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Last Updated: , Created: Sunday, June 24th, 2001

Removing mould from the siding.

Robert from Riverview, N.B. writes:

The problem relates to what appears to be mould or mildew on one exterior end of my house, showing up in what appears to be small black spots peeking through the white paint. I have been told that improper ventilation can contribute to such a situation if, in fact, it is mildew. My house faces north, therefore the two ends are east and west. For what it is worth, the end that seems to have the black specks faces east. The house, a bungalow, is 36 feet in length and has three 8x16 inch aluminum soffit vents on each side. I have been told that perhaps more vents might be useful in eliminating mildew, but I question how many vents on each 36 foot side would be appropriate. Can I install too many? With three vents on each side, spaced equally across the 36 feet, in order to keep the appearance uniform I would need four more on each side, for a total of seven on each side, which to a novice like myself, sounds like it might be excessive. Is there, in fact, any danger in having too many vents on each side? Should I, instead of additional 8x16 inch vents, install 5 inch or 6 inch round vents between each of the present 8x16s?

------- Response:

I think that we are mixing up ventilation and ventilation here.

The soffit vents feed air into the attic which should then go out the roof vents. This reduces the moisture in the attic in the summer and keeps the snow frozen in the winter. Wall ventilation is something else.

If the wall has vertical strapping, there can be good ventilation that goes up from the bottom and into the soffits and on out through the roof vents. This will tend to keep wooden siding dry by allowing both the front and the back to air out, and is the most recommended technique for use with wooden siding. Some builders in the Maritimes are hesitant to build this way because they have found that in some cases it increases air drafts into the house. The CMHC found through research that the increased drafts was due to wind going through the overlaps in the building paper. With modern full wall house wraps, this no longer presents a problem.

Without this air space, moisture gets into the wall and gets stuck there, helping to promote mould growth, and usually on the East and North walls.

If you can determine that there is strapping on the wall, making an air space between the wooden siding and the wall sheathing, you could put some little 2" round vents through the siding into this strapping air space. Place these vents high on the wall where they are sheltered by the overhang of the roof, and not above windows. If your wood is tight up against the siding there is not much you can do but to use mildew resistant paint (after having scraped off the old infected paint).

If you contact CMHC they have a number of studies on walls and siding in the Maritimes.


Keywords: Strapping, Mold, Mould, Siding, Cleaning, Environmental

Article 714