Fungal growth in walls - It could happen to you!

A CMHC/CHBA Task force noted that if wall cavities "remain warm and wet throughout a significant portion of the spring and summer months, mold, mildew and wood-rot organisms flourish on the framing and other components. Structural damage to houses has been discovered after as little as two years under such conditions."

CMHC/CHBA Joint Task Force reports fungal growth in 12 months
In 1986, the Task Force built test huts using materials(1) purchased from local building material suppliers and monitored the moisture content.

After 12 months, some studs of the test walls sheathed with waferboard or extruded polystyrene were still wet. In fact, the Task Force found fungal growth and black patches on the studs, and some corrosion of the fasteners.

(1) Lumber exceeded 19% moisture content
The Task Force observed and concluded:

Most panels that stayed wet exhibited fungal growth.

2. Permeability was the most significant factor in drying.

3. "The use of exterior sheathing materials with a very low permeability to water vapor in combination with wet framing lumber or insulation materials having a high moisture content, puts walls to a high degree of risk of moisture problems."

4. "If drying is protracted over a long period of time, rot-causing fungi may become established in the wall cavity. Once established, rotting may continue even though no moisture is added."

Test hut location had little effect

"The climate or geographical differences of the site in this experiment had little effect on the final structural moisture content values" (OBOE Engineering LTD., Consultants to the CMHC/CHBA Joint Task Force).

Other independent tests performed in Ottawa, in 1988, confirmed the Joint Task Force's findings.

Ottawa drying tests results

After 60 days in a professionally controlled test environment, wall studs in the fiberboard sheathing test panel dried from 30% moisture content to 7%, and showed no signs of fungus or decay. The batt insulation in the wall cavity was dry.

Wall studs in an identical test panel, but with waferboard sheathing, were still at 20% moisture content after 60 days, and showed definite signs of fungal growth. In fact, the upper area of the batt insulation in the waferboard test panel was so wet that water could be squeezed out by hand.
Comparison of wall drying performance of fibreboard and waferboard sheathing

Fibreboard sheathed walls dry quickly!

Fibreboard sheathing is so permeable, that wall systems using fiberboard will dry in a few weeks, without decay.

Sheathing permeability

This chart shows clearly why moisture can escape so quickly through fibreboard sheathing, compared to waferboard or extruded polystyrene.

- - Canadian Fiberboard Manufacturers' Association