Black mould in basement sign of excessive moisture

Hey there, time traveller!
This article was published 27/03/2005 (7545 days ago), so information in it may no longer be current.

QUESTION — I am having a problem with what I believe is black mould on my basement floor. It is occurring on the floor from the inside edge of my exterior walls, about eight inches into the room.

It looks like someone has dipped a sponge into grey-black ink and gently touched it onto the floor. It is dried on the floor, and easily removed with a water and bleach combination, using a mop. I believe this is a mould growth.

My basement vapour barrier was put in with a poly and acoustic sealant, caulked to the cement foundation wall, just above ground level. This poly is then wrapped under the 2-by-4 framed wall, which has fibreglass insulation between the vertical studs. The poly from behind this wall, which now wraps around the face side of the 2-by-4s, is caulked to another piece of poly which goes up the interior wall and is caulked to blue Styrofoam pieces cut to fit in between the floor joists, where these joists sit on the top of the basement concrete wall. This sheet of poly is now higher in than the piece caulked to the exterior concrete wall.

I have both unpainted and oil-based painted floors and the mould seems to be a more prevalent problem on the painted sections. Maybe this is because it is easier to see it on the paint. My basement walls are drywalled and taped, so it is hard to see what is actually going on behind the walls. I have pulled the poly off of the cement foundation wall where I could access it, at an unfinished windowsill, but I cannot get enough light to shine behind to show me the concrete.

I had thought that this might just be a problem of improper room ventilation, as until recently, I have had most of the heat ducts in this area closed off. I have a dehumidifier going in my workroom, in a basement of 1,000 square feet, and the basement air does not feel damp. The ceiling has not been installed yet so there is air moving from one room to another in this partially finished basement.

My son wants to move downstairs to this room, which is not the only room in my basement with this problem. I want to put a kanga-back type of carpet over the basement floor. When I opened up the heat vents and cleaned up the clutter, I found this problem. In the past, I have had minor amounts mould on the bottoms of basement outside walls. This I attributed to areas with little air movement, as this mould was under shelves that were packed quite full. Do I have a problem that I must deal with? I am concerned about what may lie underneath the finished walls, or what could grow underneath a carpet in this environment.

— George Hudon, Lorette, Man.

ANSWER — It does sound like there is a problem that you have to deal with. Anytime that you have mould in your home, there is usually a case of excess moisture in some area. It is very common to get small amounts on the foundation walls with excess storage, as you suggest, but I think you have a different issue going on here. I will suggest a possible source of your moisture problem.

Mould requires two main things to grow. It needs water and a medium to grow on that is cellulose-based. The cellulose is no problem, because almost everything we build our homes with fits in these criteria. Moisture is the real issue and finding the source and eliminating it is the way to solve your dilemma. This moisture may come either from an external source, such as a leaky crack in the foundation, or an internal source within the home. If there is prolonged leakage and foundation seepage, you will likely see a lot more than small amounts of mould on the basement floor. You would normally have rotten and mouldy wood framing in the walls, damaged drywall and insulation. You would be able to smell the damp material and the mould, if this were the case.

I believe that the source of your moisture problem is the warm air in the home condensing on the cold foundation wall. The foundation wall will be much cooler than the air in the house, which is made even cooler by the insulation. Once the warm air temperature is lowered past its “dew point” it will form water droplets on the cold foundation wall. This water will drip down behind the wall and dribble out the bottom on to the basement floor. This constant wetting of dirt, dust and debris on your floor will allow the mould to grow.

The way to prevent the warm air from leaking into the area near the foundation wall is to provide a fairly tight polyethylene air-vapour barrier. This is difficult to achieve, and small gaps in this membrane may allow warm air in, but normally allow some trapped moisture out. The error in your home is that the installers of this insulated wall have wrapped the wall on both sides with poly and caulked it tight. Any air that condenses within the wall cavity will not easily escape, and may drip out the bottom of the poly. What is more of a concern is the poly up against the foundation wall. If cool air leaks into this area, which will undoubtedly happen, there will be no way for the condensation to escape. It will be trapped, by the poly, up against the foundation wall. This moisture may freeze in the winter and melt in the spring, running down the foundation wall.

The solution is to disassemble the insulated wall and remove the poly from the back side. This will also allow for inspection of the foundation wall for mould, which may have formed from constant wettings. This will allow you to clean the foundation wall, and allow it to dry sufficiently, before properly reassembling the wall with a single poly air-vapour barrier on the warm side of the framing.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors — Manitoba (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com