All ten of the UK’s hottest years have occurred since 2002, and despite efforts to curb global warming it’s a trend set to continue. Consistent summer temperatures upwards of 30⁰C are becoming increasingly common in the UK, particularly in the south of the country. While those from traditionally warm countries may live in cool, shaded buildings, the chances are that most people in the UK will have to make do with a roasting flat and an uncomfortable night of trying to sleep.
Much of the UK’s housing stock doesn’t suffer so severely; especially older, uninsulated, dual-aspect properties. However, many homes built in the last 15 years aren’t so lucky. So what are the problems, and what can be done?
Glazing & External Shading
The most obvious cause of overheating in modern buildings is glazing. It lets light in, which is nice, but it also lets in a lot of heat. To be exact, solar energy can provide as much as 700 Watts of heat per m2 of glazing. That means if you have a window opening measuring 1 metre by 1 metre, it could be producing the same amount of heat as a medium sized radiator.
The modern fashion for large expanses of glass is hugely problematic. Multiply 700 W/m2 up by 6 to represent some nice big floor-to-ceiling sliding doors, and you’ve got 4.2kW to deal with on a hot sunny day. That’s the total winter heating load of a small well insulated home, entering one room, in the middle of the summer.
Older homes tend to have modestly sized windows, but these days we are obsessed with large windows, floor-to-ceiling glazing, and massive rooflights. Scaling back this desire for glass should be the first step to managing overheating in new buildings, and it’s up to architects to design light and appealing spaces without relying on vast windows.
Glazing is of course essential for natural light, views, and useful solar gain in winter, so external shading should also be employed much more widely than it currently is in the UK. Permanent shading, such as incorporating deep window reveals into a design, is one useful strategy, but temporary shading often holds the key. Shutters, awnings, and external blinds can all be used by occupants as and when required without blocking out desirable sunlight at other times. Internal blinds are common, but are nowhere near as effective as the external alternative. External shading is typically cost-effective and simple, provided it’s incorporated relatively early during design.
Ventilation
Mechanical ventilation systems are designed to remove moisture and other indoor air pollutants. It’s very rarely sufficient to provide any noticeable cooling, as the flow rates required would have to be impractically high. However, natural ventilation (opening windows and doors) can produce a cooling effect if used correctly.
Ventilation can help in two ways. Air flowing over our skin creates a cooling sensation, and replacing hot air with cooler air will reduce the ambient internal temperature. Giving occupants the ability to achieve both of these things will help reduce the risk of overheating. The introduction of cooler air can be particularly useful at night, as it allows the temperature in a building to drop quickly before the heat of the next day arrives.
Including openable windows or doors in appropriate locations is the simple way to make this work. Appropriate locations will normally constitute openings on opposite sides of a building, producing cross ventilation when air flows consistently from one side to the other. The ventilation rate from cross ventilation is almost always superior to when openings are in the same façade, where there is no pressure difference to drive the flow of air. In densely populated urban areas this can be a big issue, as single aspect apartments are commonplace to make the most efficient use of space. A single aspect, south facing apartment with lots of glazing is a recipe for disaster.
Airtightness, Insulation & Thermal Mass
New buildings must be relatively well insulated and airtight to comply with Part L of building regulations. This (and beyond) is absolutely necessary to reduce winter heating loads and energy consumption, but during summer improved thermal performance undeniably adds to overheating risk. Once heat gets in it takes much longer to escape than in traditional buildings, leading to prolonged periods of uncomfortably hot temperatures.
We must continue to insulate well and promote airtight buildings with mechanical ventilation, so there is no straightforward solution. New buildings should simply be designed with sufficient overheating mitigation strategies to compensate for the improved heat retention. This means less glazing, more shading, and better ventilation.
Some construction types becoming increasingly popular also make cool summer temperatures harder to achieve due to a lack of thermal mass. For example, a timber frame has little ability to absorb heat compared to an old stone building with thick solid walls. High thermal mass acts as a heat buffer, preventing spikes in temperature during the hottest parts of the day and slowly releasing heat during cooler periods. Integrating thermal mass into a building will often be a good idea where feasible but is rarely a key driver of design. As with insulation and airtightness, a lack of thermal mass in new buildings must therefore be kept in mind when considering the extent to which alternative cooling strategies are required.
A Question of Style
The causes of overheating and possible solutions can easily be identified in isolation; incorporating them holistically into robust design is more challenging. Current architectural style is problematic and has to change fast to avoid a major overheating crisis in the coming decades. Architects must stop relying on glazing to fulfil the estate agent buzzwords – “open”, “airy” and “free flowing”.
A return to more traditional expectations and glazing ratios will go a long way to solving the problem, particularly in urban apartment blocks. As the UK climate becomes more similar to what we associate with central and southern Europe, so must our building design. How often do you see external shutters on a new building in the UK?
Priority
Consideration of overheating usually lies somewhere between the bathroom floor finish and specification of white goods in the list of current design priorities. This needs to change if we want buildings that will be habitable in 20 years without major alterations. Ideally this would happen naturally, but often legislation is required to prompt a deviation from business as usual. Current requirements in part L, SAP and SBEM models are minimal, going little beyond very basic recommendations that can easily be circumnavigated. We are moving towards a point where cooling load needs as much attention as heating load, and compliance requirements need to reflect this.
It is finally worth noting that active cooling (air conditioning) is one way to guarantee comfortable indoor summer temperatures. The downside is that active cooling is very energy intensive and will therefore hinder our ability to meet pressing carbon emissions targets. There may come a point when the UK’s energy mix is entirely renewable and active cooling becomes widely adopted, but for now UK building design shouldn’t have to rely on it.