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The amount of loft insulation you need

Over the years, insulation has become an increasingly worthwhile investment for homes and businesses. Energy bills have soared over the last 10 years and loft insulation is a relatively cheap energy efficiency measure to install, so it makes sense that people are looking to install it where possible and benefit from lower energy bills.

Back in the eighties, just 25-50 millimetres was considered sufficient, whilst building regulations now dictate that a new build needs to have 270mm of insulation in the loft to achieve the required u-value of less than 0.16/m2R (based on standard rock wool or sheep wool insulation).

Diminishing returns of installing additional insulation

With loft insulation, there is a law of depreciating returns, which means the first few centimetres of insulation will save you many times the cost of the installation, but each extra centimetre adds less value, to the point where it is no longer worthwhile to add any more. With cheaper energy prices 30 years ago, that extra few centimetres of insulation was just not worth it, but with the recent rises in fuel costs, that extra £10 a year saving has suddenly become £30, and that starts to become a lot more attractive.

Is it worth topping up your loft insulation?

In the following table you can see the energy savings from adding additional insulation in to your loft. The energy savings often cited by publications are based on going from no insulation to a depth of 270mm, however in our experience, most homes will have some level of insulation; it is actually quite rare to find a property with no insulation at all.

The numbers in the table are based on a typical turn of the century mid terrace property. It has solid walls but a modern boiler and has an area of 86m2

Current Insulation levelSavings/year by Increasing to 270mmROI based on cost of £300

  •  None               £173         2    years

  •  12mm             £111         3    years

  •  25mm             £71           4.5 years

  •  50mm             £44            7    years

  •  75mm             £29           10   years

  •  100mm           £13           23   years

  •  150mm           £7             43   years


The actual savings will of course depend on how warm you like your home and how long you have your heating on for. A well insulated property will keep the heat in longer and therefore make your boiler work not so hard. So even if it takes 20 years to pay back, it will still pay for itself in its lifetime.

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How insulation works

During the winter, the air inside the home is warmer than the air outside. The insulation slows down the movement of energy from the warm area to the cold area, creating a thermal barrier that means your boiler doesn’t have to work as hard to keep the home warm. The thicker the insulation, and the lower the u value, the better this thermal barrier is and so the slower the heat will escape the home.

During a summers day when the temperature may hit 30 degrees, the air outside is generally warmer than the air in the home. With the heating off, the house warms up gradually through the day as the walls and roof absorb the heat of the day. But because the air outside is warmer the thermal barrier created by the insulation will again slow the movement of heat, but in the opposite direction. Warm air in the loft will not be able to penetrate into the home as easily, whilst insulation in the walls will prevent them from warming the home as quickly as well.

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