I read your column in the free press almost every Sunday. I have a problem in my basement. We have been in this home, our first house, since 1989. In the spring we noticed several wet spots on the basement floor. Last winter I, and a very talented good friend, renovated the basement. We put in a bathroom and tore out all of the dark 1965 era wall paneling and put up drywall. When we took off the old panels and the fibreglass insulation we found a lot of black mould in the areas that were wet, in one corner of the basement (north west). There was a big area of frost, above ground level, and over the years this is where all that mould grew. I read in your column that to get rid of this mould wash the walls with ammonia or bleach. Upon doing this job we were all looking for cracks in the wall.
Several days later, seeing no cracks, we reinstalled new insulation, and started the drywall work. The following spring our wet spots were back! Is this a condensation problem caused from the cold outside air hitting the warmer basement surface and running down the wall? I am starting to look into a suspended ceiling right now, so the tops of the walls are still open. Is it possible that the insulation up by the floor joists is insufficient? Could cold air seep in between the house and the basement walls? What is your opinion on this and how should I go about fixing the problems? I am also increasing the soil outside at the front of the house to cover the exposed basement wall.
The first Item to address is the mould issue on the foundation walls. I have never written that ammonia should be used to remove mould, but have suggested bleach, in the past. The current C.M.H.C. recommended method for removal of small amounts of surface mould on concrete is with unscented soap and water. Bleach is a toxic substance and using a toxic compound within an enclosed area is not advisable and normally unnecessary. If it is a small area of mould, the surface and surrounding area can be vacuumed with a central vac or other unit that vents directly outside and then cleaned with simple soap and water. Protective clothing, gloves, respirator, and eye protection should be worn to prevent contact with the mould. The area should be allowed to fully dry before covering with insulation or any wall coverings. If the mould is more than just a small concentrated amount, professional remediation and cleaning may be required.
The frost and subsequent mould growth may be due to condensation, moisture intrusion from outside, or a combination of both. Several things should be checked to determine the source of the moisture. If you eliminate the source of the moisture, you eliminate the potential for mould growth. This should be the first priority before completion of your basement renovations. Unfortunately, removal of the new drywall and insulation may be required to inspect the foundation wall and remove any new mould. If there are still puddles on the floor near the wall, there is likely a damp foundation wall, which will provide an ideal area for new fungal growth.
You have the right idea as far as re-grading around the foundation to provide a slope to channel water away, but this must be done in conjunction with downspout redirection. Because the mould and water is seen in one corner of the foundation, it is likely that a downspout from the eavestroughs may empty right near that corner. This downspout should be directed away from the foundation with an extension pipe for 2 to 3 meters to prevent seepage. I would suggest attention to these two items as the first priority. If there is water on the floor only in the spring, it may be due to seepage from melting snow or from melting of frozen condensation on the foundation wall.
If the grading and downspout extensions do not stop the water, the area will have to be stripped of the new drywall and insulation for inspection. Heavy watering directly beside the foundation in this area with a garden hose will confirm whether the water is from seepage or not. If the wall and floor get wet after this test or following heavy rains, then exterior excavation and repairs to the foundation waterproofing and weeping tiles may be required. This is the worst-case scenario and can be very costly.
If no seepage is found, condensation is the likely culprit. This should be much easier to correct than seepage. There may be an area in the foundation or floor system above this corner where cold air is leaking into the home. This should be located and filled with caulking or blow-in foam insulation to stop the draft. It may be just as likely that condensation and frost are forming on the foundation wall from poor air sealing or improper air-vapour barrier installation. The area between the floor joists is always a difficult area to insulate and air seal, and any opening in the vapour barrier may allow condensation in the heating season. This area and the newly installed insulation must have a properly installed and caulked polyethylene air-vapour barrier installed to prevent warm air leakage into the wall cavity. For details and descriptions of suggested methods of insulating and air sealing, the Office of Energy Efficiency (oee.nrgc.gc.ca), C.M.H.C., and Manitoba Hydro all have free or low cost publications for consumers.