Loft Insulation vs Cavity Wall: Which Should You Do First?

If you’re deciding between loft insulation vs cavity wall insulation, you’re asking one of the most practical questions any UK homeowner can ask. Both measures reduce heat loss, lower energy bills and make your home more comfortable — but they differ significantly in cost, savings potential, payback time and whether you can do the work yourself. In this guide, we compare the two options head-to-head and help you decide which to prioritise first.

Should You Insulate Your Loft or Cavity Walls First?

Start with loft insulation. It costs £300 to £600 compared to around £600 to £1,500 for cavity wall insulation, pays back in 2 to 4 years and can often be installed as a DIY project. Loft insulation addresses roughly 25 percent of your home’s heat loss and delivers immediate comfort improvements with the lowest upfront investment.

The exception is if your loft already has 200mm or more of existing insulation and your cavity walls are completely uninsulated. In that case, cavity wall insulation will deliver greater marginal savings since walls account for around 33 percent of heat loss in a typical uninsulated UK home. Ideally, both measures should be installed, and homeowners receiving ECO4 or Warm Homes Local Grant funding may be able to get both done at no cost.

Loft Insulation vs Cavity Wall Insulation: Quick Comparison

Here’s a summary of how the two measures stack up across the most important factors.

Factor	Loft Insulation	Cavity Wall Insulation
Typical cost	£900 – £1,200	Around £2,700
Annual savings	£180 – £340	Around £235
Payback period	3 – 5 years	8 – 12 years
Heat loss addressed	~25% (through the roof)	~33% (through walls)
DIY possible?	Yes (loft blanket rolls)	No — professional only
Disruption	Minimal	Minimal (drilled from outside)
Free under ECO4/GBIS?	Yes, if eligible	Yes, if eligible
Recommended depth	270mm mineral wool	50 – 75mm blown bead/fibre

Where Does Your Home Lose the Most Heat?

Understanding where heat escapes your home helps explain why both measures matter. In a typical uninsulated UK house, around 33% of heat is lost through the walls and approximately 25% through the roof. The remainder escapes through floors, windows and draughts.

On paper, walls lose more heat than the roof, which might suggest cavity wall insulation should come first. However, cost and payback tell a different story, and that’s where loft insulation often wins as the first step.

Loft Insulation: Cost, Savings and What’s Involved

Loft insulation is one of the simplest and most cost-effective energy improvements you can make. The current recommendation is 270mm of mineral wool insulation laid between and across the joists in your loft space. Many homes built before the 1990s have little or no loft insulation, and even those with some insulation often have only 100mm or less — well below modern standards.

Cost: Professional installation typically runs between £900 and £1,200 for a standard three-bedroom semi-detached house. If you’re reasonably handy, you can buy mineral wool rolls from any building merchant for £300 to £500 and lay them yourself over a weekend.

Savings: The Energy Saving Trust estimates annual savings of £180 to £340, depending on your home’s size, existing insulation levels and how you heat your property. Homes with gas boilers and no existing loft insulation see the largest savings.

Payback: At these figures, loft insulation pays for itself in 3 to 5 years — faster if you do the work yourself. That makes it one of the quickest-return home improvements available. For a complete overview of insulation options, visit our insulation page.

The DIY factor: Loft blanket insulation is genuinely straightforward for a competent DIYer. You roll out mineral wool between the joists, then lay a second layer across the top at right angles. The main requirements are a dust mask, gloves, adequate lighting and care around electrical cables and pipes. You should not compress the insulation or block ventilation at the eaves.

Cavity Wall Insulation: Cost, Savings and What’s Involved

Cavity wall insulation addresses a larger source of heat loss but comes at a higher cost and cannot be done as a DIY project. Cavity walls are two layers of brick or block with a gap (cavity) between them, typically 50 to 75mm wide. Insulation is injected into this gap from the outside through small holes drilled in the mortar joints, which are then sealed.

Cost: Professional cavity wall insulation for a typical semi-detached home costs around £2,700. Detached homes cost more due to the larger wall area, while mid-terrace homes can be cheaper since they have fewer exposed walls.

Savings: Annual savings are around £235 according to the Energy Saving Trust. This figure can vary depending on your home’s size, wall area, and heating system. Homes with older, less efficient boilers may see larger savings because they’re generating more heat that would otherwise escape.

Payback: At roughly £2,700 cost against £235 annual savings, the payback period is 8 to 12 years. That’s considerably longer than loft insulation but still represents a sound investment over the lifetime of the insulation, which should last the life of the building.

Materials: The most common fill materials are expanded polystyrene (EPS) beads bonded with adhesive, mineral wool fibre, or polyurethane foam. Your installer will recommend the best option based on your wall type, exposure to wind-driven rain and cavity width.

Which Should You Do First?

If budget is a factor and you can only tackle one measure at a time, loft insulation is almost always the better starting point. Here’s why:

• Lower cost — £900 to £1,200 vs £2,700, making it far more accessible
• Faster payback — 3 to 5 years vs 8 to 12 years
• DIY option — you can potentially halve the cost by doing it yourself
• Immediate impact — you’ll notice the difference in comfort within days
• No specialist access needed — no scaffolding or drilling into external walls

That said, if your walls are losing 33% of your heat and your loft is already partially insulated, cavity wall insulation may deliver a bigger improvement to your specific situation. The ideal approach is to do both, starting with the loft and following up with cavity walls when budget allows.

Can You Get Free Insulation Under ECO4 or GBIS?

Both loft insulation and cavity wall insulation can be fully funded under the government’s ECO4 scheme or the Great British Insulation Scheme (GBIS), which runs until March 2026. Eligibility is based on a combination of your household income, the benefits you receive, your property’s energy rating and your council tax band.

If you qualify, you could get both measures installed at no cost whatsoever. Even if you don’t qualify for full funding, partial grants may be available that significantly reduce your outlay. It’s worth checking your eligibility before paying full price — get a free quote through Green Reach Energy and we’ll check what funding you may be entitled to.

What If Your Home Doesn’t Have Cavity Walls?

Not all UK homes have cavity walls. Properties built before the 1920s are more likely to have solid walls — a single thick layer of brick or stone with no cavity to fill. You can usually tell by looking at the brickwork: if the bricks are laid in an alternating pattern showing the short ends (headers) and long sides (stretchers), the walls are likely solid. If you only see stretcher bonds (bricks laid lengthways), you probably have cavity walls.

Solid wall insulation is more expensive (£8,000 to £22,000 for external, £4,000 to £13,000 for internal) but addresses the same 33% heat loss through walls. If you have solid walls, loft insulation becomes even more clearly the priority first step, with solid wall insulation as a longer-term project. Upgrading your heating system with a modern heat pump can also deliver significant savings alongside insulation improvements.

Combining Insulation with Other Energy Improvements

Insulation works best as part of a whole-house approach to energy efficiency. Once you’ve addressed the roof and walls, consider these complementary improvements:

• Floor insulation — suspended timber floors can be insulated from below, saving an additional £40 to £70 per year
• Draught-proofing — sealing gaps around doors, windows and floorboards costs very little and saves £25 to £60 per year
• Double glazing — if you still have single-glazed windows, upgrading reduces heat loss through glass by up to 50%
• Boiler upgrade — replacing an old G-rated boiler with a modern A-rated model can save £300+ per year
• Solar panels — generate free electricity and reduce your reliance on the grid

The more insulated your home is, the more effective your heating system becomes — and the more you save from every subsequent improvement.

How to Get Started

The first step is to understand your home’s current insulation levels. Check your loft — if you can see the joists or the insulation is less than 270mm deep, topping up will make a noticeable difference. For cavity walls, an installer can check the cavity width and condition using a boroscope inspection through a small drilled hole.

Green Reach Energy works with approved installers across the UK who can assess your home, check your eligibility for free or subsidised insulation, and provide a clear quote with no obligation. Request your free quote today and take the first step towards a warmer, more affordable home.

Frequently Asked Questions

Should I insulate my loft or walls first?

In most cases, start with loft insulation. It costs less (£900 to £1,200 vs £2,700 for cavity walls), pays back faster (3 to 5 years vs 8 to 12 years) and can be done as a DIY project. However, if your loft already has 200mm+ of insulation and your cavity walls are empty, walls may deliver a bigger improvement.

Can I get loft and cavity wall insulation for free?

Yes. Both measures are covered under the ECO4 scheme and the Great British Insulation Scheme (GBIS). Eligibility depends on your household income, benefits received, property EPC rating and council tax band. Check your eligibility through Green Reach Energy — it takes less than two minutes.

How do I know if my home has cavity walls?

Look at the external brickwork. If you see only the long face of the bricks (stretcher bond), you likely have cavity walls. If you see alternating long and short faces (Flemish or English bond), the walls are probably solid. Most homes built after the 1920s have cavity walls. If you’re unsure, any insulation installer can check for you during a survey.

Does cavity wall insulation cause damp?

When installed correctly in suitable properties, cavity wall insulation does not cause damp. Problems can occur if insulation is installed in homes with very narrow cavities, damaged external rendering or high exposure to wind-driven rain. A proper pre-installation survey should identify any risk factors. If you have concerns, ask your installer to carry out a boroscope inspection of the cavity before proceeding.

How thick should loft insulation be?

The current recommended depth is 270mm of mineral wool insulation. This is typically achieved by laying 100mm between the joists and 170mm across the top at right angles. If your existing insulation is less than 270mm, topping it up is straightforward and cost-effective. There is no practical downside to going slightly thicker, though the marginal benefit reduces beyond 300mm. For all insulation options and to check what grants you may qualify for, visit our insulation page.