Storm windows and doors are secondary glazing or reinforcement systems built to protect existing windows and doors from extreme weather, improve energy efficiency, and reduce noise. The Energy Saving Trust states that secondary glazing can reduce heat loss by up to 40% compared to single glazing (Energy Saving Trust, 2026). Unlike primary replacements, they are installed on the interior or exterior of your current frames.
The main variable is your existing glazing. Storm windows and doors are most effective for homes with single glazing or older timber frames. They do not apply if you already have modern double or triple glazing rated to current Building Regulations (Part L), as the improvement will be marginal. They are also a temporary solution for listed buildings or conservation areas where full replacements are restricted.
Storm windows reduce heat loss and draughts
Interior storm windows are typically made from aluminium or uPVC with a glass or acrylic panel. They create an insulating air gap between the existing window and the storm unit, reducing U-values. A single-glazed window of around 5.0 W/m²K can drop to 2.5 W/m²K with a storm window (BRE, 2026). This can cut heating bills by £100–£150 per year for a typical semi-detached home. They also seal gaps, stopping draughts and condensation build-up.
Storm doors boost security and weatherproofing
Exterior storm doors are installed in front of your main entrance door. They are often made from aluminium or steel with a toughened glass panel. They block wind-driven rain and reduce air leakage, which can account for up to 15% of heat loss around doors (GOV.UK, Approved Document L, 2026). Many models include a multi-point locking system, adding security. Installation costs range from £300 to £800, depending on size and material.
Costs and grants for storm windows and doors
Interior storm windows cost around £200–£500 per window, including fitting. Exterior storm doors are £400–£1,000. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. The scheme offers up to £1,500 towards eligible measures. Always check with a TrustMark registered installer for eligibility.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates yearly heating savings of £130 for this house type when replacing single glazing with secondary glazing. Payback on this investment arrives in roughly 25 years, but the real benefit is the 40% reduction in heat loss and a 25-year product lifespan that delivers total lifetime savings of around £3,250 at current energy prices. These figures assume gas central heating and a standard 80% boiler efficiency. Homes in conservation areas or listed buildings often qualify for these upgrades without planning permission, making storm windows a practical alternative to full replacement.
| Item | Figure |
|---|---|
| Upfront cost after grants | £3,200 |
| Yearly savings | £130 |
| Payback period | 25 years |
| 25-year lifetime savings | £3,250 |
What homeowners often get wrong
The most common mistake is assuming storm windows work just as well on existing double glazing. Here are three frequent errors that cost homeowners money and miss potential savings.
- Believing storm windows suit any window type Fitting them over modern double glazing rated to Part L of Building Regulations typically saves less than £20 per year, wasting the £3,000+ installation cost. The right approach is to only install storm windows over single glazing or very old timber frames where the U-value improvement from 5.0 to 2.5 W/m²K makes financial sense.
- The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). Check your eligibility on the gov.uk website before buying.
- Ignoring condensation risks from poor installation A non-professional fit that leaves gaps can trap moisture between the storm unit and the original window, leading to rotting timber frames and mould that costs £500–£1,000 to remediate. The correct approach is to use a certified MSC installer who ensures proper ventilation and seals, protecting your 25-year investment.
Quick reference
- A single-glazed window with a U-value of 5.0 W/m²K drops to 2.5 W/m²K when a storm window is installed.
- Typical yearly heating savings for a 1930s semi-detached home with storm windows are £100–£150 according to the Energy Saving Trust.
- You do not need planning permission for interior storm windows in most conservation areas or listed buildings.
- Payback period for storm windows on single-glazed homes is roughly 25 years based on current energy prices.
- Fitting storm windows over existing double glazing usually voids the warranty on both products and saves minimal energy.
Frequently Asked Questions
Storm windows are typically aluminium or uPVC with glass or acrylic panels. Storm doors are often aluminium or steel for added security and weatherproofing.
No, they are most effective on single glazing or older timber frames. The Energy Saving Trust advises marginal improvement on modern double or triple glazing meeting Building Regulations Part L.
Yes, they are a temporary solution for listed buildings or conservation areas where full replacements are restricted. Check with your local planning authority before installation.
