French casement window, made to measure

A standard double window has a mullion that stays in place even when the sashes beside it open. A French casement doesn't have one: both sashes are hinged at the outer edges and close against each other in the middle with a draught seal, not against a fixed mullion. Closed, it looks like an ordinary double window; open, the whole width is clear, with no mullion in the way. If you're replacing an existing window without a mullion, you'll recognise this form straight away: it is the one that recreates that look.

Both sashes here are side-hung casements that open inward, not tilt and turn: you get the full opening, but not the tilt position for an ordinary gap. The standard size is 1,600 × 1,500 mm. If you want the same wide opening with a tilt position on one or both sides, look at the standard double windows (the 2-pane window with one tilt and turn sash, the 2-pane window with two tilt and turn sashes), which have a fixed mullion.

That wide opening does come at a price: without a mullion, neither sash supports the other, and the closure in the middle has to do the job alone with the lock and the draught seal. In a wide, tall French casement with heavy glass, that calls for strong hardware on the hinge sides, because those two points carry the whole weight that in a standard double window would be spread over four points and a mullion.

Within the window group, this layout differs most from the rest: no other form leaves out a fixed mullion between two sashes side by side. Don't compare it with the side-hung casement (that is one sash, not two closing against each other) but with the 2-pane window with two tilt and turn sashes: both have two moving parts, but the French casement gives you the clear opening and the 2-pane window with two tilt and turn sashes the tilt position. Which of the two you need depends on whether you'd rather have a wide clear opening or a gap without opening the sash fully.

When measuring for a French casement, the opening between the two hinge points needs extra care, because there is no mullion to absorb small deviations: the two sashes have to meet exactly in the middle to close weathertight. So, even more than with a standard double window, have the opening checked on site before you give us the final size.

Maintenance on a French casement needs a little more attention than on a standard double window: the draught seal in the middle, where the two sashes close against each other, is a part a window with a mullion doesn't have, and over time it can harden or wear. Colour and finish follow the same chart as any other pane. Because both sashes of a French casement are identical — no fixed part and no opening part that differs — most people choose exactly the same colour on both sides, which also best recreates the look of a traditional double window.

When a house with a traditional elevation is renovated, a French casement is often the form that does the existing look most justice, where a standard double window with a mullion would look more modern. In a new build people sometimes deliberately choose it to bring that same classic look back into an otherwise contemporary house, as a contrast with the rest of the elevation.

For break-in resistance, the closure in the middle, with no fixed mullion, is the point to watch on a French casement: the two sashes have to lock firmly into each other to offer as much resistance as a window with a fixed mullion. Ask about the locking points designed specifically for this form.

The weight of the glass counts for more in a French casement than in a standard double window, because the two hinge sides carry the weight without a mullion to help. The configurator checks this as soon as you enter the size and the glass.

Still not sure? Use the comparison with the other layouts in this group above to see which arrangement suits your situation best.