Ventilation: Net Free Area, Air Change and the Openings That Deliver Them

Two ways of specifying the same thing, and they do not convert

Ventilation requirements come in two forms. One states an AREA: one square unit of opening for every hundred and fifty of ceiling, so many square millimetres per kilowatt of appliance input, a proportion of a cladding's wall area. The other states a RATE: air changes per hour, litres per second per person, cubic feet per minute for a hood.

An area rule is a proxy. It says nothing about how much air actually moves, because that depends on a pressure difference the rule has silently assumed — a wind speed, a stack height, a fan. A rate rule states the outcome directly and leaves the opening to be derived from it.

The two therefore cannot be converted without supplying the missing pressure, and the most common error on this subject is treating them as interchangeable. An attic that satisfies its area ratio is compliant; whether it is drying is a different question with a different answer, and a rate calculation for a fan-assisted space says nothing about whether an unpowered opening would achieve it.

Net free area: the hole is not the opening

Every area rule in every code is written against NET FREE AREA — the clear area air can actually pass through — and almost every real opening is obstructed. Blades, frames, mesh and louvre geometry all take their share, and what remains is a fraction of the hole that was cut.

The fractions are large enough to change decisions, and they differ sharply by product. The fuel gas codes give defaults where no tested figure exists: about three-quarters of the gross area for a metal louvre or grille, and about a QUARTER for a wooden one — a factor of three between two things called the same word. Architectural weather louvres, which are built to reject rain rather than to be open, commonly sit between a third and a half. Insect mesh passes roughly half to two-thirds depending on the weave, and a louvre WITH a screen behind it multiplies the two together.

So the calculation runs backwards from the requirement: take the required net free area, divide by the assembly's free-area ratio, and the result is the gross opening to build. An opening sized on its gross dimensions instead can deliver a QUARTER of what the rule demanded, and it will look correct on every drawing.

Where a product has a tested free-area figure — manufacturers publish one from a standardised test — that is the number to divide by, and the code defaults above are fallbacks for when it does not. This is the single most useful thing on this page, because it is simple, it is routinely missed, and getting it wrong produces an assembly that passes inspection and does not work.

Balance, not total: an unbalanced roof ventilates the house

A vented roof works by the stack effect: cool air enters low at the eaves, warms, rises, and leaves high at the ridge. That requires openings at BOTH ends, and codes that allow a reduced total ventilation area almost always attach a condition that the openings are split roughly evenly between high and low, with the upper portion capped.

The reason is what happens when they are not. High vents with insufficient intake still draw the air they need, and they take it from the easiest available source — which, in a house with a leaky ceiling plane, is the conditioned space below. The roof then depressurises the building slightly and pulls warm moist indoor air into the attic, which is precisely the moisture the ventilation was installed to remove. More ridge vent, on its own, can make an attic wetter.

Baffles exist for the same reason at the other end. Insulation pushed into the eaves blocks the intake path, and a vent area that is correct on paper delivers nothing if the channel behind it is full. A baffle holds the path open and keeps the insulation out of it, which is why baffle counts are set by rafter bay rather than by area — every bay needs its own path, and an unbaffled bay is simply not ventilated.

Cavities are drained and vented, which is not the same job

The ventilation behind a rain screen or in a masonry cavity is not there to cool anything. It is there to DRY — to remove the small amount of water that gets past an outer leaf that was never assumed to be watertight, and to let vapour leave rather than accumulate against the sheathing.

That is why the areas involved are small compared with roof ventilation and are usually specified per metre of wall rather than as a fraction of it. It is also why the openings are at the top AND bottom of each drained zone: without both, there is no path, and a cavity vented only at its head has a stagnant lower half exactly where the water collects.

Weep holes carry a second function that gets forgotten. As well as draining, an open perimeter allows the cavity to PRESSURE-EQUALISE, so that wind pressure on the face of the cladding appears on both sides of it rather than driving water inwards through every joint. A cladding detailed as pressure-equalised and then built with sealed or blocked weeps behaves as a face-sealed one, which is a different and less forgiving system.

The practical failure is mortar. Droppings at the base of a cavity bridge the gap and block the weeps, which is why a cavity tray, a drainage mesh and clear weeps are specified together rather than as alternatives.

Combustion air is the case where the consequence is not discomfort

Air for a fuel-burning appliance is sized from its INPUT RATE, and the rules are written per unit of input because that is what sets the oxygen demand and the flue's requirement. The area rules differ by whether the air comes from inside the building or directly from outdoors, and by whether the duct is horizontal or vertical, because those change the driving pressure.

What changed under these rules is the building. They were written when houses leaked enough that a room borrowed what it needed through the fabric. Modern airtightness has made that assumption false, and a sealed house with an appliance drawing from an inadequately supplied space can be pulled into depressurisation by a kitchen hood or a dryer, at which point the flue can BACKDRAFT and combustion products enter the room.

That is why this is the one ventilation calculation on the site whose failure mode is not discomfort or a damp deck, and why the pages say so. It is also why direct-vent and sealed-combustion appliances have largely replaced the calculation: they take their air from outside through their own concentric path and are indifferent to the pressure in the room. Where an open appliance remains, a combustion safety test under worst-case depressurisation is the check that matters, not the arithmetic.

Where the rate rules fail: short-circuiting, and the energy bill

A rate requirement assumes the air delivered actually reaches the people or the contaminant. Supply and extract placed close together short-circuit, so a room can receive its full nominal air change while a corner of it receives almost none. Ventilation effectiveness is the correction for this, and it is why displacement and mixing layouts are not equivalent at the same flow rate.

The other failure is thermal. Every unit of outdoor air brought in has to be heated or cooled, and at the rates modern standards ask for that becomes a significant part of a building's load — which is the reason mechanical ventilation with HEAT RECOVERY exists at all. A recovery unit is rated by an effectiveness, a fraction of the temperature difference it recaptures, and that figure is net of its own fans only if the rating says so.

Two caveats belong on any recovery sizing. Effectiveness falls when the airflows are unbalanced, because the exchanger can only recover from the smaller stream; and in cold climates the unit spends part of its winter in DEFROST, during which it is not delivering its nominal rate. The nominal ventilation rate and the delivered annual average are different numbers, and a design that needs the nominal one continuously needs to say how defrost is handled.

The alternative to all of it is measurement: a blower-door test for the leakage the area rules assume, a flow hood or tracer gas for what an installed system actually delivers. The calculators here size an opening and estimate a rate, and every one of them is a design-stage tool rather than evidence about a finished building.