Blinds for Sash Windows Without Damaging the Joinery

A sash window is not a picture frame with glass in it. It is a machine with two moving parts, a pair of weights running inside hollow boxes either side, and a set of timber components that are meant to come off again. Most blind fitting instructions are written for a static frame, which is why so many blinds on sash windows end up fouling the sash fastener, blocking the shutters, or screwed into a bead that a joiner will have to break to get the window working again.

None of that means you cannot have blinds. It means the fixing points matter more than the fabric.

The one place you must never drill

The box on each side of a sash window is hollow. Behind the inner lining, a cast iron or lead weight travels up and down the full height of the window every time the sash is opened. Put a screw through that lining and one of three things happens: the screw sits in the weight's path and jams the sash, the weight abrades the cord against the screw until it parts, or you simply drill into nothing and the fixing has no hold at all.

So: no fixings into the inner lining of the box, and none into the pulley stile near the top where the pulley and the cord run. If you are not sure which timbers those are, our explanation of how a sash window actually works shows what is behind each face.

Two more components are off limits for different reasons:

• The staff bead, the moulded strip on the room side that holds the lower sash in. It is thin, it is pinned rather than fixed, and it has to come off every time anyone re-cords the window. A blind bracket screwed through it means a snapped bead and a fitter's bill.
• The parting bead, the upright strip between the two sashes, typically only 8 to 10mm thick. It is a wear part and it sits in a groove. There is nothing there to hold a screw.

Where you can fix, in order of preference

The head of the reveal. The soffit of the opening, above the window. Almost always the best answer: it is plaster over a lintel or a timber head, it takes plugged fixings, it is out of sight, and it puts the blind clear of everything that moves. Check the depth first (see below).

The face of the architrave or the wall above it. A face fix onto the moulded architrave surrounding the opening, or onto the wall above it. This is how most blinds are fitted in period rooms. Use short screws that bite into the architrave only — behind it, on most box sashes, is the same hollow box, so never drill through and plug. Keep fixings in the flat of the moulding rather than through a fine arris, and photograph the positions before you fill them if you ever take the blind down. On original mouldings, four small filled holes are a repairable change; a plugged 8mm hole through a bead is not.

The sash itself — with reservations. Lightweight blinds can be fixed to the top rail of a sash so they travel with it. This is the only arrangement that lets you open the window without disturbing the blind, and it works well for a small pleated blind. But every gram you add to a sash goes into a system counterbalanced within a few hundred grams. Each weight carries roughly half its sash; trade practice runs the upper sash marginally heavy so it does not creep down, and the lower sash marginally light so it does not creep up. Hang a kilo of blind and hardware off a sash and it will start drifting out of position, and eventually the balance has to be reset with different weights. Under about half a kilo you will usually get away with it. A lined roman blind, you will not.

Reveal depth: measure this before you choose anything

A recess-fit blind needs room front to back for the brackets, the roller or headrail, and the operating chain or cord. Published minimums from UK blind suppliers give you the shape of it:

On a sash window you do not measure to the glass. You measure to the nearest thing that sticks out, and on a closed sash that is usually the fitch fastener on the meeting rails, which stands proud of the timber by something like 20 to 40mm and sits right in the middle of the window. A blind that clears the sash face by 20mm will still catch the fastener every time it is raised. Measure the depth at the fastener, and measure the width in three places — top, middle and bottom — and work to the smallest.

Many London reveals are shallower than they look because the plaster returns to the frame quickly. If you come up short, an exact-fit blind mounted outside the recess is the honest answer: overlap the opening by roughly 70mm at the top and 50mm each side and the blind sits clear of everything.

If the window has original shutters

Thousands of London houses still have panelled shutters folded back into boxes in the splayed reveals, often painted shut for fifty years. Before you fit anything inside the reveal, open them. A recess-mounted blind will stop them ever closing again, and working shutters are worth considerably more than a blind — for blackout, for security, for heat, and for the character of the room. The comparison between original panelled shutters and modern alternatives is worth reading before you commit a reveal to a blind.

If the shutters are beyond use and you want the blind anyway, mount it on the face of the architrave so the shutter boxes stay clear for a later restoration.

Which blind actually suits a sash window

Top-down/bottom-up pleated blinds deserve a specific mention because they suit the way a sash window is used. They let you drop the blind from the top for privacy while the bottom sash is open at the sill, or raise it from the bottom while the top sash is cracked open for ventilation — which is how these windows were designed to be aired in the first place.

On thermal claims: cellular blinds hold still air in their honeycomb chambers and manufacturers publish heat loss reductions in the region of 40 to 55%, usually measured against double glazing under test conditions rather than a draughty Victorian single-glazed sash. Treat those numbers as directional. A blind reduces radiant and convective loss through the glass; it does nothing at all about air leaking around the sashes, which on an unrestored window is the larger share of the discomfort. If warmth is the goal, draught-proofing comes first and the blind second — the full ladder of options is set out in our guide to insulating sash windows.

The condensation trap

A close-fitting blind left down overnight makes the glass colder, because it cuts the room's warmth off from the pane. Moist air still finds its way behind the blind, hits colder glass and condenses, and the water runs down onto the cill where the blind is holding it against paint and timber. People then find black spotting on the reveal or along the bottom rail and blame the window.

Practical mitigations:

1. Leave a gap at the head and at the cill so air can move behind the blind. A blind sealed tight into a reveal is the worst case.
2. Raise blinds during the day, not just for the light.
3. Wipe the cills in cold weather. It takes seconds and it is the difference between sound paint and a stripping job.
4. Ventilate. The top sash exists for exactly this and almost nobody uses it.

If the glass is already streaming most winter mornings, the blind is not the cause but it will make it worse; start with why condensation forms on the inside of the glass and, if the spotting has already appeared, what actually clears black mould around windows for good.

Listed buildings and conservation areas

Internal blinds are not usually a planning matter. Article 4 directions in conservation areas control what the building looks like from outside, and a blind behind the glass is not that.

Listed buildings are a different question and worth taking seriously. Listing covers the interior as well as the exterior, and consent is required for works of alteration that would affect the building's character as one of special architectural or historic interest. In practice a blind bracket screwed to a later architrave is not what conservation officers are worried about, but drilling into original panelling, original shutters or original joinery is an alteration, and the cost of asking is a phone call. If you are anywhere near that territory, our note on when listed building consent applies to windows sets out the shape of the rule, and your local authority's conservation officer will give you a straight answer.

Before you order

Take three measurements — width at top, middle and bottom; drop from the head of the reveal to the cill; and depth at the sash fastener with the window closed. Open the shutters if there are any. Decide whether you will ever want to open the top sash, because that decides between a recess fit and a face fit. Then check that whoever is fitting understands that the beads are not fixing points.

If the sashes are painted shut, drifting, or rattling, it is worth sorting the window before the dressing goes up — everything above assumes a window that opens. Our sash window repair service covers every London borough, and getting the sashes running properly first means the blind you choose is the one you actually want, rather than the one that fits a window you cannot open.