Ventilation combats damaging humidity in houses free RSS news feed from the Electrical News Portal
(12/03/2007)

Everyone of us exhales 10 to 75litres of Co2 per hour and up to 175 grams of water per hour. A family of four produces 10-15 litres of water vapour a day through breathing washing, showering and cooking.

The ability of air to absorb this vapour is dependent on temperature: the warmer the air, the more it can absorb. Extract ventilation can remove water vapour at source. And insulation eliminates cold surfaces on which condensation can form resulting in mould growth.

Traditionally homes were built with less regard to air tightness and had a high level of background air leakage. Whilst expensive to heat, high humidity was to a great extent diluted by drafts. Over the last three decades properties have tended to have dimensionally smaller rooms and air leakages have been reduced dramatically. The result is that the same quantity of moisture is being generated by the occupants in a more confined space and with nowhere to go.

Government initiatives to reduce carbon emissions and tackle fuel poverty in social housing have improved properties through amongst other measures insulation, double glazing and low energy appliances. With the lowest income decile of the population spending over 10% of their income on fuel, an estimated 8 million households still cannot afford to keep their homes warm.

In tight buildings, a ventilation strategy is a requisite. Bathrooms and kitchens are warm or hot when in use and can hold water vapour but this quickly migrates to other colder rooms such as bedrooms and will form condensation on cold unventilated surfaces such as the back of wardrobes, corners of ceilings with poor loft insulation or windows with cold bridging. These symptoms occur where relative humidity regularly exceeds 65%.

Worse still, warm conditions and an RH of over 60% are the ideal breeding conditions for the house dust mite found in soft furnishings and bedding. Around 80% of asthmatic conditions in the home are said to be triggered by the faeces of the dust mite.

The simplest and most commonly adopted ventilation strategy is to install extract ventilation units as close to the source of the moisture generation in the ‘wet rooms’ of the house- in bathroom / shower rooms, the kitchen and utility area.

A kitchen extract fan such as an Xpelair GX6 can be installed in a wall or window. If a kitchen cooker hood is installed it must be connected via a duct to the outside. Recirculating hoods take out smells but not the humidity. The simplest axial bathroom fans are installed through a window or wall. In high-rise or exposed sites centrifugal fans such as Premier will overcome wind resistance.

Combination fans and low energy lamps are also available which can be installed directly over a shower and are ducted to a soffit vent. In all cases fans are only effective if they are On when needed, so built in automatic humidistat operation negates the need for user intervention. Rooms without windows require the fan to be fitted with an overrun timer. In this situation the fan switches On with the room light and continues to run for a preset period after the light is switched Off.

Air replacement can be from manually operated slot vents fitted to the headers of widows in bedrooms and the living room. Alternatively thermostatically operated Autoflow passive air input units can be simply retrofitted to these rooms. Important, but often overlooked, is the need for replacement air to enter toilet, bathroom and kitchen areas when the door is shut; a 10mm gap between the bottom of the door and the floor covering usually suffices.

On major refurbishments two other alternative solutions can be considered: central Mechanical Extract Ventilation (MEV) or wholehouse Mechanical Ventilation with Heat Recovery (MVHR).

Central Extract ventilation uses a multispigot single extract unit with flat or circular ducts drawing air continuously from wet rooms. Because the unit is running continuously much lower extract volumes can be used to effectively combat condensation. MEV fans such as Xvent are available with long life, low energy DC motors minimising energy use. Again air replacement comes from passive ventilators in the living spaces.

Whole house Mechanical Ventilation with Heat Recovery (MVHR) can be considered for properties built or refurbished to current Building Regulations Part F and L standards where air permeability can be down to 3 to 4 m3/h at 50pa pressure difference.

This system extracts warm moist air via ducts to the wet rooms and passes this air through a heat recovery cell before it is discharged outside. At the same time fresh air from outside is drawn through the cell where it is warmed and distributed to the living and bedrooms. Systems such as the low profile Xcell 150 feature heat recovery cells with efficiencies up to 92%.

MVHR systems are compact enough to fit into a kitchen wall cupboard. And require no additional passive air replacement. MVHR is the most expensive solution but provides the most rapid reduction in humidity and puts the least demand on expensive heat.

Dealing with air quality and humidity levels in buildings is not confined to social housing but the worst effects of damaging condensation are often most acute in this sector. Whichever strategy is being considered it is always best to seek expert advice. Xpelair offers a free CAD advice service to Local Authority and Housing Associations to provide cost effective solutions based on specific requirements.

[View all articles about Xpelair]

Related categories:  Energy efficiency   Heating, ventilation, refrigeration   Residential 

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