Thermally Broken vs Standard Aluminum Windows

Why aluminum needs a thermal break

Aluminum is strong, light, doesn't rot and holds big panes of glass in slim frames. It also moves heat very well. In a plain aluminum frame, the same piece of metal touches the cold outside air and the warm room, so on a cold morning the inside of the frame gets almost as cold as the outside. That's called a thermal bridge.

What a thermal break is

A thermal break is an insulating strip, usually glass-fiber reinforced polyamide, set between the outer and inner aluminum sections of the frame and sash. The two halves are mechanically locked to it, so the frame still acts as one strong piece. Heat has to cross the plastic to get through, and it does that slowly. Better systems use wider strips, several of them, and foam or extra gaskets in the gaps between.

Side-by-side cross-sections of a thermally broken aluminum frame with black insulating strips between two aluminum halves and a standard one-piece aluminum frame — Demo display. Left, a thermally broken frame: the black strips separate the outer and inner aluminum. Right, a standard frame: one continuous piece of aluminum.

What the frost test shows

In the photo at the top, both profiles were chilled from one side. The standard aluminum frame is coated in frost all the way through, because the cold traveled straight across the metal. The thermally broken profile in front stays clear on its room side. At home, that cold surface is where condensation forms every winter morning.

Side by side

	Standard aluminum	Thermally broken aluminum
Frame	One continuous piece of aluminum	Outer and inner aluminum joined by insulating strips
Heat through the frame	High	Much lower
Inside frame surface in winter	Close to outdoor temperature	Close to room temperature
Condensation and frost on the frame	Common	Much less
Energy code for a heated home	Hard to meet	Designed to meet it with the right glass
Where it makes sense	Unheated spaces, sheds, very mild climates	Homes and any heated space

What it means in a Seattle home

Our winters are long, damp and mild, and houses stay closed up, so indoor air holds a lot of moisture. Older aluminum windows from the 1960s to 80s often have no thermal break, and they're the ones that drip on sills, grow mold in corners and feel cold when you sit next to them. A thermally broken frame with triple glass and warm-edge spacers fixes the cold-frame part of that problem. Read more in window condensation in Seattle.

Callout (unattributed): "Plain aluminum takes the outside temperature and brings it into the room. A thermal break stops it at the strip."

Our aluminum windows

Every aluminum window we sell is thermally broken and comes with warm-edge spacers. MB-79N Pro is our value aluminum system and is NFRC certified, with a U-factor from 0.17. SlimLine Elite gives a slim steel look, is NFRC certified and has a U-factor from 0.23. MB-86N handles large, heavy glass, with a U-factor from 0.25 for casements. Your quote lists the rated U-factor for each window. See all aluminum windows.

What is a thermally broken aluminum window?

An aluminum window whose frame and sash are split into outer and inner sections, joined by an insulating strip that slows heat flow through the metal.

Do thermally broken aluminum windows still get condensation?

Much less than standard aluminum. With very high indoor humidity, some can still form on the glass, so keep up with ventilation.

Are thermally broken frames weaker?

No. The strips are glass-fiber reinforced and locked into both halves, and the frame is designed to carry large glass.

How can I tell if my old aluminum windows have a thermal break?

Look at the frame edge or a cut end. A plastic or rubbery strip separating the inner and outer metal means it's thermally broken. If the metal runs straight through, or the inside frame frosts or drips on cold mornings, it probably isn't.

Is thermally broken aluminum as efficient as uPVC?

Good systems with triple glass come close, and aluminum allows slimmer frames and bigger glass. Compare them in our windows overview.

Thermally broken vs standard aluminum windows: what's the difference?