23 Dec 2014

Condensation: Prevention is better than cure

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Condensation in the home is not just a nuisance; it can lead to significant structural problems, costly redecoration and can even affect the health of the occupants if left to develop into mould.

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Picture if you will an ice cold drink on a hot summers day. The glass is beaded in droplets of water, just waiting for you to pick it up and quench your thirst. Lovely isn’t it? Not so lovely when the same process – condensation – forms in your home. Even worse if this is left to fester, eventually leading to unsightly and unhealthy patches of mould.

There are essentially two types of condensation in the home. Condensation is the result of humidity levels in the atmosphere reaching saturation point; often referred to as dew-point. When this type of atmosphere comes into contact with a cold surface, water vapour condenses onto the surface. The colder the surface, the quicker the condensation forms.

Surface condensation will be familiar to everyone as it occurs in every home. It’s the result of breathing, cooking, heating, washing and drying clothes – all normal day-to-day activities. A house with five occupants can generate in excess of 10kg of water through condensation daily. This type of condensation forms on non-porous surfaces such as window panes and can make the household atmosphere dank and musty if not ventilated adequately. It can also lead to staining, becoming both inconvenient and costly.

The second type of condensation within the home is interstitial condensation, which is most often found in new build properties. Interstitial condensation forms within the very fabric of the building as a result of the significant amount of moisture released through excavation and building works. When a building is completed, moisture can remain trapped in the building, leading to condensation. If not dealt with, it can result in serious structural damage – such as timber decay – and can have a detrimental effect on insulation.

New build properties are built to stringent environmental guides such as the Code for Sustainable Homes and of course, Building Regulations. This has inevitably led to ‘sealing’ the building to prevent heat loss, which in turn results in dramatically reduced ventilation opportunities. As a result, such homes require a mechanical form of ventilation, often used in conjunction with a heat recovery system which preheats fresh air drawn into the building with the internal waste stale air using a heat exchanger. It’s an effective and sustainable solution.

You would think that with such measures in place, issues of condensation would be resolved. Unfortunately, this is not always the case.

Where the ducting used in the ventilation system passes from a warm room through to unheated areas and voids, such as loft spaces – where heat recovery units are usually located – condensation is likely to form on the inside of the duct. This can then run down the duct and create a potential hazard, such as dripping onto the electrical connections of a fan, as well as creating stains and mould over a period of time.

Finding a solution

One solution is to use a condensation trap that collects the moisture and releases it through an overflow port externally to address this issue. Quick and simple to install, the traps are installed in the loft space as low as possible on vertical ducts but high enough to allow for a slight fall in the overflow pipe.

Alternatively, you could aim to prevent the formation of condensation in the first place. To accomplish this, the inside wall temperature of the duct must not fall below the dew-point of the humid air entering the duct.

The first step to achieving this is to only use quality rigid ducting with a smooth internal surface area, rather than the cheaper flexible hose. This is because air itself has very good insulating properties when flowing smoothly over a surface. When the reverse happens, ripples are created and insulation is reduced. Furthermore, ensure the ducting has a uniform profile and the joints fit tightly.

Secondly, use insulation around the ducting in those colder areas that are likely to be affected. Various insulating materials are available for such purposes, such as mineral wool fitted with integral vapour barrier.

Building Regulations 2010, Document F recognises the problem of condensation and states that ducting passing through these unheated areas and voids must be insulated. The key problem faced here is that ducting insulation specification is set at a minimum standard of ≤0.04W/(m.K), with a 25mm thickness, but domestic duct insulation products on the market are generally unable to meet this requirement.

Insulation is measured on the thermal conductivity (K-value) of the material, its thickness and subsequent thermal resistance (R-value). K-values are quoted in Watts per metre per degree Kelvin (W/mK); R-values in units of degrees Kelvin per Watt (K/W).

An easy way to compare two different types of insulating material is to derive their R-values simply by dividing the thickness in metres by the K-value. Using the Building Regulations as an example, 0.025/0.04 = 0.625K/W.

Of course, whatever form of insulation you opt for, there are other considerations to be taken into account. Firstly, if an insulation system is specifically designed for use with ducting, ensure there is a full range to insulate both round and rectangular duct systems and it is available in suitable lengths. Secondly, make sure the insulation fits snugly around the duct and fittings with no air gaps. Thirdly, check how easy it is to fit and secure once on.

Condensation is a part of everyday life in this country, particularly so in the winter months, but with good ventilation practices and mechanical ventilation systems, condensation can be prevented from being a problem in your self-build.

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