Cavity wall insulation: when blown fill helps and when it harms

Most houses built from the 1920s onwards have cavity walls. A rough indicator is the brick pattern: in cavity walls you usually see only the long sides of the bricks, while solid walls show a mix of long sides and ends. Wall thickness at a door or window reveal is another clue. Small, regularly spaced filled holes in the mortar suggest the cavity has already been filled.

Timber-framed, steel-framed and concrete system-built houses can look like brick from outside but are not suitable for blown cavity fill. If you are unsure how your house was built, say so in your request; the surveyor has to establish it before recommending anything.

Before drilling, a competent installer checks the cavity with a borescope through a small hole, looks at its width and whether it is clear of rubble and mortar, and assesses the outer leaf. Cracked bricks, failed pointing, defective render and blocked or leaking gutters all let water into the wall, and a filled cavity gives that water a path inwards.

Exposure matters as much as condition. Walls facing prevailing wind-driven rain, especially on exposed coastal or upland sites, are at higher risk, and the installer should consider the exposure of each elevation separately rather than the house as a whole.

• borescope findings: cavity width, obstructions, wall ties, any existing fill
• exposure of each wall to wind-driven rain
• condition of bricks, pointing, render, sills and gutters
• airbricks and flues that must be sleeved or kept clear
• the insulation material proposed and why it suits the wall
• whether any wall is excluded, and the reason

Airbricks that ventilate suspended floors pass through the cavity. They must be sleeved before the fill goes in, or the insulation will block them. Open-flued boilers and fires need a supply of combustion air, and a house that becomes more airtight may need changes to that. A careful installer checks both and explains what it has done.

Cavity wall insulation is notifiable building work in England, which an installer registered with a competent person scheme can self-certify. Ask for the certificate. Installations by registered installers can also be covered by an independent 25-year guarantee from the Cavity Insulation Guarantee Agency; ask for it in writing and keep it with the deeds, because buyers' surveyors ask about cavity fill.

Damp patches on inside walls after cavity insulation, especially on exposed elevations, can mean the fill is carrying water across. Extraction removes the old insulation through larger holes in the outer leaf, often with vacuum equipment, and the cause of the water ingress should be fixed before any refill. If the original work has a CIGA guarantee, contact CIGA before paying anyone for extraction.

Cavity insulation is included in England's Warm Homes: Local Grant, in ECO4 until its end date and, in Scotland, in the Home Energy Scotland grant, which pays up to £1,500 for loft, cavity and underfloor insulation. The installation is zero-rated for VAT in homes. Be cautious of cold callers offering free cavity fill without a survey.

Through Offrano, your request is sold to a single installer in your area, which pays for it; you pay nothing. We email you the installer's name, address and phone number before it sees your details. No installer is obliged to take the request, and you are never obliged to accept a quote.

Source: gov.uk