Air sealing and blower door testing

Air carries more moisture than diffusion does

Two mechanisms move water vapour through a wall. It diffuses through materials, slowly, driven by vapour pressure. And it rides along with air moving through gaps, quickly, driven by pressure differences.

The second is dramatically the larger of the two in a typical house, which reorders the priorities most people arrive with. A small continuous gap moves more moisture into an assembly than the entire wall area does by diffusion, and it deposits that moisture wherever the air reaches its dew point, which is usually somewhere cold and hidden.

So air sealing is not primarily an energy measure. It is a durability measure that also saves energy.

Where the leaks are

Not the windows, mostly, which is where people look. The large leaks in a house are at the junctions and the penetrations, and they are concentrated at the top and bottom of the building because that is where the pressure differences are largest.

The recurring list is short: the joint between the foundation and the framing, penetrations for plumbing and wiring through the top plates, the attic hatch, recessed light fittings, duct and flue penetrations, chimney chases, the rim joist at each floor, and any interrupted line where an interior wall meets an exterior one.

Almost every item on that list is invisible in a finished house and trivially accessible during construction. That timing asymmetry is the whole argument for doing it properly the first time.

What a blower door actually does

A blower door is a calibrated fan mounted in an exterior doorway. It pressurises or depressurises the house to a reference pressure difference and measures the airflow required to hold it there. That flow is the leakage.

The result is a measurement of the whole building rather than a specification of its parts, which makes it the only envelope number in the project that describes what was actually built. It is usually expressed either as air changes per hour at the test pressure or as a leakage area, and modern energy codes commonly require a test with a maximum result.

The other half of the value is diagnostic rather than numerical. With the house held under pressure difference, leaks become findable: with a smoke pencil, with an infrared camera, or with a hand. A test run while the building is still open, before drywall, turns into a punch list. A test run at completion turns into a number and an argument.

The consequence nobody plans for

A tight house does not ventilate itself. That is the point of building it tight, and it is also the obligation that arrives with it.

ASHRAE 62.2 is the standard the industry works to here, and it addresses the problem in three parts rather than one: whole-dwelling ventilation, exhaust at the sources that produce moisture and contaminants, and control of those sources in the first place. [1] In a house built tight, mechanical ventilation is a requirement rather than an upgrade, and it is designed alongside the envelope rather than added to it.

Combustion appliances deserve specific attention in this context, and the correct handling is a design decision made with a mechanical professional rather than something to work out on site. Where a house is tight, sealed-combustion or direct-vent equipment avoids the problem by not depending on indoor air at all. Any arrangement where an appliance draws its combustion air from the living space needs to be assessed by a qualified professional against the tested tightness of the building.

The hazard has a name and it is worth stating plainly rather than gesturing at. An appliance that cannot draw the air it needs can spill its combustion products back into the house instead of sending them up the flue, and those products contain carbon monoxide. Carbon monoxide is colourless, has no smell, and produces early symptoms that resemble ordinary illness, which is why nobody notices it without a device that does. Working carbon monoxide alarms are required by code in most jurisdictions and are the only practical warning a household has.

If an alarm sounds, or if people in the house have headaches or nausea that clear up when they go outside, everybody leaves the building first. The call to the fire service or to the gas utility is made from outside, after everyone is out. Nobody investigates the appliance.

How to get this right on a project

Three decisions, all made before construction rather than after.

Name the air barrier on the drawings. Not the assembly, the specific material and the specific plane, traced continuously through every junction including the difficult ones at the roof-wall intersection and at intermediate floors. If nobody can point to it on a section, it does not exist.

Put a mid-construction blower door test in the contract, at the stage where the air barrier is complete and still accessible. It costs a fraction of the final test in consequences, because a defect found then is a tube of sealant and a defect found later is a ceiling.

And agree the target before anybody prices the work, since the required maximum comes from the adopted energy code and varies by jurisdiction and edition. [2] A crew that knows the number from the start builds differently to one told at the end. The same reasoning about specifying continuity rather than products runs through the front sheet under envelope.

It is worth being blunt about why this so often goes wrong, because the reason is structural rather than a matter of care. Air sealing is nobody trade. The framer is not responsible for the plumber holes, the plumber is not responsible for the electrician holes, and the insulator arrives after both and is paid to install insulation. Unless one party is named in the contract as responsible for the continuity of the air barrier, the work falls between trades, and it falls there quietly. Name the party. It is the single most effective sentence you can add to a construction contract on this subject.