On a ground floor over a cellar or an accessible crawl space, insulation goes on the underside: it is the only technique that costs the dwelling no headroom, and it reaches the 0.25 W/m²·K retrofit U-value of Approved Document L with 100 to 120 mm of mineral wool or 80 mm of polyurethane. Insulating from above is decided only when the space below cannot be reached, meaning a ground-bearing slab or a crawl space with no access, and it then forces you to rework door thresholds, stair risers and skirtings. Access from below therefore settles the choice before any performance consideration: it is measured on survey, clear height, hatch and ventilation, and written into the report step by step from the surveyor's phone. One point routinely escapes the calculation: a floor over a cellar is not an element facing outside air, BS EN ISO 13370 and the national calculation method both apply a reduction to its heat loss, and entering it as "external" inflates the saving you announce.
Understanding your ground floor before choosing an insulation technique
Identifying the substrate: slab on grade, crawl space, basement, cellar
Before any insulation work, look at what's under your feet. A slab on grade is often insulated from above during a floor renovation. A crawl space or a cellar instead allows installation from underneath. In a basement, you can aim for continuity with the walls, to avoid thermal bridges.
Identifying site constraints: ceiling height, access, moisture, services
Measure the available height. A few centimetres of insulation can block a door or a step. Check access (hatch, crawl space, clutter), moisture condition (odors, efflorescence, condensation), and ventilation. Also identify the water, gas, drainage, and cable runs. They dictate the thickness, the fixing method, and the protection needed.
Clarifying the goal: thermal comfort, reduced heat loss, acoustics
Set your direction. For comfort, you want a less cold floor and no "icy" zones. For performance, you aim for continuous insulation with careful junctions. For acoustics, favour decoupled solutions and suitable underlays.
Insulating from below: the most common technique when you have access
Installation: panels under the slab, insulation between joists, fixing, and airtightness
When the ground floor is accessible, the insulation goes on the cold side. Under a slab, rigid panels are installed (often in 2 crossed layers) with anchors or rails, then the joints are treated for good continuity. On a timber floor, the insulation is placed between the joists with mechanical support, ideally supplemented by a continuous layer under the joists to limit thermal bridges. Airtightness mainly comes down to the connections: joints, hatches, service penetrations.
Points to watch: thermal bridges at the perimeter, vapour barrier, condensation risks
Losses hide at the perimeter: wall-floor connections, joist ends, insulation returns. On the water vapour side, don't install a membrane at random. Depending on the humidity of the crawl space or cellar, the wrong choice can trap water and create internal condensation. If in doubt, a hygrothermal check and adherence to technical approvals avoid bad surprises.
Practical cases: accessible crawl space, unheated basement, vaulted cellar
Accessible crawl space: rigid panels under the slab, sealed joints, ventilation preserved. Unheated basement: ceiling insulation with mineral wool, a protective facing if needed. Vaulted cellar: moisture-tolerant materials, reversible installation, and effective airing kept in place to let the walls breathe.
Insulating from above: the right option during a floor renovation
Installation: insulation + screed, floating floor, hydraulic underfloor heating
When you're redoing a floor, insulating from above is often a quick win. Rigid insulating panels are installed, then a screed or dry boards. A common alternative, a floating floor over an underlay, limits losses and improves comfort. With hydraulic underfloor heating, the insulation goes under the pipes, then a screed encases the whole assembly, to spread the heat without it escaping into the slab.
Constraints: raised floor level, door thresholds, stairs, allowable loads
The key point is the finished height. A few extra centimetres can block a door, shrink a step, or require redoing the skirting boards. Also think about loads. Screed, flooring, and furniture add up. In renovation, check the substrate's load-bearing capacity before adding thickness.
Details that make the difference: perimeter strips, decoupling, insulation continuity
A perimeter strip prevents cracks and acoustic transmission. Decoupling the floor from the walls helps the screed work without stress. Finally, hunt down thermal bridges. The insulation must stay continuous at partitions, hatches, and services to keep a floor that's genuinely comfortable.
Comparing the two insulation techniques: cost, performance, timelines, site impact
Actual performance: insulation continuity, thermal bridge treatment, everyday comfort
In practice, insulating the underside gives a continuous layer, broken by no partition and no service run, and it leaves the finished floor level untouched. Its weakness is the perimeter: the wall-to-floor junction and the joist ends stay thermal bridges unless the insulation return is actually executed. Insulating from above handles that edge better, since the insulation turns up against the wall, but it is interrupted by every partition in place and eats into headroom.
The table below gives, for each configuration you record on survey, the technique that follows from it and the thickness to allow for a 0.25 W/m²·K element.
| Configuration below the floor | Technique that follows | Thickness for a 0.25 W/m²·K element | Governing constraint |
|---|---|---|---|
| Cellar or unheated basement, adequate clear height | underside, boards or wool under the slab | 105 to 120 mm mineral wool, 80 mm PUR/PIR | fire protection of the lining depending on how the room is used |
| Accessible crawl space, hatch in place | underside, boards bonded or fixed | 105 to 120 mm mineral wool, 80 mm PUR/PIR | through-ventilation of the void kept clear |
| Crawl space with no access | from above, insulation plus screed | 100 to 110 mm XPS, 70 to 85 mm PUR/PIR | headroom lost under the thresholds |
| Ground-bearing slab | from above, unless the floor is fully taken up | 100 to 110 mm XPS, 70 to 85 mm PUR/PIR | thresholds and stair risers to rework |
| Floor over an open passage or porch | underside, with mechanical protection | 105 to 120 mm mineral wool, 80 mm PUR/PIR | wind, water and impact resistance of the lining |
| Timber floor on joists, accessible below | between joists plus a continuous layer beneath | 120 mm between joists plus 40 mm continuous | treatment of the joist ends |
One line of that table on its own justifies a second visit before you price: the crawl space with no access. Until the clear height is measured, you do not know whether the job is an underside installation at £30/m² or a full floor rebuild, and the gap between the two runs into thousands of pounds on a house.
Site organization: phasing, co-activity, disruption, returning rooms to use
On the site side, underside work happens outside the living rooms: the dwelling stays occupied, disruption is confined to the cellar or the crawl space, and an ordinary house takes one to two days. Work from above is managed room by room, but means clearing the rooms, taking off skirtings and sometimes internal doors, then waiting for the screed to dry before any covering goes back down.
Budget: materials, labor, finishing rework, and contingencies
The overall budget leans clearly towards the underside, around £25 to £45/m² installed, lining included. Work from above starts higher on the insulation line alone, then picks up the screed, the covering, the threshold rework and the skirtings, and routinely doubles the total. Either way, plan for a 5 to 10% contingency margin for the moisture surprises, which remain the first cause of a variation on this element.
Choosing the right insulation technique in 2026: rules, subsidies, and evidence to provide
Regulatory benchmarks and common requirements: target thermal resistance and supporting evidence
In practice, eligible insulation targets the U-value set for the element. The Approved Document L retrofit values are: cold loft 0.16, rafters 0.18, walls 0.30, ground floors 0.25 (in W/m²·K). On the quote and invoice, show the U-value achieved, the area, thickness, lambda, product reference, and if possible the certificate (a BBA Agrément or equivalent).
2026 funding: conditions and documents to give the client
The rule stays simple: the application is made before the work starts, and the company must be certified to PAS 2030 in the right scope (walls, lofts, floors) and registered with TrustMark. Give the client a clear file: a dated quote, a detailed invoice, the certification mention, technical data sheets, and the client declaration to sign.
Checks and quality: photos, technical data sheets, insulation traceability, job-site sign-off
Audits do happen. Secure your job with before, during, and after photos (tricky junctions, thickness), batch traceability (labels, batch numbers), and a signed sign-off report. Keep this evidence for at least the duration of the review period. Visible quality means smoother funding.
