Summer has finally arrived and has done so with some aplomb. Wimbledon and the first Ashes test match have heralded the arrival of hot sunny days which are great if you’re in the park, but for many of us it means hot stuffy houses and sleepless nights.
Summer overheating is one of the most overlooked aspects of good low energy design, yet it can cause severe discomfort and prolonged periods of high temperatures can be dangerous for the old, young and vulnerable. It’s also going to get worse in the years to come as the long-term climate forecasts show that, even if we meet all our carbon reduction targets, temperatures in the UK are set to rise steadily over the coming decades.
If we are building homes that are to last 80 years, then perhaps we should be thinking about how they will need to function in at least the next 20 or 30 years…
How do I know if there is a problem and how can we deal with it?
Getting an idea of whether there is a problem can be tricky. A recent report from the Zero Carbon Hub highlighted that the assessment of overheating in domestic dwellings is patchy at best with no clear methodology or standardised toolset.
What analysis tool do I use?
SAP modelling provides an overheating risk indication, but this is really only a compliance indicator and doesn’t actually tell you what’s going on and how the problem is best addressed.
The PassivHaus Planning Package (PHPP) is a more useful tool which can identify an overheating risk (even if you’re not building a passivhaus!) and allows you to experiment with variables such as ventilation rates and window type/orientation in order to see the effects. The datasets within PHPP are also more flexible so if you really want to, you can plug in those climate data projections for 2050 and see how the house is going to perform in more challenging conditions.
However, PHPP doesn’t model overheating in detail, it just identifies the general risk. So if you have a large glazed area in one or more rooms or single aspect apartments within a larger building, which you suspect may be a potential ‘hot-spot’, the best solution is a Dynamic Simulation Modelling (DSM) package. This will model the actual heat movement within the building and the properties of all the building elements and thus demonstrate temperature fluctuations and frequencies in high risk areas across the year.
Once you’ve worked out there is a problem, you’ll probably want to fix it.
The worst reaction is to simply throw in some active cooling (air conditioning). This will increase capital and running costs for the building without really looking at the underlying issue. As always, the best and most elegant solutions often lie in good design that starts early in a project.
Shading is key
Excess solar energy is only an issue once it’s inside the building, so exterior shading (either permanent or temporary) is an excellent strategy. In some cases, shading alone may not be enough and so a long hard look at the amount of glazing might be required. Endless walls of shimmering glass may delight modernists, but if the occupants are cooking in summer (and freezing in winter!), then the design simply isn’t functional. Again, some careful thought can retain those amazing views and ‘clean lines’ whilst reducing the overall amount of glazing. There are a lot more design options than just brise-soleil.
Having looked outside the building, the next strategy is to see what can be done within. The first area to scrutinise is the fabric. Despite some of the recent uninformed hype in the media, well insulated homes don’t just stay warm in the winter, they stay cool in the summer, as the outside heat simply can’t penetrate through the fabric. So getting a well insulated thermal envelope is key to maintaining a comfortable internal temperature.
Next you need to consider whether the ventilation strategy is up to the job. Natural ventilation needs to be designed properly (i.e. sufficient window openings and routes for cross ventilation and/or stack ventilation). Passive stack ventilation is attractive in its simplicity and has advantages on still hot days, but it needs secure ventilation inlets on the ground floor to work effectively at night or whilst occupants are away. Critically, to work effectively, ventilation is also reliant on the occupants and opening design needs to consider ease of operation.
It is also important to note these ‘passive’ approaches will still work in a well-sealed low energy building or Passivhaus alongside a Mechanical Ventilation and Heat Recovery (MVHR) system or Mechanical Extract Ventilation (MEV) system and these systems are still important to guarantee good ventilation (and energy savings in the case of MVHR) in the winter months. We have experience of this on previous projects.
Whilst overheating is not something that we have always designed around, it is becoming an issue which we simply can’t ignore. With a bit of analysis and thought, carefully designed shading, well insulated fabric and adequate ventilation will minimise the risk of summer overheating in the majority of domestic buildings allowing us to relax and enjoy our summers without those hot, uncomfortable sleepless nights. It’s too important to leave to chance.