Insulation is the foundation of any energy improvement programme: it reduces heat loss, lowers bills, raises EPC ratings, and (in the case of loft and cavity wall insulation) is a condition of the Boiler Upgrade Scheme grant that applies to heat pump installations. But the three main insulation types have very different cost and return profiles, and which types apply to your property depends on when it was built and how it is constructed. Loft insulation for a 1970s semi can cost £400 and pay back in two years. External solid wall insulation for a Victorian terrace can cost £15,000 and take thirty years to pay back on energy saving alone.
This calculator shows all three insulation types side by side for your property, with applicable measures highlighted and non-applicable ones identified. For each applicable measure it shows the illustrative cost range, annual bill saving, EPC band improvement, grant eligibility under ECO4 and the Great British Insulation Scheme, the net cost after any likely grant, and the loan case for that net amount. It also shows the combined effect of completing all applicable measures together. All figures are illustrative UK averages. A professional survey is recommended before commissioning any insulation works, particularly for cavity wall insulation where suitability varies by property condition.
At a Glance
- Loft insulation has the highest financial return per pound of any insulation measure. For properties where the loft is accessible and uninsulated, the payback period is typically one to four years. It may be available free through the Great British Insulation Scheme or ECO4 for eligible households: the insulation calculator.
- Cavity wall insulation is the second-highest return measure for properties with unfilled cavities. It applies to most properties built from the 1920s onward. Not all properties with cavity walls are suitable: moisture issues, narrow cavities, or previously failed fills can make a property unsuitable. A professional survey is required before commissioning works: the insulation calculator.
- Solid wall insulation is expensive but produces a significant EPC improvement. Pre-1920 properties with solid walls have few alternatives for reducing heat loss through walls. Internal and external wall insulation have different cost, disruption, and planning permission profiles. ECO4 may cover the full cost for eligible households: the insulation calculator.
- Completing insulation before a heat pump installation is both financially efficient and a BUS grant condition. The Boiler Upgrade Scheme requires a valid EPC with no outstanding insulation recommendations before a grant application can be submitted. Completing loft and cavity wall insulation first also allows a smaller, cheaper heat pump to be specified.
- The Great British Insulation Scheme covers one measure per household, not both loft and cavity wall. If both measures are applicable, the second will need to be self-funded or funded by a loan. The combined cost is still modest for cavity-wall properties, typically £800 to £2,100.
Home insulation cost and savings calculator
See all applicable insulation types for your property side by side, with costs, savings, EPC impact, grant eligibility, and the loan case for the net cost. All figures are illustrative estimates.
Your property
Insulation already completed
If financing the works
Choosing Between Internal and External Solid Wall Insulation
For pre-1920 properties with solid walls, there is no cavity to fill. The two main options are internal wall insulation, which lines the inside of the external walls, and external wall insulation, which is applied to the outside of the property. Both reduce heat loss through the walls significantly and can improve the EPC rating by two or more bands, but they have very different practical implications.
Option
Internal wall insulation
Rigid insulation boards or a stud wall with insulation are fixed to the inside face of the external walls. Typically costs £4,000 to £9,000 for a whole-house installation. Reduces internal floor area by 5 to 10 centimetres per wall and requires skirting boards, radiators, and window reveals to be adjusted. Highly disruptive during installation but does not require planning permission in most cases and does not change the external appearance of the property. Best done as part of a larger renovation project when the room is already being redecorated.
Option
External wall insulation
Insulation boards are fixed to the outside walls and covered with a render or cladding finish. Typically costs £8,000 to £16,000 and requires scaffolding throughout installation. Does not reduce internal floor area and causes less internal disruption. Changes the external appearance of the property and may require planning permission, particularly for listed buildings, conservation areas, or where the render finish changes significantly from the original character. Check with the local planning authority before proceeding.
Cavity Wall Survey: What to Check Before Proceeding
Not all properties with cavity walls are suitable for cavity wall insulation, even if the construction era suggests they should have an unfilled cavity. A professional survey is required before commissioning works, and some properties will fail the survey for one or more of the following reasons.
Previously filled cavities are the most common issue. If a previous owner commissioned cavity wall insulation that has since failed, is degraded, or was incorrectly installed, a new installation is not straightforward and may require the existing fill to be removed first. Moisture problems are the second most common issue: if the external walls show signs of penetrating damp or the cavity has been bridged, installing insulation can trap moisture within the wall construction and cause damp problems inside. Narrow cavities, certain brick bond patterns, and properties in exposed locations (where wind-driven rain can enter the cavity) can also make standard cavity fill unsuitable or require a specialist product. The survey, which most installers carry out free of charge before quoting, identifies these issues. A survey that concludes the property is unsuitable should be treated as the correct answer, not an obstacle to overcome.
Grant Eligibility in Detail
Scheme
Great British Insulation Scheme
Covers one insulation measure per household for properties at EPC D or below. The scheme targets loft and cavity wall insulation primarily. Eligibility is broader than ECO4 and is not restricted to means-tested benefit recipients, though lower-income households are prioritised. Delivered via energy suppliers and local authorities. The one-measure restriction means if both loft and cavity wall apply to your property, only one can be funded through GBIS: the second must be self-funded or financed. The calculator shows the net cost and loan position for any measure not covered by grant.
Scheme
ECO4 (Energy Company Obligation)
Provides free insulation and heating upgrades for eligible low-income or vulnerable households in properties at EPC E, F, or G. Unlike GBIS, ECO4 can fund multiple measures in a single application. ECO4 is means-tested: the household must receive qualifying benefits or be referred via the ECO4 Flex route through a local authority. Solid wall insulation, which is expensive and not covered by GBIS, may be funded in full under ECO4 for eligible households. Apply via your energy supplier or through the government’s Simple Energy Advice service.
Related Tools and Guides
Tool
Shows the full recommended sequence of improvements for your property, including insulation alongside heat pumps, solar, and battery storage. Use the sequencer to plan the full improvement programme and this calculator to model the insulation component in detail.
Tool
Boiler vs heat pump cost comparison
If completing insulation opens up BUS grant eligibility for a heat pump, use this calculator to model the fifteen-year comparison between replacing the boiler and installing a heat pump with the improved EPC as the starting position.
Tool
Energy efficiency loan payback calculator
Models when cumulative energy savings overtake total loan interest. Use the annual saving figures from this calculator as the input to see the loan payback position for insulation works financed by a home improvement loan.
Guide
Government grants vs home improvement loans
Covers ECO4, the Great British Insulation Scheme, and the Boiler Upgrade Scheme in full: eligibility conditions, how to apply, and how to combine grant funding with a loan for the balance.
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Checking won’t harm your credit scoreFrequently Asked Questions
How do I know if my property has an unfilled cavity?
The most reliable ways to check are a borescope survey (a small camera inserted through a drill hole in the mortar), an infrared thermal survey of the external walls, or a check of the property’s EPC, which should indicate whether cavity wall insulation is a recommended improvement. Most cavity wall insulation installers will carry out a free survey that includes checking whether the cavity is already filled. If your property is in the standard 1920s to 1970s band and the EPC lists cavity wall insulation as a recommended improvement, the cavity is almost certainly unfilled.
Properties built from the late 1990s onward may already have factory-fitted insulation in the cavity as a standard part of construction, which would typically be reflected in a better EPC rating and no outstanding cavity wall recommendation on the certificate. Properties in the 1980s and early 1990s are a mixed picture: some were built with insulation already installed, others were not. If the EPC does not list it as a recommendation and the property was built post-1985, cavity wall insulation is likely already present.
Can I claim retrospectively for insulation already installed?
No. ECO4 and GBIS only fund new installations. Insulation that has already been completed is not eligible for grant funding under these schemes regardless of when it was installed or how much it cost. However, if your property already has loft insulation but still needs cavity wall insulation, the cavity wall measure can still be applied for under GBIS as the single funded measure, assuming the property still has outstanding EPC recommendations and meets the eligibility criteria.
For properties where some insulation work has been done in the past but the EPC still shows outstanding recommendations, the remaining recommended measures may still qualify for grant funding. Check via the Simple Energy Advice service with your current EPC certificate to identify what, if anything, remains fundable under current schemes.
Does insulation affect the Boiler Upgrade Scheme grant for a heat pump?
Yes, directly. The Boiler Upgrade Scheme requires that the property has a valid EPC with no outstanding recommendations for loft insulation or cavity wall insulation before a grant application can be submitted. If either of these is listed as a recommendation on your current EPC, the recommendation must be addressed and a new EPC issued before the BUS application is made. There is no requirement to install solid wall insulation or floor insulation before applying, only loft and cavity wall where they are listed as recommendations.
The practical implication is that completing loft insulation and cavity wall insulation first does two things simultaneously: it clears the outstanding recommendations that would block a BUS application, and it improves the property’s thermal performance so that a smaller, cheaper heat pump can be specified for the post-insulation heat demand. The heat pump will also operate at a higher SCOP in the better-insulated property, reducing running costs. Completing insulation before a heat pump is not just a grant condition: it is the more financially efficient sequence.
How long does insulation typically last before needing replacement?
Loft insulation and cavity wall insulation, when correctly installed, have expected lifespans of forty or more years. Mineral wool loft insulation does not degrade under normal conditions and can last indefinitely if the loft space remains dry and undisturbed. Cavity wall insulation, if installed correctly with appropriate products for the exposure zone, also has a very long service life. Failure in cavity wall insulation is usually related to installation defects, inappropriate product selection for a property’s exposure level, or pre-existing moisture issues rather than natural degradation of the material itself.
Solid wall insulation (internal and external) also has a long service life, typically thirty to fifty years, though external render finishes may require maintenance and repainting over that period. The insulation material itself remains effective for the life of the building if the system is correctly installed and maintained. For all insulation types, the key to longevity is correct installation by a qualified installer, appropriate product selection for the specific property, and ensuring no moisture issues are present before installation begins.
Squaring Up
Insulation is the right starting point for almost any energy improvement programme: it is the cheapest category of improvement per unit of bill saving, it may be available free through grant schemes for eligible households, and completing it is a precondition for the most valuable grants available for heat pumps. The difference in cost and return between loft and cavity wall insulation on one hand, and solid wall insulation on the other, is significant and the tool above makes it visible. For properties with accessible lofts and unfilled cavities, the financial case for insulation is almost always strong regardless of income level.
Grant funding should be checked before any consideration of borrowing. ECO4 may cover the full cost of all applicable insulation for eligible households at EPC E, F, or G, including solid wall insulation that would otherwise cost £10,000 or more. The Great British Insulation Scheme covers one measure per household at EPC D or below. Where grant funding does not cover the full cost, the net amount for loft and cavity wall insulation combined is typically modest enough for a small unsecured loan that pays back well within the expected product lifespan.
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Checking won’t harm your credit score Check eligibilityThis tool is for illustrative purposes only and does not constitute financial or energy advice. All cost and saving figures are illustrative UK averages and will differ from actual figures, which depend on property size, condition, location, and installer. Grant eligibility is indicated based on EPC rating only and does not account for income level or benefit status. Cavity wall insulation suitability varies by property: a professional survey by a qualified installer is required before commissioning works. Always verify current grant eligibility and availability through the relevant scheme administrator before relying on grant funding in any budget. Your home may be at risk if you do not keep up repayments on a secured loan.
