How Home Insulation Affects Your Heating Bills (& What to Do About It)

Up to a third of a home’s heat escapes through the roof, walls, and floor. 

This is heat you paid for your boiler to produce. 

The fix for this is often not to replace your boiler, but to add insulation.

Good insulation maintains comfortable temperatures throughout the year (both in summer and winter) and saves you money.

Why insulation and your heating system are connected

Boilers and insulation work as an integrated system. 

In a poorly insulated house, heat leaks out constantly. This forces a boiler to cycle on and off and run at high flow temperatures to replace lost heat. 

By contrast, a well-insulated home holds heat for much longer. This allows for shorter, less frequent heating cycles that burn much less gas.

Insulation and heat pumps

Good insulation is even more essential for heat pumps, which operate most efficiently at lower flow temperatures than gas boilers (around 35°C to 50°C compared to 60°C to 70°C). 

Without a well-insulated thermal envelope, a heat pump will struggle to keep your home warm during cold weather, leading to expensive, high electricity consumption.

Where heat escapes (& where to start)

Loft insulation

Up to 25% of household heat escapes through an uninsulated roof. Loft insulation (at a recommended depth of 270mm) is the main solution to this.

According to the Energy Saving Trust, installing 270mm of loft insulation can save up to £270 a year on energy bills. This means it pays for itself quickly.

It comes in a variety of materials placed between loft joists. And if your loft is easy to access, it’s simple to install.

To check your current setup, measure the depth of the existing glass fibre layer.

Cavity wall insulation

Uninsulated cavity walls (walls with a hollow space between inner and outer brick layers) account for roughly 35% of heat loss in a typical semi-detached home. 

Adding insulating material into the empty cavity blocks heat transfer. According to the Energy Saving Trust, cavity wall insulation saves around £280 a year for a typical semi-detached home.

Solid wall insulation 

Solid brick or stone walls lose twice as much heat as cavity walls, making them a top priority for energy savings. Solid walls can be insulated in two main ways:

• Externally with EWI (external wall insulation), i.e., fitting weather-proof insulating cladding to outside walls). This is generally more effective but carries higher installation costs.
• Internally with IWI (internal wall insulation), i.e., fixing rigid insulation boards or stud frames to inside walls).

According to the Energy Saving Trust, solid wall insulation saves around £380 a year.

Floor insulation 

Ground floor flooring accounts for around 10% of heat loss. Insulating timber floors addresses draughts that seep through floorboards, reducing room chill. 

According to the Energy Saving Trust, ground floor insulation saves around £80 a year.

UK government grants for insulation in 2026

The Great British Insulation Scheme

The Great British Insulation Scheme (GBIS) provides free or heavily subsidised single-measure insulation (such as loft or cavity wall insulation) for eligible homes across Great Britain. 

It targets properties in:

• Council Tax bands A to D in England
• Council Tax bands A to E in Scotland and Wales, with an EPC rating of D or below.

(Source: https://www.gov.uk/apply-great-british-insulation-scheme)

The ECO4 scheme

The Energy Company Obligation phase 4 (ECO4) scheme covers full insulation and heating upgrades for low-income households or residents receiving specific government benefits. 

Landlords and homeowners can check eligibility criteria and apply directly through the official gov.uk insulation grant portal.

Insulation types and what they are used for

Matching the right insulation material to each area of your home ensures maximum thermal performance.

Mineral wool matting

The most common insulation for lofts, floors, and internal walls, mineral wool matting is made from glass fibre or stone wool. 

It comes in flexible rolls of varying thickness and provides reliable soundproofing alongside thermal protection.

Blown and loose-fill insulation

Installed using specialist equipment that blows fire-resistant cellulose or mineral fibres into wall cavities and loose spaces. 

Blown insulation adheres to irregular joist segments and hard-to-reach partitions where matting cannot fit.

Rigid insulation boards

Cut from stiff foam plastics (such as PIR or PUR) and scaled to fit walls, floors, or roof rafters.

Insulation boards provide exceptional heat resistance per millimetre of thickness, making them ideal where space is restricted.

Spray foam insulation

Expanding foam sprayed into roof spaces or timber frames.

U-values: the number that really matters

The thickness of insulation is only part of its value. It can even be misleading. Its U-value is more important.

Think of a U-value as a “heat leak score” for a wall, roof or floor. It shows how quickly warmth escapes through it. The lower the number, the less heat you lose. 

It’s a bit like comparing winter coats. A thick one isn’t necessarily a warm one. After all, a bulky, cheap coat can let more cold through than a well-made lightweight one.

What’s a good U-VALUE number?

Building Regulations set maximum U-values for new builds, extensions and replacement parts of a building. 

For example, a roof in an extension must have a U-value at 0.16 or lower. The unit you’ll see written next to it is W/m²K. But you don’t need to worry about decoding that. Just remember that lower is better.

What to do with this

• Ask any installer for the U-value of the product and the finished job, not just the thickness.
• Compare U-values across quotes to see which delivers the best performance.
• Check the figure meets the current Building Regulations for your project.

How insulation affects the case for a new boiler or heat pump

Smaller system sizing

A poorly insulated property loses heat quickly. So, it requires a large kilowatt (kW, the measure of heating power output) boiler or heat pump to keep rooms warm. 

Improving loft and wall insulation first lowers your home’s peak heat loss rate. This allows heating engineers to specify a smaller, lower kW unit that costs less to buy and to install.

Lower operational costs

Upgrading to a modern A-rated condensing boiler or heat pump can save money. But running any heating system in an uninsulated home wastes fuel.

Combining insulation with an efficient heating system ensures your home holds heat longer, which keeps running costs as low as possible.

Get your central heating covered with Hometree

Our central heating cover includes unlimited repairs, an annual boiler service, zero parts & labour charges, and 24/7 claims support to keep your home warm. Subject to exclusions and T&Cs apply.

With no hidden fees and coverage across England, Scotland, and Wales, we’ve got you covered.

You can get covered today or contact us for a free quote.