Attic Moisture Isn’t Always Caused by a Roof Leak


Home Inspections and Attic Moisture

During a home inspection the inspector will take a look into the home’s attic if access is available. He or she is not only looking for roof leaks but also structural damage, missing insulation, vapour barriers, mould, rodents and pests. A very common issue is mould growth on the underside of the roof sheathing or boards. There are a number of circumstances which may be causing the mould but it always includes moisture.

At Ease Building Inspections recently got called to inspect an attic in a 20 year old home in a Mississauga sub division. The home owner had taken a peek into his attic (as is always a good idea for home owners to do so from time to time) and noted that there were some dark spots (mould) and damp areas on the underside of the plywood sheathing over the bedroom area. These areas had not been there before the shingles were replaced last October and the home owner was concerned that the contractor had done a poor job.

After having two other roofing contractors inspect the shingles and installation and not defects found, the home owner was no further ahead and decided to give At Ease Building Inspections a call.

When the inspector arrived the exterior of the roof could not be seen due snow and ice. When the outside temperature of -17C it is not uncommon to see condensation on the inside of the windows, however as the inspector approached the front door to meet the client he noticed that most of the windows were covered with an excessive amount of condensation. When the inspector entered, it was evident right away that the humidity in the home was way too high. The home owner had set the furnace’s humidifier to 55%.

A setting of 55% relative humidity is too high for a home in our cold winter climate. When this moist air hits a cold surface (like the windows) the humid air condenses and turns to water. Over time this moisture, if not noticed and cleaned, results in mould growth and water damage to wood, drywall and furniture.

For a guideline on controlling humidity and correct humidifier settings refer to CMHC’s “About Your House” publication No. 62027.pdf. …. But on to the attic!

When the attic hatch cover was removed moisture was noticed right away along the plywood sides of the opening. The same moist air that caused condensation on the windows was finding its way around the hatch cover and into the cold attic space. Although the attic hatch cover was well insulated, there was a 13mm gap around the attic hatch frame which was allowing the air to escape into the attic and turn into moisture. The entire plywood sheathing in the area above the hatch and the master en-suite bathroom was dotted with mould spots and actual moisture from the ongoing condensation, but there was more!

When the home owner had the roof re-shingled he had a contractor install a bathroom exhaust fan to get rid of humidity from the bathroom. The workmanship for this fan addition was subpar and the entire duct and venting system for the fan was leaking air which too was condensing in the cold attic. The exhaust fan was clearing the moist air from the bathroom but instead of taking it outside where it should be it was leaking into the attic and contributing to the condensation.

Don’t always assume that the roof is leaking as in this case but look for the less obvious. In this case the home owner was advised to reduce the homes humidity level (lower the humidifier’s setting), and to seal the gaps around the exhaust fan duct and attic hatch. Not only will the homeowner save his roof’s plywood sheathing from rot and mould but also save energy that was being lost into the attic.