The right wall and floor insulation is the ticket to a warmer, more energy efficient home. Find out more and how you could fund your improvements with the Green Homes Grant. 

  Cavity Wall wool insulation Installation - green homes grant - grand designs

Image: Installation of cavity wall insulation, Knauf

Adequate insulation is an essential component of an energy-efficient home, as between one third and two thirds of the heat loss in the average UK property can be attributed to the walls according to the Energy Saving Trust. Insulation ensures that the property maintains a comfortable temperature and reduces energy costs.

Finding a competent professional who can advise you on the most suitable materials for your property is a good first step in the process. Organisations such as the National Insulation Association, the Cavity Insulation Guarantee Agency, or the British Board of Agrément provide good advice, but the added benefit of the Green Homes Grant is that you have a regsiter of Trustmark-approved installers at your fingertips to contact for your home insulation project. 

Cavity wall insulation

 Cavity Wall wool insulation Installation - green homes grant - grand designs 

Image: Installation of cavity wall insulation, Knauf

If your home was constructed between the 1930s and 1980s, it is highly likely that it was built with uninsulated cavity walls. This can be remedied by filling in the existing cavity with insulating material such as foam or mineral wool to prevent convection and, therefore, heat loss.

"To insulate your cavity walls, the installer drills small holes around the size of a 10p coin at intervals in the outside wall of your home and then blows insulation into the cavity using special equipment," explains Neil Marshall, head of the Insulation Alliance. "Once all the insulation is in, the installer fills the holes in the brickwork, and all of this only takes a couple of hours."

Read more: Sustainable retrofit: what you need to know

Before starting the work you should submit a building notice to your local council – this is something the company carrying out the project can do for you. Expect the process to cost from around £700 for a large detached four-bedroom property.

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Solid wall insulation

solid wall external insulation application - green homes grant - grand designs 

Image: External solid wall insulation installation, The National Insulation Association

This type of construction, which is common in properties built before the 1920s, gives you the chance to overhaul the appearance of the property using exterior wall insulation and then render. You will need to seek permission from your local authority first, especially if you live in a designated area. According to the Simple Energy Advice website, you can expect an investment starting from £9,000, to which you could apply a £5,000 grant under the Green Homes Grant scheme.

"External wall insulation is the best option for retrofit from a practical point of view, as you can address these works without any internal alterations," says Adam Jannece, managing director at JCJ Construction. "It also improves the wall’s ability to withstand the elements and the insulation is fixed directly to the existing masonry structure, protecting the facade from any further weather exposure. In turn this will also stop any pre-existing penetrating damp."

Read more: Green Homes Grant: 5 tips for finding a tradesperson

Solid walls can be insulated internally as well as externally. You can also expect a lot of disruption and the loss of some floor space on a renovation. While it’s easier to apply internal wall insulation on a new-build project, there are solutions for a renovation. These may include building stud walls and filling them with insulating material, or by fitting thin rigid boards.

For some heritage buildings, solid wall insulation may cause issues. These buildings were built to be breathable, therefore impermeable insulation products may cause moisture issues, so opting for natural insulation materials is the best choice for these buildings. You will need to think about how both external and internal wall insulation will affect heritage buildings as it's possible you will see the loss of historic detail. However, specialist retrofit architects will likely have experience in how to retain detail and improve the energy efficiency of your heritage home. 

Solid wall insulation case study

instagroup external insulation before pic

Image: Instagroup

The Grand Designs magazine team spoke to Nelina Nadak, who has had exterior wall insulation installed in the past year by Instagroup. 'I have noticed several practical differences since the installation of the exterior insulation on my house,' Nelina explains. 'In winter, my home is a lot warmer without the need to increase the heat level on my thermostatic radiator valves. As a result, I can clearly see a monetary decrease on my smart meter digital screen reading. The spend per day has dropped by half. In summer, my home stays cooler when is it very hot outside. Which means I don't have to spend more electricity on running a few fans.'

instagroup external insulation 3

Image: Instagroup

This property has of solid wall construction therefore standard cavity wall insulation would not have been applicable to this site. 'The property had very good access allowing our installers to work efficiently all around the property,' says Instagroup. 'The aesthetics of the finish to our InstaClad solid wall product gave the property a lovely looking uplift while still remaining in keeping with the surrounding houses and buildings.'

instagroup external insulation 2

Image: Instagroup

The Energy Saving Trust estimate that between £200-£300 would be saved on the family's annual heating bills.

Under floor insulation

suspended timber floor insulation - green homes grant - grand designs

Image: Xthratherm insulation boards have been installed between floor joists on this home renovation project. 

Energy Saving Trust suggests investing in floor insulation in order to reduce draughts, keep your house warmer and more energy efficient  and by insulating your ground floor. Visit Energy Saving Trust and find more information on how floor insulation can benefit you.

Suspended timber floors are much easier to insulate than solid or concrete slab floors. Once the floorboards are lifted, your installer may look to use a minimum of 150mm of mineral wool between the joists to achieve U-values as low as 0.22 W/mK, which exceeds building regulations.

The National Insulation Association says floor insulation can cost about £950 for suspended floors, while the Simple Energy Advice website puts it at £1,000. Ensure you seal any gaps around the skirting boards, too, as this can cause draughts, limiting the effectiveness of the work done.

Acoustic benefits 

Thermal Floor Suspended Timber Ground Floor with rockwool insulation - green homes grant - grand designs

Image: A suspended timber floor with ROCKWOOL stone wool insulation

Don’t discount the acoustic benefits of insulating products. Products such as ROCKWOOL Sound Insulation effectively trap soundwaves and dampen vibrations.

"Insulating the floors and walls is important, especially if you’re fitting wooden floors. You don’t want the people below or in the adjoining properties getting annoyed by footsteps or noises coming from your loft, nor do you want to hear music or voices from other rooms in the house or next door," explain ROCKWOOL. "We’d recommend you fit ROCKWOOL Sound Slab in between the floor joists, and a layer of Thermal ROCKFLOOR®, high density insulation over the top of that to cut out any risk of unwanted noise transfer in the floor." 

Green Homes Grant 

Solid wall, underfloor, cavity wall insulation are each listed as a primary measure under the Green Homes Grant, meaning that you're eligible for funding of up to £5,000 for home improvements.

Unless you are receiving certain Government benefits, in which case you may be able to claim up to £10,000, you'll need to contribute one-third of the cost, with the Government issuing a voucher for the other two-thirds directly to the approved installation company. For more information and to find a local trader, head to the Green Homes Grant website

Read more: The Green Homes Grant: how does it work?

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The Green Homes Grant hub

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Green Homes Grant: a guide to ground source heat pumps

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