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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.

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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;

however…

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.

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