Condensation Solid State Home Inspections

Have you ever wondered why on a cold day the window in the bathroom is steamed up after your shower? Or, have you ever wondered why if you place a glass of cold water on your counter in summer, it is covered in water drops when you get back? This is thanks to a little high-school physics concept called condensation which Home Inspectors know can potentially cause thousands of dollars in damages to our homes.

Relative Humidity

The world we live in is full of amazing facts and one of those facts is that warm air can hold more water vapour than cold air. Condensation forms on cold surfaces like bathroom windows and water glasses as when warmer air turns cold at the surface, it can no longer carry the level of water as vapour and the water 'condensates' onto the cold surface. This is particularly visible in the shower as the warm water increases the humidity in the warm air to its maximum which then condenses very easily onto colder surfaces like the window or mirror.

Building Science has been aware of managing condensation for a long time. For example, on our pacific west coast many bathrooms up to the early 80's were built with windows but without ventilation fans and this was fine under the code of the time. The reason why is even though we have very heavy rains through the winter with cool temperatures, the outdoor air is still relatively dryer when warmed to room temperature. Keeping your bathroom window open in the winter rains actually draws dryer air into the bathroom from outside. This concept seems very counter intuitive but it is all true science based on relative humidity of the air. Of course people don't like getting out of the shower to cold air so the practice of keeping a window open was not well practiced and ventilators are now required to compensate for occupant behaviour.

Managing Condensation in Our Home

The moisture in our homes comes primarily from our lifestyles and the more people living in a home, the more moisture the home needs to be able to manage. Sources of moisture from lifestyle include:

  • Boiling Water for Tea, Coffee, Meals
  • Hot showers and hand washing
  • Combustion of natural gas (furnaces/fireplaces/hot water tanks)
  • People (and animals) breathing

The reason why bathrooms and kitchens have exhaust fans is not to control odours but to control moisture levels created in these areas. Running our bathroom and kitchen exhaust fans for up to 30 minutes after cooking or taking a shower will significantly reduce the humidity level in our homes. Excessive moisture levels that are not managed will cause damage to the home and this is commonly seen in attics by Home Inspectors.

Insulation, air, and moisture barriers in the wall and attic systems are also critical to managing hidden condensation. Good insulation levels and air barriers help separate warm and cold air zones so warm moist air does not have an opportunity to condensate in the walls or attic. Moisture barriers installed just below the drywall on exterior surfaces also helps to keep moisture from entering the wall system.

Damage Done By Condensation

Water is a natural enemy of our homes and condensation is one of the sources of damaging water. Condensing water on surfaces is common in bathrooms, kitchen windows, skylights, and other windows in the home. This surface damage often looks very bad as it will peal and blister paint finishes, leaves yellow drip markings, and in worst cases can damage drywall; however, surface damages are cosmetic in nature and while a nuisance are largely repairable.

The big problems in our home come from hidden water condensation in our walls and structure. Our homes are naturally warm and moist environments because of our lifestyle. Cold surfaces can exist on our outside wood walls, basement concrete, or in our attic and if warm moist air reaches this area, it will 'condense' and water will form perhaps behind finished areas. This water combined with organic material (wood/paper) and warmth will facilitate the growth of hidden moulds and rot damage. Often this damage can go unnoticed until it is very severe and shows through surfaces finishes.


One tool nearly all homes built in the 80's started to have to control humidity and moisture levels is a humidistat. The humidistat reads the moisture levels in the air and automatically operates a ceiling fan, most commonly in a bathroom on the upper level. The humidistat level is occupant adjustable based on the outside air temperature (remember relative humidity) to help the inside of the home stay at a healthy humidity level and prevent hidden condensation damage.

The downside of humidistats is that occupants don't like to hear bathroom fans running seemingly continuously burning up electricity and carrying warm air out of the home. As such, many home occupants leave these humidistats in the off position. As governments continue to require our homes to be more and more air tight, the issues of excess home humidity will get worse and home occupants need to be educated on controlling home humidity levels.

What do Home Inspectors Look for with Condensation?

Home Inspectors will see clues about the humidity level in a homes past during a home inspection. Surface water damage can be visible at skylights and organic growth (yes, mould) on window frames and bathroom surfaces are easily identified. Hidden damage is more difficult to see but attics provide a good clue to home inspectors of excessive moisture in the home.

Warm air naturally wants to rise in our homes and it will find ways to reach the attic around light penetrations and ceiling accesses. This warm air will carry excessive household moisture with it. It is not uncommon to see organic growth (moulds/mildew) on the underside of roof sheeting in the attic where condensing moisture from the home accumulates. When a home inspector sees this moisture, it will raise red flags that hidden moisture damage may be elsewhere in the home. It is not possible to completely prevent warm moist air from reaching the attic which is also one reason good ventilation is needed in the attic space to carry away this moist air and keep the space dry and clean.

Final Thoughts

For as long as people will want to take showers and cook foods in our homes, we will need our homes to manage humidity levels to control condensation. In the past, our homes were naturally 'leaky' and this helped to manage the humidity levels but in this era of government regulated energy efficiency, homes need more help from occupants to manage humidity levels.

Home owners need to learn to operate humidistats, to properly use bathroom and kitchen exhaust fans, and to occasionally open a window to manage lifestyle related humidity levels. Keeping our homes dry will keep them safe and solid for many years to come.

By James Bell - Owner/Operator of Solid State Inspections Inc.