Renovation & Design

A/C essential for mould prevention


Check your air conditioning unit to make sure it’s operating properly.

Question: I have my air conditioning running, with the temperature set fairly low on my thermostat, but it does not seem to cool the house properly. The air feels cooler than outside, but it still feels sort of humid inside the house.

Is there anything I can do to make it work better? Is there something wrong with my air conditioner?

Answer: I have posed this hypothetical question, because I feel this is one topic that should be discussed at this time of year. Properly functioning central air conditioning will not only make your home’s inhabitants comfortable, it will prevent issues related to high relative humidity due to summer weather.

Many homeowners don’t realize that central air conditioning (A/C) provides significant dehumidification of the indoor air in a home, while cooling it. Because of this property, a properly functioning A/C unit is important in preventing moisture related issues in your home in hot, humid summer weather. High indoor relative humidity (RH), combined with high temperature, is an ideal environment for mould growth. Because some types of mould have been linked to serious illness in susceptible individuals, maintaining a lower-humidity indoor environment can help prevent certain health issues.

While there are always mould spores in our natural environment, they may remain dormant until they are sufficiently hydrated. The moisture for this hydration may come from leaky plumbing fixtures, basement seepage, or other water spillage within the home, but can also be provided by condensation. When the moisture within the air cools and reaches its dew point, it will condense. In the outside environment, this normally occurs high in the atmosphere and the result is precipitation. Inside our living spaces, condensation may occur when warm air contacts a cold surface like uninsulated foundation walls, window glass, metal pipes and ducts, or other cool surfaces. If the surrounding air is relatively dry and cool, with low RH, this condensation may evaporate before it causes any issues. If the air surrounding that area is warm and moist, with high RH, or if air movement is restricted due to storage or other impediments, the condensation may not evaporate. Any surface that remains wet for a significant period of time may lead to moisture issues.

Many building materials will become damaged if they stay wet for a period of time, without the ability to periodically dry. This is particularly true of wall and ceiling coverings such as drywall, fibreboard panelling, or wood-based materials. These will be susceptible to wood rot or other mould growth if they stay damp. Even dust and dirt on the surface of these materials can be a good medium for mould to grow, if it stays wet for several days or longer. Because of this, we should be diligent in trying to avoid the conditions that can lead to this situation.

Everyone has experienced the lower temperatures that are present in a basement which is built below grade. Because of this naturally cool indoor environment, condensation and mould growth are often seen there first. Also, basements are often used as storage areas, sometimes in rooms with uninsulated concrete foundation walls. If too much storage is placed close to these walls, and in areas that have minimal airflow from forced air heating systems or windows, mould growth is almost a certainty. So, keeping storage away from isolated areas, foundation walls, metal pipes and fixtures may be the first item to address. Also, ensuring proper airflow and ventilation, even from screened basement windows may be enough to prevent condensation related issues. But even with these efforts, mould may appear, especially on hot, humid summer days.

The best solution for a damp, mouldy basement or home is to dry out the indoor air, so condensation is less likely to occur.

On mild days, opening windows and allowing cross-ventilation in the home may be enough to accomplish this task. But when July and August temperatures reach the high twenties and thirties, normally accompanied by high RH, this will not be sufficient. Installing and operating a central air conditioning may be the only effective way to prevent this from happening. It works because A/C units dehumidify the indoor air, while cooling. Lowering of the indoor RH may be just as important as the cooling function in providing a comfortable indoor environment for the occupants.

So how does this relate to the initial question about using air conditioning and still feeling humid air inside the home? The simple answer is that if the air does not feel dry while the A/C is running, it is not working properly. There may be several reasons for this to occur.

The first thing to check is the condition of the outside condenser unit, typically installed just outside the foundation on the North or East side of the home. This unit will have a large coil surface on the exterior, which looks similar to a car radiator, which must be kept free of debris. If this is very dirty, dusty, or covered by excessive vegetation that may be the reason for the poor operation of the system. Cutting back vegetation, or hosing down the coil and cleaning with a soft brush should be a regular maintenance item for any homeowner.

There is an inside coil inside the plenum above the furnace which can also become plugged. This may be the problem, or a plugged furnace air filter, especially if frost is seen on the ducting near the furnace.

While poor air flow through the coils may cause poor operation of your A/C system, low refrigerant levels are the most likely culprit. If the refrigerant has leaked out from somewhere in the A/C lines or system, it will not cool properly. This can often be diagnosed by checking to see if the temperature differential between incoming and out-flowing air over the inside coil is greater than 15 degrees F. While this may provide a guideline, proper evaluation will require pressure testing of the refrigerant levels, done by a professional cooling specialist. In that situation, or if the inside coil is plugged, calling a HVAC technician to clean or recharge the system should solve the problem.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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