As we head for a sub-tropical climate, how do we design heat out of our homes?

For anyone who has been around house building since the 1980s, the introduction of Approved Document O: Overheating will seem rather ironic.

Up until the 1980s we built houses rather like today. Brick and block cavity construction, but the cavity was at best 50mm wide and intended to stop damp ingress.

In fact, many a Building Control Officer, or Building Inspector as they were then known, had an argument with a bricklayer about keeping the cavity clear. The idea of a full fill cavity would have been anathema to a good brickie.

But as the 1980s gave way to the 1990s, glass-fibre Cavity Batts were invented and the process of filling the cavity with something other than composite and discarded cigarette packets began. Drylining of new builds became the norm (but more because of speed and drying time than insulation considerations). Loft spaces, up to then insulated with Christmas decorations, started to have 50mm and then 75mm of insulation rolled out in them and for the first time U Values mattered.

This all happened because of a realisation that the cost of heating fuel was rising; petrol had risen to over 40p a litre and the average heating bill was nearly £800 a year. The intention was to keep heat in. After all you’ve paid for it, why give it away?

This was a philosophy that became reflected in the design of houses as the large “picture Windows” of the 1970s were replaced with smaller, but still single glazed, casement windows.

The uninsulated cavity construction of the 1980s had a U Value of around 1.5W/m2K with an external wooden door coming it at around 3W/m2K. The equivalent house today has a wall value of around 0.3W/m2K with external doors at 1.5W/m2K. These are improvements of 80% and 50% respectively.

Air tightness testing became mandatory in 2002 as a way of removing unintended heat loss from new builds and it has worked well in stopping draughts from below window boards and around doors. The problem is that as a result, houses have lost their natural ability for air changes that came as a result of the poorly fitted back door or the gap at the side of a window jamb. Any architectural technician will tell you that most of the energy employed in the heating (or cooling) of a building is used in maintaining the temperature of the air as a result of air changes rather than via fabric heat loss or gain.

The problem that we have created with our modern houses is that we have made hot air balloons from the structures. Like the balloon we have created our houses to retain heat, designing out any natural porosity in the fabric by adding more layers of insulation to the envelope.

House builders have got what they have been striving for, perfect heat retention vessels. A vessel that in recent years, as a result of the price of building land, has grown in height from two floors to three.

The main problem with this level of perfection is that heat moves or passes from hot to cold and will keep moving in that direction until equilibrium is reached, something that should be avoided when the outside temperature is cooler than inside. 

Now we are facing increased temperatures and routine summer highs in the mid to high thirties for the first time since the inception of the Approved Documents. So it’s not a surprise that they have become concerned with the prevention of heat retention and the concomitant cooling that the retained heat will require.

Approved Document O has broadly two ways of removing heat from a property: mechanical ventilation and rapid manual ventilation. In other words extractor fans and opening a window. The problem with the former is, similar to a heating system, it uses electricity to operate it. Opening a window may require less energy but unless it is cooler outside or there is a breeze in the optimum direction it will not cool the internal environment. It will however permit polluted air in while removing any sound insulation or security benefits designed in as part of the window.

If a house once heated stays hot, the only workable solution is to prevent the heating in the first place. Part O deals with this in two ways: limiting the physical size of windows relative to their orientation and the use of shading/glass.

We have designed out any natural porosity in the fabric and added more layers of insulation to the envelope to prevent the escape of heat.

While the option of reducing the size of windows seems appealing and a simple solution, this will also reduce the amount of natural light entering a building and so increase the need for artificial lighting which, low energy or not, will then use power.

The problem is that virtually all of the materials used to construct the exterior envelope of a house have a high thermal mass. Thus, they take a long time to absorb heat but will then retain and discharge that heat long after the ambient air temperature has dropped.

Ideal for a southerly elevation in the winter months but an unwanted guest in the increasingly warmer summers of modern times. This means that the prevention of warming the entire elevation is preferable to the cure of heat removal.

Overhangs, wide soffits, brise soleil and the strategic positioning of surrounding buildings will all assist in the prevention of heating from solar gain. These are passive architectural techniques that have been used in warmer climates for centuries and are even listed in Section 2.7 of Approved Document O.

The word ‘Bungalow’ is an Anglo-Indian word derived from the Hindi word ‘Bangla’, meaning a low building often surrounded by a wide canopy or veranda for shade. In other words, not a three-floor building with no roof soffits.

Part O is a necessary response to global warming caused by excessive use of fossil fuels, in part, caused by uninsulated houses. It is also a necessary consequence of the measures put into place to mitigate energy wasteful houses. No one is saying that new houses should not be insulated but perhaps some of the solution lies not in more regulation but an understanding that we no longer live in a temperate northern climate but one that will soon be sub-tropical.