Modern construction techniques produce housing that is particularly airtight. Old houses have an air pressure of Q = 15 so you didn’t need to worry about it. Modern construction produces an air pressure of Q = 5, sometimes achieving Q = 3 or less in some really heavily insulated houses or using advanced thermal bridging. Proper ventilation is needed to prevent the building feeling unbearably stuffy and also to prevent the growth of mould in the plaster.

Natural ventilation.  This is simple and uses no electricity. Trickle vents can be placed above windows. Make sure that each of the rooms has a reasonable number of sufficiently large window and that the trickle vents, which are long plastic units allowing the air to flow through a narrow gap and through a mesh, are long enough to cover most of the window head. In really air tight houses it is still possible to use trickle vents, even below Q = 5 (see Document F of the Building Regulations) but you need to use as many as possible. SAPS4U can provide a ventilation calculation showing you have met the Document F requirements as this is sometimes requested by Building Control. In addition to the legal requirements large rooms at the rear of the house should ideally have double doors that can be opened and in all rooms it should be possible for the windows to fully open on a hot day. This will avoid stuffiness and overheating in warm weather.

Mechanical ventilation.  Traditional air conditioning systems use a lot of power. It becomes more difficult to pass the SAP test if mechanical ventilation is used as it increases the CO2. More sophisticated methods of mechanical ventilation have to be used which are more efficient. Positive Input Ventilation (PIV) draws air from the loft and distributes it into the rooms. This is more efficient than ordinary mechanical ventilation (MEV.) Mechanical Ventilation Heat Recovery (MVHR) takes air from wet rooms like bathrooms and kitchens and pumps it out of the house. It also draws cold air from outside. A heat exchanger is used to transfer heat from the warm, moist air of bathrooms and kitchens to the cold incoming air. This is then distributed all over the house to achieve good ventilation. MVHR can recycle a great deal of heat causing the building’s heating system to do a lot less work. In order for MVHR to work this efficiently the building needs to be very air tight. If not, the MVHR will produce more CO2 (by using electricity) than it will save (by you turning down the gas) and is more of a problem to passing the SAP test.

What do I think of MVHR? It would be useful if the house design was going to be very airtight, for example, hugely thick insulation in timber frame walls and advanced ACDs. Manufacturers recommend an air tightness of less than Q =1 so to work really well the building has to be more air tight than usual. At normal air tightness of Q = 5 it’s not going to work that well and it’s not much of an advantage. The traditional methods of long trickle vents above the windows are more likely to get the house through the SAP test. At the lower levels of air permeability, below Q = 1, MVHR begins to save energy by reducing the heating requirements considerably. In a very air tight house it is worth putting in.

Ventilation – Why is it Important?