Internal roof insulation: the complete guide

The Energy Saving Trust, an independent organisation working to address the climate emergency, says that about a quarter of heat lost in UK homes escapes through uninsulated roofs and loft spaces. (1)

This means, if your roof is not insulated, you have to use more energy and spend more money keeping your home warm in the colder months.

Roof insulation is essential for ensuring good thermal comfort in your loft and the rest of your home and it will also make your property more sustainable and cheaper to run.

You can insulate your roof from the inside or the outside. There are pros and cons to both methods.

The key benefits of internal insulation are:

To minimise heat loss in your home and lower your energy bills on cold days, it’s crucial that you insulate your loft, regardless of whether it's a habitable space or not. Good insulation will stop your home from feeling too stuffy on hot days, too and it helps with sound proofing.

Internal roof insulation is affordable and easy to install and, provided your roof structure is in good condition, it’s very effective.

Internal roof insulation can be installed in flat roofs, pitched roofs and roof terraces, but flat roofs are best insulated from the outside to prevent condensation.

As people now look to be more sustainable and energy efficient and want to keep their living costs down, a well-insulated home is very attractive to potential buyers. Internal roof insulation is a worthwhile investment that can significantly boost your property’s value.

Plus, when you sell your home, you must obtain an energy performance certificate (EPC), which assesses your home from A for the most efficient to G for the least efficient. The state of your insulation will be included in the report and could impact your home’s value.

Internal roof insulation can have drawbacks, including these three main disadvantages:

Some insulation is thicker than others, but when you insulate from the inside they will all take up some space on the underside of your roof. If you want to maximise the height in your loft, opt for a thin high-performing insulating material.

It will only take a few days, but if you opt for internal roof insulation, you won’t be able to use your loft while the work is going on and you may need to empty your loft to access your roof space. If your loft is a habitable space, you’ll need to replaster your loft room after the insulation is installed. If you’re doing a loft conversion, you will have to insulate the space to comply with building regulations, so it’s a good idea to do both at the same time.

This insulation method may cause a few thermal bridges to form. A thermal bridge is a cold spot in your home from where warm air can escape outside and cold air can come in. If there are gaps in your insulation they could create thermal bridges, but a professional installer should be able to largely avoid these.

The best insulation for your roof space depends on your needs. There are four main types of insulation to consider. 

Popular and cost-effective mineral insulation options include:

• Glass wool: Made from sand and recycled glass, this is available in rolls and panels and is fire-resistant and reasonably priced.
• Rock wool: Derived from volcanic rock, this is available in rolls and panels and is fire-resistant and good for soundproofing. 

Safety tip: Always wear gloves and safety glasses when handling mineral insulation, as it can be an irritant.

Natural insulation is eco-friendly, but must be treated for fire and pest resistance, it includes:

• Recycled fibre boards: Made from recycled wood, these have good thermal and acoustic properties.
• Hemp, cork and straw: These are all effective insulators and are reasonably affordable.
• Sheep’s wool: This has high insulation value and breathability but can be pricey.

Cellulose wadding, made from old newspapers and cardboard, is the most common form of recycled insulation. This sustainable and high-performing insulator can be blown or laid as boards.

Polyurethane (PU) insulation is available as rigid boards or panels, offering excellent thermal performance with a low thickness. Lightweight yet durable, it is commonly used in walls, floors and roofs to improve energy efficiency.

There are three ways to use internal roof insulation:

This is the most common method for accessible attics. Insulation is fitted between your rafters often using rolls of mineral wool or glass wool.

To prevent condensation, a vapour control layer must be installed on the warm side of the insulation.

Insulation in an uninhabited loft can take two forms: 

• Blown fibre insulation: made from materials such as glass wool, rock wool or cellulose wadding, this is often used for insulating hard-to-reach areas.
• Roll-out insulation: This simple and cost-effective method involves laying one or two layers of insulation, with varying thicknesses, over a flat floor.

Ideal for flat roofs, this technique places the insulation layer above the structural roof deck, with a vapour control layer below it, which helps to reduce condensation. This offers good thermal performance over time. 

Consider these recommended thicknesses for insulation when using the following materials:

• If you choose mineral wool, a thickness of 16 to 20cm is recommended.
• For cellulose flakes, a thickness of 16 to 21cm is sufficient.
• Polyurethane loft insulation only needs to be 12 to 14cm thick, because of its high thermal performance. This makes it suitable for tight spaces or where minimal thickness is preferred.

The cost of loft insulation varies depending on several factors:

• Type and size of the area: Whether you’re insulating a pitched roof, flat roof or loft floor will affect the cost.
• Insulation method: Your choice of material and method of application.
• Installation approach: DIY installation can reduce costs, but professional fitting ensures compliance and optimal performance.
• Comfort expectations: Converted lofts may require higher-grade insulation compared to unused spaces.

Tip: Always request a quote from an insulation specialist to ensure accurate pricing based on your home’s specific needs.

You may be able to access grants, financial support or subsidies for loft and attic insulation. In England, Scotland and Wales, financial support may be available through the Great British Insulation Scheme (2). This government initiative is designed to help households cover the cost of insulation improvements that lower energy bills and make homes more environmentally friendly. If your home is in Northern Ireland, you may qualify for a grant for insulation through the government Affordable Warmth Scheme. (3). 

Whether you insulate your loft yourself or get an installer to do it, these are the five key stages of your project:

Assessing your roof’s energy performance helps determine your home’s energy consumption and greenhouse gas emissions and shows what needs to be done to improve it.

These are the three main factors to consider:

• Your type of roof, whether it’s a flat roof or pitched roof and whether your loft is converted.
• The existing level of your loft insulation and its effectiveness.
• The general condition of your roof structure.

Although you can evaluate some of these factors yourself, you will need to enlist a specialist to carry out a comprehensive assessment.

They will identify the type of insulation that is suitable for your home and any reinforcement work needed.

Once the assessment has been carried out and the materials purchased, work can begin.

Thicker insulation or dual-layer systems may require deeper rafters. A builder can attach additional timber to existing rafters to support the insulation and maintain structural integrity.

Choose a type of insulation that is suitable for the space between your rafters. Make sure that the entire surface is covered.

Use rolls of insulation for evenly spaced rafters and insulation boards or rolls that can easily be cut for unevenly spaced rafters.

Once the insulation is fitted, add a vapour control layer to prevent condensation. It must be installed on the inside of the insulation.

Note that some insulation materials come with a vapour control layer.

If your loft is unused, then no cladding is needed, but if your loft is converted you need to protect your insulation with cladding, plasterboard is commonly used.

Internal insulation can improve comfort and energy efficiency — but only if installed correctly. Take these precautions to avoid common issues:

To avoid condensation forming, include a vapour control layer to prevent moisture penetrating your insulation and ensure your roof space is well ventilated. 

Read our article on how to improve indoor air quality here.

Thermal bridges are weak points in insulation.

To avoid these, follow the latest thermal building regulations, use the recommended material and fit type for your scheme and use a professional installer.

Internal roof insulation is a practical and affordable solution that makes your home more comfortable and energy efficient and increases its value.

However, it’s essential to select the right materials, whether that’s insulation boards or natural material insulation, follow the installation guidelines and consider your home’s unique characteristics to maximise benefits and minimise drawbacks.