Ventilation requirements

The obligation arrives with the envelope

Older houses ventilated themselves, badly and expensively, through gaps. Air moved because the building leaked, driven by wind and by temperature difference, at a rate nobody chose and nobody could predict.

Building tight removes that mechanism, which is the point: it is where most of the energy saving and most of the durability benefit come from. It also removes the accidental air exchange the occupants were relying on, and that has to be replaced deliberately.

ASHRAE 62.2 divides the replacement into three jobs, and a design that does only the first of them is incomplete: continuous ventilation for the dwelling as a whole, exhaust at the two or three places that generate most of the moisture and contamination, and control of what those sources emit in the first place. [1] All three are design decisions rather than products, and all three are cheaper as drawings than as retrofits.

Three strategies, and what separates them

Exhaust-only ventilation runs a fan that removes air from the house, so replacement air enters through whatever openings exist. It is the least expensive arrangement and its weakness is that you do not control where the replacement air comes from, which in a tight house may be a crawlspace, an attached garage or an attic.

Supply-only ventilation does the reverse, introducing filtered outdoor air deliberately and letting air leave through the envelope. It gives control over the incoming air and slightly pressurises the house, which in a cold climate pushes interior moisture toward the assemblies.

Balanced ventilation supplies and exhausts roughly equal amounts, which avoids both pressure effects and controls both directions. It costs more and it is the arrangement that recovery equipment requires.

ERV and HRV, and the climate question

A heat recovery ventilator transfers heat between the outgoing and incoming air streams, so incoming air is tempered by the air being expelled without the two mixing. An energy recovery ventilator transfers moisture as well as heat.

The distinction matters most in humid climates. In summer, outdoor air brings humidity with it, and moisture transfer means less of that humidity reaches the interior, which reduces the load on the cooling system that would otherwise have to remove it. In a cold, dry winter climate the same mechanism retains interior humidity that would otherwise be expelled, which can be an advantage in a house that runs dry.

There is no universal answer, and the honest version of the guidance is that the choice follows the climate and the house rather than a general preference. A mechanical designer will make the case for one or the other for your building. If the case offered is that one is simply better, ask which climate that answer assumes.

Local exhaust is the half that gets forgotten

Whole-dwelling ventilation deals with the general condition. It is a poor tool for the concentrated moisture and contaminants produced in two rooms.

A bathroom generates a large amount of water vapour in a short period, and a kitchen generates moisture, combustion products where cooking is by gas, and grease. Both need exhaust at the source, ducted to the outside, sized for the job and controlled in a way that means it actually gets used. A fan too noisy to tolerate is a fan nobody switches on, which makes noise rating a functional specification rather than a comfort one.

The recurring failures are mundane and they persist for years: an exhaust fan that discharges into an attic rather than outdoors, a duct run so long and convoluted that the fan moves almost nothing, and a recirculating kitchen hood in a house whose ventilation strategy assumed the kitchen was exhausted. Each is easy to specify correctly and difficult to discover afterwards.

The kitchen case carries a hazard the others do not, and it deserves naming rather than implying. Burning gas indoors produces carbon monoxide along with everything else, and a recirculating hood filters grease while returning those products to the room. That is the argument for a hood ducted to the outdoors wherever the cooking is by gas, and for carbon monoxide alarms, which most jurisdictions require and which are the only way a household learns about a gas it cannot see or smell.

If an alarm goes off, the sequence is not a diagnostic one. Everyone leaves the building, and the fire service or the fuel utility is called from outside once they are out.

How this connects to the rest of the building

The tighter the envelope, the more of the ventilation burden falls on the mechanical system, which is the argument for designing the two together. A blower door result is an input to the ventilation design rather than a score at the end of the project, which is why the mid-construction test on the contents sheet under envelope is worth more than the one at completion. [2]

It also connects to the heating and cooling decision, because a ventilation system moving air is either integrated with the distribution system or independent of it, and that is a decision with duct implications. Made early it is a layout question. Made late it is a compromise.

And it connects to how the house will actually be lived in, which is the input nobody asks for. A household that dries laundry indoors, cooks heavily, or keeps a large number of plants generates substantially more moisture than the design assumption, and a system sized to a minimum will be working at its limit permanently. That is worth saying out loud to whoever designs it, because it is the one thing about the building that only the occupants know.