December 16, 2011

Construct Canada speaker addresses moisture problems in construction

GREG MECKBACH

digital media editor

The Ontario Building Code is wrong and your crawl space is probably not haunted, a mechanical engineer told an audience at Construct Canada.

John Harris, president of DSG Home Inspections Inc. of Orillia, Ont. explained that the Ontario Building Code will change in 2012.Currently, he said, vapour barrier, air barrier and thermal insulation are all lumped in together for one inspection point under the building code. But that’s about to change with air barrier becoming separate.

“The challenge is, ‘where’s your air barrier?’” he asked. “The problem about having a separate inspection for the air barrier is it will be completed and covered off in various stages depending on what part of the building you are looking at.”

Harris addressed an audience of about 40 construction industry professionals during a presentation titled “Condensation or leak: Understanding moisture movement in the building envelope for residential buildings.” Construct Canada, which organizer MMPI says is Canada’s largest construction conference, was held Nov. 30 through Dec. 2 at the Metro Toronto Convention Centre.

In addition to building code changes, Harris emphasized the importance of having separate dehumidifiers in addition to air conditioning, the properties of concrete, sun-driven moisture and the importance of grading and capillary breaks.

He also reviewed the definitions of relative humidity (the ratio of water vapour to the maximum amount of water vapour that volume of air can hold at a constant temperature and pressure), absolute humidity (the total mass of water vapour in a given volume of air), dew point (the temperature at which a given volume of air will hit 100 per cent relative humidity) and the use of psychrometric charts, which plot temperature, dew point and humidity.

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“If you cool the air down enough until eventually the vapour can’t stay, vapour turns into water, what do you get? Fog or rain,” he said. “I’ve seen fog in crawl spaces. Literally. We get these stories of poltergeists and ghosts, a picture taken in a crawl space that looks perfectly normal but the flash reflects the moisture and you get all these shapes in your picture.”

He advised building designers not to have unheated crawl spaces because when the vents are opened in the spring, the air can bring in too much moisture. Using a psychrometric chart to illustrate his example, Harris said that in Ontario, we can have days in May where the temperature is 25 degrees Celsius and the relative humidity is 70 per cent, meaning the dew point is 19. In the scenario he presented, warm moist air is cooled below the dew point, to 15 degrees, when it flows into the crawl space from outside.

The building code says unheated crawl spaces need ventilation but people usually open the vents when the weather is warm and humid, he said, adding the code should not provide for unheated crawl spaces, he noted.

“The building code is wrong,” Harris said. “I will stand up in front of anybody and tell them that. And most people at the building science level know.”

Designing basements can be a challenge, Harris said, because concrete is not waterproof. With the changes to the building code, the air barrier now has to be around the entire building envelope, whereas in the past it only had to be around the thermally-insulated portions, he said.

Because concrete is not water tight, poor grading can be a problem, he said.

“I don’t care how you build your house, if you have water standing against the foundation, you’re going to have a problem.”

He also said sun-driven moisture needs to be addressed.

“We wouldn’t have to worry about a capillary break on the north side because it doesn’t see the sun,” he said.

“On the south, east and west faces you want to talk about capillary break, you want to talk air tight so in the winter time the humidity that’s in the house does not go out.”

Harris said a separate humidifier is a good idea, especially in a building with modern windows where the air conditioning load decreases.

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