24 June 2026

When do you insulate the cavity walls? Construction year, season and lead time

Whether your home is a retrofit candidate depends mainly on its construction year — and the best time to ask on the heating season. The planning guide.

Start with the construction year. Cavity walls came into use from roughly 1920; older homes often have solid walls without a cavity. Between about 1920 and 1975 the cavity was often left as an empty air gap — that is the classic candidate for having the cavity walls insulated. After 1975, insulation at build became increasingly common; the installer's borescope inspection shows what is already in the cavity.

Season-wise, cavity-wall insulation is possible almost all year round: only frost or persistent driving rain shifts the schedule. The smartest time to ask is therefore spring or summer — well before the heating season, while insulation companies' diaries still have room. Whoever knocks in autumn joins the queue in the fullest months.

Also allow lead time for the nature check. Cavity walls can house protected species, such as bats, and nature-protection rules require the cavity to be checked for them before insulating. Ask the company at quote stage how it handles that check and what it means for the schedule — a proper company has a clear answer.

Unsure whether the cavity is even the first step? An insulation survey with thermal scan (€ 95 to € 250) shows where your home's heat loss is greatest, precisely in the heating season when the differences show sharpest. Then set the quotes against the price index and schedule the work when it suits you.