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Watt Happens Next · Chapter 8

How Thick Should the Loft Be?

More insulation always saves heat, but each extra layer saves less. Calculus finds the thickness where one more centimeter stops paying, and the top-up you are actually pricing gets its own payback.

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Opens on the book's house. Change any number to your own and press Calculate.

The loft (US: attic floor)

The weather, the time and the money

Enter the loft and the prices, then press Calculate.

The formula

  • R (m²·°C per W) = the bare ceiling's R + thickness (m) ÷ λ; U = 1 ÷ R
  • Heat a year (kWh) = U × area × degree-days × 24 ÷ 1,000
  • Total cost per m² = cost × thickness + years × the price of a kWh of heat × degree-days × 24 ÷ 1,000 ÷ R
  • Set its slope to zero: the best R = √(years × heat price × degree-days × 24 ÷ 1,000 ÷ (λ × cost))
  • A kWh of heat from gas costs the unit rate ÷ the boiler's efficiency; payback = the top-up's cost ÷ money a year
The book's worked example, Chapter 8. With λ 0.04, 2,000 degree-days a year (48 kWh a year for each W per °C), 20 years and 30 per m² per meter, the best total R is 8.43: about 320 mm. Topping the book's 100 mm up to 300 mm takes U from 0.3413 to 0.1261, cuts 17.22 W per °C, 826 kWh of heat and 73.45 a year for 480: payback 6.5 years.
Check it. The total-cost curve is flat near its best: 250 mm and 400 mm cost nearly the same over the horizon. Look at the table, not only the best number.

Honest limits. Prices, tariffs and degree-days are illustrative. Boards across the joists, never step between them; gloves and a mask for mineral wool; never cover recessed lights, flues or vents, and keep the eaves ventilated. Insulation near electrics or flues is an installer's job. The book's house, its meters and its bill were worked out on paper from published figures, not measured by us: read your own meters and log your own night. Tariffs are illustrative, as seen in November 2026 (shaped like the GB price cap of October to December 2026, with US prices beside), shown without a currency sign; put in your own. No saving is promised. Everything stays in your browser: no sign-up, nothing stored.

These calculators go with Watt Happens Next (Make It Add Up, Book 3), where every number on this page is worked by hand.