‘Carbon Neutral’ or ‘Low Energy’ homes are promoted as being friendlier to the environment whilst reducing heating costs through a combination of better insulation, efficient heating systems and, of particular relevance here, a high degree of air-tightness.
The more one can reduce the flow of air between the inside and the outside, the less warm air you lose and the warmer the house and the lower the heating bills. And, as a bonus,for properties in areas of higher air pollution, the inability of pollutants to enter the building must surely make for a healthier indoor environment.
However, aficionados of air quality will quickly see the potential for problems. Undoubtedly, if you live next to a busy road, for example, less nitrogen dioxide and diesel particulates will enter your home if it is relatively airtight. And hay fever sufferers might see a benefit in keeping pollen locked outside. But there more to pollution and healthy air than vehicle emissions and pollen.
Pollution and other unhealthy air issues build up in the home irrespective of outside influences:
Carbon dioxide from your family’s breath builds up quickly. In an old, ‘leaky’ house, the unintentional but unavoidable draught keeps concentrations to safer levels but in a leak-free house it has nowhere to go unless you keep on top of it.
Volatile Organic Compounds (VOC) seep into the air from furnishings, carpets and numerous household products.
Particulates are produced indoors in vast quantities –cooking on gas, using a toaster, gas or oil boilers all contribute – and a open fire or wood burner adds more than any nearby road ever will.
Humidity. The amount of moisture in the air will, in a well ventilated home, largely self-regulate,with problem areas such as kitchens and bathrooms having additional driven extraction systems. But a sealed home can see humidity levels at permanently unhealthy levels. Anything over 50% is considered not ideal and levels of over 70% will risk the growth of mould and bacteria that can both damage your home and cause serious respiratory effects.
The only practical way to keep CO₂, VOC and humidity levels in the modern home down to safe levels is to ventilate, ventilate and ventilate. Open the doors and windows regularly, get those extractor fans on, and exchange the air thoroughly and often.
And yes, by doing this you do blow all your warm air outside and have to heat the air that replaces it again, and so you undo some of the savings that your modern, leak-proof house was supposed to provide. But there’s fundamentally no other way. It is, however, important how, and when, you ventilate.
As well as exchanging the indoor air for outdoor air –generally with less CO₂, VOCs and humidity – you are also letting in any external pollutants that maybe at higher concentrations, in particular traffic particulates, nitrogen dioxide, pollen, ozone and, in the country, a whole range of unpleasant, farming related gases.
Best energy practice is obviously to maintain a balance between ventilation and retaining warmed air, but best air quality practice is to exchange the air when any indoor pollutant builds up to unhealthy concentrations but not whilst your outdoor air is also unhealthy. The best times to ventilate, therefore, depend upon your own specific circumstances. In general, try to avoid times of poor outdoor air quality as a first step. If you’re by a busy road, keep the house closed up during busy times, especially if you’re down wind. If a high pollen count is forecast, wait to open windows until later. Otherwise,thoroughly change the air, ideally twice a day. Yes, at this time of year you’ll make the house colder for a while, but it’s a very good trade.
Of course it’s very hard to know what the level of any pollutant is at any time. A cheap,multi-pollutant monitor [See here]can prove extremely valuable in identifying when you should change the air and when to lock yourself in your shiny new house. Different pollutants increase at different times in different rooms. For example CO₂ rises rapidly in bedrooms at night and particulates at mealtimes in kitchens. And your teenager will be horrified if you show then the level of VOCs in their bedrooms overnight!