Eliminating window condensation

The concept of Design Dew Point could be the answer.
Friday, July 29, 2016
by Pierre-Michel Busque

Every year, our company receives calls from clients complaining about window condensation. The diagnosis procedure involves reviewing the site and documenting the severity of the problem. The adequacy of the ventilation system is reviewed and the occupants are given a report containing the obligatory lecture on limiting interior moisture loads.

Things get more interesting when the condensation problems are persistent and severe. For example, one client photographed ice forming on the interior side of the window frame during a Vancouver winter; this qualifies as a severe problem.

If the home is covered under warranty insurance, our report is usually sent to the warranty company who will review the site and issue a letter denying the claim citing “destructive behaviours” such as cooking in the dwelling or installing curtains. Frequently, their report will be accompanied by a letter from the window manufacturer stating that their products were not designed to be subjected to such abuses. Who knew that such behaviours, as installing curtains and cooking, could have devastating results on the building envelope? Invariably, the owner is blamed for making the interior environment too humid.

A monitoring program is usually necessary to determine if the condensation is indeed due to occupant behaviour. So far, roughly half of the condensation problems can be explained by occupant behaviours such as keeping the interior temperature too low, sleeping in a small unvented room with the door closed, growing a variety of “medicinal” plants in the dwelling and drying laundry in the interior space. In other cases, the design and construction of the dwelling is to blame. Typical causes of condensation encountered include, leakage of the building increasing the interior humidity levels of the dwelling, lack of ventilation, burying the window frame in interior finishes thus preventing warm air from heating up the frames and installing the windows so that the warm side of the frame rests on a cold surface. When these types of problem occur, repairs are usually disruptive and expensive.

So, in short, condensation problems on windows can be due to occupant behaviour or to improper design and construction. When the condensation problems are due to occupant behaviour, the remedial action is usually simple. When the condensation problems are due to a construction problem, the repair is usually disruptive and expensive.

Condensation on windows can lead to mould growth or to structural problems. It is therefore desirable to solve these problems.

So why are these problems still occurring in new buildings? The usual suspects are to blame: ignorance, carelessness, negligence and greed. But there is hope on the horizon. Regulations have been changing rapidly to reduce the energy demand of our buildings. These same regulations will, in time be the key to eliminating condensation problems on windows because the performance of the window installation will have to be simulated by computer, thus exposing design weaknesses.

For the change to occur rapidly, regulations will have to be streamlined and clear so that they can be easily absorbed by the construction industry. Sadly, regulations are getting bulkier and less easy to understand and change is needed.

The concept of Design Dew Point might be the answer to window condensation. The design dew point would be the minimum allowable temperature of any surface inside the dwelling. This temperature would be selected by calculating the dew point of the maximum allowable interior temperature and relative humidity.

According to the HomeOwner Protection Office titled Maintenance Matters #3 -Avoiding Condensation Problems, “As a rough “rule of thumb”, interior air temperatures should generally be maintained between 18°C and 24°C with relative humidity falling between 35% and 60% for the coastal temperate climate regions of British Columbia during the winter months.”

The interior of a dwelling can be maintained at 24°C and 60 per cent relative humidity and have normal interior conditions. The dew point of air at 24°C and 60 per cent relative humidity is 15.8°C. This temperature would be the design dew point.

Codes would be modified to incorporate the concept of a minimal surface temperature to avoid condensation. Two years ago, after speaking at a conference in Florida on the topic, a researcher from a university in Denmark said that the Danish building code had implemented such a notion several years ago. Apparently, the Danish people have learned to keep things simple, and effective.

Simulating the performance of windows as installed in the building when subject to the 2.5 per cent January design temperature would allow us to eliminate most of the occurrences of condensation on windows and will allow us to meet responsible energy utilization targets. It is an easy step to take, that should be incorporated into the design process.

Pierre-Michel Busque, P.Eng., is president of Busque Engineering.

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