The importance of ventilation

This indicative view makes visible the invisible phenomenon known as water vapour and we generate a lot of it in our homes. Excessive amounts of what in fact is actually a gas need to be ventilated out to reduce harmful effects to occupants and the building fabric itself. Image © Deeper Green

Before the 1970’s most of our homes benefitted from what is called, ‘wind-driven ventilation’ with infiltration through the windward side and displacement on the leeward side. Old forms of construction were 'leaky' with high levels of replacement outside air keeping the interiors refreshed if a little chilly in the winter months.

Then we started putting double glazing in, loft insulation and draught stripping. That in turn led to surface condensation and mould growth. Cases of occupants suffering from asthma and other respiratory illnesses started going up. In response, the national Building Regulations were updated to require ‘intermittent mechanical ventilation’ (extract fans) in bathrooms and kitchens (a.k.a. wet rooms) with background replacement ventilation, usually in the form of trickle vents in window frames.

With a new push to further insulate our walls and eliminate uncontrolled air infiltration, ensuring our homes are provided with adequate means of replacement fresh air and allowing excessive airborne moisture and toxins to escape is becoming more important than ever.

This photograph shows a typical scenario where the conditions for mould growth are initiated by persistent surface condensation forming on the interior face of an external solid masonry wall. This situation can arise with uninsulated cavity wall or a cavity wall with failed injected cavity insulation. Image © Deeper Green

Sources of water vapour

You cannot see water vapour. It is water in a gaseous form and is not to be confused with airborne water droplets like steam or fog. As a gas it can build up to unhealthy levels without notice but when it comes into contact with colder surfaces such as windowpanes or areas of uninsulated walls on a cold day, the moisture condenses out of the gaseous state and into liquid water. Without good ventilation to remove the excess water vapour, building interiors are prone to mould growth, the spores of which can aggravate respiratory conditions like asthma.

This illustration shows a typical scenario where the conditions for mould growth are initiated by persistent surface condensation forming on the interior face of an external solid masonry wall. This situation can arise with uninsulated cavity wall or a cavity wall with failed injected cavity insulation. Image © Deeper Green

In winter, as we try to conserve heat energy, our first reaction is to close windows and seal off vents. The relative humidity in rooms increases, as does the concentration of other airborne toxins, like the CO2 from our own breathing, 'off-gasing' chemicals from materials like polymer-based carpets or pollutants from outside. These will build-up to unhealthy levels making for a 'fuggy' atmosphere.

Trickle vents installed in high performance sliding sash windows as viewed internally from below. Image © Deeper Green

Replacement windows with no trickle vents

If you have a home that has replacement windows with no trickle vents and no ‘whole house mechanical ventilation system’ (see below) then you probably are not getting enough replacement fresh air into your home. In the first instance, one can try manually setting the window ‘ajar’ in the ‘night latch’ position. Most modern casement windows will have this functionality. That maybe too much ventilation depending on the size of the opening casement. Another alternative is to have trickle vents retrofitted and most window fitters should be able to carry this out. Otherwise, and especially if you are considering a more extensive low energy refurbishment of the whole house, you should consider a whole house mechanical ventilation system.

Domestic ventilation options

Here are some of the key forms or domestic ventilation but there are others such as passive stack ventilation (PSV) and positive input ventilation (PIV) but these are generally regarded as more troublesome to get right or ensure consistent performance. Image © Deeper Green

Intermittent extract ventilation (IEV)

Intermittent extract ventilation is the ventilation system that many of us will be familiar with. In the 1970s new Building Regulations were introduced requiring intermittent extract ventilation in wet rooms (kitchens, utility rooms, W.C.’s and bathrooms) and fresh air inlets (usually trickle ventilation fitted in window frames) in living rooms and bedrooms. Gaps under doors are necessary to allow airflow from living spaces into wet spaces. Such extract fans are most often activated by turning on the light switch, but a more effective means of controlling water vapour generally is by linking the extract to a humidity sensor. Some models are quieter than others.

Continuous mechanical extract ventilation (CMEV)

When increasing the standard of insulation and airtightness it may be appropriate to install a more capable form of ventilation to that which IEV can provide. Continuous mechanical extract ventilation extracts stale air from wet rooms at a background level and when relative humidity rises the fan speed increases to a boosted level. When operating at a background level, these extract fans are nearly silent. Such systems can either be decentralised with separate fans in each wet room or with a single fan connected by ductwork to wet rooms. As with an intermittent mechanical extract ventilation, living rooms and bedrooms need fresh air ventilation inlets like trickle vents (these may be wall inlets with humidity sensitive controls) and doors need to be undercut to allow air flow.

Mechanical ventilation with heat recovery (MVHR)

Retrofit projects designed to meet the Passivhaus Enerphit standard require the installation of mechanical ventilation with heat recovery. Such systems continuously extract warm air from wet rooms. This then passes through a heat exchanger. Meanwhile a second fan draws in a balancing supply of fresh air which picks up some of the residual heat from the outgoing air as it flows through separate compartments of the heat exchanger before being ducted out to the living spaces.

For the heat exchanger to extract meaningful amounts of outgoing heat energy, the overall external envelope of the building needs to be made virtually air-tight which is technically difficult to achieve even with a new build. Such an installation will involve finding space to accommodate a box containing two fans and a heat exchanger and ductwork connecting all rooms, which can be difficult to accommodate in an existing dwelling.