Why draught-proofing original sashes is worth doing properly

Original sash windows are one of the pleasures of a period or rural home — the deep reveal, the soft light, the way the upper sash drops a few inches on a summer evening. They are also, in a stiff easterly wind, one of the fastest ways to lose heat. A single tall sash can leak along six or seven separate edges, and the cold air arrives at ankle and shoulder height, exactly where you sit.

The reassuring news is that most of that loss is not the glass. It is the gaps around the moving parts. Seal those discreetly and you can take the chill off a room without touching the joinery, without replacing the window, and without losing the sliding movement that makes a sash worth keeping. The rule is simple: brushes and slim seals, nothing that jams, nothing that marks.

Find out where the cold air is actually coming from

Before you buy anything, spend ten minutes on a windy day with a slip of paper or a thin strip of tissue. Hold it against each edge in turn and watch which way it lifts. Feel with the back of your hand, too — the wrist is more sensitive than the fingers.

The usual suspects are:

• the meeting rail, where the two sashes overlap in the middle
• the gap between each sash and the staff bead, the inner bead on the room side
• the parting bead, tucked between the inner and outer sash
• the pulley stile at the top of each run, where the cord passes over the wheel
• the hollow box in the jamb itself, which can let air in at top and bottom
• the junction between frame and wall, especially where old plaster has shrunk away

Measure the widest of these gaps with a feeler gauge or folded card. Most period sashes have 2mm to 4mm of clearance, but worn frames can be closer to 6mm, and the correct seal differs for each.

Choose brush pile and slim carriers over bulky foam

Brush pile in a slim carrier is the material that suits a sash best. The carrier is typically around 4.8mm to 6mm wide, with brush lengths of 5mm, 7mm and 10mm. It compresses lightly against the face of the sash, stops the bulk of the draught, and still lets the window run.

Wide self-adhesive foam strips are the wrong tool here. They crush, crumble, turn tacky in damp weather and need clearance you do not have. Reserve rubber for the places where nothing slides — a slim silicone or EPDM bubble gasket works well on a meeting rail. On any surface that has to travel past another, use brush.

Buy a carrier with a good backing. Self-adhesive is fine on a clean, sound painted surface, but pin or glue the ends as insurance. A solvent-free contact adhesive or a thin bead of clear adhesive holds better and is still reversible with a hairdryer and a little white spirit.

Fitting brush seals without disturbing cords or weights

Work with the sashes in place and check movement after every length you fit, not at the end of the job.

• Wipe the rebate with white spirit and let it dry. Dust and old wax are why seals fall off.
• Set the brush so it overlaps the sash face by 1mm to 2mm. Any more and you add friction.
• Cut brush slightly long and compress it into the corner, or mitre neatly. Avoid gaps at the corners, where draughts sneak through.
• Run the seal right up to the pulley block, but never over the cord. A small pulley seal around the cord is fine; the cord must run free.
• Never pack the box cavity with insulation. It traps damp and stops the weights doing their job.

The meeting rail, the beads and the sash lock

The meeting rail is usually the worst offender. Fit a brush along the top edge of the lower sash, brushing against the underside of the upper sash bottom rail. If the gap there is wider than 4mm, add a slim bubble gasket in the rebate instead — but check that the two sashes still pass each other cleanly.

Along the sides, brush goes on the staff bead inboard and the parting bead between the sashes. Because sash stiles are slightly tapered, the sides often need a marginally deeper brush than the top. Once everything is fitted, close the window and test the sash lock. Added thickness can stop the cam engaging, and a lock that will not catch is a security problem rather than a draught one.

Keeping ventilation and sash balance in mind

An old window is part of the way the house breathes. Seal every gap and the moisture has nowhere to go, which shows up as condensation on cold glass and, in time, damp in the reveal. Leave trickle vents clear if you have them, and keep kitchen and bathroom extractors working.

Sash weights are matched to the weight of the sash. A brush that is too tight adds friction, and the upper sash will drift down of its own accord. If that happens, reduce the overlap rather than adding lead. Dust the brushes once a year with a vacuum brush attachment, re-test on the first cold morning, and top up any seal that has flattened. Done this way, the work is invisible, reversible, and the window still moves exactly as it should.