Why Does My House Smell?


Yes, that’s your illustrious author making the “stink face”

People often ask: “why does my house smell?” Often, this is during the winter because your house is sealed up for months, with little fresh air. In fact, with tight, energy efficient homes, this has become even more of an issue. It’s one of the reasons that there’s been a backlash against tight houses.

#1 – your house might not be adequately ventilated

First, let me address the energy efficient house issue. The problem is, many builders and architects don’t understand that a house is a complex system. You can’t just air-seal the house and have a healthy house. That’s why building best-practices call for a certain amount of fresh air circulation. So if you live in a tight house, you want to ensure you have adequate fresh air or your house will get stale and smell. If you don’t know about HRV’s and ERV’s (heat recovery ventilators and energy recovery ventilators) read this short post. Every modern home should have one of these. Once you’ve lived with one, you’ll wonder how you managed without it.

#2 – there might be a dead mouse/animal somewhere

During the winter, mice and other critters want to come in from the cold and live in your home. In my area, the field mice flood in ever winter. This can be reduced by having a well sealed house, but they’re clever and they’ll find any little opening to get in.

Invariably, some of these will die in your house, leading to a horrific smell. You’ll think the sewage lines are overflowing. It’s really nasty. The only solution is to take a good flashlight and follow your nose. Yuck! But if you don’t track it down, you’ll have to live with the odor for months.

Note: I’ve found many dead mice inside the air handlers (behind where the air filter goes). If the dead animal smell seems to be everywhere in the house, open the air handler and look for dead rodents.

#3 – sewer gases? Your drains might not be dried out or improperly vented

We all know that stink. You’ll look suspiciously at the dog but you won’t find the ‘evidence’. Sometimes the smell will disappear, other times it will remain for days. You might be the victim of sewer gasses!

In most homes, the problem may be as simple as a dried out sink trap. The sink trap is the little ‘U’ under the sink in the drain line. This is supposed to be filled with water. This acts as an odor plug that prevents gas in the drain lines from coming out the sink. If the trap is empty or too low, the odors in your drains, which are all connected together, waft out into your home, leading to the ungodly stink.

If you discover that the odor comes from your drain, simply run the water for a few seconds to fill the trap. Once the odor has diminished, this should prevent more odors from coming back in.

If the sink is in an infrequently used room, like a guest bathroom, the trap may be drying out by simple evaporation. You can greatly reduce the evaporation by adding a teaspoon of vegetable oil to the trap after filling it up with water. Don’t run the water after doing this because that will flush out the oil. You want a thin layer of oil on the surface of the water to prevent evaporation.

As a side note – if you plan on leaving your house for a while, like going on vacation, add some oil to every drain in the house to keep the traps from drying out. You’ll thank me when you return.

Improper venting:

Sometimes, the trap gets the water sucked out because the air vent is clogged or non-existent. You know those 2″ pipes that come out your roof? Those are connected to the drain lines. If you sniff them, which I don’t recommend(!), you’ll smell sewer gas. These allow the sewer gases to float up and away but they also allow the pressures in your drain lines to equalize so when you flush the toilet, or drain the tub, it doesn’t create a vacuum in the pipes which sucks the water out of the sink traps. You can hear it when this happens – other drains around the house might gurgle or toilets might mysteriously drain. That’s indicative of bad drain line venting. If you have this, explain the problem to a plumber and have them fix it. This isn’t usually an issue that a homeowner can fix.

#4 – is the smell coming from your air conditioner or furnace?

There are a few possibilities here.

Cat pee/urine/piss smell – typically, people associate ammonia with cat urine, so when they smell ammonia, they immediately scold the confused cat. Then, as the problem lingers, and they notice the smell coming from the ventilation ducts, they realize their error and start searching in the right place.

Sometimes, it’s a mouse-house inside the air handling system. They love warm places and they urinate everywhere. Gross! So if the smell comes from one vent in particular, open the vent and check for shredded material or caches of food that indicate the mouse den then clean it up with a vacuum cleaner and some warm soap-water or baby-wipes. (pro-tip: baby wipes are great for cleaning just about everything!).

If the smell comes from everywhere in the house, rodents may be living in the air handler. Usually you can open up the air handler and look behind the air filter for signs of mice.

You might also have tainted insulation. Some blown-in insulation products contained compounds that smell like this. Any leaks in the duct system allows those smells to be distributed throughout the house. You can easily determine this problem if you stick your head in the attic and sniff. If it smells the same, that’s your problem. See “Mystery smell” below for more on this.

Musty/moldy smell – This one is usually much easier to find – this smell is associated with water/moisture in the system, leading to mold growth. When air blows through the system, it circulates the smell from the mold source and your entire house gets that musty smell.

Mystery smell – dusty, “hot”, etc – Poke your head into the attic on a day when it’s hot up there and sniff. Does it smell like what’s coming out your vents? If so, you’ve got leaky ducts, usually on the return (air intake) side of the system. This is really common – when your system runs, it sucks in air through the “air return” line. If there are any holes in that line, or in the attic mounted air handler, it will suck in dust and any odors from the attic and distribute that throughout the house.

Solving this issue can be as simple as looking at the system in the attic and finding holes. Usually it’s not that simple and I recommend a professional come in with a “duct blaster” to test and seal your ducts. Sorry, but that’s the best and safest way to track these smells.

#5 – stinky bathrooms?

If the problem is the normal, um, stinky bathroom, you need better ventilation. Most homes have exhaust fans in the bathrooms, but few work optimally.

The first test is to put a sheet of paper or strips of tissue up to the fan. It should strongly suck the paper onto the vent when the fan runs. If it doesn’t, the fan doesn’t work properly. Most fans have too much duct attached which significantly diminishes the flow. You want a short, direct path from the fan to the exhaust on your roof. I’ve seen people with 50 feet of duct connected to the fan, rendering it useless. Trust me on this, the shorter the better, and best to go straight up through the roof if the bathroom is right below the attic.

You’ll want to invest in a high quality metal roof vent like this one. These last forever and are self flashing (minimizes the chance of roof leaks) and are easy to install.

In addition, the fans only work if you use them. Let them run for at least 15 minutes after you “do the deed”. This is about how long most fans take to flush the stinky air out of the bathroom.

#6 – musty basement

Musty basements mean damp, moldy basements. If you’ve got a wet basement, you need to do whatever is necessary to keep it dry. I’ve written a lot on this, as have others. A damp basement is unhealthy and can lead to serious damage to your home to the point that it becomes structurally unsound. If you try to sell your home, it will be flagged by the home inspector and you’ll be forced to fix the problem. So if you have a wet basement, fix it *NOW*. Then, you can enjoy a safer, healthier, less smelly home for the rest of the years you live there.

Got any other home smell problems? Drop me a line and together we can figure it out.


69 thoughts on “Why Does My House Smell?

  1. 1950 wood house no pets ..when room temperatures are over 85 degrees ..an ammonia smell comes .only one bed room…(this house does have termites) my only pets lol..house is on piers 2.5 ft off ground..sealled with cross air vents no animals can get in under house. This goes on for over 3 years since I’ve owned it.

    • Many possibilities, but here are some of the most common:
      – Rat and other animal leavings smell of ammonia
      – Fertilizer – ammonia is a primary component, released at high temperatures.
      – Some other rotting materials. Could be an insulation product used in construction.

      Odors release more readily under high temperatures, so that’s not a surprise.
      Do you have an air conditioning system? Or a humidifier? You can get nasties growing in such systems that release ammonia odors. Central air conditioners with ductwork are also hidouts for critters, but since the problem has existed for years, I’d have to guess some building product that is outgassing. However, with older houses, there should have been plenty of time for all such odors to have disappeared.

  2. https://polldaddy.com/js/rating/rating.jsHi Ted,
    We have been having a pungent fermented soy beans smell in the apartment. The smell moves around in our living room and one part of the kitchen that is closest to the living room. Mornings smell worse than evenings. So far, the bathrooms and rooms don’t have the smell (yet). We live in a humid country in Asia. We live in a small 3 story apartment and have the roof above our apartment. We first thought it was someone cooking the Asian fermented beans dish but it doesn’t seem to be the case as the smell is only from our apartment. It is slightly better if we have the air conditioning on. We have tried airing out the apartment as well but no change.

    There has been construction on a site next to us which could have disturbed piping. Recently we have had a lot of rain and the drainage in front of the building next door was not working properly. One side of our roof was also not draining that well.

    Could this be mold or drainage problem? But why does the smell move around the apartment? Who should we call to check this out for us?


    • Hi Kim, you’ve stumped me!
      The only thing that I can think of is that in the living room and kitchen area there may be some places where the air is getting pulled into the apartment from outside, or wherever the odor is originating. It can be quite tricky to find. Imagine that the air carrying the odor, is moving through the walls and floor, trying to find a place to come out. It reaches a hole in the wall, maybe for a pipe or power outlet or even a doorway (since behind the trim it is open to the wall cavity.) From there, it enters your living space. But the source could be anywhere.

      Now, understanding that the odor could be entering through small cracks or openings like that, look around the walls where the odor seems the strongest. Are there any openings you can see near there? As noted, this could be trim around doors with gaps between the trim and the wall. It could be power outlet plates, anything like that. You really have to use your eyes and nose and play detective.
      Once you think you’ve found the area, you can temporarily try to “fix” it with a product like “Glad press-n-seal” which shouldn’t harm your paint or painter’s tape. This is just so you can seal areas up and see if the odor disappears. If you manage to locate the area where the smell is coming in, you may be able to use caulk and seal it.
      This won’t tell you where the smell is coming from originally, and it may find other areas to enter your apartment, but it may help provide some relief.
      Rain and humid weather often bring out odors, so this could very well be what cause the smell to increase. It could also be why the smell improves with air conditioning as air conditioning removes humidity from the air.
      And you never know what is being released when they do construction. Odors are carried on air currents and spread in the air. It would seem to move from place to place as the air moves. For example, if you open a window in the kitchen or turn on a fan, it could cause the air to move from the living room to kitchen.

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