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A breath of fresh air – is your airtightness and ventilation right?

Long gone are the days of open fireplaces in every home. Not only did they serve as the main heating source but – because the drawing of air up the chimney meant a constant supply of fresh air – the primary means of ventilation too.

Ventilation is important: it removes moisture-laden air, especially in rooms like bathrooms and kitchens, as well as odours and pollutants, and replaces it with fresh air to maintain a healthy internal environment for occupants.

Traditionally, the trade off against high levels of ventilation was that buildings were harder to heat and keep warm. Today, construction is very different. The goal is high levels of insulation, combined with air and vapour tightness.

Keep the warm air in the building, and carefully manage the ventilation to refresh it enough to maintain a healthy environment.

Moisture and condensation

In our last blog post we looked at what happens when warm air containing moisture vapour comes into contact with cold surfaces. If the air contains too much moisture, or its temperature drops too much, it deposits the excess as condensation.

This explains why modern construction methods are focussed on maintaining comfortable internal temperatures and preventing moisture getting into the building fabric. If enough moisture-laden air leaks to the external side of the insulation, where surface temperatures are colder, condensation is more likely to occur.

Air and vapour tightness

Vapour barriers, such as polythene or aluminium foil layers, are designed to combat the leakage of air and moisture vapour into the structure. They must always be designed on the internal (warm) side of any insulation layers, to keep the moisture inside the building ready to be ventilated out.

Another key point is to keep them as a continuous layer. For example, if the vapour control layer is at ceiling level, and holes are cut in the ceiling to accommodate a downlighter, the resulting hole will act as a mini-chimney, allowing warm air and moisture through it into the structure. This should be avoided as much as possible, preferably through the inclusion of service voids where necessary.

What’s right for me?

Some buildings, particularly those earmarked for conservation, are constructed from materials designed to let the structure ‘breathe’ – in other words, moisture can diffuse right through the structure to the external air. In these situations, applying modern, impermeable construction products and stopping this natural transport of moisture can be very damaging to the structure.

Typically, the older the property the more ‘leaky’ it is. Over time, materials deteriorate or move, creating further paths for ‘uncontrolled ventilation’ that cause occupants to feel draughts that make them uncomfortable. Undertaking renovation work to add insulation and vapour control measures, and eliminate draughts around doors and windows, can help to seal the building – but it can reduce the available ventilation that previously refreshed the air.

The Passivhaus standard, which creates warm and comfortable buildings with minimal energy demand, is the ultimate example of this modern method of building. It is proven to work, as evidenced by ongoing studies of homes built to the standard. The key thing about Passivhaus is that the levels of insulation and airtightness are planned for from the start, with a ventilation system designed and installed to suit.

Building projects are frequently undertaken without a clear idea of what standard of airtightness is likely to be achieved. If in doubt, the best advice is always to consult an expert. It’s actually possible for a contractor to do too good a job, because if the ventilation measures haven’t been designed for an airtight building then there’s a risk to the indoor air quality of the property.

No excuse

That shouldn’t be used as an argument not to achieve good airtightness, however. On the flip side, it’s not uncommon for airtightness measures to be installed on a new-build property purely for the purposes of passing an air test, only to be ripped out again afterwards.

While that might avoid creating an hermetically sealed box, it results in paths for warm air to escape and cause uncomfortable draughts and higher energy bills. The logic behind our modern way of building is sound and scientifically proven, but too many people either see it as an imposition or are yet to get to grips with it, unaware of the important balance between insulation, ventilation and airtightness.

With a little attention to detail and some thought to airtightness and ventilation it’s possible to achieve warm, comfortable and healthy homes with low running costs – and there’s no need for homeowners to see their hard earned money go up the chimney in smoke.

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