Condensation & relative humidity-ventilation

Condensation of moisture on windows is a common occurrence in most houses in winter. However, it is a source of annoyance and if corrective measures are not taken at an early stage, serious damage from staining, rotting and mold can result. While the problem is more acute during the first winter when the house is "drying out" (many of the materials in construction contain moisture that must be dissipated) normal living habits are additional and continuing contributors to high Relative Humidities (R.H.) in many instances.

The problem is an old one and applies particularly to today's homes as they are tighter and better insulated than ever before due to new and improved building practices, increased energy costs and comfort levels commonly required by today's new home buyers. To quote from a National Research Council publication of 1963,"Humidities should be controlled so that little or no condensation appears on the inside surface of the glass (windows)". With double glazing this still permits high R.H. except during the most severe weather as indicated in the following which shows the maximum R.H. that can be tolerated if condensation is to be avoided in cold weather.

"The householder need not measure the R.H. directly; he can simply use the windows as a guide to the proper R.H. within the house (humidity indicators are readily available at hardware outlets and should be of good quality to assure an accurate reading). As soon as objectionable condensation occurs on inside window surfaces, steps should be taken to reduce the R.H. by controlling the moisture sources or by increasing ventilation.

There is no conclusive evidence that either the health or the comfort of most people will be adversely affected if R.H. is kept at a level that will prevent excessive condensation on the interior surfaces of double windows.

The homeowner frequently assumes that window condensation is a fault of construction. It is not
readily appreciated that living habits are of prime importance, nor that a well built house is often more vulnerable to excess moisture problems than one that is loosely constructed."

The NRC publication also demonstrates that under normal living conditions approximately 15 to 20 lbs. (6.804 to 9.072 kg) or 1-1/2 to 2 gallons (6.819 to 9.092 litres) of moisture per day may be introduced into a house with four occupants and that this can rise to as much as 40 or 50 lbs. (18.144 to 22.680 kg) or 4 or 5 gallons (18.184 to 22.730 litres) per day on washdays.

Ventilation is often the only effective means available to the householder for removing moisture dehumidifiers are not a practical solution except for limited areas. Exhaust fans in the kitchen and bathroorn are useful for drawing off rnoisture from cooking and bathing before the vapour can circulate through the house - these fans should ventilate to the outside and not into the attic space.

Windows are commonly relied on for general ventilation and whenever possible the windows nearest the source of moisture should be opened. Fireplaces and chimneys are useful rneans of ventilation. With a warm air heating system, it is practical to consider a dampened air duct from outside connected to the return air part of the system (fresh air intake).