Greengauge Director Toby Cambray explores the science behind a European approach to managing moisture levels in homes
While the German language gifts us wonderfully complex compound words, few are as practically valuable in buildings as Stoßlüften – literally “shock ventilation”.
This deceptively simple practice of opening windows wide for short periods, rather than leaving them cracked open continuously, represents a masterclass in applied building physics. But while German has a dedicated word for it, this practise is common throughout Europe, and – if we go back a couple of generations – in the UK too.
Aside from the fluid dynamics of natural ventilation, Stoßlüften relies on basic psychrometrics, the science of the interaction between air and moisture, and thermodynamics.
A key principle is that air has some potential to hold moisture; you can think of the water as being ‘dissolved’ in air much as you might dissolve sugar in a cup of tea. There is however a limit – at some point you can’t get more sugar in your tea, and it just sits at the bottom. Similarly with air, there’s a limit to how much moisture it can hold, and when this is exceeded, we call it condensation – the excess water comes out of ‘solution’ in the air and gathers somewhere as a liquid. The maximum amount can be worked out from first principles of chemistry, so is well known to science. For example, a 200ml mug of tea can hold 580g of sugar, sometimes referred to as standard builder’s tea.
Relative humidity
We often speak about relative humidity (RH), this is the amount of moisture in the air relative to the amount it can potentially hold. 50% RH means the air is ‘half full’ (or half empty?). A complication in the case of humid air is that the limit depends on temperature: warm air can hold more moisture than cool air. This simple fact underpins much of psychrometrics, including the basic principle of condensation. The term relative humidity implies the existence of absolute humidity, and this is the amount of moisture in the air in terms of grams per cubic meter. This is not temperature dependent at temperatures below nuclear fusion.
A typical household will release around 400g per hour of moisture on average via showering, cooking, breathing and so on, depending on the size of the household and their habits. If there is no ventilation (or infiltration) this would be enough to raise the relative humidity in a 100m2 house from 50% RH to saturation in less than 6 hours. This sounds fast but normally even a little bit of leakage or ventilation significantly reduces this effect. We might compare this with the consumption of oxygen in the hypothetically sealed house: the same family of 4 consume 2m3 of pure oxygen per day, so at 20%, it would take 25 days to suffocate. If anyone asks you if they should worry about suffocating in an airtight house, its literally more likely the place will go mouldy first (But I digress).
These principles also show it is possible to remove the excess moisture that is released within buildings. Saturated winter air might come in at 0°C, carrying just 4.8g/m³. If we warm that up to 20°C, the relative humidity drops to just 26%, but the moisture released brings it back up to more comfortable levels – at say 60%, there is an additional 6g per m3 of moisture in the air. If that air then flows out one way or another, we are removing 6g/m3 of air moved.
It is worth pointing out that when it’s really cold, this trick works well, but if the outdoor temperature is close to the indoor, the reduction in RH resulting from warming up the air is much less.
So what does this mean for Stoßlüften, or ventilation more generally? Imagine we throw open the windows of our 100m2 house (with 2.5m ceiling heights) long enough to achieve 1 air change. If the outside air is 5C and 80% RH, there is 5.44g of moisture per m3 of air. If the air inside is 20C and a bit too humid at, say 70% RH, it has 12.2 g/m3, our session of Stussluften would remove 12.2-5.44 = 6.7g per m3, or 1.7 kg of moisture in the whole volume (effectively 1.7 Litres of liquid water). For context, that is between 10 and 30% of the daily moisture generated by a 4 person household (depending on their habits).
On a mild autumn day, it might be 15C and 80% outside, meaning there is 8.5g/m3 in the outdoor air. The difference between outdoors and indoors is now much less, so the one air change removes 12.2-8.5 = 3.7g/m3 or 0.9 kg in the whole house – about half the amount removed in the cold weather. In other words, you need twice the amount of ventilation to get rid of the same amount of moisture.
As I have discussed before, many building materials as well as the things in our houses, are hygroscopic; they can store moisture, and take up and release it, constantly trying to reach equilibrium with the ambient air. If cold incoming air is warmed up so the RH reduces, as well as moisture released by occupants, the materials and contents of the building will start to give up moisture to the air, i.e. they will dry, which is usually a good thing. This also brings the RH of the air back up, which is desirable because otherwise it is a bit too low. This process is sometimes called moisture buffering.
So what does that mean in terms of energy? In the cold winter case, we are exchanging 20C air with 5C air. In the autumn, it’s 15C but we need twice as much of it. The temperature difference and the amount of air moved dictate the energy lost, which we can estimate to be 1.2 kWh in Winter and 0.8 kWh to get rid of the same amount of moisture in the Autumn. The energy cost is less in the autumn, but not as much as we might expect, because autumn air is less effective at shifting moisture. Either way, the cost of an air change or two is pennies – arguably money well spent for all households, for the benefit it gives in reducing the risk of mould.
Practical guidance
If you want to try Stoßlüften, pay attention to the outside temperature. When its mild, ventilate for longer than when its cold. It won’t cost much, and it could really help mitigate mould. These principles also explain why clothes dry so much slower in the autumn. Adding a little heat as well as ventilation (maybe in a utility room with the door closed, a window cracked open, and a radiator on low) effectively overcomes this.
The elephant in the Passivhaus room is of course MVHR (mechanical ventilation with heat recovery). A properly designed, installed and commissioned system should make regular Stoßlüften unnecessary – the flow rates are selected to control moisture, with heat recovery benefits that open windows can’t provide. But even in a Passivhaus, it may be worth remembering Stoßlüften on those 5 load washing days!
Toby Cambray is Director of Moisture at Greengauge.
This article first appeared in Passive House + magazine, Issue 51, 2025
