Ventilation blog

Ventilation and air quality in domestic properties

Dear reader, I am not writing this blog as an expert in ventilation, but as a student. My area of expertise is solid wall insulation, and if this blog generates feedback and discussion, that would be brilliant. Nick Lloyd

Air quality

With the need to make homes more airtight there is the possibility that the air quality is seriously compromised.

There is no point in saving energy if IAQ (Indoor Air Quality) is going to be detrimentally affected. Possible unwanted results of achieving a higher degree of airtightness are odours, volatile organic compounds (VOC’s) and build-up of carbon dioxide.

Additionally, higher humidity in a room is unhealthy for other reasons (see chart below). Levels of humidity between 40% and 60% are recognised as healthy for humans. Increased airtightness (which has been achieved for some time now by, for example, replacing leaky timber sash windows with UPVC ones) has caused higher levels of humidity and condensation and mould growth as a result. Dust mites breed prolifically in high humidity areas, and the waste from these creatures can give rise to asthma. I wonder how much money would be saved in the NHS if good air quality was a more serious consideration in new build and retro-fit?


Good ventilation should help prevent the build-up of moisture, and in turn reduce condensation and mould growth. It will also protect the fabric of the building by helping to prevent rot and damp within the building envelope.

There are four main ventilation systems recognised within the building regulations.

System 1 – Intermittent extract – this utilises standard through-the-wall fans in wet rooms (wet rooms being those where excessive moisture or odours are produced such as kitchens, bathrooms and WC’s). Background ventilation is then introduced through wall or window trickle vents. This system is most common as it’s the easiest option when a problem arises in an existing dwelling. This is the most rudimentary system and isn’t easy to control, whether fans are activated by turning on lights or humidity sensors (I’m not convinced the latter are reliable). Also, occupants often close trickle vents because of the draughts they generate.

 Ventilation system 1

System 2 – Passive Stack. Stacks are placed in the wet rooms and use a combination of natural convection and negative pressure caused by wind passing over the roof to draw out the moist air. It is important to ensure the inlets are of adequate size, as the system operates on a very low pressure. Inlet and extract sizing must be balanced to ensure the pressure is not reduced, preventing adequate extraction. I would use this system in a new build which didn’t require passive house accreditation as I like that it doesn’t require electricity to operate it, relying on building physics instead. The principle has been used in hot countries for centuries.

System 3 – Continuous Mechanical Extraction or MEV. This has a centralised fan ducted to the wet rooms and is designed to provide constant background ventilation, often with a boost or speed control to ensure adequate levels of extraction during periods of high moisture production. Again, trickle ventilation in the non-wet rooms is introduced via wall or window trickle vents. This system works well and as long as the fan is low wattage, doesn’t cost a lot to run. True, you are losing heat, but there are no filters to get blocked up or change and it’s a lot cheaper and easier to install than MVHR (see below).

System 4 – Mechanical Ventilation with Heat Recovery (MVHR). This also has a centralised fan but air is extracted from wet rooms and passes through a heat exchanger housed within the fan unit. Fresh air is drawn into the property via a mechanical fan and also passes through the heat exchanger to recover some of the heat energy. It is then distributed to the ‘living rooms’ via ductwork and air valves. This system is favoured in building a passive house standard home. There is a convincing argument that it’s not worth using this system in a ‘retro-fit’ as a high level of airtightness (less than two air changes per hour) is required to enable the system to work efficiently, and thereby justify the expense of the installation.

Ventilation system 2

Moisture Levels in a home. A family of four will produce anything from ten to twenty litres of water in 24 hours;  two litres while sleeping, a couple of litres bathing and showering, four litres cooking, a litre washing clothes, and so on (not to mention using an unvented tumble drier, a whopping five litres!) As previously mentioned, when the humidity level in a home is above 60%, the production of harmful bacteria and mould dramatically increases, as does the amount of dust mites (a cause of allergic reactions, in particular asthma).

The ideal ventilation system is controllable, responds to occupancy, extracts air from moisture producing areas to go outside during periods of high moisture generation and replaces it with a controllable amount of outdoor air flowing in via the other rooms.

I particularly like the DCV (Demand Controlled Ventilation) system produced by a company called Aereco. They use specially invented nylon strips in their trickle vent covers which expand when the humidity in the room rises. As the strips expand, the vent is allowed to open. When the air dries, the strips contract, closing the vent.

Mitigation of humidity with choice of building fabric

I also like to use breathable materials in the construction fabric. Wood fibre or calcium silicate internal insulation and lime plaster help to mitigate the level of humidity in a home. The idea is that these materials are vapour open and hygroscopic, allowing the walls to absorb a level of water vapour and release later when the room dries out. Some wood fibre products have a built-in vapour control layer preventing too much moisture being absorbed, especially by the end of winter months.

I always notice the improved ambience of a room after we have installed these products. Of course, a good breathable emulsion is required otherwise the performance will be affected. Hopefully one day there will be a mandatory ‘service history’ required for each dwelling to register such details for occupancy use. By the way, I’m not suggesting that a breathable fabric excuses the need for a well designed and installed ventilation strategy.

Nick Lloyd