Green Deal Insulation
Insulation Types

The better insulated your home is, the less money you’ll spend heating it. Find out more about different types of insulation, including draught-proofing, cavity wall insulation, external wall insulation, hot water cylinder insulation, loft & rafter insulation, pipework insulation & underfloor insulation.

Cavity Wall Insulation

Cavity wall insulation

If your home was built after 1920, the chances are that its external walls are made of two layers with a gap or cavity between them. Cavity wall insulation fills that gap, keeping the warmth in to save energy. It can also help reduce condensation inside the house if this is a problem on your external walls. Depending on your house type you could save up to £250 per year on energy bills.

Is cavity wall insulation suitable for your home?

Your home will usually be suitable for cavity wall insulation if:

• The external walls are unfilled cavity walls
• Your cavity is at least 50mm wide
• The masonry or brickwork of your property is in good condition
• It is more than ten years old (most newer houses will have insulation already)
• The walls are not exposed to driving rain.

Are your cavity walls unfilled?

If your house was built in the last ten years or so, its walls are probably insulated already. To find out whether they are:

• Ask a registered installer for a boroscope inspection. They will drill a small hole in your external wall to see if your walls are hollow or filled.
• Check with your local authority’s building control department – they might know if your cavity walls have been filled already.

Are your external walls easy to access?

Cavity wall insulation is blown into the cavity from the outside of a house. Every part of the wall must be filled with insulation, so it’s important that the installer can reach all your external walls.

If your home’s external walls are joined to another house, the installer will need to insert a cavity barrier to contain the insulation, so your neighbours aren’t affected.

What if there is damp?

If you have any damp patches on your internal walls then they should not be insulated until the problem is sorted out. You should speak to a builder who specialises in damp prevention.

How is insulation installed?

To insulate your cavity walls, the installer drills small holes around 22mm in size at intervals of around 1m in the outside wall of your home. With specially designed equipment, they then blow insulation into the cavity. Once all the insulation is in, the installer fills the holes in the brickwork so you’ll barely notice them.

Filling cavity walls is not a job you can do yourself: you will need to find a registered installer. A professional can do the job in around two hours for an average house with easily accessible walls; it should be simple, quick – and make no mess.

What is the insulation made of?

Cavity wall insulation can be made out of three types of materials:

• Mineral wool
• Beads or granules
• Foamed insulates

All three are manufactured according to British standards. Foam Insulation systems should be certified by the British Board of Agrément and installed according to strict guidance laid out in the associated BBA Certificates.

External Wall Insulation

External wall insulation

Insulating your solid walls could cut your heating costs considerably, because solid walls let through twice as much heat as cavity walls do. The good news is they can be insulated – from the inside or the outside. Depending on which type of property you live in you could save up to £450 per year on energy bills.

If your home was built before 1920, its external walls are probably solid rather than cavity walls. Cavity walls are made of two layers with a small gap or ‘cavity’ between them. Solid walls have no such gap, so they let more heat through. Solid walls can be insulated – either from the inside or the outside. This will cost more than insulating a standard cavity wall, but the savings on your heating bills will be bigger too.

External wall insulation process

External wall insulation involves fixing a layer of insulation material to the wall, then covering it with a special type of render (plasterwork) or cladding. The finish can be smooth, textured, painted, tiled, paneled, pebble-dashed, or finished with brick slips.

• Can be applied without disruption to the household
• Does not reduce the floor area of your home
• Renews the appearance of outer walls
• Improves weatherproofing and sound resistance.
• Fills cracks and gaps in the brickwork, which will reduce draughts
• Increases the life of your walls by protecting the brickwork
• Reduces condensation on internal walls and can help prevent damp (but will not solve rising or penetration damp)
• Is best installed at the same time as external refurbishment work to reduce the cost
• May need planning permission – check with your local council
• Requires good access to the outer walls
• Is not recommended if the outer walls are structurally unsound and cannot be repaired

Keeping the costs down

The costs we quote for installing solid wall insulation are for paying a company to come in, insulate your whole house in one go, fully redecorate and replace everything just as it was. Some people want exactly this, but it does cost a lot of money. If you’re looking for a cheaper option, the best thing to do is to insulate a wall whenever you are doing something else to it anyway. Fitting the insulation work in with your other home improvements not only saves money on the job, it also spreads the cost of the insulation as you work your way round the house.

If you’re planning a new kitchen or bathroom, this is an obvious time to fit internal insulation. But for all the other rooms, why not factor it in when you’re next redecorating? You’ll be clearing the room and making a mess anyway, so why not take the opportunity to improve your insulation while you’re in there?

External insulation will also cost less if you do it when you’re having other work done to the outside. If you’re having a new roof, or painting the windows, or even having solar PV panels fitted, then you will probably have scaffolding up already, which can save a bit on the costs. And if you have rendered walls with damaged render, or brick walls that need re-pointing, external insulation may not cost you much more than you would need to pay for the repairs.

Lots of older houses have an attractive frontage which wouldn’t be suitable for external insulation, but a much less impressive rear where external insulation could be just the thing. The front wall can then be insulated internally, one room at a time.


If you insulate a solid wall, you have to make sure it complies with the current Building Regulations. The main condition to meet is the thermal performance of the insulated wall – if you live in England or Wales then it must have a U-value of no more than 0.30 W/m2K. The U-value is a measure of how quickly heat will pass through the wall – as a rough guide you will need around 60mm to 120mm of insulation to achieve this, depending on what insulation material you use.

Normally your installer will ensure that the insulation is up to standard and will arrange approval from the local Building Control Office for you. If they are not going to do this, you should contact Bulding Control at an early stage to make sure you comply.

If you are planning to remove and replace more than half of the internal plaster or external render of a wall, or if you are dry lining a wall, then you have to insulate to this standard whether you were planning to insulate or not.

If you live in Northern Ireland or Scotland you should contact your local building control office for information on what you need to do to comply with the regulations.

Hot Water Cylinder Insulation

Hot Water Cylinder Insulation

Insulating your hot water cylinder is one of the easiest ways to save energy and therefore money. If you already have a jacket fitted, check the thickness, it should be at least 75mm thick. If not, it’s worth treating your cylinder to a new winter coat. Fitting a British Standard jacket around your cylinder will cut heat loss by over 75% and could save you around £20-35 a year – more than the cost of the jacket! A hot water cylinder jacket costs around £15, and fitting it is a straightforward job.

Loft & Rafter Insulation

Loft & rafter insulation

In an uninsulated home a quarter of your heat is lost through the roof. Insulating your loft or attic is a simple and effective way to save that waste and reduce your heating bills. Loft insulation is effective for at least 40 years, and it will pay for itself over and over again in that time. If your loft is already insulated, it’s worth checking that you’ve got enough insulation to get the maximum saving. If everyone in the UK installed 270mm loft insulation, we could save around £500 million – and nearly three million tonnes of carbon dioxide every year Depending on the type of property you live in you could save up to £250 per year on energy bills.

Choosing loft insulation

Easy access and regular joists – If your loft is easy to access and has no damp or condensation problems, it should be easy to insulate.

If access is easy and your joists are regular, you can use rolls of mineral wool insulation. The first layer is laid between the joists – the horizontal beams that make up the ‘floor’ of the loft – then another layer is cross-laid at right angles to cover the joists and make the insulation up to the required depth. This can be done by a competent DIY-er or a professional installer.

Storage or living space – If you plan to use the loft or attic for storage, you will want to lay boards over the joists. Unfortunately, if you only insulate between the joists before doing this, the insulation won’t be thick enough. To get enough insulation you can:

• Insulate between the joists with mineral wool and then lay rigid insulation boards on top, with wooden boarding on top of that. You can buy insulation board pre-bonded to floor boarding to make the job easier.
• Raise the level of the floor so you can fit enough mineral wool beneath the new floor level.
Either way, make sure you don’t squash the mineral wool when you fit the boards on top – this will reduce its insulation value.

If you want to use your loft for living space, you can insulate the roof of the loft instead of the floor by fixing rigid insulation boards between the roof rafters. Boards must be carefully cut to the right width so that they fit snugly between the rafters. They can then be covered by plasterboard.

Rafters aren’t usually very deep, so to get the best performance you may have to insulate over them as well, using insulated plasterboard. If there isn’t room to do this, make sure you use the highest performance insulation board available.

Difficult access – If your loft is hard to access, you can have blown insulation installed by a professional, who will use specialist equipment to blow loose, fire-retardant insulation material made of cellulose fibre or mineral wool into the loft. This doesn’t usually take more than a few hours.

Irregular joists – If your loft space is irregular, the joists are the wrong distance apart for rolls of mineral wool, or there are lots of obstructions that make laying matting tricky, you can use loose-fill insulation. This is sold in bags as cork granules, vermiculite, mineral wool or cellulose fibre, which can be poured between the joists to the right depth. This can be done by a compete DIY-er or a professional installer: you may need to increase the height of the joists to get the insulation deep enough.

Flat Roofs – A flat roof should preferably be insulated from above. A layer of rigid insulation board is added either on top of the roof’s weatherproof layer, or directly on top of the timber roof surface with a new weatherproof layer on top of the insulation. This is best done when the roof covering needs replacing anyway.

It is possible to insulate a flat roof from underneath, but this can lead to condensation problems if not done correctly.

Damp lofts – Insulation stops heat escaping from living spaces, so it will make your loft space cooler – which could make existing damp or condensation problems worse. Get professional advice before installing insulation to see if you can fix the damp problems first.

Pipes, water tank and loft hatch

Insulating between the joists of your loft will keep your house warmer but make the roof space above colder. Pipes and water tanks will be more likely to freeze, so you will need to insulate them. If your water tanks are some distance from the loft hatch, you will also need something to walk on for safe access.

The cooler air in your insulated loft could mean that cold draughts come through the loft hatch. To prevent this, you can fit an insulated loft hatch and put strips of draught-excluding material around the edges of the frame.

Pipe Work Insulation

Pipe Work Insulation

Lagging water tanks and pipes and insulating behind radiators reduces the amount of heat that escapes, so you spend less money heating water up, and hot water stays hotter for longer. By slipping pipe insulation around your exposed hot water pipes you’ll keep your hot water hotter for longer. Fitting insulation to pipes is easy if the pipes are accessible; if your pipes are hard to reach, you may need professional help.

Under-Floor Insulation

Floor Insulation

Insulating under the floorboards on your ground floor will save you around £60-£75 a year, and you can seal the gaps between floors and skirting boards to reduce draughts too. Insulating and draught-proofing your floor can be a cost-effective energy saving measure.

Gaps and draughts around skirting boards and floors are simple to fix yourself with a tube of sealant bought from any DIY store. Floorboards will rot without adequate ventilation, though, so don’t block under-floor airbricks in your outside walls.

Older homes are more likely to have suspended timber floors. Timber floors can be insulated by lifting the floorboards and laying mineral wool insulation supported by netting between the joists.

Many homes – especially newer homes – will have a ground floor made of solid concrete. This can be insulated If it needs to be replaced, or can have rigid insulation laid on top.

You don’t need to insulate the floors of upstairs rooms in your house if they’re above heated spaces (like the living room). But you should think about insulating any floors that are above unheated spaces such as garages, as you could be losing a lot of heat through those.

Professional floor insulation

For some jobs, you might need to get some professional help. If you don’t feel confident lifting your floorboards yourself you can get a professional in to do this, fit the insulation and replace them afterwards. Costs will vary depending on how big your house is and how easy the floorboards are to lift and to replace.

For solid concrete floors, make sure that if they ever need to be replaced, your builder puts in insulation – in fact, you have to insulate a floor when it is replaced in order to comply with Building Regulations. Solid floors are insulated using rigid insulation foam, which can be fitted either above or below the concrete. If the concrete is above the insulation it can sometimes store heat during the day which helps keep the room warm at night. On the other hand, if the insulation is above the concrete, the room will heat up more quickly in the morning.

If your concrete floor doesn’t need to be replaced, it can still be insulated. Rigid insulation can be laid on top of the original concrete floor, then chipboard flooring put on top of that. This will raise the level of the floor, so you will need to make sure doors are trimmed shorter to make room for the insulation, and skirting boards and some electrical sockets might need to be moved.

Complying with building regulations

If you are adding extra insulation to your floors, the work will need to comply with the relevant Building Regulations for where you live. Your installer will normally arrange this for you but if you are doing it yourself, it is your responsibility to comply.

If you live in England or Wales, the floor should achieve a U-value of 0.25 W/m2K or less, if possible. The U-value is a measure of how quickly heat will travel through the floor. To achieve this standard you will normally need at least 70mm of high-performance foam insulation, or 150mm of mineral wool, but this will vary depending on floor type, shape and size.

If you are replacing at least half of a floor then you have to insulate to these standards whether you planned to or not.

For further information, and for regulations in Northern Ireland and Scotland, we recommend that you contact your local Building Control Office before starting work.

How do I tell what kind of floor I have?

If you have a basement or cellar beneath your house that you can get into safely, go and take a look. If the floor is a suspended wooden floor, you will probably be able to see wooden joists and the undersides of the floorboards. Also, if you have air bricks or ventilation bricks on the outside wall(s) of your house that are below floor level, you probably have a suspended timber floor. (If you do have these airbricks in your walls, don’t block them up. They are needed to help ventilate the space under your floor and stop your floorboards from rotting.)

If you don’t have access to the space underneath your house, you will need to lift a corner of the carpet and underlay and have a look. If you live in a flat (apart from the ground floor) then you will also need to lift the carpet to see what kind of floor you have. However, you don’t need to insulate your floor if there is another flat beneath you.