Insulating with
wood fibre.

Wood fibre is obtained by defibring softwood, most often sawmill offcuts and thinnings, then reconstituting boards or a loose fill. According to the data sheet of one of the main manufacturers, its flexible board also contains a small proportion of polyolefin fibre that gives it its stability, and ammonium sulphate. It comes in three forms, which are not used in the same places:

The values above are those published by one manufacturer for its products; from one brand to another, the conductivity of a flexible board varies from 0.036 to 0.038 and that of a rigid board rises with its density. The figure that counts is the one on the certificate or declaration of performance of the exact product written on the quote: it is the one used to calculate the grants.

The much-searched term "wood wool" (laine de bois) refers in practice to the flexible board. Loose fill is blown or injected by machine: the manufacturer cited reserves it for companies it has trained and approved.

The thermal resistance R of a layer of insulant is calculated by dividing its thickness by its conductivity: R = e / λ. The thresholds to keep in mind are those that govern grants: R ≥ 3.7 for a wall, R ≥ 6 for a roof slope, R ≥ 7 for an unconverted loft floor, R ≥ 3 for a ground floor. With wood fibre, this gives:

Compared with a common mineral wool at 0.035, wood fibre therefore needs a few extra millimetres for the same R: about one centimetre on a wall, two on a roof slope. Under a roof where every centimetre of height counts, this is a point to check before choosing, together with the height of the rafters. For loose fill in an unconverted loft, the thickness to blow is greater than the settled thickness: it is read from the manufacturer's table and checked on handover with height gauges.

In winter, what counts is conductivity, and wood fibre is on a par with mineral wools. In summer, another quantity comes into play: the amount of heat the insulant can absorb before transmitting it. It depends on its specific heat capacity and its density. Wood fibre manufacturers' data sheets state 2,100 J/(kg·K), whereas mineral wool manufacturers declare about 1,030 J/(kg·K), referring to the standard NF EN ISO 10456. And a fibre board weighs 60 to 160 kg/m³.

The result is a time lag (déphasage): the heat of the roof, warmed in the middle of the afternoon, passes through the wall later, often when the outside air has already cooled. We do not give a number of hours: it varies with the thickness, the density of the product, the roof covering and the orientation, and the values circulating online almost never state their assumptions.

This gain is mainly noticeable under a roof: converted lofts, attic rooms, bedrooms under roof slopes, where summer overheating is most frequent. It is more modest behind a thick stone wall, which already plays this role. And it replaces neither shutters closed in the hot hours nor a house that is ventilated at night.

Wood fibre is highly permeable to water vapour: its resistance factor μ is 1 to 2 as loose fill, 2 for a flexible board and 5 for a rigid board according to the data sheets consulted, that is, barely more than air. It is also hygroscopic: the manufacturer states that it absorbs a temporary excess of moisture and then releases it. This is what makes it valuable in a wall that has to dry out.

This does not remove the need to restrain vapour on the heated side. In winter, the air in the house is more humid than the air outside; without a continuous membrane, it enters the wall and can condense on contact with the cold face. In our specifications, roof slopes insulated with wood fibre receive a humidity-variable vapour-control layer, and timber-frame walls a vapour barrier with taped joints. These membranes are explained in detail in our guide to insulating your roof from the inside.

On the outside, it is the opposite: the aim is a wall that becomes progressively more open towards the outside. A rigid wood fibre board laid over rafters or a frame can thus act as a rain barrier, provided the system allows for it.

Under the roof, from the inside. Two crossed layers of flexible boards between and under the rafters, a vapour-control layer, a plasterboard sheet on a frame. This is the use where summer comfort most often justifies choosing wood fibre.

On the roof, as sarking. When the roof covering is redone, rigid boards laid over the rafters insulate without touching the rooms. The technique, its constraints and its singular points are the subject of our guide to sarking.

On the façade, under render. On a masonry wall, rigid boards are bonded and plugged, then rendered. On a timber frame, a high-density board serves both as a rain barrier and as a support for the render: this is the solution we priced for an extension in 2026. The local rules, notably in protected areas, are detailed on our page on external insulation in Annecy.

Against old walls. On stone, rammed earth (pisé) or rubble stone bedded in earth, a frame filled with wood fibre lets the wall breathe where a closed insulant could trap water. The full process, from the moisture diagnosis to the choice of membrane, is in our guide to insulating walls from the inside.

In timber frames. Extension walls, floors and timber flat roofs: the fibre fills the space between studs or joists. This is the most frequent use in our quotes, as our article on the timber-frame extension shows.

Wood fibre is combustible. The boards we consulted are classed E for reaction to fire according to the standard NF EN 13501-1, like most bio-based insulants; one loose-fill manufacturer reports a laboratory classification of B-s2,d0 for its product. Rock wools, for their part, are non-combustible.

In a house, the fibre is never left exposed: a plasterboard sheet, a render or a cladding protects it, and on a façade it is the whole system — insulant, fixings and render — that undergoes a technical assessment. In an apartment building, fire regulations for façades require solutions assessed according to the type of building; they sometimes lead to a preference for rock wool, at least on certain areas.

Wood fibre appears in 12 of our quotes drawn up between 2024 and 2026 in Savoie and Haute-Savoie, over about thirty lines. That is few: these figures are rounded orders of magnitude, for work that is supplied and fitted, excluding VAT.

For external insulation, we do not publish a range specific to wood fibre: on the quote where we priced both, at 160 mm under render, the wood fibre variant was no more expensive than rock wool. Budgets for external insulation under render, all insulants combined, are given on our pages on external wall insulation (ITE).

For loose fill blown into an unconverted loft or injected into a cavity, none of our quotes from the period contains any: we give no price.

VAT. In a home completed more than two years ago, insulation that reaches the performance thresholds falls under the 5.5% rate. New construction, or an extension of more than 10% of the existing floor area, falls under the 20% rate. The borderline cases are detailed in our article on VAT on renovation work.

When thickness is lacking. In an apartment where every centimetre of a room counts, an insulant that performs better at equal thickness makes it possible to reach the grant threshold with a thinner lining.

When liquid water is a threat. Against a buried wall, under a slab on ground or in an area exposed to untreated rising damp, a wood-based insulant has no place.

When fire regulations rule it out, on an apartment building façade in particular. And sometimes, simply, when the budget calls for a mineral wool and the wall would gain nothing decisive from the fibre.

Two other guides in the series complete this one: glass wool or rock wool, then which insulant to choose, which compares all the insulants with one another.