Question 1

How is insulation payback calculated?

Accepted Answer

Payback is calculated by dividing the installation cost by the annual energy saving. Annual saving = (old U-value - new U-value) x area x degree-days x 24 / 1000 x fuel cost / heating efficiency. Fuel price inflation accelerates the payback period over time.

Question 2

What U-values should I aim for?

Accepted Answer

Current Building Regulations Part L targets: walls 0.18-0.26 W/m2K, roofs 0.11-0.16 W/m2K, floors 0.13-0.18 W/m2K, windows 1.2-1.4 W/m2K. Uninsulated solid walls can be 2.0+ W/m2K, and single glazing 5.0+ W/m2K, so the savings potential is significant.

Question 3

Which insulation measure has the best payback?

Accepted Answer

As a rule of thumb (typical figures from Energy Saving Trust guidance — your own payback depends on the U-values, area and fuel price you enter above), loft insulation has the best payback (roughly 1-3 years) due to low cost and large heat loss reduction. Cavity wall insulation (around 3-5 years) is next. Floor insulation (roughly 5-10 years) and external wall insulation (15-25 years) have longer paybacks but are still worthwhile over the lifetime of the property.

Question 4

What are degree days?

Accepted Answer

Degree days measure how cold a heating season is. One degree day occurs for each degree the daily mean temperature is below the base temperature (15.5C). The UK average is approximately 2,100 degree days per year. Northern Scotland can reach 2,800+, while southern England may be 1,800.

Question 5

Does insulation payback improve with fuel price inflation?

Accepted Answer

Yes. If fuel prices rise each year, the annual monetary saving increases each year, shortening the payback period. As an illustration, over 20 years at a steady 3% annual increase the cumulative saving works out roughly a third higher (around 35%) than at static prices; at a steeper 4-5% it climbs to roughly 50-65% higher (the compounding effect of the inflation rate you enter above). This makes insulation an increasingly good investment, though future fuel prices are uncertain.

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