With their vapour barriers, high-energy-efficiency doors and windows, and tightly sealed joints, today’s houses don’t let much air out or in. That airtightness is a good thing, but can be a shortcoming in some cases: when it causes negative pressure, for example.


A few explanations

When the mechanical ventilation units in your home (e.g., range hood, dryer vent, bathroom fan) are turned on, they remove air from rooms, venting it to the outside. In principle, the partial vacuum displaced by these appliances is immediately replaced due to natural infiltration of fresh air through gaps in the building envelope. This is called compensating air, or make-up air. In very-well-sealed houses, though, that rebalancing doesn’t occur naturally. This creates negative air pressure in the house—your house is literally sucking air to the outside, an effect that is also called depressurization.


Fuel-burning appliances—fireplaces, wood stoves and central heating systems fired by natural gas, heating oil or wood—and their chimneys often amplify the negative-pressure effect; the problem tends to worsen if these appliances aren’t sealed, or if they lack dedicated combustion air inlets connected to the outside.


Impacts of depressurization

Negative pressure can cause various problems in a house, some more serious than others:


  • Inefficient ventilation – Fans in the various rooms vent air less efficiently if the source of make-up air is weak. This results in poor indoor air quality.
  • Chimney backdraft or poor combustion – When heating appliances draw indoor air, there can be a danger of backdrafting. In an extremely airtight building, just turning on the dryer or range hood can reverse the pressure in a chimney, pulling smoke, fumes or cold air into the house.

Tip: If you are insulating your property to improve energy efficiency and you have a fuel-burning appliance, you should equip it with a combustion air inlet; if this can’t be done, you should change appliances. Also ensure that the duct connecting the air inlet is insulated, to reduce the risk of condensation.

  • Iced-up doors and windows – Cold air forcefully drawn inside the house can cause ice formation on warm surfaces around door and window frames, which can stop them from opening.
  • Increased radon infiltration – This radioactive gas, which is naturally present in soil, can seep into basements. Depressurization in a house can result in higher levels of infiltration—which is not something you want, because exposure to radon is hazardous to lung health.

What does the code say?

Current construction code standards require that a make-up air unit be installed if there is an unsealed fuel-burning appliance (gas, oil or wood) in the house. To work properly and prevent backdrafting of combustion gases into the house, the appliance must not only heat the air, but deliver a volume of air equal to that vented from the house.


A make-up air unit is also required in homes located in regions where underground gas seepage (e.g., radon) is problematic, and if there is no active system for attenuating those gases.


A range of solutions

A problem with negative pressure needs to be taken seriously. Here are some methods for monitoring and correcting the situation.


Rules for living in an airtight house

Learning to live in an airtight home means:


  • resisting the temptation to install a range hood or fans that are more powerful than they need to be. With a range hood that’s too powerful, you may even need to open a window to allow the unit to draw enough air.
  • not running too many fans at the same time.
  • staying alert to the quality of your indoor air and, if necessary, remedying the situation—sooner rather than later.


Degraded indoor air quality is not only a threat to the health of a house’s occupants, but to the “health” of the house itself.


Good to know

If you have an air exchanger, don’t count on it to compensate for air vented outdoors by other fan units. That isn’t its job: an air exchanger is designed to remove a given volume of stale air and replace it with a similar volume of fresh air.

Adding a simple exterior air inlet (i.e., not equipped with a fan) is not a solution either. For one thing, it isn’t code-compliant. For another, you would need a duct with a diameter of 25 to 30 cm (10 to 12 inches) to even begin to meet the demand—and that’s if the wind is blowing in the right direction!


Air compensators to the rescue!

In some cases, eliminating a problem with negative pressure requires adapted equipment that can compensate for the volume of air removed from the house. In Quebec, some heating appliance manufacturers have studied the issue and already offer solutions, such as make-up air fans with built-in heating elements.


Variously called “fresh air heaters” or “fresh-air intake units,” these compensating appliances draw in air from outside and pre-heat it. Depending on the type, they may distribute that pre-heated air to the house’s interior directly or through a central-heating duct network. The temperature of the heating elements adjusts to the volume of air flowing through the appliance, to save on energy. A ventilation specialist can advise you on whether you need this type of system, and can install one if you do.


Whatever solution you are planning, the important thing is to remain vigilant. For your health and that of your loved ones, don’t let negative pressure cause damage in your home!


Our thanks to Patrice Lévesque, of Novamech, a mechanical engineer specialized in ventilation and the co-author of the Guide des bonnes pratiques en ventilation, produced by the CMMTQ (Corporation des maîtres mécaniciens en tuyauterie du Québec) and CETAF (Corporation des entreprises en traitement de l’air et du froid).