Blackout curtain lining: choosing blackout, dimout and thermal linings

A lining controls light, reduces heat gain through glass, protects the face fabric from UV and adds body to the drape. Unlined curtains let light through and reveal the weave and colour unevenly in strong sun.

In bedrooms, media rooms and hotel guestrooms the goal is darkness. In living rooms and west-facing offices across humid, tropical Southeast Asia including Vietnam, the goal is usually to cut glare and heat while keeping some daylight. Start with choosing window treatments by window and room before deciding on the lining.

There are five main groups: standard, dimout, coated blackout, woven blackout and thermal. They differ in light control, drape, weight and behaviour in humidity.

Confirm light-blocking performance from the manufacturer's data for each fabric code, since products sold as dimout vary between suppliers. When comparing lining codes on SEAMATER, check the manufacturer's published light-blocking data.

Use blackout where occupants sleep or watch screens during daylight; use dimout where the aim is less glare and heat without full darkness.

• Master and children's bedrooms: blackout, especially on east-facing rooms with early sun.
• Hotel and resort guestrooms: blackout plus light-gap control, following brand standards where they exist.
• Apartment and townhouse living rooms: dimout or standard lining with a sheer layer.
• Offices and meeting rooms: dimout or screen roller blinds to reduce screen glare.
• Home cinemas: blackout with a closed ceiling pocket and generous side overlap.

Yes, to a limited extent. Internal curtains only reduce part of the heat, because much of the solar energy has already passed through the glass. Performance improves when the window-facing side is light-coloured or reflective.

On west-facing glazing, a lined curtain reduces radiant heat on occupants and furniture and slows fading of timber floors, rugs and upholstery. Glazing choices (low-e or insulated glass units) and external shading usually have more effect on cooling load. Present curtains as a supporting layer, not a substitute for glazing design.

One practical caution: dark, tightly closed curtains close to glass in strong sun can trap very hot air against the pane. Check the glazing supplier's guidance where thermal stress is a concern.

Light usually enters around the curtain, not through it. The four points to control are the top, sides, centre and hem.

Heading choice matters: wave headings leave slightly more gap at the track than pinch pleats; see curtain heading types and hardware. Blackout roller blinds fitted inside the recess always leave side gaps; outside-recess fixing or side channels reduce them, as covered in measuring blinds inside vs outside the recess.

A sheer for daytime privacy plus a blackout layer for night is the standard two-layer set in Vietnam and much of the region. It needs two independent tracks and a pocket wide enough for both, typically about 25–30 cm internally.

• Layer order: the sheer commonly sits next to the glass with the blackout on the room side, so the sheer gives a uniform elevation by day; some projects reverse this to seal the blackout closer to the glass. Decide based on the intended look and motor positions.
• Pocket width: typically about 25–30 cm, to accommodate two tracks, the fold depth of each curtain and any motor head.
• Roller plus curtain: a blackout roller at the glass with a sheer curtain in front is a compact option where pockets are shallow.
• Controls: with motorised systems, give each layer its own group and scenes.

Guestroom blackout needs documented light-blocking data, tolerance for regular laundering and compliance with the project's fire requirements. Many hotel brands specify room darkness levels, so check the brand standard before selecting fabric codes.

• Confirm fire performance requirements with the project's fire consultant and ask the manufacturer for the test report against the applicable standard; some blackout linings have dedicated flame-retardant versions.
• Prefer detachable linings or woven blackout where curtains are laundered often.
• Test light leakage in a mock-up room at peak sun before bulk ordering.
• Record lining codes in the handover file.

On SEAMATER, lining and face-fabric swatches are ordered inside the same project so drape and colour can be checked as a combined layer. Cut swatches carry a deposit, refunded when the order is confirmed.

Coated linings should be kept dry, rolled rather than folded, and never left in damp storage for long periods. Folded coatings can stick, crack or develop mould in humid conditions.

• Store fabric off the floor in dry areas, particularly during northern Vietnam's damp season.
• Transport made-up curtains hung or loosely bagged, avoiding sharp folds in the lining.
• Clean according to the manufacturer's instructions; many coated linings tolerate only gentle washing or dry cleaning.
• In rarely used rooms, open curtains and ventilate periodically to limit condensation between curtain and glass.

For broader guidance, see choosing interior materials for hot, humid climates.