9-Step Plan to Line Curtains With Bloomsbury Interlining for 25% Less Heat Loss

Bloomsbury interlining is the soft brushed bump hidden between the face fabric and the cotton lining. Its weight sits in a practical middle range: dense enough to slow the convection current beside cold glass, yet manageable under a domestic sewing machine and a standard pencil pleat tape.

The fabric has loft, and that loft traps still air. Cotton sateen lining alone adds a neat back to the curtain and protects the face cloth from light, but it barely changes a heating bill. The bump is the working layer.

Measure before cloth is cut

Use a 5 metre steel tape for the recess and the drop. A fabric tape sags across a 2.4 metre drop, and a lost centimetre at this stage can show later at the sill or floor.

Take the window width at three heights and work from the narrowest reading. Old reveals are often out of square, and the tightest point decides how the curtain will cover the opening.

For a pencil pleat heading, the flat fabric should finish at about 2.25 times the track width. A 1.5 metre track therefore needs roughly 3.4 metres of flat face fabric per pair after side turnings are allowed for. That fullness matters because the curtain still has to close generously once the pleats are pulled.

Why loft changes the heat loss

Air meets the cold pane, cools, falls, runs across the floor, and is replaced by warmer room air at the top of the window. A thin lining has too little thickness to interrupt that loop.

Bloomsbury bump is a brushed cotton or cotton-blend, felt-like cloth. Its pile holds pockets of still air, and still air provides the insulation. Glazing studies carried out for the Edinburgh World Heritage and Changeworks report on traditional windows found that a well-fitted interlined or heavy curtain can cut heat loss at the window by 20 to 35 per cent compared with bare glass. Single glazing sits toward the top of that range. Modern double glazing gives a smaller gain because the glass unit is already doing much of the work.

If an interlined curtain hangs 4 centimetres away from the wall on a slim pole, warm air can spill over the top and travel down the back. The same cloth performs better when the curtain returns to the wall at both ends and the top is covered by a pelmet board. Heavy interlined curtains on a hidden track behind a pelmet can outperform the same curtains on an exposed pole, even where the pole looks more expensive.

Cutting, locking and lining the curtain

Cut the face fabric to the finished drop plus 20 centimetres for the hem and heading turnings. Cut the bump to the finished drop with no turnings. Trim the interlining so it sits just inside the fold of the hem.

Lay the face fabric wrong side up on a clean floor. Smooth the bump over it without stretching either layer. The two layers then need to be held together in vertical rows about 30 centimetres apart.

Use a loose, long catch stitch in matching thread for those rows. This lock-stitching stops the bump from sagging inside the curtain through repeated winters. If that stage is skipped, the interlining can settle down into the hem within two years.

Fold the side turnings over the bump and slip-stitch them in place. Then fold the bottom hem, mitring the corners so three layers do not form a hard wedge at each lower corner.

Lay the cotton lining across the back. Turn its edges under and slip-stitch it down 1.5 centimetres inside the curtain edges. That small setback lets the face fabric roll slightly to the back, so the lining is hidden from the front view.

Apply the heading tape last, stitching through all three layers. Pull the cords to the finished width only after the tape is secure. A 2.4 metre drop interlined pair weighs several kilograms, and that weight has to be considered before the track position is treated as a purely decorative choice.

Fixing a track that will hold

Interlined curtains expose weak fixings quickly. A pair across a 1.8 metre track can weigh 6 or 7 kilograms, with the load pulling downward and outward on the bracket screws every time the curtains are drawn.

Into a solid masonry lintel, use a 7 or 8 millimetre Fischer wall plug, such as the grey SX or the red UX universal. Drill with a matching masonry bit and blow out the dust before inserting the plug. A plug pushed into a hole full of brick dust holds only a fraction of its rated load.

Plasterboard causes many failures because the screw may bite only into 12.5 millimetres of gypsum. Find timber studs with a detector and fix brackets into them where the bracket spacing allows. Studs at 400 millimetre centres seldom fall exactly where every bracket is wanted, so the remaining positions need a metal spring toggle or a self-drilling plasterboard fixing rated for the curtain load. The small grey plastic plugs sold for picture hooks are not suitable for this job.

For a heavy interlined pair, spread the load across at least three brackets on a 1.8 metre track. Use four brackets once the track goes beyond that length, so the pull is shared instead of being concentrated on one fixing.

A deep reveal with hollow dot-and-dab plasterboard over masonry needs longer fixings. A standard 30 millimetre screw can finish inside the void. Use a frame fixing long enough to pass through the board, cross the cavity, and seat 35 millimetres into the masonry.

A heat-loss calculation for one sash

Take a single-glazed sash window measuring 1.2 metres wide by 1.6 metres high. That gives 1.92 square metres of glass. Single glazing has a U-value around 4.8 to 5.0 watts per square metre per kelvin.

On a winter night with the room at 20 degrees and the outside at 2 degrees, the temperature difference is 18 degrees. The bare glass loses roughly 1.92 times 4.8 times 18, which is about 166 watts through that one window.

A well-fitted interlined curtain that closes the convection loop acts, in rough terms, like adding another resistance in series with the glass. The measured improvement of a sealed interlined curtain comes in around a quarter off the window heat loss. On 166 watts, that is about 40 watts saved per window while the curtains are drawn.

Across a room with three such windows, drawn for fourteen hours of a cold day, the retained heat is well over a kilowatt-hour. Repeated through the cold season, that small hourly saving accumulates because the curtain uses no energy once it is hanging.

The same curtain on a modern double-glazed unit with a U-value near 1.4 saves far fewer watts in absolute terms. Interlining therefore earns most of its keep on old single glazing and listed sash windows where secondary glazing is not allowed, while its effect is much harder to notice in a new build.

Pelmet, sides and the lower edge

A pelmet board fixed 15 centimetres above the window reveal hides the track and helps seal the top of the convection loop. The board is part of the thermal detail, because a clear gap above the curtain lets warm air pass behind the fabric.

If the curtain finishes short of the wall at each end, cooled air can escape around the edges even when the fabric itself is thick. Run the return right back to the wall on both sides.

The gap the bump cannot cover

Interlining does nothing for an unsealed gap where the curtain meets the floor or sill. A 3 centimetre opening at the bottom can let the cooled-air stream move out into the room, leaving the loft in the cloth with much less to hold back. Finish the curtain so the hem just kisses the sill or floor, and the air the bump traps stays trapped instead of draining out under the lower edge.

Whether the same hem treatment will pay off on a draughty timber sash, where the frame itself leaks air around the rattle, is a separate question this build does not settle.