Ventilation in the loft and new buildings

Ventilation in the loft and new buildings

Ventilation is the answer. This can be done by opening windows, installing extractor fans, venting exhaust air from clothes driers to the outside air and the use of balanced flue gas heaters. Water vapour moves about. It doesn’t only condense on cold surfaces in the room where it is produced; it can penetrate all parts of the home, and is likely to condense in any colder area it reaches. It also rises by convection to cooler bedrooms and the space under the roof. Warm, moist air gets into the roof space through ceiling cracks, holes used by pipes and electric wiring and gaps around the trap door.

It doesn’t matter how small the gap – it can still get through as it’s a gas. It also passes through porous plaster or plasterboard ceilings unless they incorporate a moisture barrier. Unless there is sufficient ventilation for it to escape to the outside air, it will condense on the roof covering and roof timbers. The severity of the condensation depends on the roof construction, how well the loft is insulated and ventilated, and how easily moist house air can get into it. However, it can very quickly build up in a poorly ventilated loft, saturating the insulation and making it quite useless. In the end it can soak through the ceiling too.

Loft insulation certainly makes a house warmer, but means that the roof structure will be colder. This exaggerated difference in temperature enables the water vapour to pass more easily from the house itself to the roof space. Tiles on loosely laid felt will ‘breathe’ and allow the moisture to disperse, but fully-lined roofs tend to trap moist air. Even worse are flat roofs having a lead, bituminous felt or asphalt covering; these cannot breathe at all.

Vapour inside the walls

The better draught-proofed, and more airtight a house, the more likely it is that moist air will force its way into the structure during the winter, possibly leading to condensation. While ‘superficial’ condensation is a nuisance and can spoil decoration, it is visible, and serves as a warning to the householder to provide better ventilation. But interstitial condensation can cause serious and lasting damage to a building and, unless it is so severe that damp shows through on a ceiling or outside wall, it can go unnoticed for many years.

In older draughty houses, risk of ‘interstitial’ condensation is slight, though superficial condensation will sometimes occur in unheated rooms. Risks increase when fireplaces are blocked up, windows double-glazed, external doors draught-proofed, and lofts and external walls better insulated. Builders of new, well insulated air-tight houses should guard against moist air getting into walls and the loft by using air barriers, called vapour checks, and by ensuring that any air leaking through is easily vented to the outside.

Loft insulation

Loft insulation is yet another way of retaining heat inside the main part of the house and also contains the risk of condensation, but it must be coupled with the provision of proper loft ventilation, or condensation may become a problem in the loft.

Loft ventilation

This is vital to protect the timbers in your roof from rot attack. The better the loft insulation the greater the temperature contrast between the loft and rooms below, and the greater the risk of condensation.

Central heating

Usually installed as the source of heat, central heating will also reduce the risk of condensation as the water vapour is not allowed to cool and the temperature differential between different parts of the house is reduced. But effective central heating is rather expensive to run for long periods these days.

Double glazing

This is a highly effective way of retaining heat as 20% is lost through windows which are single glazed. It also ensures that the inner pane of glass is not cold, which is usually the case with single glazing, and thus eliminates a major source of condensation.

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