Understanding the cold roof configuration and its points of attention
Principle: ventilated loft and insulation at floor level
A cold roof keeps the loft "outside the heated volume." Insulation is laid at loft floor level, not under the rafters. Air circulates in the loft via air inlets and outlets (eaves, vent tiles, ridge depending on the case). The goal: remove moisture and limit heat build-up under the covering, while concentrating the effort on the ceiling's airtightness.
Site risks: condensation, stray air entry and thermal bridges
The classic pitfall: poorly controlled ventilation. Too little air and condensation appears on the underside of the covering. Too much air in the wrong place and you create draughts in the insulation, with a drop in performance. Take care with the vapour barrier or vapour check layer on the warm side according to best practice, treat hatches, spotlights and ducts, and maintain insulation continuity at the verges and around the truss feet.
When a cold roof makes sense for your configuration (unconverted loft, accessibility)
It's often worthwhile for an unconverted loft, when access is simple and the roof space isn't intended for conversion. It's an effective solution for reaching a good insulation level without touching the roof structure. Conversely, if the loft is meant to become habitable, insulation under the rafters and careful vapour management are generally more suitable.
Warm roof: insulation under the rafters or from outside, depending on your configuration
Principle: continuous heated volume and elimination of air movement in the loft
The idea is simple: insulation is placed as close as possible to the roof to create a continuous heated volume. The loft moves to the warm side. The result is fewer stray draughts, a more stable temperature under the rafters and better-controlled condensation risk, provided airtightness is carefully executed.
Choosing the right method: under-roof insulation vs sarking (depending on structure and covering)
Under-rafter insulation is done from the inside. It suits cases where the covering is in good condition and you're willing to reduce the habitable volume slightly. Sarking is installed from the outside, when the roof covering is being redone. It provides continuous insulation above the rafters, useful on an irregular structure and to limit thermal bridges.
Key execution points: vapour barrier, airtightness and insulation continuity
A warm roof mainly depends on installation quality. The vapour barrier must be continuous on the interior side, with bonded joints and treated duct penetrations. The insulation must never be interrupted at the base of the slope, at the verges or at the ridge.
- Treat the wall-roof junctions and the loft hatches.
- Plan for the roofing underlay and ventilation according to the covering system.
- Check continuity after installation, before closing the finishes.
Comparing cold roof and warm roof: performance, cost and installation constraints
Thermal performance and summer comfort: what really changes depending on the roof
On a cold roof, the insulation is laid at ceiling level, with a ventilated, unheated loft. This is effective if ventilation is well controlled, otherwise the moisture risk increases. On a warm roof, insulation follows the rafters (or goes above them with sarking). Continuity limits thermal bridges and improves summer comfort, provided the airtightness and vapour barrier are carefully executed.
Impact on space: headroom in the loft, conversion potential and access to services
A cold roof keeps the loft "outside the volume," handy for storage but hard to convert. A warm roof brings the loft back inside the insulated envelope. You gain habitable volume, but under-rafter insulation can eat into the height, unless you use an external solution. Services also run more easily on the heated side.
Budget and job duration: deciding based on configuration and the condition of the covering
A cold roof is often faster and cheaper because it's done from the inside, without touching the covering. A warm roof costs more, especially if it requires stripping the covering or sarking. It becomes the logical choice during a full roof re-covering or a loft conversion project.
Special cases that tip the roof choice (2026)
Roof under major renovation: covering to be redone, roofing underlay, treatment of specific points
When the covering is starting from scratch, the roof choice often comes down to the details. Use the job as an opportunity to install a breathable underlay (HPV) if the insulation is in contact, and to redo the ridge, verges, valleys and flashings. Well-treated specific points avoid water ingress and costly rework.
Roof with a converted loft: managing purlins, dormer windows, doghouse dormers and hatches
With a converted loft, the roof structure dictates the method. Check purlins, rafters and bearings before adding insulation. Around dormer windows and doghouse dormers, aim for continuity of insulation and airtightness. An airtight hatch often makes the difference.
Damp or damaged roof: diagnosis before insulating and suitable solutions
Moisture, mould, blackened wood. You don't insulate without a moisture diagnosis. Track down the cause: leak, condensation, insufficient ventilation. Dry it out, repair it, then choose a solution compatible with vapour migration and correct underside ventilation.
Grants and requirements to know in 2026 for your roofing jobs
RGE and documents to prepare: quotes, technical datasheets, photos and traceability
To secure grants for an insulated roof, the company must be RGE-certified in the right domain. Prepare a simple, complete file. Quotes and invoices should detail the treated area, the technique (unconverted loft, under-rafter, sarking) and the performance. Also keep the technical datasheets, proof of marking and dated photos before, during and after. Traceability of materials (references, batches) avoids blockages in case of an inspection.
MaPrimeRénov' and CEE: points of attention for roof/loft insulation
In 2026, MaPrimeRénov' and CEE often remain combinable if you follow the rules of each scheme. The quote must be signed at the right time, and the CEE forms must be started before ordering. Check that the targeted operation matches the roof insulation concerned and that the stated thermal resistance is consistent with the products installed.
Checks and common mistakes: ventilation, vapour barrier and consistency with the energy audit
Checks focus as much on the paperwork as on the installation. A classic mistake is insulating without addressing ventilation and airtightness, which opens the door to condensation. Another sensitive point is a missing or poorly positioned vapour barrier. Finally, make sure the work aligns with the energy audit when one exists, especially regarding the order of work items and moisture management.
Key figures
insulation under a ventilated air gap
Cold roof
insulation above the waterproofing
Inverted roof
insulation under the waterproofing
Warm roof
Frequently asked questions
In renovation, aim for at least R ≥ 6 m²·K/W under the rafters (warm roof) and R ≥ 7 m²·K/W in an unconverted loft (cold roof) for the main schemes. In practice, prepare the supporting documents (invoice with R, area, thickness, lambda) and have the work carried out by an RGE-certified company if you're building a grant application.
Pierre-Louis Guhur
CEO of Argile
