Stop the rot from condensation

Klober outline ventilation needs throughout the house, with pointers to builders in the face of ever changing regulations affecting ventilation, and how to avoid condensation.

The sight of water running down windows, forming pools on sills is still the image we most commonly associate with condensation.

But though modern ventilation and insulation standards have pretty much eradicated this problem, we create more moisture in our homes today than ever before - 15 litres of water vapour each day isn't unusual! As creatures of habit we also make matters worse by concentrating activities into relatively brief periods at particular times of the day, only then to leave our homes unoccupied or unheated for hours on end.

The key question for any builder is "Am I guilty of underestimating the damage condensation can cause?" Air inside a heated building usually contains more moisture than it does outdoors.

Due to its higher pressure, warm air is forced through the structure, taking moisture with it.

Most building materials have a degree of permeability so can't totally obstruct movement of moist air.

As it cools, the vapour reaches its "dew point", causing water droplets to form within the fabric of the building.

This is what is meant by the term "interstitial condensation", which presents a far greater hazard than surface condensation.

In today's well insulated homes, natural roof space ventilation has been all but eliminated while, because of thicker insulation, lofts, batten cavities and other spaces above it are colder than ever.

This provides the ideal environment for condensation and though corrosion of materials such as metal roof truss nail plates, resulting from surface condensation may be readily identifiable, high moisture content within the structure can go undetected for long periods, even beyond the point where structural damage has occurred.

So what steps and precautions can be taken to make life easier with Building Inspectors? There are a few elements of a building where early advice from a specialist manufacturer will pay dividends, both in time and money.

Ventilation is one.

When it comes to the chosen ventilation method, many house designs today, particularly among self-builds, differ greatly from typical housebuilder stock.

Building Regulations and BS 5250 (Control of Condensation in Buildings) were simply not designed to cover such structures, which invariably have unique ventilation needs.

As a ventilation specialist, Klober might be expected to take such a line, but advice is free and whichever ventilation method is recommended, the solution normally straightforward and inexpensive.

A good example of an "untypical" building is one which is "steep or wide", with a roof slope greater than 35andordm; or width greater than 10 metres.

In addition to eaves vents, increased ventilation must be provided by adding high level openings to avoid stagnant air pockets.

A design may be complicated further by changes in the roof slope, a common point for ventilation restrictions to occur.

Care must be taken to ensure the minimum free air space is provided throughout.

None of this will do much good if simple safeguards aren't taken.

Moisture extraction, for example when building or resiting bathrooms (and wet rooms in particular), is all too often assessed wrongly when costing a project, by the client if not by the builder! Prevention of vent blockage by debris and paint or access by insects, birds rain or snow, is also a key precaution to take.

A simple 4mm mesh will provide a balance between blockage prevention and maintaining air flow.

We mentioned one of the better known products designed to provide guaranteed air flow into the roof space - the eaves vent.

This simple product maintains a clear air path from the gutter over insulation, preventing birds and large insects from gaining access.

Klober sells an economic solution to eaves ventilation through a "3 in 1 vent pack", including fascia vent, felt support tray and roll out rafter tray.

From within the loft, lightweight loft vent trays can be used in new build or refurbishment situations to provide a consistent ventilation area between rafters at the eaves.

Self-adjusting to the pitch of the roof, they are simply held in place by the loft insulation and fitted without disturbing the roof covering.

By contrast rafter trays, which are nailed or stapled to rafters, ensure correct positioning of insulation, providing a clear ventilation channel above it.

For all builders, products designed for universal use can only make life easier, given greater likelihood of frequent fixing opportunities, cost, and local availability.

Vents which are also unobtrusive takes away any lingering objection from clients to the unsightly nature of old style profiles.

Products such as Klober's newly launched Uni-line Vent, usable with most common interlocking concrete tiles, is a good example.

A word about use of vapour permeable underlays.

Products such as Klober's Permo Forte are now in common use.

Suitable for both cold and warm roof installations, most are fully waterproof in their own right, while allowing the structure to "breathe".

It very much pays to check the water vapour permeability and resistance rates in particular when choosing these membranes.

This information may be an element of a British Board of Agrandeacute;ment certificate, but as over 30 products have one today, compared to just a handful five years ago, you will soon see that some are very limited in their scope.

Whatever the choice, vpm's tend to be used to eliminate roof space condensation without additional ventilation.

As you read this, changes to BS5250 are being considered, and it remains to be seen whether this practice will be confirmed as an acceptable one.

To ensure condensation doesn't cause deterioration of the fabric to the point of structural damage, a combined assessment of building use and design will quickly determine whether any particular ventilation precautions should be taken.

The golden rule is "if in doubt then check".

Water vapour can penetrate timber floors and pass freely up cavities of brick walls into the roof space, so caution is rarely misplaced.

And with condensation having such an adverse effect on the performance of roof and wall insulation too, such a check could prove doubly beneficial.

Back to top