Friday 25 November 2022

How warm should your house be to keep it healthy?

During the gas shortage some of us are turning our heating down to save money and some are turning it down to reduce demand and keep prices down generally. How low can we go? There is our own health to consider but fit adults wrapped up warm and with personal heating devices such as jackets and gloves can be comfortable in surprisingly chilly conditions (see the next post). What about the house? Central heating keeps the house dry as well as keeping us warm. How much heating do we need to keep the house safe?

As a rule of thumb - keep humidity below 70%

Oxford City Council has some excellent advice on the causes of damp and mould and how to avoid it. They recommend keeping humidity at or below 70%. (At higher levels, in cold weather, condensation is likely at cold spots on walls and windows.) Using this limit, I prepared a chart, showing the minimum internal temperature at different external temperatures. I assumed 90% humidity outside, which is common especially in winter, and I assumed some additional water vapour generated by people breathing in the house, equivalent to one person moderately active (3 x the sleeping rate) in a room 4x3m.

The minimum safe temperature keeps humidity below 70%. The green line represents the minimum ventilation level to meet building regulations: 0.5 air changes per hour (ACH). External humidity is 90%. Additional water vapour sources inside come to 2g/hour/m3 - three times the rate of a sleeping person in a room 4x3m.

With the minimum ventilation from building regulations, you need to be about 8°C warmer inside than out.

Ventilation is key and the different lines in the chart represent different rates. ACH stands for air changes per hour. The green line, shows ACH = 0.5 which is typically regarded as a minimum for human health. At this level, the minimum temperature needed to keep humidity below 70% ranges from 9°C when it is close to zero outside, up to 17°C when it is 10°C outside. The minimum safe temperature is lower when it is colder outside because cold air holds less water vapour. On average, you need 8°C warmer inside than out.

With less ventilation, you need more warmth.

With less ventilation - lower ACH - the minimum safe temperature increases rapidly because the moisture being added indoors takes longer to clear away. At higher air change rates the safe temperature decreases but even with infinite ventilation you still need enough to reduce the humidity from 90% to 70%. 

Unoccupied rooms with no additional sources of moisture can be cooler. However, there will be some exchange of air with other rooms in the house and outside. In practice the safe temperature is similar to the blue line, for ACH=1.0

With more sources of moisture, you need more warmth. 

Increasing the rate of water vapour generated also increases the minimum safe temperature. This chart varies the mass of water vapour generated instead of the ventilation rate. ACH = 0.5. The green line is the same in both charts. With double the rate of adding moisture you need to be about 12°C warmer inside than out.


Assumptions as before except that ACH=0.5 and the rate of moisture generated inside the house varies. The green line is the same as before: 2g/hr/m3 or one moderately active person in a fairly small room.

With persistent rain you need another 1°C inside to be safe.

These charts were based on 90% humidity because that is common in winter in my area. When it is raining, humidity is close to 100%. This will not do much damage unless it is persistent – but in some areas persistent rain is common. 100% humidity adds another degree or so to the minimum safe temperature.

On the other hand, shifting the limit up to 80% humidity would get you another 2°C leeway. You could go as low as 7°C when it is 0°C outside, or 15°C at 10°C outside. This might be acceptable if your house is well insulated and you keep a very close eye on potential cold spots behind furniture and around windows.


Keeping internal humidity below 70% is a good rule of thumb for keeping the house safe from condensation and mould.

This means that in the heating season you need the room temperature about 8°C warmer than outside. (This is not such an issue in summer as humidity is generally lower and we have more ventilation. Also the cold spots in the house are not so much colder as they are in winter so the risk of condensation is much less.)

This assumes:

  • You have reasonable ventilation (at least 0.5 air changes per hour). If you are relying on trickle vents on windows, you need to keep them clear. The same goes for extractors in bathrooms and kitchens.
  • You are not overcrowded because more people breathing etc generates more water vapour.
  • You avoid drying clothes on radiators. If you must do this, open the windows.
  • You do not have structural issues such as a breached damp course, water running down the outside walls or holes letting water in. If this is the case, no amount of heating will keep your house dry. 

At a typical winter temperature of 5°C the house would be OK at about 13°. That is way too cold for me. My next post discusses different ways to keep people warm in a cold house.


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