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How to deal with odor coming up from basement?

I live in Nova Scotia. Moderately hot and sometimes fairly humid summers, cold winters although very variable. Similar to US North East.

House is 120 years old. Basement has two areas--main one about 1600 square feet and other under rear extension about 200 sq. ft. Half of main basement is 6 ft high, rest is 3 ft. Extension is all 3 ft. crawl space. Basement has vapour barrier on floor and spray foam on walls. No insulation below floor of living area. Access is via steps outside house. Basement has moderate odour (worse in extension) but is quite dry with humidity around 50%. Old well in basement is sealed off except for small hole to allow sump pump float.

12 months ago, I had heat pump and all new duct work installed. Immediately got basement odour in house apparently coming through ducts. No odour problem with air conditioning on or neither heat or air conditioning.

I am now thinking that problem is caused not by leaks in the heating system but by uneven pressure between basement and main house that is causing odours to be drawn up. Have closed all vents allowing warm air into basement and am experimenting with extractor fan in window. Definite improvement but not completely.

Do you think i am on the right track? If so, what is best kind of fan to install permanently? Or should I install a Humidex? Would it be wise to add a second fan in the extension area as this is closed off from main basement apart from small entrance (about 2 by 3 ft)? Will running two fans cause any problems with pressure etc? Also, should I provide for make up air as required by US code (not sure about Canadian requirements). I am wary about opening up basement to living area into smells get up through vent. Blogs on this site offer varying opinions on the need for make up air. The main house has been retrofitted for improved energy efficiency but still has rate of 5.3 air changes an hour. Basement is pretty tight especially when I seal up the two small windows.

Asked by David Monaghan
Posted Sun, 12/08/2013 - 18:55
Edited Tue, 12/10/2013 - 12:01


1 Answer

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The best solution to your problem would be to find the source of the odor and to mitigate the conditions that allowed the odor to develop. That would be a permanent solution.

A temporary solution would be to seal air leaks in your basement ceiling and to depressurize your basement with an exhaust fan, to limit the amount of basement air that reaches your living space. This approach wastes energy, and is therefore undesirable.

An odor in your basement can have a variety of causes. It might be sewer gas, due to a plumbing problem. It might be a dead animal. Or it might be mold. You need to do a little detective work to find the source of your odor.

If your basement odor is mold, then you need to take all of the steps required to dry out your basement. Here is an article that might help you approach that task: Fixing a Wet Basement.

If your odor has another source, you need to locate the source and fix the problem.

If you want to depressurize your basement to reduce the odor, you can install an exhaust fan in a basement window. It sounds like you've already tried that remedy, but you still have an odor. That leads me to believe that your really have a major odor problem, and it's time to find out where the smell is coming from.

Of course, an unbalanced HVAC distribution system can cause problems. If you suspect that your supply ducts and return ducts aren't balanced, you should hire a home performance contractor to check your duct system. All of the seams in your duct system should be sealed with mastic, and you need to be sure that the air flows through your supply system and return system are balanced.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/09/2013 - 06:31

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