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March 2, 2024

How Much Loft Insulation Do I Need?

Loft Insulation Costs

Reducing heat loss makes homes more energy efficient, and the loft is a good place to start. Every loft or attic in the UK needs effective insulation to cut heating costs and reduce carbon footprints. 

The crucial factor is to ensure your home has enough insulation to do the job effectively. So how much loft insulation do you need and how do you figure this out? 

You can find out here and learn about other important considerations when it comes to insulating your loft.

Why You Need Loft Insulation

Warm air rises and will continue to do so in your home until something stops it. Without loft floor insulation, heat escapes into the space and out through the roof.

Millions of homes in the UK have lofts that are uninsulated or lack sufficient insulation and these lose approximately a quarter of the heat they generate.

Loft insulation is a simple, cost-effective energy efficiency measure. It pays for itself many times over by reducing heat loss, saving up to £445 a year (for a typical detached property) on energy bills.

Furthermore, insulating your loft stops cold air from descending into your living areas. This means cooler, more comfortable temperatures during hot weather.

Deciding How Much Loft Insulation You Need

Recommended minimum depths for loft floor insulation have increased significantly over the years to the current 270mm. This has been the standard since 2003 and is what you should aim for if your loft is uninsulated.

If you have loft insulation but live in an older property, it will probably need topping up to give you the full benefit. For example, if your existing insulation is 120mm deep, topping it up to the recommended 270mm could reduce heating costs by a further £25 a year. And increasing insulation depth from 50mm to 270mm could save you up to £50 more a year.

Checking the depth of your existing insulation will help you determine how much extra insulation you need. This will avoid buying too much or having to purchase more later.

If your insulation doesn’t cover the loft floor joists, you may need to add more. You can use a different type of additional insulation than the existing insulation material if you want to. Bear in mind that some thinner, advanced insulation materials can provide the same level of insulation as older, thicker types of insulation.

When deciding how much loft insulation you need, you also have to take into account loft size and insulation efficiency.

How to Calculate Loft Size and Amount of Insulation Needed

Measure the length and width of your loft in metres and multiply the two numbers to give you its floor space in square metres.

Blanket rolls of mineral wool or natural wool are the simplest and most common method of insulating lofts.

Based on a standard blanket insulation roll size (5.68 metres long, 1.14 metres wide, 170mm deep), you‘ll need:

  • Eight rolls to cover 50 square metres.
  • Ten rolls for 60 square metres.
  • 11 rolls for 70 square metres.
  • 13 rolls for 80 square metres.
  • 14 rolls for 90 square metres.
  • 16 rolls 100 square metres.
  • 17 rolls for 110 square metres.
  • 19 rolls for 120 square metres.

Rolls of blanket insulation can vary in length, so you need to check for this.

How to Calculate Loft Insulation Efficiency

Insulation efficiency is measured by its level of thermal flow resistance – its R-value. Thicker insulation has a higher R-value number.

The current recommended insulation depth of 270mm is considerably higher than previous guidelines. In 1965, for example, the advised insulation depth was only 25mm.

As well as the thickness of the material, insulation performance is also influenced by thermal conductivity – how heat flows through the material. So, in many cases, thicker insulation doesn’t necessarily mean better insulation.

Regulations on insulation in buildings have no set requirement for R-Values. However, home insulation professionals currently recommend a combination of depth and conductivity that achieves an R-value number from 6.1 to 7 for loft insulation.

You can calculate the R-value of insulating material by dividing its depth in inches by its thermal conductivity. But it’s a lot easier if you can find the R-value stipulated on the product label or in manufacturers’ guides.

Insulation efficiency can also be measured by its U-value, which takes into account more potential heat loss than R-values. The lower the U-value, the better the insulation.

When to Replace Loft Insulation

Loft insulation generally lasts 40 years or longer, depending on the type of material. The most common kind of insulation, blanket rolls, can last up to 50 years.

It’s important to realise you can only add extra loft insulation to existing insulation material that’s remained in good condition. In other cases, the best solution will be to replace the insulation completely. 

Your loft insulation is more likely to need replacing if it was installed a long time ago. Although insulation material is highly durable, it gradually degrades. 

It can also lose thermal efficiency as it settles over time, and cheaper insulation material won’t last as long as quality materials.

Other signs a loft may need replacement insulation include:

  • The existing insulation has been compressed by the weight of the floorboards.
  • Draughts or condensation caused by gaps in the insulation.
  • The insulation has become damp due to water leakage or lack of ventilation.
  • Damage caused by household pests such as rodents.
  • Dust and airborne pollutants settle on the insulation.

Can You Have Too Much Loft Insulation?

Realistically, the risk of over-insulating a loft is minimal. There’s generally not enough space to install too much insulation material.

The depth of loft insulation in some new-build homes is being increased above the government-recommended 270mm to 300mm.

That said, the amount of insulation needs to be balanced with levels of airflow and ventilation. Too much insulation without sufficient ventilation can cause condensation, dampness, and mould. 

Buildings rely on constant air circulation to “breathe”. If loft insulation compromises the flow of air, moisture can’t evaporate or has no outlet without adequate ventilation.

Similar problems can arise if insulation is too tightly packed together in multiple layers.  Instead of over-insulating one part of your house, it’s better to spread the correct amounts of insulation throughout it.

If you hire a professional loft insulation installer, you won’t have to worry about potential problems resulting from impaired ventilation due to over-insulation.

Neither will you use too much insulation by insulating the wrong areas of the loft.


Which Parts of Your Loft Need Insulating?

You may have thought about insulating loft rafters instead of the floor.

The standard method of loft insulation is to insulate the floor only – between and across the timber joists. This is known as cold loft insulation. It stops heat from the rest of your home from escaping into the loft.

Warm loft insulation, on the other hand, entails insulating the top of the loft – between and across the timber rafters of the roof. This is a more complex and costlier job. It keeps your loft warmer because heat is allowed to flow into the loft from the living space beneath.

To compensate for this, your heating system will need to work harder to maintain a comfortable ambient temperature throughout the house. And you’ll see your energy bills rise accordingly.

So, unless you’re planning a loft conversion, or want to create a storage area for temperature-sensitive items, warm loft insulation would be a waste of insulation material and money. 

Loft Insulation Costs

Loft insulation costs vary widely, depending on considerations such as loft size and type of insulation material.

Blanket insulation is the most affordable way to insulate your loft, with the material costing around £5 a square metre.

Excluding installation costs, this works out at roughly:

  • £125 to £160 for a terraced house.
  • £200 to £250 for a semi-detached house.
  • £300 to £375 for a detached house.

Loft insulation installed by professionals will result in maximum heat retention in your living space. Labour charges can vary from £150 to £400 a day, but installing blanket insulation usually only takes professionals a few hours. 

There’s no VAT on professional insulation installation until March 2027, when it’s expected to revert to its reduced rate of five per cent.

Loft Insulation Grants

Various government-backed loft insulation grants are available that significantly cut the cost of loft insulation or even pay for it completely. Eligibility for funding depends on your property and your circumstances.

The Great British Insulation Scheme (GBIS) provides funding for loft insulation in homes in certain Council Tax bands with a low energy performance rating.

Under Local Authority Flexible Eligibility (LA Flex), local councils have their criteria for loft insulation grants for households on low income.

The Home Upgrade Grant (HUG) pays for loft insulation for some homeowners in certain postcode areas in England off the gas grid.

The Affordable Warmth Scheme covers loft insulation costs for households with an income below £20,000.

National home insulation specialists Effective Home can check whether you’re eligible for a loft insulation grant.

Call us on 0333 003 0703 or contact us online to arrange a free, no-obligation home survey.


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Justine Effective
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