Ask three people the main difference between a casement window and a double hung window and you will get the same sentence three times. One cranks outward. The other slides up. It is true, it is the first line of every buying guide on the internet, and it tells you nothing you can put on a purchase order.
Here is the difference that decides everything else. A casement sash is held shut by being pressed into its frame. A double hung sash is held up by a spring and held shut by little more than a sliding contact. One is a compression problem. The other is a counterweight problem that also has to keep the weather out.
Everything else people argue about flows from that one sentence: how much air gets in, where the insect screen lives, who cleans the outside glass, how wide the sash is allowed to be, whether you can escape through it, whether a room air conditioner will ever fit.
| The question | Casement | Double hung |
|---|---|---|
| How the sash is held shut | Pressed into a continuous perimeter gasket by the locking hardware | Held up by a balance spring, sealed by contact against the jamb channel |
| What has to be rated | Friction stay, rated by sash weight at a stated sash width and height | Balance, rated per jamb and matched to a weighed sash |
| What wears out first | Operator gearbox, friction stay, perimeter gasket | Balance, sash lock, meeting-rail weatherstrip |
| Is the wear item adjusted or replaced | Adjusted. Eccentric cams and hinge geometry re-tighten the seal | Replaced. Tired track weatherstrip is a new part, not a tweak |
| Which side of the wall the screen sits on | Inside, because the sash swings out past it | Outside, because both sashes move within the frame |
| How much of the hole can ever be open | The whole opening | About half, because two sashes share one plane |
| What you can never remove from the view | Nothing, unless you ordered a mullion | A horizontal meeting rail across the opening, permanently |
A casement sash closes against gasket that runs the full perimeter of the frame. The multi-point lock does not simply latch the sash, it pulls it in, so that the gasket is under compression all the way round. On an outward-opening sash the weather then works in the same direction as the lock: wind from outside presses the sash harder onto the gasket. The harder it blows, the tighter it shuts.
A double hung has no such arrangement, and it cannot have one. Both sashes must slide, so whatever seals them to the jamb has to be loose enough to let them slide. That means a contact seal, usually a pile or a flexible wiper, riding in a channel. On top of that, the two sashes meet each other in mid-air at the meeting rail, with nothing between them but an interlock profile and a cam lock. There are two sealing problems in a double hung, not one, and the second of them is a joint between two moving parts.
This is where the ownership experience diverges, and it is worth stating as plainly as possible: a casement that has started to leak is usually an adjustment problem. A double hung that has started to leak is usually a replacement problem. North American trade guidance puts it in almost those words: casement seals can be re-tightened through the hinge and locking hardware, whereas a hung window with tired seals needs new weatherstripping in the tracks. One is a screwdriver on a wet Tuesday. The other is a part number, a colour match and a visit.
The direction of the performance difference is not in dispute. Casements routinely post lower air leakage than hung windows of the same size and the same glass. What is not standardised is the number. Air leakage is measured under a named standard at a named pressure on a named specimen, and a fixed light will test better than an opening sash in almost every system ever built. So the useful request is never "what is your air leakage". It is: the report, the standard name, and confirmation that the thing on the test rig was the operating configuration and not a fixed panel that happens to use the same profile.
This is the difference that almost nobody writes about, and it is the one that decides what you are actually buying.
A casement never has to hold its sash up. Gravity goes through the hinge into the side jamb and from there into the wall. The hardware has exactly two jobs: carry the sash at the open position, and turn the force from your hand into enough compression to close the gasket.
A double hung has to cancel gravity before it can do anything else. Let go of the lower sash and it falls. The part that stops it is the balance, a spring mechanism hidden in the jamb channel, one per side, each sized to roughly half the sash weight. Take it away and the window is not a window, it is a guillotine with a latch.
An Australian supplier of aluminium double hung windows publishes the working ranges by balance type, and they are the most useful numbers in this whole comparison:
| Balance type | Sash weight it suits | What it is good at | What it gives up |
|---|---|---|---|
| Spiral, also called tube balance | Light to medium sash, commonly quoted to roughly 15 kg | Slim, fits a narrow aluminium jamb, tension adjustable with a tool | Ceiling on sash weight, so it is the first thing a heavier glass build breaks |
| Block and tackle | Medium to heavy, quoted to 30 kg and above | Pulley advantage carries real weight, parts stay available | Bulkier, cord routing makes replacement fiddlier |
| Constant force, coiled flat spring in a cassette | Light to medium, around 15 kg | Force stays even across the whole travel, smooth, corrosion resistant | Force is set at manufacture, so it is the least forgiving if the glass changes later |
Ranges as published by an Australian aluminium double hung window supplier. Treat them as that supplier's ranges, not as a standard.
A Chinese hung-window manufacturer publishes the sizing protocol even more explicitly, and it is worth reproducing because it changes how you read a quotation. Every finished sash is weighed after fabrication: profile, glass unit, hardware, glazing bead, all of it. That measured weight sets the balance specification. A sash that weighs 14.2 kg takes two balances rated 7.1 kg each, one per jamb. Their published stock of balance ratings runs from about 2.5 kg to 18 kg per unit, which is how you match a small upper light all the way up to a heavy triple-glazed lower sash.
Balance failure also has a signature, and it is nearly always misreported. A sash that slides down on its own. A sash that is suddenly stiff to lift. A sash that travels smoothly on one side and drags on the other, which usually means one balance has gone and the other is still working. A sash that slams shut, which is a finger trap in a house with children. None of these is a frame defect or a glass defect. All of them are one replaceable part inside the jamb.
Here the two window types stop being different at all, and this is the part that belongs on every enquiry form.
Both are limited by sash weight, and sash weight is dominated by glass rather than by aluminium. A working figure used across the industry is roughly 2.5 kg per square metre for every millimetre of glass thickness. A 6+12A+6 insulated unit therefore carries about 30 kg per square metre of glass on its own, before a single millimetre of profile is counted.
Take a 36x36 casement window. Thirty-six inches square is about 914 mm square, roughly 0.84 square metres. At that glass build the glass alone is around 25 kg. Add the aluminium sash and you are comfortably past 30 kg before anybody has asked a question.
Now read that one number against the two rating systems that exist to carry it.
Because a casement sash swings outward, its insect screen has to be on the inside. Because both sashes of a double hung move within the frame, its screen has to be on the outside. Every window guide states this in a single sentence. Almost none of them say what it costs you, which is strange, because it is the difference you will live with every day of the year.
| What follows | Casement, screen inside | Double hung, screen outside |
|---|---|---|
| Who cleans the mesh | Unclip it and wash it at a sink, on any floor | Reach the outside face. Screen specialists note openly that upper-level units may need a ladder or scaffold |
| What it is exposed to | Room air, nothing else | Sun, hail, wind-driven grit, all year round |
| What you look through | Mesh sits between you and the glass, always, and the crank handle passes through or beside it | Mesh sits outside the glass; you also have to choose half or full height |
| The choice nobody mentions | Whether the handle needs clearance through the screen frame | Half screen keeps the upper view clear but only vents the lower opening; full screen lets both sashes vent but puts mesh across the whole view |
| What wears first | The screen is opened far more often than the sash is | Same, on the other side of the wall, plus the track it sits in collects grit |
The practical instruction is simple and it is routinely skipped: specify the screen at the same time as the window, in the same enquiry. On an interior screen you need to know where the handle goes and whether the panel can still be released around it. On an exterior one you need to know how it comes out, and whether the second floor can be reached to do it. Retrofitting a screen decision after installation is where most of the small, permanent annoyances come from.
Two sashes sliding over each other in a single plane can never expose more than about half of their own opening at once. That is geometry, not marketing. A casement exposes the whole thing. This is the fact every comparison table leads with, and it is usually filed under ventilation.
The more interesting half of the same geometry appears when the window is shut, which is most of the time. A double hung permanently carries a horizontal meeting rail across the opening, because that is where the two sashes interlock and lock. You will look at that bar for the life of the building. A casement has no such bar unless you order a mullion, and if you do, it is vertical and you chose it.
The free-area difference is also why the two types come up in the same sentence as escape requirements. Where a room needs a second exit, a casement is frequently specified because the whole sash swings clear of the opening. But that requirement is local, and it is written as clear-opening dimensions, never as a product name. A basement that needs an escape is usually a casement in North America and a tilt and turn basement window across much of Europe. Same requirement, entirely different product. Write the clear opening in millimetres into the order and let the supplier tell you which of their products meets it.
A fully open casement sash stands clear of the wall. North American dealer guidance commonly quotes a projection in the region of 150 to 250 mm, but the real number comes from your sash width and the opening angle of the stay you are buying, so take it off the drawing rather than off a website. Everything inside that arc is then a design problem: a walkway, a porch railing, a shutter, an exterior light fitting, or another casement swinging the same way.
A double hung projects nothing. It is the only one of the two you can put over a public path, on a boundary, on a narrow balcony, or behind planting. It is also the only one of the two that will take a room air conditioner, because the lower sash closes down onto the unit and the unit sits in the opening. If a project brief has window air conditioners anywhere in it, that single fact settles the window type for those rooms, and every other argument in this article stops applying to them.
A casement puts its dead load and its wind load into the two side jambs, and the hinge side carries more of it. A double hung puts its balance hardware into the jamb channels and takes the upper sash load up at the head. So replacing one with the other in an existing opening is not a like-for-like swap: the load has moved to a different part of the frame. The person who should confirm the new path is your engineer, not your glazier.
A double hung's tilt latches let you release the sash and bring the outside face into the room. A standard outward casement cannot do that; the outside face is outside. On a ground floor that is a hose and a squeegee. On the eighth floor it is a rope-access contract. There are easy-clean and egress stays that change this, and they carry their own width ratings, which is the sort of thing worth asking about before the profiles are cut rather than after.
We make thermally broken aluminium windows and doors. Our published window line is outward-opening casement in three series, inward-opening tilt-and-turn in two, and one side-press sliding window. There is no double hung in it.
That is not a claim that nobody builds one. Aluminium double hung windows exist and are built properly, including by suppliers who publish their balance sizing by weighed sash, as described above. The claim is narrower and more useful than that: if your schedule says "double hung", the first question is not which double hung, it is whether "double hung" is the requirement or the habit. In our experience most briefs that use the word want one of three things.
| What the brief usually means | The aluminium answer | Why |
|---|---|---|
| Nothing may project past the wall | A slider, not a casement | Our side-press window exists precisely because a plain sliding sash must keep a gap in order to slide, and the side-press mechanism moves the sash out of the sliding plane to compress the seal instead. The comparison worth reading there is sliding versus casement, not hung versus casement |
| It must open inward and be cleanable from inside | Tilt-and-turn, not a hung window | This is the European answer to the same two wishes. The search that actually helps is the operation, not a brand: someone typing tilt and turn windows near me and someone typing double hung very often want the same thing |
| It should look like a hung window | A casement frame with a transom, ordered as such | A horizontal rail can be produced. Say so in writing, because it is a different product with a different load path and a different set of sealed joints, and discovering that after the profiles are cut is expensive |
So the most useful thing you can take away is not a preference for one window over the other. It is a short list of requirements to settle before anybody writes a product name: may anything project, who cleans the outside, how much free area the room needs, and what clear opening your local code demands. Answer those four and the label tends to pick itself, whether or not the word "double hung" appears anywhere in the document.
| Item | What we publish | What you still need to ask for |
|---|---|---|
| Thermal transmittance | K 1.3 for AW90, 1.6 for C120, 1.7 for C116 | These are system values, not whole-window values and not NFRC ratings. Our sliding window carries no published K. Ask for a calculated whole-window figure for your exact size and glass build |
| Air, water and wind resistance | No performance class published for any series | The test report, the standard name, and written confirmation that the specimen was the operating configuration rather than a fixed light |
| Corrosion and strength | 240-hour salt spray and 5,000 N shear resistance, both for AW90 | Do not transfer these to any other series. Ask each series for its own evidence |
| Hardware cycle count | 30,000 cycles on CMECH hardware, tested | A rig number, not a service life, and not comparable with a loaded full-sash cycle test on a different rig |
| Alloy, temper and thermal break | 6060-T66, PA66GF25 multi-chamber, 1.8 to 2.0 mm wall on windows | These are the same across our range, so they are not what distinguishes one series from another. Section depth, wall thickness, corner construction and glazing are |
| Rated sash weight | Not published | The single most useful number you can request, and it is meaningless unless you state the glass build it was rated with |
| Ask this | This answer sounds fine and tells you nothing |
|---|---|
| Rated sash weight for this sash width and height, and at which glass build | "It is heavy duty" |
| Which standard and which class were the air, water and structural tests run to | "It meets international standards" |
| Was the tested specimen the operating configuration | "The series is tested" |
| For a hung window: balance type, rating per jamb, and the weight stamp needed to reorder one | "Standard balances" |
| For a casement: friction stay rating, and the opening angle the projection was calculated at | "German hardware" |
| Which side does the screen mount on, and how does it come out on the second floor | "Screens included" |
| Net free opening and clear opening, in millimetres, on the drawing | "Standard size" |
| What is adjustable after installation, and who performs the first adjustment | "Fully adjustable" |
| Replacement parts by part number, and whether they are stocked or made to order | "Spare parts available" |
| Exterior clearance required at full open, in millimetres, shown on a section | "It opens fully" |
In direction, the casement, and it is not close, because compression beats contact as a principle. In practice, the more airtight window is the one that was adjusted correctly on site and still has its seals intact. Ask for the number, the standard it was measured under, and confirmation that the operating sash was the thing on the rig.
It depends which floor you are standing on. A double hung tilts in, so the outside face comes to you. An outward casement does not, so the outside face stays outside. On a ground floor that difference is twenty minutes. On an upper floor it is the whole decision.
Only in the hung window. The lower sash closes down onto the unit and the unit sits in the opening. A casement has nothing to close down onto, which is why portable units sold for casement windows need a panel kit and a hole.
Not in our published catalogue. We build outward-opening casement windows, tilt-and-turn windows and a side-press sliding window. If what sits behind the words "double hung" in your schedule is that nothing may project, or that the glass must be cleaned from inside, one of those three is usually the honest answer, and we would rather tell you that than quote you the wrong product.
Neither, in the sense people usually mean. Both are limited by their load path, and in both cases glass weight sets the limit. State the glass build you intend to use and ask for the rated sash dimensions at the same time. Asking for the largest sash without naming the glass produces a number that stops being true the moment you upgrade the glazing.
Only if the standard, the test pressure and the specimen are the same. A fixed light tests better than an opening sash in nearly every system, so if the report does not say which of the two was tested, it is not a number you can put next to anyone else's.
