Eco homes & overheating
Nigel Griffiths looks at the design factors that will ensure your new house is as comfortable and efficient in summer as it is in winter
Nigel Griffiths looks at how to manage thermal efficiency and solar gain in your property to create a comfortable year-round living environment, that won't get too warm
In recent decades, the big focus for homeowners and for Building Regulations has been on improving thermal efficiency. As we have come to understand heat transfer through building fabric, we have insulated our homes better to reduce heat loss. This approach has been pretty successful – but another problem is emerging in modern buildings: overheating.
This might seem a little counter-intuitive. After all, the same insulation that keeps warmth in should also keep excessive heat out. So what’s going on? To design out this issue and create a comfortable environment, we need to understand why new homes overheat in the first place.
Our focus in this article is on architectural design and the passive solar approach. These days, there’s rightly a major focus on natural light and all its benefits. Bringing in plenty of daylight (and maximising surrounding views) through generous glazed openings is known to improve wellbeing. Plus, warmth from the sun enters a building most quickly through glazing – leading to potentially beneficial free heat (solar gain). The problem comes when this heat gain is excessive, which can be caused by several factors: Substantial glazing on the southern elevation Insufficient thermal mass
Lack of solar shading
Poor ventilation
None of these elements alone will necessarily cause overheating, but the wrong combination can do so. So let’s look at each of these issues and examine how the risks of overheating in eco homes can be eliminated.
Passive solar versus Passivhaus
Before getting into too much detail, it’s worth clearing up a potential confusion between passive solar construction and Passivhaus. It is possible to design and construct houses with large areas of glazing on the southern elevation, which make the best of the solar resource in winter and which need no space heating at all until the temperature drops below 0°C. This is known as passive solar construction – it’s a great concept and I’ve carried it out successfully myself, while avoiding overheating.
By contrast, Passivhaus is an approach to construction that seeks to drive down heat losses from buildings to meet exacting target levels, based on relatively narrow metrics. Some Passivhaus projects make significant use of solar gains, but it is possible to build to this standard with any orientation of the building. With this methodology, windows tend to be modestly proportioned, as they are regarded as relative weak points in the thermal envelope.
Building in thermal mass
The successful creation of passive solar buildings is quite a science. You must consult an expert to get the calcs right, but it’s well worth the effort if you have an appropriate site. As mentioned above, a key part of the equation will be to orientate the structure towards the south and put substantial glazing on that elevation (as heat passes much faster through windows than it does through walls).
The trouble comes when this is done without due consideration to overheating, especially in houses where there’s a lack of thermal mass. Heat that enters a building via solar gain needs to be stored, ideally in dense masonry or earth walls (offering thermal mass), and released later as the internal environment cools after sunset.
Many modern homes, however, are built of lightweight construction and feature little or no masonry inside the thermal envelope. So if you want a passive solar house and are using a lightweight system, the designer should look to incorporate elements such as screeded floors and internal blockwork in key locations. If you’re not trying to heat the building in this way, then simply decrease the amount of glazing on the southern elevation and you will reduce the risk of overheating accordingly. If the walls of the property are super-insulated (above current Building Regs) and the structure is relatively airtight, then heat losses will be minimal and you can achieve an acceptable level of thermal comfort with a very low heat input.
Solar shading
Another key consideration when seeking to make use of passive solar design is to employ shading. Windows on a
southern elevation will let in plenty of light and heat, both of which will be welcome in winter – but if this is unfiltered, it will lead to overheating in summer. There are two main ways to address this risk: designing in solar shading that’s fixed to the building, and tree planting.
Fixed solar shading is set at an angle that admits light and heat when the sun is low in the sky (during the winter months) but excludes excessive warmth when it’s higher (in summer). There are various options here. You can have a slatted screen covering an entire window, for instance; a horizontally-projecting structure, sometimes known as a brise soleil; or go for roof overhangs similar to the Build It Education House (buildit.co.uk/ourhouse).
If you’re using a screen, the slats will be set at a precise angle and distance apart so that heat is reflected when the sun is above a certain elevation, but allowed to penetrate into the core of the building when it is low in the sky.
Solar shading is often installed on the upper stories of a property but omitted at ground floor level. With the latter, the exposure is usually less and there is often more thermal mass in the floor construction (which might be a concrete slab or beam and block system).
Clever positioning of trees can be one of the best ways to provide solar shading. Deciduous varieties are naturally green during the summer when screening is needed, and admit more light and heat in the winter. Planting trees is also one of the most positive things that you can do for the environment, in the widest sense.
However, on many sites there will not be sufficient room to plant trees at the requisite distance or scale to provide
the necessary shading. It also takes time for them to grow, so unless you already have a bank of appropriately located deciduous trees then you will need some other form of screen (whether temporary or permanent).
Blinds
Blinds are remarkably effective at excluding excessive solar gain while allowing diffuse light in, though they will still admit some heat. But it’s important to understand they aren’t a permanent designed-in solution, unlike solar shading.
Modern internal blinds can have manual or motorised control, with or without automation, and can even be operated via a phone app. Some external blinds can be automated to respond to levels of solar irradiation – and this is an optimal solution in many instances. However, if blinds malfunction for any reason then you risk losing your shading altogether, and when they are closed you’ll have compromised or lost your view out.
In some circumstances internal blinds can be a lowimpact, partial solution to an existing overheating problem. But good architectural design of your new eco home should really eliminate the risk in the first place.
Glazing specification
The amount of heat that can penetrate through windows and other glass features is measured as the solar heat gain coefficient – known as the g-value. A g-value of 0 would mean no heat is transmitted, while a result of 1 indicates all the solar energy would pass through.
One of the key methods of altering the g-value of glazing is to apply special finishes during manufacture. Low-emissivity (low-e) coatings are frequently advertised
as a route to minimising heat loss from inside the building through double and triple glazing. But they can also be used to prevent heat entering in the first place.
When keeping warmth in, the microscopically thin low-e coating is applied to the inner leaf of the double or triple glazed unit (on the side that faces in towards the cavity). If you want to reduce the amount of heat coming in, however, it can instead be applied to the inner face of the outer pane – creating what’s known as solar control glass.
Normally, soft low-e coatings are used for this purpose, as they are more effective at preventing solar gain than the hard-coat technology used to reduce heat loss. To give you a rough guide, a typical double-glazed unit might have a g-value of approximately 0.7, while a version with solar control glass might be around 0.2-0.3. Getting the right balance between this and the U-value (a window’s ability to retain heat inside the building) is crucial.
There are also much thicker solar control coatings available that can be used on the outer face of glazing, but they’re generally more suited to commercial construction rather than domestic projects.
Ventilation & cooling
Overheating has become more common in modern buildings partly because they are so airtight. Georgian houses often had large south-facing windows, but did not overheat too badly thanks to their inherent thermal mass and high air permeability. These older homes can naturally accept heat, store it effectively and cool down relatively easily – though, of course, they are not as good as new builds at keeping the heat in during the colder months.
We aim to avoid air leakage in modern homes, but we can introduce controlled ventilation to prevent heat from entering into a building – and to purge it if it does get too hot indoors. One example would be the use of a solar space or sunspace on a southern elevation, which could perhaps double up as a conservatory. In this situation, if there’s no call for heat internally, when the temperature in the solar space rises above a pre-set level, vents automatically open at the base (to draw in cooler air) and at the top (to allow the warmest air to escape). This is an effective use of the passive stack natural ventilation effect.
Alternatively, a whole-house ventilation system can be used to expel warm, stale air and provide purge ventilation overnight. As with the above, this is specialist passive solar design and should be considered as part of an integrated approach across the whole building fabric and services.