Overheating: A new angle on the performance gap

We’ve seen in the last few years how the difference between the planned and actual energy performance of new homes can vary considerably.

What we’re now also seeing is an increasing tendency for new homes - and in particular urban apartment buildings - to overheat. Climate change predictions show how this phenomena can only worsen in the next 20 years.

This is somewhat confounding for an industry that is struggling to achieve the standard of building performance that regulations require. Indeed, there is the real prospect that higher carbon intensive cooling systems will be retrofitted into relatively newly built apartments to offset summertime overheating.

What has gone wrong? Have we been installing too much insulation, and making buildings too air tight, which is turning a winter ‘under heating’ problem into a summer overheating legacy?

The answer is no. What we’ve not done is to heed the basic ‘building performance in reality’ challenges that super insulation in a temperate climate - allied to ever increasing internal heat gains - requires.

In general terms the under performance of ventilation systems, and MVHR systems in particular, has resulted in very low levels of ventilation effectiveness - a critical aspect in mitigating summertime overheating.

Let’s make sure we don’t carve out a new and imponderable niche in the unwelcome performance gap problem

This has been added to by our rather obsessive culture towards risk and the dire consequences of allowing otherwise perfectly capable human beings to open windows. Far better to fix restrictions so that if any hapless person managed to fall out of a window it couldn’t possibly be the fault of a designer or contractor. The result of which is that the area of window openings assumed by designers for ventilation purposes are often nowhere near achieved in reality - resulting in super insulated homes becoming swelter boxes.

The underperformance of systems allied to design assumptions that are nowhere near the mark - core aspects of the performance gap - rear their heads in summer as much as in winter.

So, as with the majority of challenges in our industry, the issue is a systemic one and potentially leading to considerable uncertainty as to how to fix it.

Going forward, we need to get much smarter is assessing overheating potential during the design stage, with energy and comfort modelling a necessity for higher risk buildings such as urban apartment projects in noisy areas.

Clients need to bite the bullet and to make sure that their designers and indeed contractors have demonstrated expertise and experience in identifying, assessing and deploying - often through convoluted supply chains - effective overheating mitigation measures. Given how nascent the problem and its identification is, the best start is an energy model. Designers and contractors will need to get up to speed and ready for tricky questions from clients. I don’t think it’s going to be long before the first lawsuits happen in any event, given the severity of some of the problem projects.

We were somewhat surprised to learn about a BRE study going back to the early eighties that assessed overheating risks in homes built to the then far superior Swedish Building Regulations. This made plain the requirement to think about the impact of summer conditions on better insulated buildings - even in the case of simple two up two down house design - and to design with overheating risk in mind.

Let’s learn from this eighties and subsequently published research and guidance - the NHBC guide ” Understanding overheating - where to start ” is an excellent first start - as well as your own experiences and make sure we don’t carve out a new and imponderable niche in the unwelcome performance gap problem.

Shading devices may be required and effective ventilation using a combination of windows and mechanical systems certainly will be. Energy modelling may be needed too. Dare I say but some first principles thinking as well. Opportunities for new products and services as well as wider innovation beckons.

Pete Halsall is chief executive of Beattie Passive