How do I know if my cavity wall insulation has slumped?

Slumping is one of the most common cavity-fill failures, and it has a recognisable signature because settled material always leaves the gap at the top. Here is how to read the signs and confirm them.

What slumping is and why it leaves a top gap

Slumping is the downward settlement of cavity fill so that it no longer reaches the full height of the wall. Some early or poorly installed mineral-wool and bead fills can compact, settle or degrade, especially if they became wet, and gravity pulls the material down. Because the loss is from the top, the tell-tale cold zone is high on the wall — near ceilings, around upper window heads, and at the top of gables — while the lower wall, where the fill has collected, may still feel reasonably warm. This top-down pattern is what distinguishes slumping from a wall that was simply never filled, where the whole elevation reads cold.

It helps to picture how the cavity was filled in the first place. Loose fill is injected under slight pressure through a grid of drilled holes and is meant to knit together into a stable, evenly distributed blanket that grips both leaves and stays put. Slumping is the slow failure of that stability: instead of a continuous blanket from wall plate to damp proof course, you end up with the fill redistributed downward, leaving an air gap at the head of the wall. The amount that drops varies — a few centimetres barely matters, but a settlement of several courses of brick creates a wide uninsulated band that the heating cannot disguise. Because the inner leaf above the slump line is now backed by nothing but air, it behaves exactly like a single-skin cold wall in that zone, which is why the first sign is so often a cold strip and mould along the top of an upstairs external wall rather than anything lower down.

What you can observe yourself

Without tools you can still gather useful clues:

• Hand test: on a cold day, compare the temperature of the wall near the ceiling with the same wall lower down — a marked difference suggests a void above.
• Condensation and mould appearing high on outside-facing walls, where the surface is now coldest.
• Higher heating bills with no change in habits, as heat escapes through the uninsulated band.
• Drill-hole grid in the brickwork confirming the cavity was filled in the first place.

None of these is conclusive on its own, but a high-level cold band plus condensation is a strong pointer toward settlement.

Indicative comparison. Source: CIGA / surveyor guidance.

Confirming with a borescope and thermal camera

Definite confirmation needs to look inside the cavity. A surveyor drills a small borescope hole, typically near the top of the suspect wall, and inserts a camera to see whether there is a void, how far down the fill has dropped, and whether the remaining material is dry, clumped or wet. Several holes at different heights build a profile of where the fill sits. Thermal imaging taken from inside on a cold day shows the cold band as a clear cool zone, helping target where to drill and how widespread the slumping is. Together these give a reliable verdict — and, if extraction is being considered, a map of what is actually in the cavity.

The thermal camera deserves a word of caution, because it shows surface temperature, not the fill itself. A cold band high on a wall is consistent with a slump void, but a concrete lintel, a run of wall ties, or a poorly fitted loft hatch above can read cold for unrelated reasons. That is why the borescope is the decider: the thermal image says where to look and how big the cold zone is, but only the camera inside the cavity confirms there is genuinely a gap where insulation should be. For the survey to be meaningful the conditions matter too — there needs to be a reasonable temperature difference between inside and outside (a cold morning with the heating on) for the cold band to stand out, so a thermal survey booked on a mild day can miss a void that would be obvious in winter.

Why slumping happens and what follows

Understanding why fill slumps helps you judge how likely it is in your home. Loose-fill materials rely on staying evenly packed across the cavity; over years of thermal movement, vibration and — critically — repeated wetting and drying, some early or poorly installed fills lose that even distribution and settle downward under gravity. Mineral-wool fibres can mat and compress once damp, and some bead systems without an adequate bonding agent can flow and re-settle. The result is the same: a denser, sometimes damp mass at the bottom and an uninsulated void at the top. That void then behaves like a wall that was never filled in its upper reaches — a cold band that condenses moisture and grows mould near the ceiling — while the over-packed lower section can hold water against the inner leaf. Because slumping tends to be progressive, the cold band often widens season by season, which is why a problem that seemed minor one winter can be markedly worse the next. None of this means every settled wall needs urgent work; a dry void is a performance loss rather than a damp emergency. But it does mean a borescope check is worth doing once the high-level cold-and-mould pattern appears, so you know whether you are dealing with simple settlement or settlement plus saturation.

Frequently asked questions

Sources & further reading

• CIGA — Cavity Insulation Guarantee Agency
• Energy Saving Trust — cavity wall insulation

Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.