Casement vs Sash Windows: Which Costs More to Keep

Most people do not choose between a casement and a sash. The house chose, two hundred years ago, and the question that actually arrives is whether to keep what is there or change it. Sometimes the question is sharper than that: a builder has suggested casements because they are cheaper, or a rear extension needs windows and nobody can agree what should go in them.

The two are not versions of the same thing. One swings shut on hinges; the other slides past itself on cords and weights. Almost everything else follows from that, including the question most people are really asking, which is what each one costs to keep.

The casement came first, and then came back

The English casement is the older form by a long way. Sixteenth and seventeenth century windows were mullion-and-transom frames with leaded lights, and where an opening light was wanted it was formed in a flat iron frame hinged into the rebate, closed with a hook and stay. Timber casements gradually superseded the iron ones and have been in continuous use ever since.

The box sash arrived later and arrived here first. The counterbalanced sliding sash, with weights and pulleys hidden in a hollow box frame, came into use in London in the second half of the seventeenth century and spread to the rest of the country through the first half of the eighteenth. The earliest examples are recognisable by heavy sections, glazing bars up to 50mm wide, and frames often worked out of solid timber rather than built up from separate linings. Only later did the design lighten into the sash box you see across most of London, and the number of panes fall as glass got cheaper and bigger.

Casements then came back. The revivalist architecture of the late nineteenth and early twentieth centuries reached for older window forms deliberately, which is why an Arts and Crafts house in Bedford Park or a 1930s semi in Kingsbury has leaded casements while the terrace two streets away has sashes.

What each one has to do to stay shut

This is the part that explains the rest of the article.

A casement is hinged. When you close it, the leaf swings into a rebate and stops against it. Everything the window needs to do about weather it does by compression: the leaf presses onto a seal, or onto bare timber, all the way round. Modern casements exploit this with a compression gasket in a routed groove, and with multi-point espagnolette locks that pull the leaf tight at three or four places along its length rather than just at the handle. That is genuinely good sealing, and it is available because the leaf is not going anywhere once shut.

A sash slides. The two leaves have to run past each other and past the frame for the whole life of the window, so nothing on the sliding faces can be compressed. The best you can do is a wiping seal that the sash brushes over: a pile of nylon or wool brush set into a routed carrier in the staff bead, the parting bead and the sash stiles. There is only one place on a box sash where genuine compression is possible, and that is at the meeting rails, where the two sashes push together at an angle when a fitch fastener draws them up.

That asymmetry is why a properly draught-proofed sash will still not test as tight as a good modern casement, and why the perimeter detail matters so much more on a sash. Get the pile height wrong on a brush seal and you either leave a gap or make the window too stiff to lift. We go through the sizes and the routing depths in our guide to brush seals for sash windows.

Casement types you will actually meet

Not all casements are the same, and the difference matters when someone quotes you.

• Flush casement. The leaf sits within the frame, its face level with the frame face. This is the traditional form and the one that looks right on a period building. It relies on the rebate and the seal for weather protection, so the joinery has to be accurate.
• Stormproof casement. The leaf has a rebated lip that laps over the face of the frame, throwing water clear before it reaches the joint. A mid-twentieth-century development, common on postwar houses. It weathers better and looks heavier.
• Top-hung and side-hung. Side-hung gives a bigger clear opening; top-hung sheds rain when open. Many casement windows combine a fixed light, a side-hung leaf and a small top-hung vent light over.

The sash equivalents run just as wide: counterbalanced box sashes, spring-balanced sashes with no weights at all, horizontal sliders, and mock sashes that are really casements dressed up with a false meeting rail.

What goes wrong, side by side

The pattern is that a casement fails at its hinge line and its bottom rail, and a sash fails at its balance and its cill. Casement faults are visible from the room. Sash faults are hidden inside the box, which is why people are surprised by them. If yours is a sash and it will not stay up or will not move at all, the diagnostic sequence is in our article on the stuck sash window.

Cost to keep, over twenty years

Nobody can give you a figure that is right for every house. What follows is the shape of it in London, on original timber, with the usual caveats about access, condition and how much scaffolding a borough forces you into.

Twenty years is about one overhaul, one draught-proofing job, one rot repair and three coats of paint. Added up, with the casement taken as a two-leaf window so the two are comparable in size:

So the honest gap is around a hundred pounds a window over two decades, and about a thousand across a ten-window house. Real money, but not the gulf people imagine. Two things move the total more than the window type does. Fixed glass costs nothing to maintain but paint, so a casement that is mostly fixed light with a single opening leaf halves both moving-parts lines above. And access dominates: a third-floor elevation needing a scaffold can cost more to reach than the joinery costs to do.

Timing matters too. Book the overhaul and the draught-proofing as one visit and you pay less than the two lines added together, because the sashes come out once and the beads come off once. That is what the full restoration line represents. Our breakdown of sash window repair costs goes job by job if you want to sanity-check a quote.

Set against all that, the sash has two genuine advantages. It is repairable almost indefinitely, because everything on it is a separate replaceable member: a bead, a bottom rail, a pocket piece, a cord. And a well-set-up sash needs almost nothing between overhauls beyond paint and a drop of oil on the pulley spindles. It is not a high-maintenance window. It is a window that gets ignored for forty years and then needs a proper day's work.

Thermally, they are closer than the salesman suggests

An original single-glazed timber window, casement or sash, sits somewhere around 4.8 W/m²K as a whole window. The glass itself is worse than that, at roughly 5.4 for plain 4mm float, and the timber frame is what drags the average down to something less awful.

The real difference between the two types is air leakage rather than conduction, and air leakage does not appear in a U-value at all. Where a modern casement genuinely wins is that its deeper rebate will swallow a full 24mm sealed unit without anyone noticing, while a period sash rebate is typically 10-12mm deep and needs a slim unit, a rebate deepened by a joiner, or a vacuum unit. That is a joinery constraint, not a physics one.

Where the regulations bite, and where they do not

Repair is not building work, whichever type of window you have. Approved Document L treats a controlled fitting as the entire unit including its frame, so splicing a rail, re-cording a sash, re-puttying or easing and rehanging a casement is not caught, and no U-value applies. Take the frame out and fit a new window and you are providing a controlled fitting, which does have a figure to meet; that figure, and the alternative the guidance allows where a building's character rules it out, are in our guide to what a window U-value actually means. FENSA and Certass certify replacement, not repair.

Swapping one type for the other is a planning question rather than a building-control one, and that is where the refusals come from. Replacing sashes with casements on a front elevation in a conservation area, or on any listed building, alters the proportion and the reading of a facade more than almost anything else. The detail on Article 4 directions and what they actually restrict is in our guide to windows in a conservation area. If the building is listed, consent is needed for the work regardless of what the windows are made of. As always, the borough is the authority on its own designations; ring the conservation team before you order anything.

Choosing, if you genuinely have a choice

You have a real choice on a rear extension, a loft conversion, an outbuilding, or a modern infill. Everywhere else the building has already answered.

1. Look at the head of the opening. A segmental or flat brick arch over a tall, narrow opening with a recessed reveal is a sash opening. A wide, low opening with a timber lintel or a soldier course usually wants a casement.
2. Look at the reveal depth. A box sash needs a box, and the box needs somewhere to sit. Thin modern walls make a genuine sash frame awkward and often force a mock sash, which fools nobody.
3. Count the storeys. Sashes come into their own on tall elevations, because they open without swinging anything into wind or over a pavement.
4. Ask what the neighbours have. In a terrace, matching the run is what a planning officer looks at first.
5. Be honest about the budget over time. If you will never repaint them, neither timber option is right, and you should read our comparison of uPVC sash windows against original timber first.

If what you have is a sash, and it is tired

Start by finding out whether the problem is the timber or the mechanism, because they cost very different amounts. Push the bottom sash up to about waist height and let go. If it drops, the balance is out and that is cords, pulleys or weights. If it will not move at all, that is paint, swelling or a fallen weight. If it moves fine but the room is cold and noisy, the timber is probably sound and you are looking at a sealing job rather than a restoration.

Press a bradawl or a penknife into the bottom rail, the cill and the base of each pulley stile. Sound timber resists; rotten timber takes the point with no effort. Those four spots will tell you most of what a surveyor would.

Our sash window restoration work covers the whole sequence from splicing timber to rebalancing and draught-proofing, and the wider sash window repair service runs across every London borough. If it turns out to be casements you are dealing with, the same principles apply: repair the leaf, keep the frame, and only replace when the joinery has genuinely gone past saving.