Multifoil under beam-and-block floors — the thin retrofit

If your home was built between the mid-1980s and the early 2000s, there's a very good chance your ground floor is beam-and-block — you've probably lived on one without ever knowing its name. Roughly 70% of new-build homes since 1985 use precast concrete beams spanning the foundation walls with concrete infill blocks between. Strong, fast to build, and — when the technology was first rolled out — usually fitted with minimal insulation underneath.

Retrofitting one with traditional PIR or mineral wool usually isn't an option: you'd lose 120mm of ceiling height to do it, which on a 2.4m room makes the space non-compliant. Multifoil is the 50mm answer. Here's how we fit it, what it costs, and when it's worth doing.

Retrofitting a beam-and-block floor from below is almost never practical: there is no crawl space. Retrofitting from above in a traditional build-up (100mm PIR plus 18mm chipboard) steals 118mm of headroom. On a 2.4m ceiling, that matters. Multifoil is the thin-retrofit answer we install on around 25% of underfloor jobs across Essex, Suffolk and Cambridgeshire — usually homes built 1985-2005.

Why beam-and-block is a retrofit problem

A beam-and-block floor sits on the foundation walls with no ventilated crawl space beneath. The underside of the floor is solid ground or a shallow oversite. There is no void to insulate from below. The only retrofit route is from above: lift the floor, add insulation, re-lay the floor surface.

Traditional PIR buildup (100mm Kingspan Kooltherm K103 plus 18mm chipboard plus 10mm finish) eats 128mm of floor-to-ceiling height. For homes built after 2000 with generous 2.5m ceilings, fine. For 1985-1995 builds with 2.35-2.4m ceilings, it often pushes ceilings below building regs (2.1m habitable minimum) once you factor doorheads and door trims.

The multifoil method — step by step

1. Lift the existing floor surface. Carpet up, existing chipboard lifted, exposed beam-and-block cleaned and checked for cracks.
2. Lay Actis Hybris H30 directly on the beam-and-block surface — 30mm quilted multifoil, seamed and taped at every joint.
3. Fit 25mm battens across the multifoil, creating a reflective air gap beneath the new floor surface.
4. Re-lay 22mm chipboard or engineered board on top of the battens.
5. Total build-up: 55mm including finish layer. 68mm less than the PIR equivalent.

U-value hit at this build-up: 0.22 W/m²K. Building regs Part L for new-build ground floors is 0.13 W/m²K — but that is a new-build target. For retrofit work, 0.22 is the practical ceiling and lifts the floor from an original ~0.50-0.60 to acceptable territory. SAP-point gain is typically 5-7, and comfort is dramatically better: cold-foot complaints disappear within a week.

What it costs

Multifoil pricing sits slightly below equivalent PIR once you factor the reduced floor-rise work — no door-trimming, no stair-landing adjustment, no skirting relift. For homes where headroom is not a constraint, PIR is still a valid choice; where headroom matters, multifoil wins on installed cost as well as on headroom retained.

Payback on an unfunded install

Unfunded payback is 9-11 years. That is longer than cavity wall or loft top-up, because the heat-loss percentage through a modern-era beam-and-block floor is smaller than through an uninsulated cavity wall. But there are two factors that often flip the numbers.

• Comfort uplift — the coldest part of a modern home is the ground-floor tile or laminate in January. Multifoil retrofit changes that overnight.
• Heat pump compatibility — if you are planning an ASHP install, the floor U-value matters for heat pump COP. Lifting from 0.55 to 0.22 W/m²K is worth 0.4-0.6 COP in real winter conditions. That is £180-£240 a year of extra running-cost saving attributed to the ASHP, but enabled by the floor.
• Underfloor heating — if you are planning wet underfloor heating, the floor insulation is non-negotiable. Multifoil retrofit pays itself back through enabling UFH, independent of the heat-loss maths.

What an install looks like on site

1. Day 0 — free survey. Lift a small section of chipboard to confirm beam-and-block (not timber joist), measure headroom in all rooms, check for existing services (water pipes, UFH).
2. Day 1 — strip. Clear rooms, lift existing chipboard and any skirting board that needs to come up.
3. Day 2 — insulate. Lay Actis Hybris H30 with taped seams, fit 25mm battens, chalk-line to verify flat plane.
4. Day 3 — re-lay. New 22mm chipboard fitted, glued and screwed, skirting refit if needed.
5. Day 4 — finish. You return rooms to furnished state. We leave you the BBA certificate and a thermal-imaging photo pack.

Total job length for a 3-bed semi: 3-4 working days. Two rooms lost to the works at a time — most households can work around it. OMEGA van, labourers, product, paperwork, all included in the fixed-price quote.

Book a free floor survey

If your 1985-2005 home has a cold ground floor and you cannot lose 120mm of ceiling height, the thin-retrofit multifoil method is the answer. Book a free home energy survey online or ring freephone 0800 229 4094 — Essex, Suffolk, Kent and Cambridgeshire, Mon-Sat. No cost. No obligation. No pressure.