Sarking insulation and internal insulation are the two main ways of insulating a pitched roof. They share the same goal, reducing heat loss through the roof, but they do not apply in the same situations and do not cost the same. Choosing the wrong technique means either paying too much or missing out on performance. Here is what you need to know to make an informed decision.
What is sarking insulation?
The word “sarking” originally referred to the boarding laid over the rafters. Today it describes an external insulation technique: rigid insulation boards are laid directly on the rafters in a continuous layer, then covered with counter-battens, battens and the new roof covering. The insulation wraps the roof structure like a coat.
The advantages of sarking insulation
- No thermal bridges: the insulation is not interrupted by the rafters, which improves real-world performance.
- No loss of space: the internal volume stays intact, a real advantage for converted lofts or lofts to be converted.
- Exposed timbers are possible, for a warm look.
- Protected timbers: they stay on the warm side, sheltered from temperature swings and condensation.
- Work done from outside: the house remains habitable and the interior stays clear.
The limitations
- The covering has to be removed: the technique only makes sense when the roof needs renovating anyway.
- The budget is higher, since it includes a new covering.
- The roof is raised by a few centimetres, so the verges, the junctions with neighbouring roofs and sometimes the cornice must be adapted.
What is internal insulation?
Internal insulation, or insulation between and under the rafters, is installed from inside the loft. A first layer of flexible insulation, mineral wool or wood fibre, is placed between the rafters. A second, crossed layer is fixed under the rafters to cut out thermal bridges. A continuous vapour barrier or vapour control layer is then fitted on the inside, before finishing with plasterboard or panelling.
The advantages
- The covering stays in place: ideal when the tiles or slates are still in good condition.
- A lower budget, with no heavy scaffolding and no new covering.
- Work can be phased, room by room if necessary.
The limitations
- The thickness of the insulation reduces the height and volume of the loft.
- The vapour barrier must be perfectly continuous: an installation error can lead to condensation in the insulation and the timbers.
- Without an underlay, the insulation may be exposed to dust and water getting in under the tiles.
Sarking insulation or internal insulation: the comparison
| Criterion | Sarking | Internal |
|---|---|---|
| Covering | To be replaced | Kept |
| Thermal bridges | Practically none | Limited by the second layer |
| Internal space | Intact | Reduced |
| Risk of condensation | Low | Depends on the quality of the vapour barrier |
| Indicative price excluding VAT | €90 to €160/m² excluding covering | €60 to €110/m² including finishes |
| Duration of the work | 1 to 3 weeks with covering | A few days |
How to choose
Your covering needs replacing
If your roof is more than forty or fifty years old, the underlay is missing or damaged, or the tiles are becoming porous, the decision is almost made for you: sarking insulation is the obvious choice. Since the covering is coming off anyway, external insulation costs relatively little extra and offers the best performance. It is the solution we recommend as part of a roof renovation.
Your covering is in good condition
If the roof was redone fifteen or twenty years ago with an underlay, stripping it just to insulate makes no economic sense. Internal insulation is then the right answer, provided it is carefully installed. If the loft is not lived in, insulating the loft floor is simpler still.
Your loft is already converted
Insulating from the inside means dismantling the existing finishes. If the covering is nearing the end of its life, it is better to wait and opt for sarking insulation at the time of the renovation, so the interior does not have to be touched.
Which insulation for sarking?
PIR boards offer the best performance for a small thickness. Wood fibre boards are thicker but provide much better summer comfort and a favourable environmental footprint. We help you choose according to your priority.
Points to watch
- Thermal resistance: aim for at least R = 4.5 m²K/W, the level generally required by regional grants.
- Airtightness: insulation without airtightness loses a large part of its effectiveness.
- Junctions: with gable walls, roof windows, the chimney and the neighbouring house.
- Ventilation of the covering, under the tiles or slates.
- Permits: the slight raising caused by sarking insulation may, in some municipalities, require a planning check, especially for terraced houses.
Sarking insulation and summer comfort
An often underestimated advantage of wood fibre sarking insulation is comfort on hot summer days. Dense insulation delays the heat passing through the roof by several hours, so it only reaches the loft in the evening, when you can open the windows. With light insulation of the same thermal resistance, the effect is much smaller.
What about grants?
In Flanders, Mijn VerbouwPremie for roof insulation remains open to income categories 3 and 4, with a required thermal resistance of at least 4.5 m²K/W. In Wallonia, Rénopack and Rénoprêt have replaced the grants since 1 October 2026. In Brussels, Renolution grants are no longer awarded for recent invoices. See our article on roof grants.
Frequently asked questions about sarking insulation
How thick should sarking insulation be?
To reach R = 4.5 m²K/W, allow around 10 to 12 cm of PIR, or 18 to 22 cm of wood fibre. We calculate the exact thickness according to the chosen insulation and the goals of your project.
Is sarking possible on a terraced house?
Yes, but raising the roof means the junction with the neighbouring roof must be carefully detailed, often with a zinc upstand. We check this point at the first visit.
Sarking insulation: our view
If your roof needs renovating, sarking insulation is almost always the best choice: performance, comfort and preserved volume. If your covering is sound, well-installed internal insulation offers excellent value for money. In both cases, the quality of the installation makes the difference. Our teams carry out both techniques in Brussels, Walloon Brabant, Hainaut and Flanders: discover our roof insulation service and request a free visit.
