Your heating bills have crept up over the past few years. There are cold patches on walls that used to be fine. Or you've noticed damp spreading in places where it wasn't before.

Your cavity wall insulation might have failed - and it's more common than you'd think.

Cavity wall insulation should last 25+ years when installed properly. But poor installation, damp getting in, or ventilation problems can cause it to fail much sooner. When it does, it doesn't just stop working - it can actively cause problems like damp and mould.

Here's how to spot if yours has failed and what to do about it.

Do You Even Have Cavity Wall Insulation?

First, work out if you have it.

If your house was built after 1990, you should have cavity wall insulation - it became mandatory in building regulations.

If your house was built between 1920 and 1990, you might have cavity walls but the insulation might have been added later (or not at all).

Quick visual check:

Check the thickness at a door or window - cavity walls are typically around 260-300mm thick, solid walls are usually thinner.

Having cavity walls doesn't automatically mean they're insulated. Older properties often have empty cavities. Check your EPC or ask the previous owner if you're not sure.

Warning Signs Your Cavity Wall Insulation Has Failed

Cold patches on walls - Walls that used to feel fine now feel noticeably cold in certain areas. This suggests the insulation has degraded, settled, or got wet and stopped working.

Damp patches or mould - Black mould appearing on internal walls, particularly around the middle or lower sections. Damp patches spreading where they didn't exist before. This often means water's getting into the cavity and the insulation is acting as a bridge, pulling moisture through to your internal walls.

Rising heating bills - If your bills have increased without you changing your usage, failed insulation could be the culprit. Your heating system is working harder to compensate for heat escaping through the walls.

Condensation on walls - Not just windows - actual condensation forming on wall surfaces, particularly external walls. This can happen when insulation fails and cold surfaces meet warm, moist air.

Musty smell - Damp insulation trapped in your walls creates that distinctive musty odour that won't go away.

If you're seeing multiple signs, your insulation may have failed.

What Causes Cavity Wall Insulation to Fail

Damp getting in - The most common problem. The insulation material needs to stay dry to work. Once it gets wet, it stops insulating and starts conducting moisture from the external wall to your internal wall.

Water gets in through cracks in brickwork, failed mortar, damaged render, or blocked gutters that overflow onto walls. In exposed locations or areas with driving rain, even small defects let water in.

Poor installation - If the cavity wasn't filled properly, there are gaps where cold air gets through. If it was overfilled, it can block essential ventilation or bridge the cavity entirely, creating a direct path for cold and damp.

Sometimes insulation gets caught on wall ties or pipes during installation, leaving voids nobody spotted. Other times installers used the wrong material for the property's exposure level.

Ventilation blocked - Cavity wall insulation reduces airflow through walls. If proper ventilation wasn't added when it was installed (trickle vents on windows, adequate extractors), moisture builds up inside your home with nowhere to go. It condenses on cold surfaces and eventually soaks into walls.

Wrong material for your area - Properties in exposed locations or high-rainfall areas need specific types of cavity insulation. Standard blown fibre or bead insulation can fail quickly in these conditions if water regularly hits the walls.

Age and degradation - Older insulation materials break down over time. Houses insulated in the 1970s or 80s often used materials that have degraded or, in some cases, materials like Urea Formaldehyde that release harmful gases as they break down and need removing.

A customer came to us after three different damp specialists couldn't solve their mould problem. Black mould kept appearing on bedroom walls despite treatment, and they were getting condensation they'd never had before. Turned out their cavity wall insulation had got wet and was actively pulling moisture through from outside. Once we identified the cause - failed mortar letting water into the cavity - and removed the saturated insulation, the mould problem disappeared.

What Happens If You Leave It

Failed cavity wall insulation doesn't just stop working - it actively makes things worse.

Wet insulation pulls moisture through to your internal walls. Damp patches appear, mould grows, and plaster starts deteriorating. The damp creates perfect conditions for black mould, which affects air quality and health.

Your heating system works harder trying to warm a house that's losing heat through failed insulation. Bills climb steadily whilst comfort drops.

What starts as removing and replacing insulation can escalate to repairing water damage, treating mould, and replastering walls if left long enough.

How Cavity Wall Insulation Gets Fixed

Fixing failed cavity wall insulation isn't a DIY job - you need someone who understands what went wrong and how to prevent it happening again.

First, an expert surveys your walls to identify what's failed and why. They check for damp, assess the existing insulation, and work out whether you need partial or complete removal.

Before touching the insulation, fix whatever's letting water in - repoint brickwork, repair render, sort guttering. Otherwise the same problem comes back.

Then the failed insulation gets removed. Loose materials (beads, fibres) get vacuumed out through drilled holes. Foam or bonded materials require removing sections of wall to break them up. If ventilation was part of the problem, it gets sorted before new insulation goes in.

New insulation is installed using appropriate materials for your property, with proper checks throughout to ensure no cold spots or gaps.

The whole process typically takes a few days to a week, depending on how much needs removing and whether walls need partial rebuilding.

What It Costs

Removing failed insulation:

• Whole house: £1,500-£3,000 for most properties

Reinstalling cavity wall insulation:

• Typical semi-detached house: £2,500-£4,500

Total for removal and replacement: £4,000-£7,500 typically

If scaffolding is needed or the work is complex, costs increase. Repairing the original defects that let water in (repointing, render repairs) is additional but essential - otherwise you're just delaying the next failure.

This sounds expensive because it is. But leaving failed insulation means paying higher bills indefinitely whilst living with damp and mould problems that keep getting worse.

Getting It Properly Assessed

If you're experiencing warning signs - cold patches, damp, rising bills, mould - get your walls assessed properly. The signs overlap with other issues (ventilation problems, damp from other sources), so trying to diagnose it yourself usually wastes money fixing the wrong thing.

When we survey your home, we check your cavities and can spot obvious issues. If you're concerned about your cavity wall insulation specifically, let us know and we'll investigate further. For detailed assessment, we can arrange specialist cavity surveys if needed.

Sometimes the problem is more complex than just failed insulation - issues with ventilation, structural problems, or damp from multiple sources can create similar symptoms. A whole house survey identifies everything that's contributing to the problem.

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