How to insulate a loft or roof

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Around 25% of heat lost from an un-insulated home goes through the roof. The good news is you can easily install roof insulation

So, how much you save by insulating your loft or attic? The uSwitch guide makes it easy to find out how to insulate your roof and save money on your energy bills.

How much could I save by insulating my loft?

Numbers from the Energy Saving Trust indicate that you could save, depending on the size of your home, up to £250 a year simply by adding roof insulation.
Loft Insulation Detached house Semi-detached house Mid-terrace Detached bungalow
Cost (one time cost) £395 £300 £285 £265
Potential savings (each year) £250 £150 £140 £200

Estimates from Energy Saving Trust; based on insulating a home that is gas heated with a completely uninsulated loft or adding to existing insulation from 100 mm to 270 mm. Average unsubsidised professional installation costs, although these will vary

roof insulation

Does the type of roof I have affect my options for insulation?

Your options for roof insulation will depend on the type of roof you have. Pitched (sloping) roofs are more straightforward and there are more options to choose from, while flat roofs and dormer roofs are more of a challenge to insulate.

What is the difference between a warm loft and a cold loft?

If you have a typical house with a sloping roof, or have the top flat in a block and are responsible for the loft space and roof above, you can either choose to have a warm loft or a cold loft:

  • A warm loft is where you insulate immediately under the roof, which means your loft space is warm as well.

  • A cold loft is where you insulate immediately above the ceiling of the top storey, so no heat from your home gets into your loft.

Cost-wise, there are a range of options available for roof insulation, some of which are relatively inexpensive. Some can be achieved with little DIY experience, while others require more do-it-yourself expertise. Some roof insulation options require specialist equipment and need to be carried out by a professional. Energy-efficiency grants are currently available for cold loft roof insulation.

Warm roof solutions are generally more expensive, but they can provide a greater level of heat retention. Also, you can more easily use the loft to store temperature-sensitive items, as a 'cold roof' can get very hot in the summer.

What type of loft insulation is easiest to install?

The simplest and cheapest loft insulation solution is the classic cold loft option. This involves insulating between and over the wooden joists immediately above the ceiling of your top floor. This is generally the only grant-funded option, although it is also fairly easy to do as a DIY project.

If you use your loft for storage, you will need to clear it before any grant-assisted work can be done, as grants, whether partial or 100%, will not cover the cost of clearing a loft. It could also be the perfect opportunity to sort through the items you're storing to see if you still need them.

It's a relatively simple process, as you can see in this video:

  • Read the Transcript

    Hi my name is Kevin Sears. I’m the business development manager at uSwitch.com, price comparison site. Insulation is one of the best and cheapest ways to make your home more energy efficient. With the help of E.ON we’re going to show you just how easy it is to insulate your walls and your loft in your home.

    We’ll start off on with loft insulation. The installer will always make sure that your carpets are covered to stop any mess coming through.

    Hi my name is Bruce (second name). I’m at a property in Leicester to install loft insulation and cavity wall.

    Then it’s up to the loft and down to business. In this example the customer’s existing insulation will be topped up. This is laid between the joists above the ceiling of your top floor. It’s a relatively simple process but it’s well worth doing. Estimated savings for an uninsulated loft, a whopping £175 a year.

    Now the loft insulation is complete we’re going to move on to cavity wall insulation. Cavity wall insulation means the space between the outer and inner walls of your house is filled up, keeping your house much warmer. This isn’t available for all houses, so take a look at our insulation page to see if your house has cavity walls. If your house was built after 1930, but before 1995, it probably has cavity walls.

    The installer will first drill a few holes onto the outside of your home, through which the insulation is pumped in. Once completed, the holes are then filled back up with new mortar. It may sound like a big job, but it’s actually really simple, and it’s well worth doing.

    Cavity wall insulation will save you around £135 a year on your bill. And that’s it; in one or two hours your home is now much more energy efficient, you’re reducing your carbon footprint, but more importantly you’re saving money on your energy bills. The whole process is easy peasy and can save you up to £310 per year on your energy bill.

What depth of roof insulation should I install?

The recommended depth of blanket style insulation (glass or mineral wool) for a loft is 250 to 270 mm.

If you already have insulation, but it was put in some time ago, it is worth checking the depth, as only a few years ago the recommended depth of insulation was 200mm, and before that it was as low as 100mm.

What if there is already some insulation in my loft?

If you do have existing roof insulation, the first thing to do is to check the depth. If there is only 25mm of insulating material, then this is likely to date back to the 1970s, as this type of insulation was subsidised then. In fact, any insulation that is less then 100mm is likely to be old and is worth disposing of; it is also difficult to top-up older insulation to the recommended depth as modern insulation is sold in standard depths of 100mm and 170mm.

Most typically, especially where insulation has been put in since the 1980s, this will fill the approximately 100mm depth of the joists, and this is easy to top-up. The commonly available, low cost and subsidised products tend to come in either 100mm depth, to go between the joists, and 170mm to be laid (at right angles) over and across the joists, to take the depth up to the recommended 270 mm.

Are there any other jobs that need to be done in my loft to complete the insulation work?

If you are having your insulation installed professionally or under a grant, there are a number of additional tasks that are normally carried out. If you are doing a DIY installation, you mustn't ignore them.

  • Remember to cover the pipes with pipe insulation. Your loft will be colder due to the insulation keeping the warmth in the floors below, so you're more likely to get burst pipes in freezing weather.

  • Insulation should go up the side and over any tanks, or special tank insulation can be used, but you mustn't insulate under any tanks, as without some heat flowing up from below, these are also likely to freeze.

  • If your tank is in a raised position (at least 10cms above the uppermost layer of insulation) then you can insulate the underside of the tank .

  • If there are electric cables in the loft, try to leave these exposed so they can remain cool. If there is enough slack, they can be gently raised and the insulation put underneath. In practice, lighting cables are unlikely to be a problem, especially if you use low energy lightbulbs.

    Shower cables are most likely to need attention, although the fire risk is fairly minimal as showers tend to only be used for short periods. If you see any cables or junction boxes that appear to be in a poor state, you will need to get an electrician to put them right, anyway.

  • If you have recessed halogen lights in a room below the loft, they must be protected before they are covered in insulation. Halogen lights give off a lot of heat that is concentrated in a small area, and they pose a serious fire risk if insulation is placed directly against them. Protective cylindrically shaped fire protectors, which are called loft caps or loft covers, are available and it is essential to invest in these before you install your insulation.

  • Don't forget to insulate the loft hatch. This usually involves attaching a block of polystyrene insulation to its upper side, and ensuring any gaps around the side of it are sealed by applying draught proofing strips around the frame

Can I still use my loft for storage once the roof insulation is in place?

If you have the recommended level of loft insulation, you will no longer be able to store items by resting them on the joists. Putting items on insulation weighs it down and reduces its effectiveness, and you won't be able to put boards onto the joists either.

There are a couple of options you can consider though, although these wouldn't be included in any grant funding. You could put in some cross-joists so that any boarding will be able to accommodate the full 270mm of insulation. This is probably a job for a professional joiner. There are some firms which specialise in providing a fully (raised) boarded loft, along with a loft ladder and suitable loft lighting, providing a very convenient storage space.

Alternatively, and probably more cheaply and easily, if you already have insulation in up to 100mm, you can add one of an increasing number of insulation products that have a polystyrene or a wooden layer on top of them. Those with a polystyrene top layer will not be as hard wearing as those with a wood finish, but polystyrene provides the best insulation performance. In theory you won't be able to achieve the performance of 270mm of mineral or glass wool, but you will come close to it. If you want to match or exceed the performance of 270mm of a wool product, then you could consider a warm roof solution.

How does a warm roof solution work?

For a warm roof solution you will need to install insulation in the plane of the roof pitch, that is, immediately below the sloping roof. This will help to avoid your roof-space becoming excessively hot in summer or cold in winter, and will reduce heat losses from your roof. Assuming you don't have a room in the roof, and it is just a 'loft', then you should also implement all the 'cold-roof' insulation measures described above, including any raised boarding as appropriate.

how to insulate a loft

One important point to remember with a warm roof solution is the need for ventilation immediately below the roof tiles. This is to prevent any condensation build up, or water getting in through or around the tiles. You need to create a space for air to flow parallel to the rafters and immediately under the tiles from one side of the roof (the eaves) to the apex and down to the other side, otherwise the roof structure may begin to rot.

What types of insulation can be used for a warm roof solution?

The materials used to insulate under the roof include batts of mineral or glass wool insulation , held in place by 'battens' of wood attached to and across the rafters. Alternatively, polystyrene slabs , which are sometimes supplied foil-covered, can be fitted. These usually have to be cut to size. Another option is EPS (expanded polystyrene) 'squeeze' products; these are manufactured with a 'concertina' shape and spring-like effect and can be pushed into place.

There is an exception to the ventilation rule, and that's when applying polyurethane spray foams. These solutions, mainly professionally installed but also now available as a DIY measure, can generally only be applied where the underside of the tiles is bare and there is no roofing felt. Spray foam may also provide additional physical strength where a roof is not in the best condition, by holding the structure together. However missing or slipped tiles must be attended to prior to applying the foam insulation.

What insulation solutions are available for a flat roof?

There are three types of insulation solution if you have a flat roof:

  • Warm deck or warm roof refers to a situation where the 'deck' of the roof, which is usually made of wood, is below the insulation.

  • Cold deck or cold roof is where the insulation is below the roof deck (and the associated joists). Typically a gap will have to be left for ventilation - because this is a 'cold' area, condensation may form which can lead to rot. In both these cases the weather membrane, typically formed of roofing felt and bitumen, will be the topmost layer, protecting against rain.

  • Inverted roof is where the insulation goes above the weather membrane, effectively protecting it from heat and cold which can shorten its life and that of the roof deck - it can even protect against wear and tear if there is access to the roof. With an inverted roof, the top-most layer is generally gravel or similar.

The lowest layer with any of these solutions - especially with the cold deck solution - is likely to be a vapour membrane, which is used to stop warm air rising and bringing moisture that will condense somewhere in the roof structure.

The insulation of flat roofs nearly always requires professional assistance, and there are no associated energy-saving grants. A good time to insulate a roof is when it is being replaced, although in many cases a roof that is in good condition can be retro-fitted with insulation.

Do dormer bungalows and houses with attics in the roof space need a different approach to insulation?

Dormer roofs are roofs that contain rooms and can present considerable challenges when it comes to insulation. Dormer bungalows, also known as Dutch bungalows, were built quite widely in the UK, particularly in the 1960s and 70s. They have a conventional lower storey, along with an upper storey, with a smaller floor area, in the roof.

Three storey town and detached houses have been built in the last few years, as builders attempt to build bigger houses in limited plots of land. Their top storey is often 'in the roof', like an attic. However, due to modern building regulations, they are well insulated. But if you have an older property of this type, Victorian or Edwardian, many of which had attics, you'll need to install insulation; this may not be as straightforward as in a modern house .

How do I insulate the dormer window area of my dormer roof?

Dormer roofs nearly always have 'dormer windows' which are not flush with the pitched roof, but instead have window structures that protrude from the roof. These structures have a front containing the window, triangular shaped side walls (which generally don't have a cavity) and a flat roof (or occasionally a miniature pitched roof). So the advice that applies to insulating windows, walls and flat roofs applies . However, if the window is the full width of the structure, insulated dry lining of the walls may be difficult as it will narrow the area. If this is the case, you may want to explore external cladding, or creatively use curtains to provide insulation.

How do I insulate the 'voids' behind the dwarf walls in my dormer roof?

Perhaps the most unusual aspect of a dormer bungalow is the triangular roofspaces or voids between the edges of the main roof and the walls at the ends of the rooms (these semi-internal walls are known as dwarf walls, because they are often not the full height of the room, on account of the pitched ceiling). With these voids, if there is no existing access, it may be worth putting in a hatch or door because the void is likely to provide a good storage area. If there is access, or access is created, there are two broad options. You can either insulate the pitched roof , or you can insulate the void side of the dwarf wall and the 'floor' of the area within the void .

Insulating the 'floor' of the void (or, the ceiling of the storey below, depending which way you look at it, is straightforward, with a number of options available, and is in effect the same as creating a cold roof solution. If space allows, you can even add raised boarding to create a storage area.

However, in order to insulate the dwarf wall with the same materials, you will need to support the mineral or glass wool, without compressing it. One solution is to use hooks and rot-proof nylon string, but it may be easier to use polystyrene, urethane or another rigid product, in which case insulating the pitched roof is probably going to be the simpler option anyway.

The most challenging aspect of a dormer roof is likely to be pitched (sloping) ceilings. The same challenge will apply to providing better insulation to any loft rooms put in before the current building regulations.

Part of the solution is similar to creating a 'warm roof' (as discussed earlier). But this will be slightly complicated by the need to take down the ceiling to gain access. A vapour membrane is also a likely requirement, to stop moisture, which is naturally generated in an occupied room; without a membrane, vapour would rise and penetrate into the roof and insulation structure, where condensation could occur, resulting in rot.

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