A brick vault floor on steel joists combines brick arches laid on edge between steel sections, and behaves thermally like an uninsulated heavy wall, with a U-value of 2.0 to 3.0 W/m²·K. Insulating brick vaults is almost always handled from below, at 8 to 12 cm on the underside, which keeps the existing finishes and avoids loading the arches and their bearings. Insulating from above stays possible where the cellar is inaccessible, but it raises the finished level and means reworking thresholds, steps and skirtings. Before any installation, the condition of the mortar joints, the corrosion of the sections and the efflorescence are surveyed arch by arch: closing a damp vault behind an insulant speeds up the decay of the iron and the brick instead of slowing it.
Recognising your brick vault floor before insulating
Identifying the vault and brick in an old floor (visual, sound, bearings)
From a cellar or crawl space, look for a brick vault shaped like an arch between load-bearing elements. The bricks are often laid on edge, with visible mortar joints. When tapped, sound areas ring solid, a detached brick sounds hollow. Also check where the arch bears, at the edge, on the metal beam or on an angle iron.
Understanding the load-bearing structure: steel beams, spacing, joint condition
The load-bearing elements are steel beams (I-sections, IPN or similar) that carry the floor. Record the spacing and orientation, then check the bearings in the walls. Look at the condition of the joints between bricks and where they meet the metal. Crumbling mortar often signals a need for repair before insulating.
Spotting common defects: sag, cracks, moisture, salts and corrosion
Watch for sag (a dipping floor), cracks following the curve of the arches or at the edges. Signs of moisture and white efflorescence (salts) weaken the joints. On the steel side, look for active corrosion (thick rust, flaking). If in doubt, get it checked before closing up with insulation.
Choosing an insulation strategy suited to an old floor
Insulating from below: when it's the safest solution for a vaulted floor
On a vaulted floor, insulating from underneath is often the safest solution. You keep the existing floor coverings and avoid stressing the vaults. Aim for a continuous, well-fixed installation, with clear moisture management if the space below opens onto a cellar or a crawl space.
Insulating from above: level constraints, floor coverings and added loads
Insulating from above makes sense when the space below is inaccessible. But the finished floor level rises. Anticipate door thresholds, steps, skirting boards and service reworks. Choose insulation suited to compression and check the added loads, especially on old joists.
Preserving thermal mass and limiting thermal bridges at the floor's edges
In old buildings, the floor's thermal mass helps smooth out temperatures. Keep the mass on the heated side where possible, and pay attention to continuity at the edges. An insulation upstand against the walls and good airtightness reduce thermal bridges without trapping moisture.
Managing water vapour and moisture on a brick floor
Assessing condensation risk: ventilation, room use and cold spots
On a brick floor, water vapour follows the path of least resistance. If ventilation is weak or the room sees heavy use (kitchen, bathroom, utility room), humid air eventually meets cold spots at the edges, at the walls or bearings. A hygrometer, an inspection for mould in corners, and identifying cooler areas (at floor level, near skirting boards) already give a useful reading.
Choosing materials compatible with older construction: breathability and capillarity
Old buildings work like a wick. Favour breathable solutions capable of managing water by capillary action (lime renders, compatible screed, capillary-active insulation). Conversely, an overly sealed facing or a resin can trap moisture and shift the problems onto the walls or timber.
Treating rising damp and infiltration before work: diagnosis and on-site action
Before insulating or redoing finishes, treat the cause. A moisture diagnosis (measurements, salts, source) avoids "sealing in" a floor that's still damp.
- Check gutters, downpipes, leaks and ground level against the facade.
- Treat the base of the walls, improve water drainage, ventilate a crawl space if present.
- Repair cracks and joints, then let it dry before reinstating finishes.
Installing without weakening the floor structure
Fixings, hangers and spacing: avoiding pull-out in the vault's brick
Before drilling, identify where the floor is working structurally. On a brick vault, avoid random fixings in thin areas. Favour anchoring into identified load-bearing sections, with suitable hangers and regular spacing. When the substrate is uncertain, a pull-out test and advice from a structural engineer stop the installation turning into a lottery.
Managing services and penetrations: precautions on a historic floor
Plan services before closing up. Limit penetrations, keep diameters reasonable and avoid "making do" by enlarging an opening on site. If a passage is essential, prefer non-structural areas and keep clear distances from the bearings. In renovation, the simple rule is to minimise irreversible interventions.
Site safety and checks during installation: flatness, loads, hard points
Check flatness as you go. A hard point or a badly placed shim can concentrate loads and create cracks. Spread out material storage, limit stacking, and check that the hangers hold after each phase. A visual check, plus a set of measurements, secures performance and lifespan.
Framing the job in 2026: incentives, requirements and records to keep
Incentives available in 2026 for floor insulation: MaPrimeRénov' and CEE (check per project)
In 2026, floor insulation can qualify for MaPrimeRénov' and CEE, if the project is eligible and applied for in the right order. MaPrimeRénov' depends on the household's profile and technical criteria. CEE go through a standardised operation sheet, with an incentive offered by an obligated party or a delegate. Plan ahead for applications before invoicing. The timeline is easier to hold when grants and energy saving certificates are built into the costing, with what is left to pay shown to the client.
RGE and useful documents: quotes, technical data sheets, thermal resistance, photos
To secure the incentives, the company must be RGE-certified in the right category. Keep detailed quotes and invoices, technical data sheets (thickness, lambda, R, marking), treated surface areas, and before/during/after photos. On the CEE side, also keep the signed sworn statement. A paper trail means an easier job to justify.
Special cases in older buildings: co-ownership, heritage protection, and alignment with the energy audit
In a co-owned building, check the vote at the general assembly and any available collective incentives, with an application managed by the property manager. In a heritage-protected area, sign-off from the Architecte des Bâtiments de France (ABF) may be required. The energy audit helps prioritise and avoid an insulated floor that's "out of sync" with the walls, ventilation and heating. Consistency before haste.
