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Roof and Loft Insulation

In this post we are looking specifically at Roof and Loft Insulation. Visit our Wall Insulation page to find out how to insulate your walls.

Roof and Loft insulation standards

The UK has some of the oldest and least energy efficient housing in Europe. If your property was built from the 1990’s onwards, any loft or flat roof areas should be insulated. If it was built before this, there is a good chance that it was only partially insulated or it has no insulation at all. There are many thousands of Victorian, Edwardian and Regency properties across Brighton and Hove that were built with no insulation and many that have not had any insulation retrospectively added.

So what can be done?. A quarter of heat is lost through the roof in an uninsulated home. Insulating your loft / roof will significantly reduce heat loss which means your rooms will stay warmer for longer thereby saving you money on your heating bills. According to the Energy Savings Trust, loft insulation with an expected lifespan of 40 years should pay for itself many times over.

Types and Thickness of Insulation

You can use mineral wool insulation which comes in rolls approx. 1.2m in length or rigid foam insulation boards which are large rectangular panels usually 2.4m by 1.2m.

Both types are light weight, easy to handle, easy to cut into the desired size and shape and they come in a range of thicknesses.

Rigid Insulation Board
Mineral Wool Insulation
The rigid insulation boards are usually more expensive but they have a better thermal performance which means you can achieve better insulation with thinner layers. Mineral wool has better sound proofing qualities. Ideally you would lay a total depth of 270mm of mineral wool insulation. This is the thickness recommended by the UK government. Again you can adjust this amount if you are using rigid board insulation. The thickness and type of the insulation you use is a key factor in EPC calculations. For example. 270mm of mineral wool insulation generates the same score as 135mm of rigid insulation boards.

Insulating at the joists

If your loft has not already been converted into a liveable room and you have no plans to convert it, as long as it is easily accessible this is the easiest type of loft area to insulate. It’s definitely a job for a keen DIYer, alternatively there are lots of professional trades people out there who will find it very straight forward.

Joists are the horizontal timbers that you can walk on when you enter the loft area. Rafters are the timbers that support the pitched roof. Whatever you do don’t step on the area in between the joists! If you do you are stepping on the ceiling of the room below and it is not designed to take any weight. Your foot and possibly the rest of you will go through.

The area between the joists is where you fit the first layer of insulation. The joists are usually 75mm or 100mm in depth and the gap in between them is usually 400mm or 600mm wide. Mineral wool rolls tend to be 1.2m (1200mm) across and they come in a variety of thicknesses. They often come with pre-cut perforations so they can easily be cut into halves or thirds to suit either a 400mm or 600mm gap.

Start by placing insulation in between the joists up to the top of the joists and then add more insulation at right angles on top of the joists so they are entirely covered. You should ensure the insulation does not completely block the eaves area, i.e. where the joists meet the pitched roof as this allows ventilation which is required to prevent condensation issues.

With regards to how thick the insulation should be, the more the better. The thicker the insulation the better the job it does at trapping heat in the property. That said, from an EPC score point of view, the score you achieve does not keep increasing if you keep increasing insulation. For example. In our experience 400mm thick of mineral wool insulation does not necessarily achieve more points that 270mm of insulation.

Boarded Loft Areas

Many lofts have been ‘boarded’, i.e. ply or chipboard sheets have been laid across the top of the joists so the loft area can be used for storage. This is often done after insulation has been inserted in between the joists. You can simply lay more insulation on top of the boards to increase the total amount of insulation to the desired depth. However, if you are using mineral wool insulation and you then sit the items you are storing on top of the new insulation, this will squash the insulation and it will no longer be as thick and as effective as what it was.

Ideally you would lift the existing boards, lay all of the insulation and then re-fit the boards. To do this you would need to raise the height of the joists to allow for the desired depth of insulation. This can be done using ‘loft legs’ or blocks fixed to the top of the existing joists. Always ensure there is a small gap between the top of the insulation and the boards to allow for ventilation.

Insulating at The Rafters

If you plan to convert your loft area into usable living space at some point in the future, your builders will insulate the pitched roof / sloping ceilings of you new loft room when they do the conversion. They will fit insulation in the rafters and should follow building regulations for the required thickness. If you are not converting the loft into a room and you just prefer to insulate at the rafters rather than the joists then you may be limiting the amount of insulation you can fit and it can be a more difficult and job.

The first thing to determine when insulating a pitched roof is if you can have a ‘warm roof’ or a ‘cold roof’. The main difference between the two is where the insulation is located. A warm roof is where the insulation is above and between the rafters so the rafters are kept warm. A cold roof is where the insulation is between and below the rafters so the rafters remain cold. If it’s a detached property then is it usually straight forward to ‘build-up’ i.e. add insulation on top of the rafters. This means the height of the roof will increase. If the insulation is being added to a terraced property then a cold roof is probably a better option as you may not be able to insulate above the rafters as you may not be able to change the roof height.

Warm Roof

To create a warm pitched roof your builders will need to remove the roof tiles and any batons they are sitting on / attached to and then remove any old roofing felt. Fit insulation between the rafters and then across the top of the rafters covering them completely. Fit a new breathable membrane, new batons and then replace the tiles. The height of your roof will have increased due to the new insulation across the top of the rafters.

Cold Roof

This can be done from inside the property so you may not need to remove the roof tiles and add a new waterproof membrane although it is worth making sure the existing tiles and membrane are in good condition. You would fit insulation in between the rafters and then to hold it in place and ensure the rafters themselves are covered by insulation you would fit insulated plasterboard to the rafters. You should leave a gap above the insulation to allow for ventilation.

Always seek advice form a professional roofing company or experienced builders. Replacing a roof and converting a loft into a usable room is not really a DIY job.

Spray Foam

Spray foam insulation is a liquid polyurethane foam that is sprayed into the the area you want to insulate. When the foam sets it forms an insulating layer. As it’s a liquid, it is very quick to install and it can get into tricky areas easier that may be difficult to insulate with rigid insulation or woolly insulation.

Spraying foam into the rafters is not without risk. It has been known to cause condensation and rot in the timber rafters and it can be very difficult and sometimes impossible to remove. Sometimes the rafters will need to be completely replaced to fix any issues. You must make sure you use accredited installers and they fully assess the property to ensure it is in a fit state and that spray foam is suitable.

Research conducted by the BBC in 2024 found that a quarter of the UK’s biggest mortgage providers will not lend against homes with spray foam in the roof.

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