What does Building Control in Scotland recommend for new domestic buildings?
The recommendation is a staggered approach, depending on the air tightness of the building envelope:
- If the building is very airtight (0-3 m3/(h m2) @50Pa), the only choice is Heat Recovery Ventilation (MVHR)
- If is it semi-airtight (3-5 m3/(h m2) @50Pa), either MVHR or dMEV can be used
- If it is lesser airtight (5-7 m3/(h m2) @50Pa), either dMEV or natural ventilation can be used. MVHR is possible, too, as there are different reasons for the use of this technology, other than the air tightness of the building.
A problem arises when the measured air tightness of the building is very different from the target air tightness. In such cases often the ventilation strategy needs to be revised. What complicates the matter is that the air tightness test needs to be carried out when the building is finished. At this point remedial measures are very difficult to implement. E.g. the architect specified 5 m3/(h m2) @50Pa with natural ventilation and trickle vents. Now the trades have done a good job and the building achieved 2.5 m3/(h m2) @50Pa. This means MVHR needs to be retrofitted into the finished building.
Looking at the reality of the building industry, it is often difficult to predict the real air tightness of the building, as much depends on the workmanship of the trades. It is probably the better approach to plan for a solution that complies with the tighter standards, e.g. MVHR.
We find the best choice of ventilation strategy is MVHR – if it designed, specified, installed and maintained right. There are some cost implications, but the comfort and air quality will pay dividends (over and above the energy savings).
Alternative ventilation strategies:
dMEV is a much cheaper solution in the initial expenditure, but as these don’t recover any heat, with the expelled air all the heat energy is lost. Such fans are better versions of extractor fans, that run constantly on a low level and boost when necessary, e.g. when they detect increased humidity. Modern dMEV fans are very quiet.
recent research in Scotland has shown, that most dMEV systems do not deliver the desired indoor air quality? The reason is, that the ventilation path through the building is much less controllable than with MVHR systems. E.g. some bedrooms might have very little to nearly no ventilation at all. dMEV systems need to be carefully specified to avoid such downfalls of the system.
Natural ventilation via background (trickle) ventilators with mechanical intermittent extract fans is still an allowed ventilation strategy for ‘leakier’ houses, but…
that in practice hardly any new built house will achieve the legal requirements for indoor air quality, unless occupants open window regularly? As such natural ventilation alone is not fulfilling the demands of Building Standards and therefor it is not a viable ventilation strategy. Occupants need to co-operate, e.g. keeping windows of bedrooms in use tilted all night (without curtains or blinds blocking the ventilation path) and cross-ventilate during the day every couple of hours.