John Lewis Curtain Linings That Cut Heat Loss by Up to 25%

The quoted saving is measured at the window

The Energy Saving Trust has long treated an uninsulated single-glazed window as a real source of heat loss in a room. Lined curtains remain one of the lower-cost ways to reduce that loss, especially in older homes where the windows are kept for cost, planning, or appearance reasons.

The upper figure of 25% often attached to thermal curtain linings belongs to the glazed opening itself. It describes the drop in heat escaping through one window when a close-fitting lined curtain is drawn across it. A loose panel with open edges may land near 10%. The strongest result comes from a curtain that traps the air beside the glass and keeps that air still.

A John Lewis thermal lining hung behind cotton or linen face fabric works by creating a still layer between the room and the pane. Still air provides much of the insulation. The lining is commonly a tightly woven polyester with an acrylic or aluminium-flecked coating on the reverse. The dense backing helps, although the trapped air pocket carries a large share of the effect.

Two curtains cut from the same fabric can give different results. Fix one close to the architrave, add a pelmet, and the warm room air has fewer routes into the cold space by the glass. Hang the other on a pole with a 4cm gap at the head and the same open margin down each side, and warm air rises behind the fabric, cools on the glass, then escapes at the top. The cavity has become a moving air channel.

Measure the window and the surrounding wall before comparing product descriptions. Thermal performance starts with the completeness of the seal between room and glass. A curtain ending 10cm above the sill leaves a continuous slot. Warm room air travels behind the fabric, chills on the pane, and falls into the room as a low draught. Colder glass speeds that loop.

Three details interrupt it. A pelmet or deep covered track across the top blocks the exit route for rising air. Returns, where the outer curtain edges bend back to meet the wall, close the sides. A drop that reaches the floor, or pools by 2cm to 5cm, closes the lower opening and slows the cold spill below the window.

A made-to-measure curtain with bump interlining between the face fabric and lining behaves as a heavier soft barrier. It puts extra load on the track, needs a sturdier pole or corded system, and carries the per-pair cost well beyond the ready-made lining band. In a north-facing room with single glazing, that construction is the one most likely to approach the upper end of the measured range.

Deep folds need somewhere to stack. In a bay, there may have to be enough wall or recess width for the curtain to clear the glass during the day. The thickness that helps overnight has to sit to one side by morning. When the interlined pair closes again, it creates a deeper air pocket and fewer circulation paths against the pane.

Stock drops change the seal

John Lewis sells both made-to-measure and ready-made ranges, with standard drops of 137cm, 182cm and 228cm. Choosing the shorter stocked drop can reduce the purchase price, although it also leaves a larger exposed opening and weakens the thermal result.

A three-pane bay in numbers

Take a three-pane bay with each pane roughly 0.6m by 1.2m. The glazing area is about 2.16 square metres. With a U-value near 4.8 W per square metre per kelvin, a room at 20 degrees, and outside air at 2 degrees, the 18-degree difference means the bare glass loses on the order of 187 watts continuously through that bay alone.

Draw a well-fitted thermal-lined curtain across the bay and the effective U-value of the window plus curtain assembly drops. Field measurements collated by the Energy Saving Trust and by independent retrofit studies put the improvement from heavy lined curtains with pelmets in a band of roughly 15% to 25% at the window. Using the midpoint, 20%, the same bay loses about 150 watts. The saving is 37 watts while the curtain is closed.

That 37-watt difference is modest in a single hour. Across the September to April heating season, with a UK living room heated for several hours each evening, it builds. At current unit rates, the saving can pay back a £60 pair of made-to-measure linings inside a few winters, since the lining trims the loss during every closed hour when the room is warm.

Leave a 4cm head gap and the result changes sharply. The convection loop starts again, the improvement falls back toward 10%, and the saving is roughly halved. From the room the curtain may look unchanged, yet the air movement behind it changes the measured performance.

Weight, light, and the edge at the floor

The same sealed fit that improves the evening heat figure changes the room once the fabric is closed. A thermal lining is opaque by design, so a thin curtain’s dusk glow disappears. That can alter how Farrow and Ball colours read after dark. Deep shades such as Hague Blue or Railings behind a heavy interlined curtain can feel flat and enclosed under a single ceiling pendant, because the window no longer contributes diffuse light when covered.

The sill changes too. A sansevieria or ZZ plant that had tolerated low winter sun can spend the darkest months in functional shade behind closed fabric. The same curtain that protects the room during heated hours cuts off the morning light if it stays drawn after the heating has clicked off. In an older single-glazed sash, it also blocks the slow trickle of air that used to leak through the window.

Farrow and Ball’s own pairing logic offers one way to keep the room from feeling heavy after the curtain closes. A chalky off-white such as Wimborne White on the ceiling, paired with a mid-tone on the lower wall, leaves more reflectance in the room. Low lamps and wall lights spread light across the fabric and paint more gently than one central fitting can manage on its own.

The reverse side matters from outside. John Lewis thermal ranges commonly show ivory or white on the back, and that pale backing is visible from the street at night. Behind a strong face fabric, the ivory reverse reads as a neutral band from outside. On a terrace, a row of matched ivory reverses gives the frontage a calmer rhythm than a run of windows with clashing curtain backs.

The finished curtain carries more weight than the face fabric suggests. A 228cm interlined pair across a wide bay can run to several kilograms, which rules out the lightest tension wires and the slimmest extendable poles. The pelmet or deep track that improves heat retention also needs to carry the load without sagging in the middle. John Lewis sells corded and motorised tracks rated for heavier drops. A metal pole of 28mm or 35mm diameter with a centre bracket helps the curtain draw square and keeps the side return tight.

Dressing the curtain after hanging affects performance as well as appearance. Training the folds with soft ties for 48 hours after first hanging helps the heading pleats settle at a consistent depth. Even, deep folds preserve the air pocket across the width. Shallow uneven folds can press the lining flat against the glass in places, reducing the trapped layer.

Tie-backs belong to daytime use. At night they pull the leading edge away from the side return and reopen the gap that the return was cut to close.

Where a curtain pools onto the floor, the floor covering changes the feel of the coldest part of the room. A vintage Persian rug laid under the curtain pool warms the band below the window where cool air tends to fall. A thicker hand-knotted pile holds that improvement better than a flatweave.

Restoration of a worn antique rug is worth doing before it sits permanently under a heavy curtain. Reweaving a damaged selvedge or refixing a loose fringe protects the edge that will take repeated contact. Once a curtain pools onto a rug night after night through a heating season, the contact line collects wear and dust, and a fragile edge there fails faster than one in open floor. That hidden contact line is the part no heat-loss calculation can price cleanly.