Can a House Be Too Airtight? No, But It Can Be Under-Ventilated: Bath Fans, the HRV, Combustion Appliances, Backdrafting and What to Check After Air Sealing an Ontario House

What changes when the house gets tight

A leaky house at 8 ACH50 swaps about half its air every hour on a windy winter day and almost none on a still one in May. That is not ventilation, it is weather. It dries the house out in January, which is why old houses get static shocks, and does nothing in the shoulder seasons when the bathroom is steaming. Seal it to 4 and the winter air change drops by half; the humidity comes up, the windows start to fog at the bottom corners, the cooking smells hang around, and anyone with a natural-draft water heater is now running an experiment on which way the flue flows.

None of that is a reason to leave the holes. All of it is a reason to look at the fans before the first cold snap. The blower door guide has the benchmarks by house age; the line where ventilation stops being optional is about 4 ACH50 for an existing house, and every new house under SB-12 is below it from day one, which is why the packages carry an HRV.

The five checks the week after sealing

1. Every bath fan actually moves air, and moves it outdoors. Hold a square of toilet paper to the grille with the fan running; it should stay there. Then go to the attic or outside and find where the duct ends. A fan that ends in the attic insulation is wetting the attic you just sealed, and it is common; the bath fan page covers the fix. A fan on a timer or a humidistat, 20 minutes after every shower, is the cheapest ventilation there is.
2. The range hood vents outside, not through a charcoal filter back into the kitchen. A recirculating hood is a noise machine. Cooking is the largest source of moisture and particulate in a house, and in a tight one it has nowhere to go.
3. The combustion appliances draft with the fans on. See the next section. This is the one that matters for carbon monoxide, and it takes ten minutes.
4. Winter humidity stays between 30 and 40%. A $15 hygrometer on the counter. Over 45% in January means condensation on the windows and, out of sight, on the cold side of whatever insulation the air reaches. The leak or condensation page is for when the stain shows up.
5. The HRV, if there is one, is running and clean. Filters every three months, the core washed yearly, the outside hood clear of snow and lint. An HRV with a blocked core is an expensive hole in the wall.

Backdrafting: the one that can hurt you

A natural-draft water heater or an older furnace sends its exhaust up a flue on nothing but the buoyancy of warm gas. That works when the house is at the same pressure as outdoors or slightly above. Run the bath fans, the range hood and the dryer at once in a house that is now tight, and the fans pull the house pressure down below outside; the easiest place for make-up air to come in is the flue, and the exhaust comes with it. That is backdrafting, and in a leaky house the top plates supplied the make-up air so nobody noticed. Seal the plates and the flue is the hole that is left.

The test: close the windows and exterior doors, run every exhaust fan and the dryer, run the water heater until the burner has been on a minute, and hold a smoke pencil or a lit incense stick at the draft hood, the gap at the top of the tank where the flue starts. The smoke should go up the flue. If it drifts into the room, the appliance is backdrafting under the conditions the house now runs in. The fixes are a sealed-combustion or power-vented water heater and furnace, which take their air from outside through a pipe and are immune to house pressure, or a make-up air supply interlocked with the big fans. An energy advisor does this test as part of an EnerGuide evaluation; the home energy audit page explains what else is in it.

What an HRV does and why SB-12 wants one

A heat recovery ventilator is two fans and a core. One fan pushes stale air out, the other pulls fresh air in, and the two streams cross in the core without mixing, so the outgoing 20 C air warms the incoming minus 15 C air to about 10 C before it reaches the ducts. The house gets a controlled, balanced air change, roughly a third of an air change an hour, without the pressure swings that cause backdrafting and without throwing the heat out the wall. It is the only device that makes a house under 3 ACH50 a good place to live, which is why every SB-12 package carries one and OBC 9.32 requires mechanical ventilation in every new house.

The size of it comes from the ventilation design, which comes from the heat-loss calculation; a house that was sealed from 9 to 4 ACH50 has a different ventilation requirement and a smaller heating load, and both are inputs. OntarioHeatLoss.ca produces the stamped design that the HVAC contractor sizes the equipment and the HRV from, and the heat-loss page explains when one is required. Adding an HRV to an existing house is an HVAC job priced from that design, not an insulation line, and a bath fan on a timer is the first step if an HRV is not in the budget this year.

Radon, mould and the other things people blame on tightness

Radon comes up through the slab and the foundation cracks, and a tighter house with less air change holds more of it, which is the honest half of the “too tight” worry. The answer is not leaks; it is sealing the slab edge and the penetrations, which is part of air sealing anyway, and testing. The OBC requires a radon rough-in in every new basement, and the radon requirements page covers what that is and what a test costs in an existing house.

Mould in a tight house is a humidity problem, which is a ventilation problem, not a tightness one. The same house at 45% humidity and 8 ACH50 grows mould on the cold corners too; the leaks just dried it out a little more often. A house at 35% with the fans running and the HRV on does not. Where the mould was already there, in wet batts or on the back of old insulation, it does not get buried under new insulation; it gets removed, dried and fixed first. The mould page has what that looks like.

The last one: the old argument that a house that cannot breathe rots its walls. Walls rot from water, and in an Ontario wall most of that water arrives as air leakage, warm house air condensing on cold sheathing. The tight house with a vapour barrier on the warm side and a wall that can dry outward is the one that lasts; the air barrier checklist is how it gets built that way.

Questions people ask

Sources and further reading

• Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
• Ontario’s Building Code: the official page with the current regulation; vapour barriers are 9.25, attic ventilation 9.19, thermal barriers over foam 9.10.17, radon 9.13.
• Addressing moisture and mould in your home: Health Canada: what to do about wet insulation and mould before anything gets covered up.