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Ventilation & Heat Recovery

Air moves inside your house by the stack effect. This is the movement of warm air upwards, being replaced by cool air at the bottom of the house. This cool air could be drawn into the house through cracks and holes, gaps in floorboards or poorly fitting seals around windows and doors.

Air can also be moved via flue effect, i.e. exhaust fans. If the ventilation is not balanced, an exhaust fan will de-pressurise the house, encouraging cool air to enter through leaks, and possibly causing back draught in open hearth fireplaces and other non-sealed combustion appliances.


Moisture is moved by gravity (leaks in the roof), capillarity (the tendency of water to fill small spaces in porous materials such as brick and concrete), and diffusion (moisture moving from an area of high concentration to an area of lower concentration).

Capillarity is controlled by erecting a barrier that either has very large holes (so the surface tension cannot support the weight of the water) or no holes at all (such as a damp proof course).

Water can move into walls by the sun creating vapour drive inwards after a rain. Wind-driven rain will often
penetrate the exterior finish of a wall, so ensuring the wall can dry to the outside and can drain any accumulating water is very important.

Mechanical Ventilation and Heat Recovery (MVHR)

An air tight house, with balanced mechanical ventilation, properly placed air and vapour barriers and plenty of insulation will be very energy efficient, provide a healthy indoor environment and run almost silently.

Mechanical systems should be considered when the building envelope is built to high standards and will provide heat, cooling, water heating and ventilation. The efficiencies of a mechanical systems should be considered with a view to low energy consumption and creating healthy indoor environments. They allow regulation of relative humidity, to discourage mould growth. In an air tight house, the MVHR system provides all the air in the house, so the addition of a high-performance filter will enhance indoor air quality.