Vent it Right..!

Once you have removed any unwanted air leakage from a building, its a good idea to provide some sort of controlled ventilation to ensure sufficient fresh air – so everyone can breathe – but also to allow the excess moisture arising from human activity to be exhausted from the building.

Why do we Ventilate..?

We ventilate our homes for a number of different reasons, but the most common are:

a) Remove Pollutants (CO₂, VOCs, & Smells)

b) Control Humidity (remove water vapour)

c) Control Temperatures (remove heat in the summer)

The traditional approach to providing ventilation was to provide air bricks in each room – normally at high level – and chimneys within the main rooms to allow for natural ventilation. The warm wet air is exhausted at high level and fresh air is drawn through the other gaps in the building fabric to replace it with cold air which must then be heated. Houses relying on this approach to ventilation feel cold, are draughty and consume a lot of energy to keep them warm.

A typical 20th Century house will have a background ventilation rate of between 1-2 air changes per hour depending on wind speed and external temperature – that is to say that the volume of air equivalent to the house will be changed once or twice per hour.

This does not mean that the air you want to get rid of will be removed from the places you need to remove it, just that there will be that amount of air movement within the building.

This kind of ventilation loss will normally amount to around 30% of the overall heat loss from your home.

The higher the wind, the more quickly the building in ventilated. Similarly the colder it is, the more windows will be closed, and the slower the building will be ventilated.

These approaches, whilst extremely inefficient did work to a degree when fossil fuels were cheap and abundant, and no-one knew about their global warming potential. Without a similarly cheap heat source their running costs are huge.

Because of this, many properties designed with passive ventilation have subsequently had their high level air bricks blocked up. Sometimes they have been replaced with trickle ventilators over window frames, which in combination with intermittent fans in bathrooms, kitchens and utility rooms etc are supposed to provide adequate ventilation.

However, not only does this cause the same problem – all the warm air in the house is lost – but leads to another – trickle ventilators can be closed, and fans can be switched off – which leads to poor indoor air quality and increases the potential for mould growth.

Why does mould grow?

Mould growth will occur on any surface where the Relative Humidity is consistently above 80%. For typical indoor air at 20ºC with a Relative Humidity of 50% this critical 80% limit is reached when the surface temperature goes below 12.6ºC.

What is Relative Humidity?

Relative Humidity (RH) is the measure of how much moisture the air contains relative to its temperature. As air is warmed up its ability to absorb moisture increases. Conversely as its cooled down, it can hold less, so it releases it in the form of condensation.

The graphs above demonstrates how the ability of air to absorb moisture deteriorates with temperature. In the example shown, a given amount of air can hold the the same absolute amount of moisture at 6ºC and 20ºC but at 6ºC it is has a relative humidity of 100% (it can’t hold any more) but at 20ºC it has a relative humidity of around 43% meaning it has the capacity to absorb the same amount of moisture again with space to spare.

The humidity within a building is controlled by changing the warm saturated say 20ºC @ 60% RH, and replacing it with new cooler – say 12º @ 80% RH – air through ventilation and warming it up to decrease its RH to around 40%.

Some people have tried to resolve this by installing Positive Input Ventilation within their loft in combination with a heating coil.

What is Positive Input Ventilation (PIV)?

Positive Input Ventilation pushes fresh air into the building and forces the stale air out through the building fabric. The problem with this approach is that in pushing the saturated air through the building fabric, we have no way of seeing where the air will reach 12.3ºC and start to condensate. We also have no way of knowing whether it will be on a surface and if mould will start to grow, and we may not be able to see the unintended effects of this until significant damage has been done – for example – to the floor joists.

If you are considering fitting a PIV unit in your home, check out my post on the things to consider first here

So what can we do instead?

Mechanical Ventilation with Heat Recovery (MVHR)

Mechanical Ventilation with Heat Recovery or MVHR can be both centralised (one unit with ductwork) or decentralised (single room through the wall units), but both of these systems do the same thing, they extract the air from your house and pass it through a heat exchanger which recovers almost all of the heat from the air, then they provide fresh air, which is run through the same heat exchanger to pre-heat it. This process is normally over 97% efficient and will save more energy than they use over the course of a year. The systems are always running providing fresh filtered air to your home and can be boosted if necessary should there be a need for it (eg smells).

How can I stop mould from coming back?

You need to increase the surface temperature of the internal surfaces by insulating the building better to ensure the temperature does not drop below 17ªC, and ensure the RH remains between 40-60% all year round.

If you have issues with under or over ventilation – or an issue with mould which you would like to discuss, please get in touch and we will see if there is something we can help with.