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ARCHIVED – Does use of enclosed floor-joist space as return air ducts affect indoor air quality? v4n4-11

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A joint study completed by IRC and Canada Mortgage and Housing Corporation (CMHC) on the effects of using enclosed floor-joist space as return air ducts has yielded findings that will ultimately result in recommendations for property owners and builders.

A long-time practice – a modern problem
Traditionally, forced-air heating systems used ducts made entirely of galvanized sheet metal to distribute air within our houses. Somewhat later, builders began to use floor-joist spaces as return air ducts by covering the space between floor joists with a sheet of galvanized metal. This type of air duct has three wooden sides – the subfloor and two joists – and the galvanized metal on the bottom.

Beginning in the 1980s, builders started to use oriented strand board (OSB) for subfloors, and more recently, plywood and OSB for engineered joists. Researchers were concerned that the practice of using the floor-joist space as return air ducts might promote the distribution of various chemicals emitted by the OSB and plywood throughout the house.

The project
To address this concern, IRC conducted tests to measure emissions of formaldehyde and volatile organic compounds (VOCs) in nine houses with four different return air duct constructions.

The four return air duct constructions were:

Case 1 – rectangular duct including one side with OSB subfloor, two sides with OSB joists and one galvanized sheet metal side;

Case 2 – rectangular duct including one side with OSB subfloor, two sides with solid wood joists and one galvanized sheet metal side;

Case 3 – rectangular duct including one side with plywood subfloor, two sides with solid wood joists and one galvanized sheet metal side;

Case 4– rectangular duct, all four sides made of galvanized sheet metal.

Measurements of formaldehyde and VOC levels were taken in four locations at each house: supply air duct, return air duct, living room and outdoors. A variety of other factors, including weather conditions, indoor air temperatures and ventilation rate, were also recorded, and conditions in the houses were controlled to improve the validity of the data.

Results
Researchers found that, in general, the Case 1 houses had the highest concentrations of formaldehyde, total volatile organic compounds (TVOC), terpenes and aldehydes. (The latter two are the major volatile compounds in wood products.) Nevertheless, the differences among the various types of duct were small and may even be attributable to other factors such as the relative newness of the Case 1 houses.

The limited results suggest that the effects on indoor air quality of using the joist space as ducts are insignificant, or put another way, that enclosed floor-joist space is an acceptable option for distributing air within our houses.

IRC will continue to monitor this situation.