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Heating & gas calculator

Insulation savings calculator: U-value upgrades, energy saved & payback

Enter the area and U-value of walls, roof, windows and floor before and after insulating. Using heating degree days for your climate, it shows the heat loss avoided, kWh and money saved, CO₂ and payback for each measure.

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Results

The upgrades cut fabric heat loss by 49% and save about 7,220 kWh of fuel (722 €) per year, paying back in ~19.2 years. Best value: Walls (6.3-year payback).
Money saved per year
€722
7,220 kWh of fuel per year
Payback (all measures)
19 yr 2 mo
cost 13,850 €
Heat loss before
13,192kWh/yr
fabric only, 298 W/K
Heat loss after
6,693kWh/yr
125 W/K
Reduction in fabric loss
49.3%
CO₂ saved per year
1.32t
30-year savings
€34,350
with 3%/yr price growth
Design heat loss cut
4.14kW
at 21 °C in / −3 °C out, so a smaller boiler or heat pump will do
Heating degree days
1,844K·d (base 15.5 °C)
London, UK
Annual heat loss by element
Monthly fuel saving

Savings per measure

ElementArea (m²)U before → afterkWh fuel savedSaved (€/yr)Cost (€)Payback
Walls1101.5 → 0.554,3694372,7506.3 yrs
Roof / loft600.7 → 0.151,3801381,2008.7 yrs
Windows & doors222.8 → 1.21,4721479,90067.3 yrs
Ground floor600.7 → 0.7000—
  • Degree days: London, UK (1991–2020 normals, approximate), base 15.5 °C.
  • Ground floor losses use a 0.7 factor because the ground is warmer than the outdoor air in winter. Ventilation and air-leakage losses are not included here.
  • A 15% performance gap is applied to all savings (thermal bridges, workmanship, warmer indoor temperatures after insulating).

How this insulation calculator works

Heat leaks out of a building through its fabric (walls, roof, windows, doors and floor) at a rate set by each element’s area and U-value. Over a heating season the total loss is proportional to the climate’s heating degree days. This calculator applies that standard engineering method, the same principle behind EN ISO 13790/52016 and national energy-rating software, to each element before and after insulation:

  1. You enter the area and the U-value before and after for walls, roof, windows and ground floor. The help text lists typical values.
  2. The calculator gets heating degree days for your climate: from 1991–2020 normals for 16 preset cities, from NASA’s satellite climatology for any location, or from your own figure.
  3. For each element it works out the annual heat loss before and after, applies a performance gap to the difference, turns the heat saved into fuel saved using your heating system’s efficiency (or a heat pump’s SCOP), and prices it.
  4. With your installed cost per m², it gives the payback of each measure separately, so you can see which upgrades to do first.

The formula

Heat loss coefficient H = A × U (W/K) Annual heat loss Q = H × HDD × 24 / 1000 (kWh/yr) Fuel saved = (U_before − U_after) × A × HDD × 24/1000 × (1 − gap) / η_heating Money saved = fuel saved × price Payback = installed cost / money saved Design heat-loss reduction = ΔH × (21 °C − T_design)

Ground floors get a factor of 0.7, because the ground under a house stays warmer than winter air. Ventilation and air-leakage losses are left out: insulation doesn’t change them (draught-proofing aside).

Worked example: semi-detached house in London

The defaults describe a typical inter-war or post-war house in London (≈ 1,840 heating degree days to 15.5 °C), heated by a 90% efficient gas boiler at 0.10 €/kWh:

Measure Area U before → after Fuel saved Saved per year Cost Payback
Cavity wall fill 110 m² 1.50 → 0.55 4,369 kWh 437 € 2,750 € 6.3 years
Loft top-up to 270 mm 60 m² 0.70 → 0.15 1,380 kWh 138 € 1,200 € 8.7 years
Old double → new double glazing 22 m² 2.80 → 1.20 1,472 kWh 147 € 9,900 € 67 years

Fabric heat loss falls from about 13,190 kWh to 6,690 kWh a year (298 W/K to 125 W/K), a 49% reduction. After the 15% performance gap, the household saves about 7,220 kWh of gas and 722 € a year, plus 1.3 t of CO₂. All three measures together cost 13,850 € and pay back in about 19 years. The walls and loft on their own pay back in about 7 years; the windows take 67. Over 30 years, with gas prices rising 3% a year, the package saves roughly 34,000 €. Design heat loss drops by about 4.1 kW, enough to allow a noticeably smaller heat pump.

How to read the results

  • Savings per measure is the most useful table. Work through the measures in order of payback, quickest first.
  • Reduction in fabric loss of 40–60% is typical of a solid retrofit; 70–80% is deep-retrofit territory.
  • Design heat loss cut translates directly into a smaller boiler or heat pump.
  • Monthly fuel saving shows that most of the saving lands between November and March.
  • 30-year savings matter because insulation usually lasts 30–50 years with no maintenance. Lifetime value stays high even when payback looks long.

Typical U-values

Element Before (typical) After (good practice)
Solid brick wall (225 mm) 2.0–2.1 0.25–0.30 with 100 mm EWI
Unfilled cavity wall 1.4–1.6 0.45–0.55 filled
Timber frame (1970s) 0.5–0.6 0.18–0.25
Loft, no insulation 2.0–2.5 0.12–0.16 at 270–300 mm
Loft, 50 mm 0.6–0.7 0.12–0.16
Single glazing 4.8–5.8 1.2–1.4 double / 0.7–0.9 triple
Old double glazing (pre-2000) 2.8–3.1 1.2–1.4 double / 0.7–0.9 triple
Suspended timber floor 0.7–1.0 0.20–0.25

Tips to get the most from insulation

  • Start with the loft, the cavity and draught-proofing. They’re cheap and pay back fastest.
  • Mind ventilation. As the house gets tighter, make sure trickle vents or mechanical ventilation keep indoor humidity in check.
  • Combine it with other work. External wall insulation is cheapest when the scaffolding is already up for re-rendering or a new roof.
  • Avoid thermal bridges. Insulate window reveals and junctions; uninsulated edges can cut the benefit of wall insulation by 10–20%.
  • Re-size your heating. After insulating, check what heat pump you now need with the heat pump calculator, and compare fuels in the heating cost comparison.

Limitations

This is a steady-state degree-day method. It ignores solar gains through windows (which make glazing upgrades slightly less valuable on south façades and more valuable on north ones), thermal mass and changes in airtightness. Degree days derived from monthly climate data are estimates, within about ±10% of station records. Results hinge on the “before” U-values, so if you’re unsure, get a survey. Many countries offer grants or reduced VAT on insulation; enter them as a lump sum in the advanced settings.

Method & assumptions

Fabric heat loss of each element: Q = A × U × HDD × 24 / 1000 (kWh/yr), where HDD are heating degree days to the base temperature. Saving = (Ubefore − Uafter) × A × HDD × 24/1000 × (1 − performance gap). Fuel saved = heat saved ÷ heating-system efficiency (or SCOP). Payback = installed cost ÷ annual money saved (no discounting).

Frequently asked questions

How much money does insulation save?
It depends on the U-value improvement, the area and your climate. As a rough guide for a gas-heated semi-detached house in a UK or German climate: a loft top-up from 50 mm to 270 mm saves 100–200 per year, cavity wall fill 250–450, external wall insulation on a solid-brick house 400–700, and replacing old double glazing 100–200. Payback runs from 2–5 years for loft and cavity fill to 30+ years for windows.
What is a U-value?
The U-value (W/m²K) is the heat flow through one square metre of a building element for each degree of temperature difference between inside and outside. Lower is better. An uninsulated solid brick wall is about 2.0, a filled cavity wall about 0.5 and a new-build wall 0.15–0.25. Single glazing sits at 4.8–5.8, modern triple glazing at 0.7–0.9.
What are heating degree days?
Heating degree days add up, day by day, how far the mean outdoor temperature falls below a base temperature (commonly 15.5 °C, which allows for internal gains). London has about 1,800–2,000 a year, Berlin about 2,400–2,700 and Stockholm about 3,200–3,600. Heat loss through a wall is proportional to them.
Which insulation measure has the fastest payback?
Almost always loft (attic) insulation and cavity wall fill. Both are cheap per m² and tackle big losses, and draught-proofing is similarly cheap. External or internal wall insulation and new windows save a lot of energy but cost far more per m², so they take longer to pay back.
Why are real savings often lower than calculated?
This is the 'performance gap': thermal bridges at junctions, imperfect installation, and occupants keeping the house warmer once it is insulated (comfort take-back). Studies typically find realised savings 10–50% below engineering estimates. The calculator applies an adjustable 15% gap by default.
Does insulation help with a heat pump?
Twice over. It cuts annual kWh, and it lowers the design heat loss, so a smaller, cheaper heat pump can run at a lower flow temperature and higher efficiency.

Sources & references

  1. ISO 6946:2017 — Building components and building elements: thermal resistance and thermal transmittance, calculation methods
  2. CIBSE TM41 (2006) — Degree-days: theory and application
  3. Degree Days.net — degree-day data and methodology
  4. UK DESNZ — Greenhouse gas reporting: conversion factors 2024
  5. Energy Saving Trust — Home insulation

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