How Thick Should Loft Insulation Be?

Insulation, Membranes & Felts
product guide
mineral wool loft insulation installed between rafters and joists

With a number of important practical and economic benefits, loft insulation is undoubtedly a key part of any pitched roofing structure. Loft insulation is installed between the joists and rafters of a pitched roof to help slow the transfer of heat between the outside and inside. This ensures that the property stays warm in the winter and cool in the summer. What’s more, according to Which?, loft insulation can save you up to £315 per year on energy bills.

There are many types of loft insulation to choose from however, each with their own advantages and disadvantages. Before you ask yourself how thick your loft insulation should be, it’s vital to understand the differences between your options, so that you can make the right choice for your budget and your property. Read on to find out more!

Types of Loft Insulation

Here is a helpful rundown of the most common types of loft insulation.

  • Blanket Insulation: The most common form of loft insulation is blanket insulation, also known as loft roll insulation, often made from glass mineral wool. This is a reliable, affordable form of insulation that’s simple to install. Insulation rolls are ideal for accessible areas of the roof space, such as between joists or in exposed wall cavities.
  • Loose Fill Insulation: Loose-fill insulation is made from lightweight materials such as fibreglass, cellulose fibres, cork granules, or mineral wool. Containing a high percentage of recycled materials, loose fill insulation is a highly sustainable way to insulate your loft. Plus, it’s much easier to install in less accessible areas or lofts.
  • Spray Foam Insulation: Often made from polystyrene, spray foam insulation is sprayed in liquid form into areas such as wall cavities and can expand up to 100 times its original size. This form of loft insulation can only be installed by a professional, due to the specialist equipment required and potential risks involved.
  • Insulation Boards: A popular way to insulate a loft space, using a insulation board offer the best thermal performance, along with simple installation. The boards are designed to fit snugly between joists and rafters and are easy to cut to size by hand if required. We’d recommend insulation boards for large lofts with good accessibility.
  • Insulation Slabs: Available in a variety of sizes, thicknesses, and materials, slab insulation offers a fantastic combination of thermal and acoustic performance. This form of loft insulation is ideal for homes in busy or noisy areas, as it will help to save money on energy bills as well as combatting excessive noise from outside.

What is the Recommended Thickness for Each Type of Loft Insulation?

With so many different types of loft insulation available, it’s important to be aware of the different thicknesses required for each to achieve the thermal performance you want. For example, the recommended thickness for loft insulation is 270 millimetres for glass wool, 250 millimetres for rock wool, and 220 millimetres for cellulose.

Can I Use More Than One Layer of Insulation?

Yes, new rolls of insulation can be laid on top of existing insulation. In fact, combining more than one layer of loft insulation is the most common way homeowners and developers meet the 270mm thickness recommendation. One popular method is to install a 100mm thick layer of insulation between the joists in your loft, and then adding a layer of 170mm insulation at right angles over the joists to get the total thickness to 270mm.

What U-Values Can Different Thicknesses of Loft Insulation Achieve?

Considering U-Values is highly important when calculating how much loft insulation you need. As a general rule, the lower the U-Value you achieve, the more heat your loft space will retain. Learn more about U-Values.

Insulation Boards

On average, a 50mm loft insulation board will provide a U-Value of 0.4, whereas a 150mm thick insulation board can achieve a U-Value of 0.2 or lower. Boards of insulation are available in thicknesses from 20mm to 200mm, so you’re sure to find the right thickness of insulation to meet the requirements of your loft space.

Multi-Foil Insulation

Multi-foil insulation is made from a combination of reflective foil and insulating wadding. This highly modern form of loft insulation can achieve a U-Value as low as 0.04 with a thickness as low as 35mm. This makes multifoil insulation a highly space-efficient choice, and a very popular way to top-up existing layers of loft insulation for higher performance.

Spray Foam Insulation

Spray foam insulation can achieve a U-Value of 0.4 when installed at a thickness of 40mm, or even 0.16 or lower when using 150mm thick spray foam insulation. According to the National Insulation Association, 100mm of spray foam insulation is equivalent to 170mm of mineral wool insulation, making it the more space-efficient choice.

Check Thickness of Existing Loft Insulation

Before you work out how thick your new loft insulation needs to be, it’s vital to check the thickness of any existing insulation in your loft space. It should be noted that if you find that your current loft insulation is less than 100mm thick, it is unlikely to still be effective, and we’d advise that you dispose of it. If however your existing loft insulation is 100mm thick between the joists, it may be possible to top it up with new insulation materials.

As the years have passed, loft insulation thickness requirements and recommendations have increased significantly. This is to enhance the levels of insulation in British homes, to make them more energy-efficient and reduce energy bills. For example, back in the 1960s, the thickness of loft insulation recommended by building regulations was just 25mm!

Below, you can see a table of the recommended levels of loft insulation since the 1960s.


Recommended Thickness















Can Loft Insulation Be Too Thick?

Although it can be tempting to believe more is always better, exceeding recommended loft insulation thicknesses can actually have a number of negative consequences. The most common of these is poor ventilation, which can lead to issues such as damp, condensation, and mould. The ideal is to use a thickness of loft insulation that maximises thermal performance, whilst allowing enough ventilation to prevent issues with airflow in the loft.

In Closing

Ensuring that you have the right thickness of loft insulation is important for maintaining a comfortable environment within your home and keeping energy bills down. It’s key to take the material you choose into account and selecting the correct thickness to meet the recommended 270mm. Take a look at our range of roof insulation and take that first step towards reducing bills and making your home a more pleasant living space. Or, if you’ve already purchased some insulation, maybe it’s time to learn how to insulate a loft.

If you’ve got any further questions, don’t hesitate to get in touch with our award-winning customer service team. Call them on 01295 565 565, email [email protected], or leave us a message in our handy live chat below.

Discover More