Ventilation and Heat Loss

Table of Contents


A typical house may lose up to 15 per cent of its heat through draughts. The warm air inside the house is replaced by cold air from outside coming through the gaps that inevitably exist in any building around doors and windows. The incoming cold air must then be heated if the temperature of the house is not to drop. The more cold air that comes in the more energy must be used to heat it, so clearly it is worth trying to reduce draughts. Draughts are a form of accidental ventilation, so the heat loss they cause is called ventilation heat loss. A completely sealed building would have no ventilation heat loss but after a while the occupants would die through lack of oxygen!

Ventilation rates, whether intentional or accidental, are measured in terms of ‘air changes per hour’ (ac/hr). If a house is said to have a ventilation rate of 1 air change per hour, it means that in one hour all the air in the house is exchanged for air from outside. It is not easy to measure air change rates without a lot of very complicated equipment, and even if you can measure the rate it won’t be much use, as ventilation varies with wind speed, degree of exposure, type of house, standard of construction and other variable factors. The BRE have measured ventilation rates in apparently identical houses on a new estate and have found that one house could have twice the rate of another, even with all the windows and doors shut. As a very rough guide, a Victorian house might have a ventilation rate of 2ac/hr or more, a modern house a rate of lac/hr, and a well draught sealed house a rate of 0.5ac/hr.

In spite of the belief left over from Victorian days that fresh air is a Good Thing, a well sealed building will be much easier to keep warm, and provided the windows can be opened if necessary, the building regulations will not be contravened. In a house with good structural insulation the loss of heat due to ventilation may well be as much as that lost through the structure, and this is as good a reason as any for doing all you can to reduce draughts.

You are very unlikely to be able to seal a house so well that there would not be enough air to breathe; you need only 6.8m3 per person per hour of fresh air, which in a house with a volume of 410m3 would be about 0.016ac/hr. A large closed wood burning stove with a fuel consumption of lkg of wood per hour needs 20m3 of air per hour, so even five people plus a big stove would need only 0.13ac/hr. You are unlikely to be able to achieve a lower ventilation rate than 0.5ac/hr, so there is little need to worry unduly about being unable to breathe, and if you have a party you can always open a window if people start gasping.

However, you must not reduce the ventilation rate by draught sealing any room which has a heating appliance with no flue, such as a gas geyser, paraffin stove or calor gas heater. You run the risk of being suffocated if you use one of these, particularly the gas types, in a room with insufficient oxygen. Open fires, although they have flues, are so inefficient that they need a lot of air to work successfully, and may not burn well in a building with too low a ventilation rate.

Ventilation can be reduced first and most simply by keeping doors and windows shut; even a window that is just ajar can let in a lot of cold air. The second simple control is to draught seal all doors and windows, for the gap round an average door is equivalent to having a whole brick missing from your wall. Finally, draughts can be controlled by porches, which act as air locks.

The formula

The formula used to calculate the heat loss through ventilation is Qv=0.36 x V x N.

  • Qv is the ventilation heat loss in W/degC.
  • 0.36 is derived from the ‘volumetric specific heat of air, which is the heat required to raise the temperature of one cubic metre of air by 1 deg C. The specific heat of air is 1300 Joules per cubic metre degree Celsius (J/m3 degC); but if this figure were used in the equation the result would be in Joules per hour. To obtain an answer in Watts, which are Joules per second, 1300 is divided by 3600, the number of seconds in an hour, to give M3=0.36.
  • V is the volume in cubic metres of the space being heated.
  • N is the number of air changes per hour. This will have to be guessed, but as stated before, a very rough guide would be to take a house that feels cold and draughty as having a rate of 5ac/hr, a Victorian house as having a rate of 2ac/hr, a more modern house a rate of 1.5 or lac/hr, and a well draught sealed house a rate of 0.5ac/hr.


As an example consider a bungalow, recently built, with an internal floor area of 81m2 and a floor to ceiling height of 2.3 metres. The volume is 81 x2.3=186.3m3 (the roof space is not considered part of the volume, as you are not trying to heat it). As the bungalow was recently built, and assuming that it does not feel too draughty, N could be taken as lac/hr. Putting these values into the ventilation equation gives the following:

  • Q=0.36 x V x N
  • Q,=-0.36 x186.3 x1
  • Q=approximately 67W/degC

Thus for every degree of temperature difference between inside and outside it would take about 67W to replace the heat loss caused by ventilation. If it were freezing outside and you wanted the house to be at 20°C you would need more than one kilowatt in order to replace the heat being lost to cold draughts (67 x20= 1340W).

Condensation and smells

It can be argued that reducing ventilation will lead to condensation and stale air. Provided that draught sealing is combined with insulation which incorporates the correct vapour barriers, the reduction in ventilation should not cause condensation. If condensation does occur it can be dealt with at source, usually in the kitchen or bathroom, by installing an extract fan controlled by a dew stat. This is a device which turns the fan on only if condensation is likely to form, thus avoiding unnecessary ventilation. An easier solution is to open a window a little bit if you find the condensation a nuisance, but remember to shut it again as soon as the condensation has cleared. Smells can be dealt with in the same way.

Finally, it is not worth doing all this draught sealing to cut down ventilation loss if you leave windows or doors open. As your awareness of the ways a building loses heat increases you may find that you develop a greater ‘energy consciousness’ and that habits like shutting doors and turning off lights become automatic.

Selecting a heating system

Once you know the structural heat loss and the ventilation heat loss the two figures can be added together to give the total heat loss from your house. As an example, consider a house which has a structural heat loss rate of 225W/degC and a ventilation heat loss rate of 75W/deg C. By adding together the two figures you reach a total heat loss rate of 300W/deg C. This figure can then be used to work out the size of the heating system and the amount of energy that will be consumed in a season’s heating.

Your heating system, be it gas, solar or wood powered, must be able to supply enough energy to keep the building warm in the worst possible conditions: it is no good trying to heat a room with a lkW fire if the room loses heat at a rate of 2kW in very cold weather. The energy which the heating system must be capable of supplying is therefore based on the maximum likely difference between inside and outside temperatures. In southern England the value used is often 20°C or 21°C, but in the north of Scotland or other areas known to be cold it might be 25°C or more. The use of too high a value for the temperature difference would result in a heating system that is too big for the job it has to do; it would work at under capacity and inefficiently almost all the time.

Taking the example of a house with a total heat loss of 300W/deg C, and assuming a temperature difference of 20°C, the chosen heating system must be able to supply 300 x 20=6000W or 6kW.

A 6kW heater does not supply heat at a rate of 6kW all the winter; if it did you would be too hot most of the time, and the fuel bills would be enormous. A properly designed heating system supplies enough energy to maintain the temperature of your house at the desired level; if it is warm outside the system will provide little energy, if it is cold it will provide a lot.

Degree days

To estimate the energy you might use over a whole heating season (usually assumed to be from 1 October to 30 April), you need to measure the way the outside temperature changes from day to day because it is this variation that determines the energy required. The daily change in temperature can be allowed for by the use of ‘degree days’ which give a measure of the variations between inside and outside temperatures.

The degree days used by the Department of Energy assume a base temperature of 15.5°C: if the average outside temperature over 24 hours is 14.5°C (I.e., one degree less than 15.5°C), that is one degree day. The figures given by the Department of Energy are actually based on a constant indoor temperature of 18.3°C, but the figure of 15.5°C used to calculate the number of degree days allows for the heat gains to the building from the sun, electric lights, cookers and of course people.

These so called ‘casual gains’ are not insignificant, particularly in a well insulated house. As an example, a 100W light bulb gives out 100W of heat, an adult male is worth 114W if seated and at rest, 144W if eating, and 264W if dancing. The figure of 18.3°C may seem slightly low as an average temperature for a house but the Department says it is ‘considered comfortable for normal domestic purposes’. In fact, most houses are not heated to this level at night but might be kept warmer in daytime, so the temperatures average out.

In the UK, degree day figures are calculated by the Meteorological Office, and if you want to know how they do it, all is explained in a free Department of Energy booklet called Fuel efficiency booklet 7, Degree Days.

Leave a Comment