Insulating your Roof

This page answers questions about roof insulation. For other types of insulation, go to the Insulation FAQ.

Do I really need 270mm insulation in my loft? I already have 150mm – isn’t that enough?

The current recommendation is for 300mm of loft insulation which reduces your heat loss down to 0.13 W/m2/K. This is better than most walls even with cavity wall insulation and in fact is on a par with Passiv Haus standard – the most rigorous standard around.

You may have noticed that the upper floors of your house are warmer than below. This is simply because warm air tends to rise and it is difficult to keep it downstairs unless you have a door closing off your stairwell. This makes it doubly important to insulate your loft effectively because the difference in temperature between inside and out is greater. Also the sky can be very cold at night which increases heat loss from upward facing surfaces.

However, firstly, the recommendation of 300mm is for 300mm of a standard insulation material such as mineral wool – if you are using a high performance material you will only need 150-200mm to achieve the same level of insulation.

Secondly, if you have 150mm already, even of mineral wool, your heat loss is already quite low. Topping up to 300mm is probably economic because loft insulation is cheap but it probably won’t make more than 2-4% difference to your bills, depending on the insulation in the rest of your home. This tool will help you estimate your savings.

Should I insulate at floor level or under the tiles?

You can either insulate your loft at floor level, between and over the joists, or you can insulate under the tiles, between and under the rafters.

Insulating under the tiles gives you a warm roof space which may be an advantage depending on what you want to do in it. However, you will need more insulation because the rafters have a larger area, and you will need to take extra care with ventilation. You should use a breathable membrane instead of standard roofing felt to avoid problems with condensation above the insulation (cf. advice on Multifoil insulation below). Condensation is bad for the insulation and for the felt.

Insulating the loft at floor level is more usual but if you use the loft for storage then you will need to use some rigid insulation above the joists, (not mineral wool!) or extend the joists and lay boards over them. Joist legs are available which can provide a relatively easy diy solution to providing a raised loft floor and provide that extra space for insulation and provide a solid floor for storage. You will still need to take care to allow room for ventilation at the edges under the eaves and you should have venting at ridge level too. The more insulation you have, the more important the ventilation is, because the loft will be colder and the moist air more likely to condense.

I have sloping ceilings in the top floor rooms. What can I do?

In a lot of homes, there are sloping ceilings at the edge of the top floor rooms where the roof cuts off the corner. It can be difficult to insulate this bit but if you don't you may find you get mould on those parts of your ceiling from condensation. The problem is that if you just stuff mineral wool insulation down the gap between the rafters you block off the ventilation that prevents your rafters from rotting.

Sometimes you can put a thin layer of rigid insulation boards (wood fibre or phenolic foam, for example) between the rafters and still leave a ventilation gap, as shown in the diagram. This will fix the condensation problem and reduce heat loss substantially, though not as much as elsewhere. If you really are stuck you can insulate from the inside, but then you have some more decorating to do.

Rigid insulation between the rafters leaving an air gap

Is spray foam insulation any good?

Spray foam insulation has been available since 1986, and is used in new construction projects because it offers comfort and energy efficiency benefits. It can help to fix issues with inadequate insulation, reduce utility expenses, and retrofit hard to access crawl spaces or attics. A good description of the pros and cons of spray foam is available here.

Which on-line also has a good overall summary

Both articles warn about potential problems with mortgages following the application of spray foam. More here. The Residential Property Surveyors Association (RPSA), released a statement on spray foam in August 2022 recommending its removal. Then in October they retracted it - their advice is now under review.

Is Multifoil insulation any good?

If you have limited depth to install insulation, e.g. if you need to install insulation immediately under the roof tiles, then Multifoil insulation may provide a solution in combination with a thinner-than-normal standard insulation layer. Multifoil is very thin and consists of layers of reflective foil interleaved with high performance insulation layers.

The thermal performance of Multifoil now seems to be accepted, if somewhat cautiously by the building industry. Standard thermal tests, where Multifoil does not perform as well as other types of insulation, are not seen as appropriate by the Multifoil manufacturers, as the product acts in a different way to other insulation products, by reflecting heat. (This approach is well proven - multiple foil layers the size of a tennis court are used on the recently launched NASA James Webb Telescope to shield it from the sun and allow the telescope and instruments to be cooled to very low temperatures). Multifoil can provide a condensation barrier, and it also has the advantage that the reflective surface can help against heat gain in the summertime.

There are Multifoil products that have Agrement Board certificates. According to advice from the Building Control Alliance, Multifoil products must be carefully installed strictly in accordance with the conditions on the Certificate. If you do go this route, we suggest you choose a knowledgeable and reputable supplier/installer. Further information is available from a number of UK manufacturers, including TLX Insulation, SuperFOIL and Thermic Technology.

I can't get into my loft - what can I do?

You can make a small access hole and blow insulation into it. This must be done by a professional using special equipment and the material must be fire retardant. Contact the National Insulation Association for more information on installers that can do this.

I have an attic room - what can I do?

This also applies if you have sloping ceilings in your upper floors.

You should have insulation in the ceiling of your attic room and also in the eaves, which are often accessible as a storage area. We have seen many homes with no insulation at all in the eaves and just a thin partition separating them from the room! The guidelines for insulating in your attic are the same as for the loft - you need to allow for ventilation for the rafters, and have a breathable membrane to allow moisture to dry out.

For the eaves you can either insulate the rafters or the floor and the partition separating them from the room.

Insulating the attic is easy to do when it is being constructed or when you are replacing the roof. It can also be done from inside, though this will involve removing the plasterboard to expose the rafters. You might consider this if you are redecorating.

I have a flat roof - what can I do?

If your flat roof was installed before 1976 and has not been insulated since then it is likely to be a good target for improvement. The best time to do this is when you are re-roofing it. There is some good guidance here from the National Federation of Roofing Contractors. It is important to have correct ventilation above the insulation (external vents) so that moisture can escape. You should not just cram insulation into the existing space.

Alternatively, if you have enough room you can insulate on the inside by lining the ceiling, rather like a solid wall.

If there are leaks in the flat roof and water gets into the insulation then it might as well not be there as wet insulation does not work very well. If you think this may be the case then the CCF thermal imaging camera should be able to locate wet areas. Then you need to find the leak, fix it, and possibly replace the wet insulation.

I have insulation in my loft but there are gaps around the edges. Is this shoddy workmanship?

The gaps at the edges of your loft are required to allow ventilation to the wooden structure in the eaves. This is not shoddy workmanship unless the gaps are excessively large. There should be at least a 25mm gap between the insulation and any part of the eaves.

What should I do about my cold water tank in the loft?

Do not insulate underneath your tank! You don’t want your water tank to freeze in the winter time and the same goes for the pipes leading to and from it. The best thing is to give your cold water tank its own jacket and be sure to insulate the pipes too.

Can I still keep stuff in my loft when it is insulated?

Yes – as long as you have enough head space and you have a load bearing surface to put things on, see above. Do not put anything directly on top of soft insulation such as mineral wool because it will compress the material and reduce its efficacy. It is also possible to add loft legs to the joists or otherwise build up the height of the joists to allow more insulation between them.

What is the best value stuff to insulate my loft?

This will vary depending on your needs and the level of subsidy you can get. Insulation that you buy in DIY stores like Homebase and B&Q is often subsidised by energy companies as part of their CERT scheme. (Carbon Emissions Reduction Target). There is more about the scheme here

Where can I get more advice about loft insulation?

The Energy Saving Trust is a good place to start.

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