Canadian Contractor

Dave Gray   

Under Pressure, Part 1 of 2: The danger of depressurization

Renovation Contractor

As our customers get their houses ready for winter or decide to make their houses more energy efficient, it is appropriate to have a discussion about depressurization – a situation that can make a house dangerous.

By Gary Sharp, CHBA

As our customers get their houses ready for winter or decide to make their houses more energy efficient, it is appropriate to have a discussion about depressurization – a situation that can make a house dangerous.

What is it? Depressurization occurs when more air is being exhausted from a house than is coming in, either through air leakage or mechanical means. In that state, the house is under a negative pressure, relative to the outside.

In cold weather, houses lose heat through the building envelope (convection, conduction, and radiation) and because of unintentional cold air leakage into the building, and warm air out of it. Heat loss due to air leakage is one of the easiest heat loss mechanisms to fix so new homebuilders and renovators are spending more time and effort to make homes airtight. If you’re renovating a house that’s more than 30 years old, chances are good that most of the heat loss from that home is due to air leakage; in the 1970s, when we first started to address energy efficiency, it was estimated that up to 50 percent of the heat loss from a house was due to uncontrolled air leakage.


Renovators use air-tightening techniques to improve an existing building’s thermal performance. Insulating and air sealing is a common practice, and will continue to be important techniques in making our buildings more energy efficient and reducing greenhouse gas emissions.

In addition to contributing to heat loss, moving air can carry moisture (water vapour) into a wall or ceiling system. When the water condenses inside of the structure and does not dry out, it leads to mould and rot of structural members, which is not only bad for the home, but dangerous to its occupants. Air leaking into a wall and carrying water vapour is a far greater source of moisture entry into that wall than by simple vapour diffusion – up to 100 times more of a problem.

There is a lot of equipment in houses that exhaust air and can lead to depressurization.


Exhaust Device L/S (litres/second) CFM (cubic feet per minute)
Bathroom Fan 20 – 50 40 – 100
Range Hood 25 – 600 50 – 1,200
Indoor BBQ Grill 60 – 600 120 – 1,200
Clothes Dryer 40 – 55 80 – 110
Central Vacuum 45 – 65 90 – 130
Wood Fireplace (at full burn) 150+ 300+

When homeowners run equipment simultaneously – imagine a clothes dryer and range hood on at the same time – it increases the amount of air being exhausted and can result in depressurization.

Where does the air come from to balance the extra air being exhausted? Much of it comes in through unintended leakage points, but some of it comes in through the chimney. After all, the chimney is really just a hole in the building envelope. Chimneys are intended to carry smoke and combustion gases to the outdoors. But if the house reaches a certain point of depressurization, air will begin to move in the opposite direction: it will flow into the house, and can bring combustion gases with it. These gases include carbon monoxide (CO), an odourless, colourless gas that can kill people.

Depressurization can create other problems as well. Soil gases leaking into the house – including radon – is also a threat. And don’t forget about the real possibility of rain or water on the outside of the building envelope being pulled into the building by the depressurization.

Carbon monoxide is generated by any appliance that burns fuel – whether that fuel is oil, natural gas, propane, or wood. Under normal conditions, when the house is not depressurized, the CO simply goes up the chimney and exits the house. But if the house is depressurized, the CO can’t vent properly through the chimney and it “spills” back into the house. Any appliance that has a chimney is considered to be a “spillage-susceptible” appliance. Renovators should take them into consideration when air tightening a home.

In an attempt to limit the potential danger of CO not venting properly, many new houses use sealed combustion appliances. These appliances get their combustion air from outside, and they exhaust to the exterior, which means that they’re not spillage-susceptible. But that doesn’t mean we’re off-the-hook about depressurization. Soil gases (primarily radon) can still be a problem, and a CHMC study found that exhaust fumes from an attached garage can enter a depressurized house.

So how do you know if the house is depressurized? The best way is to hire a qualified mechanical contractor or certified energy advisor and have them do a test. The test for depressurization basically involves closing up the house (windows and doors), turning on the exhaust appliances, and measuring the pressure difference between indoors and outdoors. From this information, the amount of “make-up air” required can be determined. Make-up air is supplied by installing an active system that blows air into the house (i.e. a fan) whenever the house is depressurized.

How do we fix depressurization? Have a mechanical contractor install a make-up air system to supply the replacement air when the exhaust appliances are turned on. This keeps the house in “balance” and greatly reduces the likelihood of depressurization.

A less accurate, but still valid method to determine if spillage is occurring is to examine the appliances with chimneys and look for soot in places where it should not be if the appliance was working correctly. On water heaters this is the space on the top where the chimney connects. Look for soot or discolouration on the top of the unit near the chimney. For fireplaces, look for smoke stains on the face of the fireplace above the firebox and ask the homeowner if they need to open a window to get the fireplace to draw properly. While this doesn’t tell you how much depressurization is occurring, it does tell you that you need to be concerned about depressurization in this home.

As renovators, we’re all trying to do the best job we can for our customers. Making the house more energy efficient by insulating and air sealing will make the house more comfortable and will reduce the client’s energy bills. Keeping your customer safe in their home is also your responsibility, so don’t take a chance with depressurization. Check it out, install a proper make-up air system, and don’t forget that all homes need carbon monoxide detectors.


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