Sniffing out source of odour
QUESTION: I am writing to you with a question regarding an occasional strange odour in my house. It smells like sour stagnant water or rotting wet wood. It doesn’t smell like mould or sewer gas and it’s not coming from any drains or the sump pit. I have had some improvements done in the last few years and hoped they might also eliminate the problem, but nothing has helped.
I have had my furnace ducts thoroughly cleaned. I have had a new high-efficiency furnace installed and the cold air intake was left in place. The roof has been checked for leaks and has been re-shingled. The roof stack has also been checked to see if anything is blocking it. The neighbours do not have anything nearby that would smell.
I would think if it were any of these things, the smell would be noticeable at all times, but the odour is only noticeable when it is foggy outside or if the humidity is high and it is not windy. I have also noticed it is most prevalent in the master-bedroom heat vent, which is the room farthest from the furnace, although it is faintly noticeable in all of the rooms.
My home is a 26-year-old split-level and the lot backs onto a retention pond. The odour started quite a few years ago, and the only thing I can think of is the cold-air intake is under my deck, which is cedar, and there is no air flow when it is foggy and I am getting a stagnant smell. As soon as the wind starts to blow, the odour disappears. I’m thinking I should have this vent extended so it’s above the deck surface. If so, who would do this type of work? I would greatly appreciate your opinion and advice regarding this problem.
ANSWER: In attempting to determine the cause of an issue in a home, especially something as subjective as a specific odour, I try to look at all the possibilities and then rule them out one at a time.
Your description of the type of the smell is the natural place to begin. While I’m quite familiar with the odours associated with rotting wood, I’m not sure what a “sour stagnant water” smell is. I’m glad you don’t feel it’s waste or sewer gas you’re smelling, but don’t be too quick to rule out mould.
While many moulds can give off what can be described as a “musty-basement” smell, wood rot is also a type of mould, so your description may indeed be fungus-related. This could be coming through the fresh-air intake under your deck if the deck is rotting or there are decomposing leaves or debris.
If the underside of the deck is accessible, locate the vent hood or screen where the fresh-air intake duct terminates. If not, you may have to remove a few deck boards. Once found, the vent hood should be cleaned with a damp cloth, brush, or other tool to remove any dust or debris blocking it. Given its location under your deck, I’d be very surprised if the screen on your vent is not partly or completely blocked. If so, the dirt and debris may be the source of the smell. This material may be slowly decomposing, but may only be noticeable on very wet days, or when there is little wind or air movement to dry it out.
Mice, stinging insects and other pests like to take refuge in areas like this, especially if they can come and go without being easily detected. If they gain access and create a nest or perish inside the ducting, rotting-type odours are likely.
If you find the fresh-air intake is not plugged or damaged and has reasonable airflow, relocating it may help prevent a return of the problem. Because your lot backs onto a retention pond, without much cover from vegetation or other buildings, your home is likely more susceptible to variations in wind and weather changes.
Your observations about the smells occurring on wet or calm days may be another clue to the source. If the odour is due to moisture damage in the deck or house framing, this may not happen on sunny and windy days, but more likely when it is rainy, calm or foggy. Another possible explanation is our olfactory system works much better when the environment is wet or damp, so the smell may always be present but only noticeable at these times.