00:00 Saturday 28 February 2009  Written by Michael Kiernan

Reducing dampness and condensation in the home

In this week's article, the problem of condensation and dampness is discussed, which I alluded to in the previous article. After our prolonged period of very cold weather I have noticed an increase in enquiries on this subject. People are noticing condensation problems that weren't readily noticeable in the past, or the problem has been a lot worse this year.

The problem

All air contains water vapour. The quantity contained depends on the temperature of the air. This ratio is called the 'relative humidity'. Hot air is able to carry much more moisture than cold air, so as the temperature of air increases it is able to hold a greater volume of water.

At any temperature, when the air becomes saturated, it will deposit beads of water (condensation) on to any surface that is cold enough. The temperature of the surface at which this moisture will form is called the 'dew point'. (For an example, take a glass bottle out of the fridge.)

If this happens near the ground, to a small layer of air, dew or frost will be formed, if a larger amount of air is involved, mist or fog will arise. If this happens to air that is rising in the atmosphere and expanding, clouds will form. If it happens in the home, it's called condensation.

Research has shown that if relative humidity levels exceed 70% for prolonged periods of time there is a high probability that the condensation occurring on cold surfaces will lead to mould growth.

Conditions for condensation

In Ireland, condensation in houses is mainly a winter problem, particularly where warm moist air is generated in living areas and then penetrates to the colder parts of the building. The effects are often seen on the internal surfaces of external walls and ceilings, and also on the northern and eastern sides of the house. Condensation is generally noticeable where it forms on non-absorbent surfaces (i.e. windows or tiles) but it can form on any surface and it may not be noticed until mould growth or rotting of material occurs. For example, this mould growth can occur at the back of internal insulation or multiple layers of wallpaper on a cold external surface.

Where does the moisture come from?

The moisture in the air comes from a number of sources within the house. Water vapour is produced in relatively large quantities from normal day-to-day activities (breathing, washing, cooking, etc) - a five-person household puts about 10 litres of water into the air every day (without taking into account any heating).

Moisture can also be drawn from the structure of the building into the internal air; from below the floor or through the walls and ceilings. Problems with the structure of the building can mean that its moisture content is unnecessarily high. This can either be due to the method of the original construction or as a result of structure failures.

Older houses may not have a damp-proof course (DPC) in walls or under solid floors, to prevent soil moisture from rising up into the living areas. Buildings may also lack or have insufficient airbricks or vents to allow adequate under-floor ventilation.

The effect of moisture generation is made worse by keeping the moist air in the house - it is theoretically possible to avoid condensation by ensuring adequate ventilation. Usually in certain areas of a house, such as bathrooms and kitchens, where the warm air contains a lot of moisture, if that air then spreads to cooler parts of the house, it will condense on any colder surface.

Up until the middle/late part of the twentieth century, most houses had high natural ventilation because the level of air tightness was low. Conservation then became popular and natural ventilation was greatly reduced by the introduction of double-glazing, draught excluders, fitted carpets (which prevent air movement up through suspended wooden floors) and the removal of open fireplaces with the introduction of central heating.

Houses have become more effectively sealed, keeping any moisture produced within the house and providing better conditions for condensation to occur. Ventilation is only effective if it is provided consistently throughout the whole envelope of the house. Condensation is encouraged by poor air circulation where stagnant air pockets form (behind furniture and in cupboards) and the first evidence is often the appearance of mould growth.

Modern lifestyles mean that many houses remain unoccupied and unheated throughout the greater part of the day, allowing the fabric of the building to cool down. The moisture-producing activities are then concentrated into relatively short periods (morning and evening) when the structure is relatively cold while the building is still warming up.

How to eliminate or alleviate the problem

First of all, you need to ensure that the amount of moisture in the air is not excessive. Check the structure of the building: Check that the walls are not suffering from rising damp. Check the roof - make sure that it is sound and directing rain into the guttering, not into the structure of the building. Check the guttering and downpipes - make sure that they are carrying the water away and that there are no damaged/blocked guttering or drainpipes causing the external wall to become soaking wet. Check solid floors to ensure that damp is not coming up through them; if it is, you may need to introduce or replace a damp-proof membrane underneath it (potentially a big job) or fit a more suitable floor covering. Check that there are no leaking water pipes or tanks within the house.

Once you are happy with the structure of the building, look at your lifestyle within the building. Try not to breathe. (Only joking!) After a bath or shower, try to ventilate the room to the outside, not to the rest of the house - just opening a window (and closing the door) will help. Dry clothes out of doors or if you must dry indoors, then opt for a cool area of the premises - this latter suggestion may sound strange, it will take longer but less moisture will be held in the air at any one time. While drying clothes indoors, ventilate the room. When people come in with wet coats, hang them outside the living area to dry (perhaps in a porch).

To prevent moisture spreading around the home, some form of local extraction will be required (fans in the bathroom and kitchen), with the overall objective being to keep relative humidity levels below 70% for most of the time. Extractor fans are available with an air-moisture switch so that they operate automatically when the moisture in the air reaches a certain level. Trickle vents can also be installed in double-glazed windows, which can be opened or closed depending on the conditions.

Other units (more expensive and complicated) are available which remove the moist air but reuse the thermal energy that would otherwise be wasted (MHRV). Increase the insulation standard and background heating levels throughout the house, especially in rooms not normally occupied.

Heating, insulation, heat loss, air tightness, ventilation and condensation are all inter-related and should all be considered whenever major renovations are undertaken on a house. Seeking the advice of an experienced energy adviser before the work begins will be well worth it in the long term.

Michael Kiernan is a qualified engineer with over 20 years' experience. He is the managing director of Carhoo Technical Services based in Clonakilty. He specialises in the provision of audit and survey services of domestic houses for energy and heat loss in both new and existing housing. He is fully insured and is registered with Sustainable Energy Ireland for the provision of Building Energy Rating certificates. If you have any queries in relation to this article or any other energy matter, please contact Michael on tel: 023-8821690, email: info@rebelenergy.ie or website: www.rebelenergy.ie.

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