Don't Forget...

We are often contacted by home owners, who are renovating existing properties, either to extend them or to make them more energy efficient. They install new glazing, add insulation and draft proofing in order to reduce heating costs and cold drafts.

 that by upgrading an existing dwelling the risk of black mould triples?

Why is the endeavour to improve a property potentially endangering the health of the occupants? Because often it is overlooked to apply an appropriate ventilation strategy. Whilst living in a drafty dwelling, there seems to be no need to think about fresh air. However, if most of the drafts have been blocked off and new air tight windows have been installed, the air infiltration rate is often greatly reduced. Especially in older properties this is more critical than in new builds, as inevitably there will be more cold bridges – parts of the external shell which could not be insulated as much as other parts. These areas have a colder inner surface temperature and attract condensation, which over time leads to mould growth.

Therefore it is important to think about a suitable ventilation strategy when older dwellings are being upgraded, either through new glazing, draft proofing or added insulation.

Following points can be indicators that MVHR is feasible:

• If the building’s air permeability will be dropped to 5 m³/(h m²) at 50Pa.
  This can, in many cases, be achieved when new air tight windows are being installed (without trickle vents) and the property is being draft-proofed (floor, walls, penetrations into the loft).
• If the renovated building is close to a busy road, railway line or flight path, which is too noisy to keep the windows open at night.
• If there is existing moisture issues in the dwelling (other than structural ones), MVHR systems have an additional benefit as they passively de-humidify in the colder seasons.
• If occupants suffer from respiratory issues or certain types of allergies.

We have learned over time what works, what works better and what does not really work. Whilst we pass on our experience with you, let this not hinder you to do your own research. Push-pull MVHRs are quite common in Europe and are being looked at here in the UK, too. They work on the principle of breating in and out as they change from extracting air to supplying air in regular short intervals. They normally work in tandem with another system and are used to fulfill regulatory requirements for background ventilation. We see three major problems with them:

• They are very restricted in their ventilation rate before they become noisy. Even for a master bedroom, we would not deem one system enough for good indoor air quality.
• They cannot be used in wet rooms, as they would push humidity into the dwelling.
• Case studies have shown that the indoor air quality within the room is not corresponding with the ventilation rate that these systems provide. In simple terms, they just push the air a meter to the left and then back to the right. Some convection in the middle will aid the air quality, but not as good as with a continuously operating MVHR system.