Windows are the largest single heat-loss point in most flats, and rental tenants cannot replace them. The good news is that two reversible window treatments, shrink film and heavy thermal curtains, can cut window heat loss substantially without any landlord permission or permanent alteration. This guide covers what each does, when it is worth the cost, and where both quietly fail.
Assume the goal is to reduce heat loss in winter without changing the fabric of the flat. Solar-heat-gain reduction (summer overheating) is a different problem addressed briefly at the end.
The physics in one paragraph
A single-glazed window loses heat at roughly 5.0 to 5.5 W/m2K. Standard double glazing is 2.5 to 3.0. Modern low-e triple glazing is 0.9 to 1.2. For every 1 W/m2K reduction on a 2 sq metre window, roughly £8 to £15 of heating cost is saved per year in a UK climate. Window film approximately halves the single-glazing U-value by trapping a still-air layer between the film and the glass. Thermal curtains cut evening heat loss by roughly 25 percent by preventing warm room air from cycling against the cold glass.
Peel-off shrink window film
What it is: a clear plastic film applied to the window frame with double-sided tape, then shrunk taut with a hairdryer. Creates a secondary sealed air layer roughly 30 to 50 mm in front of the existing glass.
- Cost: £12 to £20 per window for a decent kit (Duck Brand, Frost King, Stormguard).
- Install time: 20 to 40 minutes per window for a first-timer.
- Best for: Single-glazed windows, drafty old double-glazing with perished seals, north-facing bedrooms in older conversions.
- Reduces heat loss by: Roughly 45 to 55 percent on a single-glazed window; 10 to 20 percent on a leaky double-glazed window.
- Downsides: Slightly reduces optical clarity (barely visible except in direct sun), prevents opening the window for the season it is in place.
Apply in autumn, remove in spring. Some renters install on bedroom windows only (which stay closed for the winter anyway) and leave living-room windows filmless because they need to open them.
Heavy thermal curtains
What they are: multi-layer fabric curtains with a face fabric, a thermal lining (heavyweight interlining), and a back lining. Full-length, reaching the floor, and wide enough to cover the frame plus 10 cm each side.
- Cost: £40 to £150 per window for mid-range ready-made; £150 to £400 for custom or high-end. Charity shops sometimes carry heavy lined curtains for £15 to £30.
- Best for: Any window in a room that stays cold in the evening, especially east and north-facing living rooms and bedrooms.
- Reduces heat loss by: 15 to 25 percent overnight when closed.
- Requires: Curtain track or pole rated for the weight (heavy curtains can weigh 5 to 8 kg per side).
The critical dimensions: curtains that stop at the sill or dangle 5 cm above the floor lose most of their benefit because warm air cycles below the fabric and against the cold glass. Full-length curtains that touch the floor make the air layer sealed. This is the single biggest fitting mistake.
Related: Winter Heating Savings Tips covers the whole heating strategy alongside these window treatments.
What does not work as advertised
- Bamboo or slat blinds. Attractive but negligible insulation. Reflect a little radiant heat; do not seal an air layer.
- Standard roller blinds. Almost no thermal benefit. Thermal-lined roller blinds (specifically labelled) provide roughly 10 to 15 percent.
- Bubble-wrap secondary glazing. Actually works optically (traps air), but looks terrible. Consider it only for garden sheds, workshops, or rooms nobody sees.
- Standard voile curtains. Zero thermal benefit; light-filtering only.
- Adhesive-back “insulating” window films. Mostly cosmetic. The thermal benefit is minimal because they lie flat against the glass rather than creating an air gap.
Combining the two treatments
The most effective approach in a rental winter setup:
- Shrink window film on bedroom windows that stay closed all winter.
- Full-length heavy thermal curtains on all living-room and bedroom windows.
- Simple foam draught strips around any window frames that visibly leak air.
Combined cost for a typical one-bedroom flat: £80 to £150. Combined effect: a 3 to 5°C perceived-warmth increase in the coldest room, without changing heating settings.
Summer solar-heat gain
The opposite problem in summer:
- External blinds are best but rarely renter-installable.
- Interior reflective roller blinds (silver-backed) reflect solar heat before it enters the room; effective for west-facing windows.
- Solar-control window film (different product from thermal shrink film) reflects 30 to 60 percent of solar heat while remaining clear. £20 to £40 per window.
Quick reference: what to buy for which window
| Window situation | Best treatment |
|---|---|
| Single-glazed bedroom, closed all winter | Shrink film + full-length thermal curtain |
| Double-glazed living room, well-sealed | Thermal curtains only |
| Double-glazed with visible frame gaps | Foam draught strips + curtains; film optional |
| Kitchen or bathroom window opened daily | No film (needs to open); simple lined curtain if any |
| West-facing summer overheating window | Reflective roller blind or solar-control film |
| Modern triple-glazed windows | Nothing; already efficient |
The break-even calculation
A £15 shrink film kit on a single-glazed bedroom window saves roughly £10 to £20 of heating per season. It breaks even in year one. A £60 pair of thermal curtains on a north-facing bedroom saves roughly £15 to £30 of heating per year, plus any perceived-warmth improvement that reduces the temptation to turn the heating up. Payback is 2 to 4 years, well within any rental tenancy.
Nothing about window treatments is glamorous or new. What matters is that they are the single largest reversible retrofit available to a renter, and both types cost less than one month’s heating bill. Install once at the start of a winter, forget about them until spring, and the flat is measurably warmer for a decade of tenancies to come.
Where these treatments interact with damp
Reducing heat loss increases relative humidity in the room, sometimes to condensation levels on the coldest surfaces (usually the top corners of exterior walls). This is not a fault of the film or curtain; it is what happens when a leaky flat becomes an airtight-ish flat without matching ventilation. Pair any winter window treatment with the shock ventilation habit from the air-quality guide, twice a day, all windows fully open for 3 to 5 minutes. Skip that step and mould appears in month two.
Sources and further reading
Frequently asked questions
On single-glazed windows: yes, meaningfully. A 12-pound kit typically drops window U-value from around 4.8 to about 2.5 W/m2K, comparable to some double glazing. On modern double glazing it does very little because the frame is already sealed.
For visible impact, at least 350 gsm fabric weight with a thermal lining. Interlined heavy curtains (three layers: face, thermal, lining) at 500+ gsm can cut window heat loss by roughly 25 percent overnight, but they need to reach the floor and cover the frame.
Only marginally. They reduce radiant heat exchange but do not seal the air layer against the window. Roller blinds with a thermal backing are better but still less effective than heavy curtains reaching the floor.
If applied to the window frame (not the glass) with the double-sided tape kit provides, no. It peels off cleanly with a hairdryer at end of tenancy. Never apply directly to glass with adhesive; that leaves marks.
