Insulating a bay window costs between £50 and £1,200 depending on the method you choose
Bay windows are a distinctive architectural feature but their large glazed area makes them a significant source of heat loss in a typical UK home. The cost to insulate one depends entirely on whether you opt for a quick DIY fix or a more permanent professional installation.
Insulating a bay window costs £50 to £1,200. The cheapest option is thermal curtains (£50-£200), which can reduce heat loss by up to 25%. For better insulation, secondary glazing or cellular blinds are more effective but cost more.
- Thermal curtains cost £50-£200 and cut heat loss by up to 25%.
- Secondary glazing ranges from £200 to £1,200 per bay window.
- Draught-proofing strips add £10-£50 and boost other insulation methods.
- Cellular blinds trap air, achieving R-values similar to double glazing.
- A pelmet above curtains prevents warm air escaping, doubling effectiveness.
- Insulating a bay window costs between £50 and £1,200 depending on the method you choose
- Thermal curtains can reduce heat loss through a bay window by up to 25%
- Cellular blinds provide a higher insulation value than standard roller blinds
- Secondary glazing is the most effective DIY insulation for an existing bay window
- Quick numbers, cost, savings, and payback for bay window insulation methods
- To insulate a bay window effectively, combine draught-proofing with a thermal barrier
- To qualify for a UK government grant, your bay window insulation must be installed by an MCS-certified or TrustMark-registered installer
The cheapest option, thermal curtains, costs £50 to £200 per window and can be fitted by any homeowner without professional help (Energy Saving Trust, 2026). Secondary glazing, either acrylic or glass panels, ranges from £200 to £1,200 per bay, installed by a professional or as a DIY kit. Draught-proofing strips and sealants add £10 to £50 to the total and are often recommended alongside other methods (Energy Saving Trust, 2026).
Thermal curtains can reduce heat loss through a bay window by up to 25%
Thermal curtains with a thick lining and a close-fitting track or pelmet create a still-air layer that slows heat transfer from the warm room to the cold glass. The reduction in heat loss is measured against an uncurtained single-glazed window; double-glazed windows see a smaller benefit because the existing insulation is already better. Energy Saving Trust data shows that fitting thermal curtains across all windows in a typical home can save £15 to £25 per year on heating bills (Energy Saving Trust, 2026).
For a bay window specifically, the savings will be larger if the bay is single-glazed and exposed to prevailing winds. A pelmet or cornice above the curtain rail prevents warm air from rising over the top of the curtain and escaping, which can cut the effective insulation value by half.
Cellular blinds provide a higher insulation value than standard roller blinds
Cellular blinds, also known as honeycomb blinds, trap air in their cells and achieve an R-value equivalent to double glazing in some designs (Energy Saving Trust, 2026). They are most effective when fitted tightly within the window recess to minimise convection currents that would otherwise carry heat past the blind. A 2026 comparison found cellular blinds reduce heat loss by up to 40% compared to uninsulated windows, but savings depend on fit and fabric quality (Energy Saving Trust, 2026).
Standard roller blinds typically reduce heat loss by around 10% because they leave a gap at the sides and top. Cellular blinds are more expensive, typically £80 to £300 per window, but offer a better insulation-to-cost ratio for homeowners who want a neat, permanent solution without the visual bulk of curtains.
Secondary glazing is the most effective DIY insulation for an existing bay window
Secondary glazing adds an extra pane of glass or acrylic inside the existing window, creating a sealed air gap that cuts heat loss by up to 60% (Energy Saving Trust, 2026). Acrylic kits cost £200 to £600 and can be fitted with magnetic tape or clips; glass panels cost £500 to £1,200 and require professional fitting (GOV.UK, 2026).
The payback period on secondary glazing for a bay window is typically 5 to 10 years based on average gas heating costs and the current energy price cap (Ofgem, 2026). For bay windows with unusual angles or curved sections, a custom-made acrylic panel is often the most practical option because it can be cut to shape without the expense of bespoke glazing.
Quick numbers, cost, savings, and payback for bay window insulation methods
| Method | Typical cost (range) | Annual heating saving (estimate) | Payback period (years) | DIY difficulty |
|---|---|---|---|---|
| Thermal curtains | £50–£200 | £15–£25 | 3–8 years | Easy |
| Cellular blinds | £80–£300 | £20–£40 | 4–8 years | Easy |
| Acrylic secondary glazing | £200–£600 | £40–£80 | 5–10 years | Moderate |
| Glass secondary glazing | £500–£1,200 | £50–£100 | 6–12 years | Professional |
| Draught-proofing strips | £10–£50 | £10–£20 | 1–3 years | Easy |
All figures sourced from Energy Saving Trust guides and DESNZ 2026 household energy efficiency data (Energy Saving Trust, 2026).
To insulate a bay window effectively, combine draught-proofing with a thermal barrier
Draught-proofing seals gaps around the window frame and sash, preventing cold air infiltration that undermines other insulation methods (Energy Saving Trust, 2026). Self-adhesive foam strips cost £5 to £10 per window and last 1 to 2 years; silicone sealant is more durable but requires careful application to avoid trapping moisture.
Adding a thermal barrier, such as curtains, blinds, or secondary glazing, on top of draught-proofing delivers the greatest overall heat loss reduction. A bay window that is draught-proofed and fitted with either cellular blinds or secondary glazing can achieve a heat loss reduction of 50% or more compared to an untreated window, according to EST modelling.
how to draught-proof sash windows
To qualify for a UK government grant, your bay window insulation must be installed by an MCS-certified or TrustMark-registered installer
The Great British Insulation Scheme and ECO4 fund secondary glazing and draught-proofing only if installed by an approved scheme member (GOV.UK, 2026). MCS certification covers secondary glazing; TrustMark covers draught-proofing and curtain track fitting under the scheme (MCS Certified, 2026).
Check the installer’s registration on the MCS register or TrustMark website before agreeing to any work. If you install thermal curtains or cellular blinds yourself, you will not qualify for grant funding, but the materials themselves are low-cost enough that the payback period remains short without subsidy.
how to check if a builder is TrustMark registered
Frequently Asked Questions
Costs range from £50 to £1,200 depending on the method. Thermal curtains start at £50, while professional secondary glazing can reach £1,200 per bay, according to the Energy Saving Trust (2026).
The best method depends on your budget and DIY skills. For maximum heat loss reduction, secondary glazing or cellular blinds offer the highest insulation value, per Energy Saving Trust guidance.
Yes, thermal curtains can reduce heat loss through a bay window by up to 25% when fitted with a close track and pelmet. The Energy Saving Trust confirms savings of £15-£25 per year on heating bills for all windows.
Secondary glazing for a bay window typically costs £200 to £1,200, installed professionally or as a DIY kit. Acrylic panels are cheaper, while glass offers better insulation, per the Energy Saving Trust (2026).
Yes, several DIY options exist. Thermal curtains and draught-proofing strips are easy to fit without professional help, costing as little as £10-£50 for strips, according to the Energy Saving Trust.
