What decides window and glazing performance?

A window does several jobs at once: it carries the light, it is usually the weakest point in the thermal envelope, it is a significant acoustic path, and it is a security and compliance element.

Most of the attention goes to the specification. Most of the lost performance goes to the installation.

What the specification controls

**Frame material.** Aluminium conducts heat readily, which is why a standard aluminium frame can undo good glass. Thermally broken aluminium, timber and composite frames each behave differently, and the frame is frequently the limiting element rather than the glazing.

**Glass.** Single, double or higher, with coatings and gas fills changing the result substantially. The right answer depends on orientation rather than on a single best option.

**Orientation.** North, west and south-facing glass have different problems. West glazing is about summer heat gain and shading. South is about heat loss. Treating all elevations the same is the most common specification error.

**Shading.** External shading outperforms anything applied to the glass, because it stops the heat before it arrives. On the west elevation it matters more than the glazing upgrade.

What the installation controls

The gap between the window frame and the wall.

That gap is sealed, insulated and flashed, or it is not. Where it is not, air moves through it and water can enter, and the assembly performs well below its rating regardless of what was specified.

This is the most commonly compromised detail in Australian residential construction, because it is executed at the edge of a component, frequently in poor conditions, and it is covered immediately afterwards.

In a controlled environment it is done at a station, to the same detail every time, and inspected before anything conceals it. That is the same argument as [thermal performance](/blog/energy-and-thermal-performance-in-modular): a building achieves its designed rating only if the execution matches the drawing.

Acoustics

Glazing is usually the dominant path for external noise.

Thicker glass helps. Two panes of different thickness help more than two of the same, because they do not resonate together. A wider cavity helps.

And again the seal decides it. Sound travels through an air gap far more readily than through glass, so a well-specified acoustic window with an unsealed perimeter performs like an ordinary one.

Where the dwelling sits on a main road or under a flight path, this is worth resolving at design stage alongside the rest of the [acoustic strategy](/blog/fire-and-acoustic-performance-in-modular).

Where compliance overrides preference

**Bushfire.** At higher attack levels the glazing and the frames both carry requirements, and screening may be mandatory. That comes out of the [site assessment](/blog/bushfire-and-flood-resilience) rather than the design brief.

**Fall protection.** Openable windows above a certain height need restriction or protection.

**Safety glazing** in specified locations, including doors and areas of low-level glass.

**Accessibility.** Door widths, threshold heights and operable hardware. A level threshold at an external door is an accessibility requirement and a waterproofing detail at the same time, which is why it is designed rather than adjusted on site.

Doors take the load

External doors are used constantly and they carry hardware, weather and security duty at once.

A door that drops out of alignment stops sealing, and the thermal and acoustic performance goes with it. That is a frame and installation issue rather than a door issue.

Sliding doors are the common weak point, because the seal is a brush rather than a compression gasket and it wears at the ends first.

The summary

Specify for the orientation rather than uniformly. Shade externally where the sun is the problem. Then treat the perimeter seal as the item that decides whether any of it was worth paying for.

Discuss the glazing on your project.

MADEmodular / MADEbetter.