Ventilation And Air Quality In A Sealed Dwelling

A factory-built dwelling is sealed better than a site-built one, for the same reason its tolerances are tighter: it is assembled indoors, in sequence, with the junctions accessible when they are made.

That is a genuine performance advantage. It also removes the uncontrolled air leakage that older houses relied on without anybody deciding to, which means the ventilation has to be designed.

Where the moisture comes from

A dwelling generates a surprising volume of water vapour daily: showers, cooking, washing, drying and the occupants themselves.

In a leaky building most of that left through gaps. In a sealed one it stays unless something removes it, and what it does instead is condense on the coldest surface it can find. Windows first, then wall junctions, then inside the wall.

Which is a durability problem as much as a comfort one, and it is the mechanism behind most mould in otherwise well-built houses.

What actually removes it

**Extraction at the source,** which means the bathroom and the kitchen, because that is where most of the vapour is produced.

**Ducted to outside.** An extraction fan discharging into the roof space moves the moisture from the room into the structure, which is worse than not extracting at all. This is a common and consequential shortcut, and it is one the [waterproofing](/blog/waterproofing-in-a-factory-built-building) discipline should never permit.

**Running long enough.** A fan on the light switch runs while somebody is in the room and stops before the moisture has gone. A run-on timer or a humidity sensor fixes that for a few dollars.

**Sized for the room,** rather than whatever was in the box.

**With a replacement air path,** because a fan cannot extract from a sealed room. Which means an undercut door or a vent, and in a tight dwelling a deliberate path from outside rather than from the roof space.

Natural ventilation

Openable windows positioned to give a cross flow, which means openings on more than one orientation rather than all on the same wall.

In an accessible dwelling window operation is a reach and force question: a resident who cannot open a window does not have natural ventilation, whatever the plan shows. Which puts window hardware in the same category as [switch positions](/blog/switches-outlets-and-reach-ranges).

Security and insect screening both affect real airflow, and both are frequently the reason a window that opens is never opened.

Balanced systems

Where a dwelling is built tight and occupied continuously, which describes most supported accommodation, a balanced system with heat recovery is worth costing.

It supplies fresh air and extracts stale air continuously, recovering most of the heat from the air it removes, so the dwelling ventilates without losing what the [thermal performance](/blog/energy-and-thermal-performance-in-modular) work achieved.

The arguments for it are strongest where residents are home all day, where a resident has a respiratory condition, and where the dwelling is heated for long periods.

It needs ducting space, a unit position, filter access and maintenance, all of which are design items and none of which are afterthoughts. Filters nobody can reach are filters nobody changes.

Why supported accommodation raises the stakes

Occupancy is higher and longer. Residents may be home all day. Some have respiratory or immune conditions. Drying may happen indoors more often. Bathing may be more frequent and involve more water.

All of which increases the moisture load and the consequences of not managing it.

What good looks like

Extraction in every wet area, ducted outside, on a timer or a sensor. A designed replacement air path. Windows a resident can open, on more than one wall. Filters somebody can reach. And, where the dwelling is tight and continuously occupied, a balanced system rather than an assumption.

Our standards.

MADEmodular / MADEbetter.