Advanced options
Secondary glazing = adding an inner layer to existing single glazing: the trapped air gap does the insulating, roughly halving heat loss through the glass: same physics as double glazing at a fraction of the cost. The film kit (cling-film stretched drum-tight with a hairdryer) is the famous GBP 8 version: one winter, startlingly effective, invisible from a metre away. It's also the answer for listed buildings and conservation areas where replacement windows are refused: fitted secondary glazing is explicitly conservation-approved.
What The Secondary Glazing Calculator Works Out
- Cost: the number of single-glazed windows multiplied by the method's typical price: about GBP 8 a window for a shrink-film kit, GBP 60 to GBP 120 for magnetic or clip-on acrylic panels, and GBP 250 to GBP 450 for professionally fitted sliding secondary units.
- Heating saving: a typical GBP 13 a year per window on gas heating (GBP 10 to GBP 15 is the usual range), because the trapped air gap roughly halves heat loss through the glass. The payback line divides cost by saving: film pays back inside one winter, panels in a handful of years, fitted units in 20-plus but still ahead of replacement windows.
- What each method buys: film is heat-only and reapplied yearly; panels are reusable each winter with decent noise reduction; fitted units are permanent, openable, best for noise, and the explicitly heritage-approved route for listed buildings and conservation areas where replacement windows are refused.
- Your quoted price per window (advanced): overrides the typical figure with a real quote. Leave at 0 to keep the method default.
- Heating saving per window (advanced): edit the GBP 13 default for large windows, draughty sashes or a cold exposed wall, where the saving can be higher, or small windows where it is lower.
- Years you will stay and expected life (advanced): the net position card adds up the savings over your stay and subtracts the cost of buying the method as many times as its life requires: film every winter, panels about every ten years, fitted units lasting 25 or more.
- Gas price change (advanced): applies a percentage change to the saving so you can see how higher or lower gas prices move the payback.
- Cost per window per year (advanced): the price of each window's treatment spread over its life, for a like-for-like comparison with the yearly saving.
- Gas saved (advanced): the money saved converted to kilowatt-hours at your unit rate, defaulting to the current Ofgem price cap, with the CO2 avoided at 0.183 kg per kWh.
- Versus replacement double glazing (advanced): the payback on new double-glazed units at about GBP 700 a window, which save only a little more heat than secondary glazing. Keep the inner layer well sealed: moist room air leaking into the cold gap condenses on the original glass.