How to Save Energy 2 - Many people believe that living in a draughty house is good, since the fresh air prevents problems with damp and mould. But too much air coming into our homes can also mean a loss of heat and higher energy bills.  In his second article about reducing our energy use, local architect Rob Rickey explains the importance of reducing draughts while adequately ventilating your house. He also provides some useful tips on how to do this.


When outside air or draughts leak into a building we call it infiltration.  It may be through gaps in building fabric, poorly fitted windows, floor boards, recessed ceiling lights or joints between main building elements. It is generally driven by wind pressure, so when there is no wind there is little infiltration of air.

Such gaps and cracks can account for up to 50 % of all heat losses through the external envelope of a building, so it should be of interest to everyone.

Put simply, if you turned on your gas boiler on a windy night in winter and heated the volume of air in the house entirely to 20 C, then turned the gas boiler off again:

● the Victorian home would be cold again within 90 minutes

● the failing new build home would be cold again within two hours

● the average new build home would be cold again within four hours

● the Passivhaus home would be cold again within 17 hours


Building regulations in the UK require controlled ventilation in different rooms.

This is provided by extractor fans in wet rooms (kitchen, bathrooms, laundry rooms and W.C.s). In a modern house, there are design criteria in the building regs for each of these rooms. If air is forced out of a house, there must be a way for air to come in to replace it, which is usually provided by trickle vents built into windows in modern houses.

There are two reasons for requiring a minimum level of ventilation; indoor air quality and moisture control.

Indoor air quality

People spend a substantial fraction of their lives indoors (often 80-90%) and so these locations can represent a significant fraction of exposure to air pollution. Indoor air pollutants (including nitrogen dioxide, damp, mould, particulate matter, and VOCs) can trigger or exacerbate asthma, other respiratory conditions or cardiovascular conditions.

Most enclosed spaces have a wide range of indoor emissions including from buildings materials, furnishings, gas and solid fuel cookers, boilers and stoves, solvent-containing products, cigarette smoking, cleaning and personal care products.


There has been a lot in the news recently about mould growing in poorly built and maintained housing. Mould is evidence of two factors: relative humidity and surface temperature.

Relative humidity is how much water is in the air compared to the temperature of the air. It is written as a percentage; when the relative humidity is, say, 50%, that means the air is holding half the amount of water that it could hold. Warm air holds more moisture than cool air, which you may have experienced stepping off an airplane (cool, dry air) onto the tarmac in a tropical country (warm, humid air).

You can find out the relative humidity in your house with an inexpensive digital thermometer with a humidity readout. 


When warm, moist air passes over cool surfaces (like your bathroom window) and the air cools down, the water vapor in the air begins to condense, or form into small drops. The temperature at which this condensation begins to happen is known as the dew point.


Where does this moisture come from? Us!

As the chart below shows, a family of four with two teenagers will release about 280 full buckets of water into the air in a year. It is important to get this moisture out of the house before it condenses on surfaces. Note that all these activities involve warm water.


From Moisture In New Homes - a Guide for Occupants

Air tightness

Once poorly understood by the mainstream building industry, air tightness , (the amount of air leaking or infiltrating), is now increasingly seen as one of the most crucial objectives on any building project.

Not only is it vital for energy efficiency, it is also key for thermal comfort and for protecting a building’s structure from dampness and mould. Not only can incoming moist air condense in a house, outgoing warm moist air passing through cracks and gaps can condense inside wall construction, leading to more mould or dry rot.

What can I do about it?

Reduce infiltration

Chimney balloon - Chimneys act like giant hoovers, sucking air out of your home. This causes cold air to be pulled in from other gaps, causing the cold draughts we notice. A chimney balloon effectively blocks the chimney, stopping warm air from escaping and stopping cold air from getting in.

Install weather stripping - This applies to windows and doors. When the wind is blowing you can easily detect gaps. The best product depends on the type of opening, so consult your builders’ merchant.

Caulking - Use this to fill cracks not associated with the moving parts of doors and windows.

Add window insulation film - these are provided as kits that include the film and double-sided tape to fit to the window frame. The film is tightened by using a hair dryer to shrink it. If you are using the windows for ventilation, you need to consider how to replace the lost ventilation.


Add insulated curtains or roller shades - Heavy curtains that cover the window, or well-fitted roller shades will help. Curtains that are made with reflective foil as an

Close blinds and curtains that you don’t need for view or daylight when the outside temperature drops below 15° C. This is the so-called balance point temperature where internal heat gains from people, appliances, etc. offset the envelope heat loss to the atmosphere in a well-insulated house.

It is at the balance point temperatures where no indoor heating will be required to maintain the temperature of the home at the thermostat set point. See if you can determine your balance point.

Increase controlled ventilation.

For most existing houses, controlled ventilation consists of individual extractor fans in bathrooms and kitchens. If these are correctly specified and operated they are effective at removing moisture from the rooms, and to a lesser degree providing fresh air to other rooms that have trickle vents or open windows.

They are often noisy and not operated with a long-enough override to really clear the air. There are other options, including MVHR (mechanical ventilation with heat recovery), PIV (positive input ventilation) and MEV (mechanical extract ventilation) which I will cover in the next article.

Rob Rickey MArch Msc is a local architect and energy expert and a member of the Crediton u3a Energy Action Group.  

If you have any queries or feedback about this article, please email us.

May 4, 2023

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