How to insulate a loft or roof

Insulating a pitched roof or loft is one of the most cost-effective ways to improve your home's energy efficiency. 

Uninsulated homes lose around a quarter of their heat through the roof. Even if you already have some insulation, you will still benefit from topping it up to the recommended minimum amount.

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Insulating a flat or pitched roof 

Most roofs can be insulated, but there are different methods for each.

• Pitched roof – there are two options: cold or warm loft insulation.

• Flat roof – there are three options available: warm deck, cold deck, or inverted roof. 

Cold loft insulation

This is what most of us imagine when we think about loft insulation:

Cold loft insulation means insulation is installed between and across the timber joists that form the ‘floor’ of a loft space under a pitched roof. It’s generally the simplest and cheapest way to prevent heat escaping through your roof.

It’s called cold loft insulation because it leaves the loft space itself uninsulated, which means it can get very cold in winter and hot in summer. It’s a good choice if you don’t plan to use your loft space or use it to only store items that are unaffected by extremes of temperature.

It's simple to do as a DIY project, and you can buy loft insulation from most high street DIY stores.

Warm loft insulation

If you plan to use your loft regularly, or to store items that need a stable temperature, then warm loft insulation might be a better choice.

The insulation is installed directly under the roof slope, between and across the timber rafters. It reduces heat loss in the same way as cold loft insulation, while maintaining a comfortable temperature in the loft space itself.

It's not as easy to install as cold loft insulation. You will generally need a professional installer, as doing it wrong can cause damp and condensation. As well as insulating the underside of the roof, you’ll also need to insulate any gable walls and/or party walls. 

If you choose warm loft insulation, note that some of the heat from your home will rise up into the loft space. Your home's central heating system will be heating a larger volume of air within your home, so you can expect higher heating bills than you would have with cold loft insulation.

Can I insulate my loft myself?

There are several different types of loft insulation material. Each has its advantages and disadvantages.  

How much does loft insulation cost?

We’ve found out how much it costs to install and how long loft insulation will take to pay for itself. 

The payback time is particularly short if you started out with none at all, but even homes with existing loft insulation can save money by increasing the thickness.

The government and energy companies currently have a number of grants available for either free insulation or a contribution towards the costs. 

Find out more about what insulation grants are available.

We've worked with the Royal Institution of Chartered Surveyors* (RICS), which publishes average building work and repair costs, to bring you average costs for loft insulation. 

It's split into the average costs for a terraced, semi-detached and detached house, fitted by a professional. Bear in mind that costs will vary, depending on where you live in the country. 

Only logged-in Which? members can see the costs in the table below. If you're not a member, join Which? to get instant access.

Loft insulation costs

Table notes: Price based on glass mineral wool insulation quilt laid over and between the joists, 270mm thick.  Data copyright RICS 2022, reproduced with permission. Data is current as of October 2022.  

Loft insulation savings

Professionally installed loft insulation will typically take less than five years to pay for itself through the savings you’ll make on your heating bills if you go from no insulation to the recommended amount.

Log in or join Which? to see how quickly loft insulation will likely pay for itself through savings on your energy bills.

How to prepare a loft before you install insulation

Whatever type of insulation you choose, you’ll need to clear out the loft space before you begin. If you've hired an installer, ask them if they have any specific requirements.

Wiring in the loft space must be dealt with safely. Wires should be kept above the insulation but not stretched if they don’t comfortably reach. An electrician will be able to reroute any problematic wiring. You can find a recommended local electrician by visiting Which? Trusted Traders. 

Downlights in the ceiling below the loft should be protected from overheating. Special covers can be installed, which create an air pocket below the insulation, and reduce heat loss and draughts around the light fitting.

Pipework and tanks in the loft should also be insulated correctly. If you install cold loft insulation, the loft space will become much colder in winter, so you'll need to protect pipes to prevent them from freezing.

Ensure you have the correct protective clothing if you plan to DIY. You’ll normally need a face mask, goggles, and gloves, as a minimum.

As well as escaping upwards through your loft, heat can escape downwards through your flooring. Find out more about floor insulation options and costs in our comprehensive guide. 

How to install DIY loft insulation

Installing blanket or loose-fill insulation is a straightforward DIY job that can be tackled by most competent DIYers. Here's how to install these two popular types of loft insulation.

How to install loft insulation rolls

Blanket insulation is the most common type of insulation and is sold in rolls, which are laid down between and across the horizontal joists in a loft.

To meet current Building Regulations standards, you should install a minimum depth of 270mm. For example, you might lay a 100mm thick roll in between the joists and then lay a 170mm or 200mm roll across the top, at right angles to the joists.

Rolls of blanket insulation come in different widths, so start by measuring the spacing of your joists and buy rolls of the correct width to fit neatly between joists.

Unroll the insulation blanket and lay it flat between the loft joists. Don't stretch or tear the insulation – use scissors if it needs to be cut. Add a further layer of blanket insulation across both the joists and the lower layer of insulation, at right angles. Be careful not to compress the insulation. 

If you want to use your loft for storage, you may need to raise the level of the floor to allow room for at least 270mm of insulation plus a ventilation gap. You can buy special spacers or ‘legs’ that screw onto the joists, or simply add timber battens to raise the floor height.

Refer to the manufacturer’s instructions to ensure you’re fitting it correctly and getting the most out of your insulation. If you’re not sure of the best option, contact an installer for a professional opinion.

How to install loose-fill loft insulation

Loose-fill insulation material comes in bags, and because you simply pour the material from the bag to lay it, it's an easy task to complete yourself. Just make sure your loft is properly prepared first.

Before laying your loose-fill loft insulation, ensure the space between the joists is relatively dust-free and there are no cracks or holes in the ceiling. You should also check the depth of the joists is sufficient to hold the correct level of insulation.

Calculate the loft area in square metres. Check the manufacturer’s specifications to work out how many litres/bags you’ll need to cover the space to the recommended depth.

The material should be poured between the joists in the roof space, brushing or raking between the joists to level it. If you’re not fitting boards over the loose-fill insulation, check the level of the material regularly. If your loft is draughty, high winds can unsettle loose-fill insulation and blow it around in the roof space, so you may need to brush it back into place.

How to insulate a loft hatch

When insulating your loft, don’t forget about the loft hatch. The access hatch should be both insulated and draught proofed to prevent heat loss.

To DIY insulate an existing hatch, cut blanket insulation to fit, then glue and/or staple it to the hatch and cover with a sheet of plastic or other material to keep it in place. Make sure you can still open and close the hatch easily.

If you’re installing loose-fill insulation, you can make a ‘pillow’ filled with insulation and fix this to the hatch instead. Make sure it’s well sealed so the insulation can’t escape. 

If your loft hatch is past its best, you can replace it with an all-in-one, fully insulated and draught-sealed replacement frame and lid. 

Can you still use an insulated loft for storage?

One reason some some of us don’t insulate our lofts is because we want to use it for storage. The required 270mm of insulation material over the joists would make it impossible to lay boards on top. 

But there are ways around this:

• Insulate between the joists with blanket or loose-fill insulation, then lay rigid insulation boards on top and timber floor boarding on top of that (or insulated loft deck boards, as mentioned earlier).
• Alternatively, raise the level of the floor using timber battens or special loft legs so you can fit 270mm insulation beneath the new floor level. 

Either way, make sure you don’t squash the mineral wool when you fit the boards on top. If you do, this will reduce its insulation properties. 

RICS cost calculations

*To arrive at the average prices above, RICS uses cost data from its Building Cost Information Service (BCIS) database, where costs are collated from a variety of sources and analysed. 

Materials costs are based on the best trade prices from a range of suppliers across the UK, which are then benchmarked to reveal the best national average. Labour rates are based on the current Building and Allied Trades Joint Industrial Council wage agreement. Data copyright RICS 2023, reproduced with permission. Data is current as of October 2023.