Spending 20 grand on insulating a home is pointless if you leave the windows open. In how far does this obvious logic apply to air bricks, trickle vents or other cracks in the building envelope? The two main sources of heat loss in buildings are conduction and convection, whereby the former is the transfer of heat through solid materials like brick or insulation, and the latter is the loss of warm air through intentional or unintentional leakage points. (The third way of heat transfer is radiation, and its most important effect on buildings is passive solar gain through the windows.)

If a large part of a wall, floor, ceiling or window is insulated fantastically, but there is an air gap around the edges or a cold bridge, i.e. section of the wall that conducts heat very well, as is often the case in the eaves or at the wall/floor joint around the damp proof course, the performance of the whole installation gets seriously undermined.

Our current building modelling and rating methods tend to take target U-Values and apply them to the entire building element (wall, floor, ceiling, windows), ignoring edges and joints because of their relatively small size. However, despite their size, the impact on the energy efficiency of the whole house is dramatic. By funnelling the heat loss onto a smaller surface area, cold bridges and air gaps also cause condensation hot spots leading to mould growth and rotting timber.

Some research has been undertaken on the building performance gap and the likely factors contributing to it – the most blatant ones being improper design consideration and poor workmanship. There seems to be surprisingly little knowledge about how air tightness and cold bridging interrelate. This recent GreenBuildingAdvisor blog article from Maine argues (qualitatively) that air leakage is the elephant in the draughty room, i.e. that convective losses will outweigh conductive losses by quite a margin. The stack effect (or chimney effect) is partly to blame for this, whereby the whole building forms the stack.

This is particularly relevant if you have a loft hatch at the top of your stairwell, that is inadequately insulated and draught proofed. If you had your loft insulated by an installer, e.g. under CERT, CESP or ECO, the draught proofing and insulation of your hatch may in our experience well be inadequate. Putting in a product that effectively drought seals and insulated your hatch may well be the easiest and cheapest next step to lowering your heating bills and improve comfort by reducing draughts.

Drought proofing the quickest win for any building in terms of keeping your precious heat in. However, improved air tightness, and thus less fresh air in the living space, also leads to an increase in the so-called sick building syndrome, respiratory afflictions caused by VOCs, mould spores, toxins, radium etc. being trapped in the building. The longer term development will go toward well drought-proofed buildings combined with mechanical ventilation with heat recovery (MVHR) or even better, passive ventilation with heat recovery (PVHR), designing buildings that ventilate themselves, again using the chimney effect but combining it with heat exchangers. Nifty entrepreneurs at Ventive have also developed a system retrofitting PVHR into existing fireplace chimneys.

With our current housing stock with its air bricks, trickle vents, cracks and openings all over the place, we are far from getting too air tight. It makes sense to drastically reduce the air permeability of our dwellings and then start to think about MVHR or PVHR.

insulation and air tightness