The ultimate guide to roof and loft insulation
27 August 2020 | OVO Energy
You can always tell who hasn’t insulated their loft space because of the pigeons and seagulls sitting on their roof, enjoying all the warmth coming up from the house below. Heat rises, and those wily birds are quick to take advantage.
What is loft insulation?
Loft insulation is a barrier of material within your roof space. It can either be laid between the joists (the horizontal beams along the floor of your attic) or the rafters (the angled beams that support the roof).
Either way, it slows down the transfer of heat between your living space and the outside world, creating a warmer home in winter and a cooler one in summer. Insulation between the joists keeps warmth in your living space below and creates a cold loft, while insulation in the rafters allows you to keep warmth in the roof space as well.
Other benefits of roof insulation include:
Loft insulation for an average 3-bedroom, semi-detached house with gas central heating can start at about £50 for DIY installation, and could take less than a year to pay for itself, thanks to lower energy bills.
Why is loft insulation needed?
In uninsulated homes, a quarter of the heat lost wafts out through the roof. So fitting insulation in your loft, attic or roof space is a great way to improve your home’s cosiness factor. And there’s even more good news: it lasts for more than 40 years and should pay for itself many times over in energy bill savings, as the cost of loft insulation is comparatively low.
In terms of the environmental benefits, having good loft insulation also helps you reduce your home’s carbon footprint. That’s because the better insulated your home is, the more efficient your heating system will be – reducing emissions as well as saving you money on bills.
Loft insulation costs and savings
These figures are from the Energy Saving Trust website, and show the benefits of installing roof insulation of up to 270mm.
How much could I save by insulating my loft?
This table shows the benefits of increasing your loft insulation thickness from 0mm up to 270mm.
|Type of residence||Detached||Semi detached||Mid terrace||Bungalow|
|Fuel bill savings per year||£240||£140||£135||£200|
|Typical cost of loft insulation installation||£395||£300||£285||£375|
|Carbon dioxide savings per year||990 kg||580 kg||550 kg||820 kg|
This table shows the benefits of increasing your loft insulation thickness from 120mm up to 270mm.
|Type of residence||Detached||Semi detached||Mid terrace||Bungalow|
|Fuel bill savings per year||£25||£15||£15||£20|
|Typical cost of loft insulation installation||£310||£250||£240||£295|
|Carbon dioxide savings per year||95 kg||55 kg||50 kg||80 kg|
Both tables are based on insulating a gas-heated home. The installation costs are unsubsidised averages and will vary.
Can I insulate my loft myself?
It’s possible (in fact quite simple) to lay roof insulation yourself, as long as:
- It’s easy to get into your roof space.
- There are no damp or condensation problems.
- Your loft joists are regularly spaced.
- You’re not trying to insulate a flat roof.
For anything more complicated than this, you’ll need to be an experienced DIY-er or bring in a professional installer who knows how to insulate a loft in the most appropriate and effective way.
Flat roofs should always be insulated by a professional, and if the space is damp you’ll need to sort that out before starting any loft insulation work.
If you live in a period home, or one that was built using local traditional stone or other materials, make sure you use appropriate loft insulation materials and methods. Otherwise your home may not be able to retain heat, or stay dry, in the way it was meant to. In fact, it may be best to consult or employ local craftspeople who are trained in the relevant techniques.
Thinking of transforming your loft into another bedroom or bathroom? For that, you'll need professional help. Have a read of our complete guide to loft conversions and how to do them.
How to insulate a loft
If you do have a dry attic with easy access and evenly-spaced joists, the cheapest and simplest way to insulate it is between the joists, with rolls of blanket roof insulation. This can be made of mineral wool, glass fibre or recycled materials.
- First, clear your loft of any old boxes and anything else that may have been left up there.
- Lag any pipes and water tanks in the roof space to prevent them freezing, as the attic will become much colder once it’s insulated from the living space below.
- Check whether there’s any electrical wiring up there. If there is, it will need to be above the insulation material, but it mustn’t be stretched to achieve this. If you’re not sure what to do, consult a professional electrician.
- Measure your loft and buy enough blanket insulation product to fill it to a depth of at least 270mm.
- Unroll the insulation and lay it along between the joists. Most joists are around 100mm high (but measure yours first to confirm this), so you’ll need 100mm thick wool for this first layer. Loft insulation rolls are usually 1140mm wide, so you’ll probably need to cut it to fit. Use scissors, as tearing or stretching the material will compromise its effectiveness.
- Create a second layer at right angles to the first. If your first layer is 100mm deep, you’ll need 200mm of wool for the second layer.
I need to use my loft as storage space, can I still insulate it?
If you want to store bits and pieces safely in your attic you’ll need to fit boards over the joists. But if you simply fill the spaces between the joists with insulation and then board over the top, you won’t achieve an adequate depth of loft insulation.
To increase the thickness of your roof insulation layer, you can:
- Lay mineral wool between the joists and then fit rigid insulation boards across the joists, with floorboards on top. You could even buy insulation and floor boards that are already bonded together, which makes the job much faster and easier.
- OR use your carpentry skills (or someone else’s) to create a higher floor level, so you can fit in the required 270mm of loft insulation.
Whichever you decide to do, don’t put in 270mm of mineral wool and then just squash it down, as this will reduce its efficiency, and you might as well have used a smaller amount.
Remember, insulating between the joists will create a cold roof space, so if you’re keeping anything up there that needs to stay warm, it’s better to insulate between the rafters rather than the joists.
How do I stop warm air leaking into the loft through the hatch?
Make sure the hatch fits snugly. If it doesn’t, you may need to replace it. If it seems to fit more snugly in the winter than in summer, you may have damp in your loft, and you’ll need to tackle this before thinking about roof insulation.
Once you’ve got a close-fitting hatch, apply draught-stripping between the hatch and the frame.
Check there aren’t any other holes going up into the roof – above the airing cupboard, for instance, or around pipes. If there are, you should seal these before you insulate your attic.
I’ve converted my loft to living space. How do I insulate it?
If you’re using the loft as a bedroom, games room or study, you’ll want to keep it as warm as the rest of the house – so there’s no point in insulating the attic floor. Instead, you need to insulate the roof. The usual way to do this is by inserting rigid insulation boards between the rafters.
They should be cut to exactly the right size, so they fit snugly and don’t allow any draughts through. To get the required 270mm thickness of insulation, you may have to insulate over the rafters as well, using insulated plasterboard.
Unless you love the look of insulation boards (unlikely) you can then cover them with plasterboard, and paint or wallpaper it.
You can also use this method if you want to keep the loft warm for other reasons.
My loft hatch is too small for an adult to get through. How can I insulate my attic?
You’ll have to use a professional installer, who can blow granules of insulation into your loft with specialist equipment. The roof insulation material will be cellulose fibre or mineral wool, treated to make sure it’s fire retardant. This should take just a few hours, depending on the size of your loft.
How does loft insulation work if you’ve got a flat roof?
If you’ve got a flat roof you can still insulate it, but it’s better to insulate it on the outside rather than the inside. Insulating it from below can cause condensation problems.
When your roof is next due for replacement, you’ll have to insulate it anyway, to comply with government regulations. If that date’s coming up soon, it makes sense to do both jobs together. You can add a layer of insulation between the timber roof surface and the outer weatherproof layer.
But if your flat roof isn’t due for replacement for a few years, you can add the insulation board directly on top of the weatherproofing.
Our loft’s a bit damp, will insulation make it worse?
Insulation between the joists will make your attic much colder, so this could worsen your damp problem. Even rafter insulation can encourage damp, as it stops draughts and reduces natural ventilation. So if you decide to insulate your loft you may also need to increase the amount of ventilation, and in that case you’ll need Building Control Approval.
We recommend getting professional advice from a builder or damp expert before you start on this project. Talk to them about sorting out the damp before you think about insulating the loft.
My cold water tank and pipework are up in the loft. Will insulation affect them?
Yes. If you’re laying insulation between the joists, the roof space will become extremely cold in winter. So you need to insulate (lag) your tank and pipes individually to make sure they don’t freeze and burst. If they do, the repairs will cost far more than the savings you make from loft insulation.
If you insulate between the rafters rather than the joists of your attic, the space will be warm and your pipes should be fine.
How do I find an insulation installer?
Visit the National Insulation Association (NIA) website to find NIA members in your area. Installers listed on the site have agreed to follow a code of professional practice.
If you’re in Scotland, call Home Energy Scotland on 0808 808 2282. You can also visit Historic Scotland's Technical Conservation knowledge base for information on how to insulate traditionally constructed buildings.
Green Homes Grant: Government help to pay for insulation improvements
With the Green Homes Grant scheme, you can apply for some very useful money-saving and energy-saving vouchers. The Government scheme is designed to help homeowners and landlords in England with the cost of installing everything from insulation to heat pumps to solar thermal.
The vouchers are worth up to two-thirds of the cost of making your home more energy efficient – up to a maximum of £5,000 per household.
Find out more about the Government’s Green Homes Grant scheme
More ideas to help you insulate your home
We’ve put together a range of user guides to give you ideas for ways you can improve your home’s insulation levels for roof, loft, walls, windows and doors.
Cavity wall insulation
Insulating your cavity walls could be a very cost-effective way to retain heat in your home and save money on your bills. It’s estimated that cavity insulation could save you up to £160 a year in heating bills.
Find out more in our guide to cavity wall insulation
Solid wall insulation
Fact! Solid walls let twice as much heat escape as cavity walls. For a detached house, insulating solid walls could save you as much as £455 a year.
Read more in our guide to solid wall insulation
Draught-proofing windows and doors
Draught-proofing your home’s windows and doors and you could save between £25 and £50 on your heating bills each year – not to mention the environmental benefits.
Find out more in our guide to draught-proofing windows and doors
Looking for other ways to save money on your energy? Check out our most affordable energy plan Better Smart and start saving on your gas and electricity bills today. Read less
*Source and notes for graphs and table: https://energysavingtrust.org.uk/