified and shaded (summer), can indeed present a problem with excessive solar gain all by itself.

In addition to this, variables such as the size of average UK dwellings (yes this does have a significant impact for the very small) and reliance on ‘modelled’ occupant behavior in terms of appliance numbers and habits with these also have a serious impact on performance in the overheating stakes.

The Passivhaus methodology can make smaller Passivhaus buildings appear difficult to achieve the standard given the relation to the Form Factor (the buildings compactness) and the assumed internal gains (people and appliances). As a result, in order to meet the heat load requirement, it is common for designers reduce the heat loss elements (U-Values), which can have a detrimental effect on the overheating.

Prolonged periods of elevated temperatures are largely of a function of inadequately designed ventilation. Fundamentals like the provision for sufficient cross-ventilation, including night purge ventilation is not often fully considered. Ventilation through MVHR intake duct positioning is also often overlooked, as if placed on a southerly aspect, the ventilation will continue to effectively suck in the warm air radiating up from the building as it cools. Even with true summer bypass, some MVHR units will start the heat recovery process when the outside temperature drops to 12oC to prevent condensation build up in ductwork.

Thermal mass or heavyweight buildings are often cited as the answer for overheating in small residential dwellings. It is however debatable whether the thermal mass of a building, when it is occupied at it’s highest levels, has the ability to counter the excess of internal heat gains (IHGs) alone. A high thermal mass strategy generally works best in commercial application where the utilisation patterns are more favourable to significant night-time secure purge ventilation as are no occupants to experience any thermal or noise discomfort.

What is needed is greater attention paid to overheating with occupancy stress testing, real ventilation scenarios and appropriate design. We should also be designing and contracting buildings for a future climate ensuring that buildings can be adapted accordingly. The strength of Passivhaus design (rather than ‘eco’) and methodology is that once these variables are known and correctly planned and implemented the results are guaranteed.

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