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How do I optimise ventilation without losing too much warm internal air?

January 9, 2013
Not opening windows leads to high internal humidity

Not opening windows leads to high internal humidity

Anyone who lives in a small enclosed space such as a 2 bed flat needs to open their windows. However, the temptation is to keep the windows shut, since heating a property is now very expensive.

Opening windows is important for several reasons.

1. It removes household contaminants such as CO2, H2O and body odour.

2. It helps prevent mould growth and reduces dust mites.

3. If windows are opened daily, the cooler fresher air will help keep your home drier.


Take a look at the example below.

Air at 20oC at 50% relative humidity will contain 7.28g of water vapour/kg of dry air;


Air at 10oC and at 50% relative humidity will only contain 3.80g of water vapour/kg of dry air.

This means that by replacing 100% of the internal air you can halve the amount of moisture inside the property. However, the big problem is that while you have halved the moisture content of the air you will have also halved the internal air temperature!

If you can afford to re-heat the new cooler air by switching the central heating on then fine, but what if you can’t!!

How do I ventilate the house without losing too much warm air?

Well, why not give the house a short period of ventilation in the morning and once again in the evening?

If you open the windows for 15 minutes just before you go to work in the morning and again in the evening (at around 8pm) you will hopefully be getting the Ventilation-Heat Loss balance about right.

The peaks in net vapour pressure are the best times to open the windows.

The peaks in net vapour pressure are the best times to open the windows.

Why open the windows in the morning and evening?

Well, having carried out some monitoring of vapour pressure fluctuations in a domestic environment recently, I‘ve found out that by identifying the maximum net vapour pressure levels from 00:00hrs to 12:00hrs and again from 12:00hrs to 24:00hrs, I can determine the best time to open the windows.

The reason why the best time to open the windows is when the net vapour pressure is at a maximum is simple. It’s the time when you can expel the most amount of water vapour in the shortest time, thus limiting the loss of centrally heated internal air.

One Comment
  1. Alex permalink

    I find your articles really good and I thank you for sharing them, however I must strongly disagree with you on this occasion, opening windows to deal with cold surface condensation is fundamentally outdated and incorrect advice.

    Let’s assume it’s winter (when houses tend to suffer the worst from condensation) and there is therefore a large indoor/outdoor temperature differential, and the occupant has just had a shower; to open a window and let in cold air, will chill the internal wall surface temperatures and the air in that room (which is already at saturation point) to below dew point temperature, causing the air in the room to release a large amount of its moisture through a process called transient condensation. Opening a window makes all that steam disappear, giving the illusion that the problem has been dealt with, but actually that humid air has merely condensed on the walls due to the sudden chilling effect from the influx of cold air. That example is to the extreme, but the principle is exactly the same throughout the house, and in fact no where in the technical document BS 5250 “Code of practice for control of condensation in buildings” does it advocate the opening of windows as a means of preventing condensation.

    Ventilation is only one part of the jigsaw however, and to effectively deal with humidity and prevent cold surface condensation, you must ensure a number of things.

    1 – The property is properly heated and kept above dew point temperature. Large temperature fluctuations can change the relative humidity of the air, causing it to release some of its moisture (remember, warm air can hold onto more moisture than cold air). Educating a client of this is often half the battle. Sometimes heating systems require upgrading with TRV’s fitted to radiators to give more control.

    2 – The building is properly ventilated. We find that extractor fans wired to run via the lighting circuit are almost always unable to cope with modern life, and offer no extraction for 99% of the day. We produce humidity constantly through breathing, showering, cooking etc so surely it makes sense to have some form of constant background extraction (no not opening windows!); for our clients, we regularly specify extractor fans wired to run continuously on trickle speed 24 hours a day (with boost speed wired to the lighting circuit for showering etc). We sometimes specify one in the kitchen too where we deem it necessary. They are silent and cost peanuts to run (far less than opening windows in winter and paying to heat outside), and they are fantastic at managing humidity, they really make a difference! If every house had this form of extraction system fitted then chronic condensation issues would be few and far between.

    3 – The building fabric has good thermal values. Even with good levels of extraction and a properly heated house, if there are really poorly insulated cold wall/ceiling surfaces, then you are still likely to get condensation in these areas. Solid walled properties tend to be more prone to this and thermal imaging is a very effective way of finding poorly insulated areas (N.B. winter is the best time to do thermal imaging as you need a big temperature differential between indoors and outside, so turn your heating up and make sure it’s cold outside!).

    Managing humidity and preventing cold surface condensation is a balancing act that can only come from an appreciation of how the above 3 points impact upon one another.



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