Have you , like me, heard people who live in recently built houses complaining of damp? Such conversations have grabbed my attention especially where the houses have been recently built, according to SAP 2009 or SAP 2012 specifications (2010 or 2013 building regulations).

When I say damp, there might be some difficulty drying clothes on a clothes horse which can be overcome by opening a window. There might be condensation on some of the windows that can be overcome by keeping a small window open. These are the indicators of the problem. Sometimes there are more serious problems like black mold growing on the walls and ceiling. This is much more serious as it potentially effects the physical structure of the building if dampness gets into the insulation and it may also effect the health of the residents.

Today we are building houses to a very advanced level of air tightness

Today we are building houses to a very advanced level of air tightness. SAP 2012 requires an air permeability equivalent to Q = 5. In the early days of SAP the air permeability used to be Q = 10. Many builders are capable of achieving an air permeability of less than Q = 3. This is good and causes the house to use less fuel and to perform better in the SAP calculation. The problem is that at these levels of air tightness the quality of ventilation becomes significant. A building with an air permeability of less than Q = 3 must be properly ventilated. The residents might achieve that level of ventilation by keeping a small window constantly open. Obviously, this will cause the house to become cold if the window is left open in cold weather. This increases the fuel bills and defeats the object of having a well designed, highly insulated and airtight building.

The correct procedure for ventilation of buildings with a low air permeability is described in Document F of the building regulations. Most new houses will either use MVHR (Mechanical Ventilation with Heat Recovery) or intermittent extract fans with background ventilators. MVHR is more efficient than normal mechanical ventilation as it recycles the heat and allows the central heating in the house to be turned down. But unless the house is built to an extraordinary level of air tightness (Q = 1 or lower) MVHR will still consume more energy than it would actually save. SAP assessors dislike it because, if you are trying to get a house of relatively simple construction to pass the SAP test by a narrow margin, the addition of MVHR can cause it to fail. MVHR must be installed by a specialist installer – by the manufacturer or by someone who has been trained – and it also needs proper maintenance. In Cumbria the ventilation system in a public toilet caused an outbreak of legionnaire’s disease when the council didn’t renew the maintenance contract. Intermittent extract fans in kitchens and bathrooms and background ventilation are preferred by most building designers as they avoid these problems.

Background ventilation is achieved by trickle vents at the window heads. Trickle vents are often installed by double glazing companies who have no specialist knowledge of building design. The most common cause of dampness is that the trickle vents are too small. I used to have a trickle vent on my bedroom window that seemed to work well when a gale force wind was blowing into it. Document F contains tables showing how to calculate the exact area of trickle vents required according to the size of the house, the air permeability and the number of bedrooms. SAPS4U can carry out ventilation calculations to ensure that the trickle vents installed are big enough to ventilate the house properly, avoiding any problems with dampness and avoiding any need for the residents to keep a small window open in freezing cold weather.

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Why new houses get damp