Balancing energy saving and ventilation

By Nicola Terry, Cambridge Carbon Footprint and Transition Cambridge

Indoor air quality is important for our health and wellbeing. If your bedroom is stuffy this can disturb your sleep and if you are like me you do not function very well the day after a bad night. Poor quality air can exacerbate all sorts of health issues too. So what do we do? Leaving the window open wastes energy. Are trickle vents sufficient? – not always! How much ventilation is enough and how do we know? CCF now has some logging air quality sensors you can borrow to find out if you have a problem. Read on to find out if this could apply to you, and what you can do about it.

Good ventilation needs a door or a window on at least two sides

You do not need to worry about ventilation if you live in a draughty home. However if you have stopped up most of your draughts to save energy, or if your house was not draughty to start with, then it is worth checking if you have enough. In most homes we rely on trickle vents in windows for minimum natural ventilation, though old windows may not have vents at all. In any case you need air flow across the room, either from windows on two sides or from a door on one side to a window on another. The diagram below shows how cross ventilation is (usually) good at ventilating most of a room. The direction of the wind makes a difference too. I have met with people who closed their trickle vents, for example if they think they are getting a draught. This is not a good idea.

Bad air can creep up on us slowly without us noticing.

We are quite quick to notice if we are too cold or too hot but we often do not notice bad air, especially if it creeps up on us slowly. There are lots of different sources: you can get solvent gases coming off furniture and wall coverings, fumes from cooking appliances, or odours from people. We ourselves produce CO2 and moisture, just by breathing. However, both of these are bad for you if allowed to accumulate.  You also get CO2 from cooking with gas. CO2 levels are generally considered a good guide to ventilation quality. 


Humidity needs to be in the middle – too much or too little both cause problems.

Humidity is another important measure. Too much or too little is a problem: air that is too dry can exacerbate problems with lungs and airways and allergies while air that is too humid can trigger asthma and allow mites and moulds to grow. In the middle is best – about 40% to 60% relative humidity. Unfortunately, it is not always possible to control this through ventilation because it depends on outside conditions too. Humidity is a good guide to general ventilation levels during the heating season, when we need to balance ventilation and energy saving. 


Trickle vents are often insufficient, especially in modern homes.

Chart from Occupant Interactions and Effectiveness of Natural Ventilation Strategies in Contemporary New Housing in Scotland, UK, Tim Sharpe, Paul Farren, Stirling Howieson, Paul Tuohy and Jonathan McQuillan in Int. J. Environ. Res. Public Health (2015)

Modern homes can be particularly bad for ventilation. This chart is from a study of homes in Scotland, that were built after 2009. It shows CO2 levels in bedrooms overnight. In these homes, having the trickle vents open hardly made any difference. Opening the window was effective. Below the red line, at 1000 ppm, is generally regarded as ideal.

Tom and I both tested our homes using the new CCF CO2 monitors - with interesting results

Tom’s house is not at all modern but he has put a lot of work into it to save energy including stopping draughts. He tried out one of the CCF monitors to see what happens in his bedroom overnight. The monitor records CO2 level, relative humidity and temperature hourly and stores the data in a file that you can upload to your computer via a USB connection. Then you can view it in a spreadsheet program such as Excel. (It is a csv file so compatible with many applications,) Tom did some experiments – one night with the bedroom door closed and one night with it open. The CO2 level went over the ideal level on both nights but with the door open it was just a bit over. With them closed it went way over – above 3,000 ppm. In my professional work I sometimes get datasets with CO2 levels monitored from homes as a measure of ventilation, I have seen much higher levels than this in some homes – even 10,000 ppm!


I borrowed a monitor too. My house has MVHR (mechanical ventilation with heat recovery) so I did not expect there to be a problem and there was not. The CO2 level hovered around 750 ppm for most of the night, well below the 1000ppm mark.


If you have too little ventilation – check trickle vents, check air flow under doors, ...

If you decide you do have a problem, what should you do about it? Well, the main thing is to make sure that you have some ventilation. If you have trickle vents make sure they are clean. Leave the door open if you can as that helps get a cross draught across the room. If that is not possible, at least make sure there is plenty of room under the door for air flow. Building regulations require a gap of 10mm below internal doors to allow air mixing between rooms for most ventilation strategies. If you have a thick pile carpet it could be blocking the ventilation. If you find you need to open windows, some windows can be securely fixed slightly open – effectively a larger trickle vent.


... watch our video, borrow a CO2 monitor

Alternatively, you could consider a powered ventilation strategy, with or without heat recovery. There was an Open Eco Homes talk about this last year – you can watch the video here

If you think you would like to borrow a CO2 sensor, click here.