Insulating your loft: method, materials and insulation steps
Up to 30% of a house's heat losses pass through the roof, and often more in old, uninsulated homes. Insulating your loft is therefore the most cost-effective job for a DIYer: little technical skill required, a moderate investment and a quick return on the heating bill. You still need to choose the right material and fit it correctly, because a poorly done insulation loses a large part of its effectiveness, in particular because of thermal bridges.
Unused or convertible loft space?
- Unused loft (not converted): floor insulation between and over the joists, ideal in rolls or loose-fill
- Convertible loft: insulation under the roof slopes with semi-rigid panels and a vapour barrier
- The height under the beams and the state of the roof structure determine the choice of technique
- Easy access (hatch, ladder) is necessary to lay floor insulation
- A finished walkway floor allows storing belongings above the insulation
The distinction between the two configurations is essential: insulating unused loft space consists of laying insulation on the floor, whereas insulating convertible loft space requires working between the roof rafters. The first job is simple and quick, the second requires more care for the ventilation and the vapour barrier.
Choosing the right material
- Glass wool in rolls: the most common price-performance ratio
- Rock wool: fire-resistant, better shape retention
- Wood fibre or cellulose loose-fill: ecological, ideal for blowing between the joists
- Semi-rigid panels: easy to cut, perfect for convertible roof slopes
- Cellulose wadding loose-fill: good acoustic and summer thermal insulation
- Polystyrene panels: resistant to moisture but unsuited to irregular surfaces
What thickness and thermal resistance?
In France, the regulations recommend a thermal resistance (R) of at least 7 m².K/W for roof insulation, i.e. about 30 cm of mineral wool, more in cold regions. For unused loft space, the ideal is to lay a first layer between the joists (about 20 cm) then a second perpendicular layer over the joists (10 cm), which removes the thermal bridges created by the joists themselves. A lesser thickness reduces the energy gain; a greater thickness provides extra summer comfort.
Laying unused loft insulation
- Check the state of the roof structure and the absence of leaks before any installation
- Install a vapour barrier on the interior side if the old insulation does not have one
- Unroll the first layer of wool between the joists, leaving them visible
- Lay the second layer perpendicular over the joists, with staggered joints
- Cover the inaccessible areas with loose-fill if the access is too narrow
- Also insulate the bottom of the hatch with a glued rigid panel
- Mark out the cable runs and recessed spotlights before covering
Insulating convertible loft space
- Lay a vapour barrier on the warm side before the insulating panels
- Insert the semi-rigid panels between the rafters, without compressing them
- Fill the gaps with offcuts cut with a knife
- Make sure there is a ventilated air gap between the insulation and the roof covering
- Repeat the operation on each roof slope before fitting the cladding
- Check the continuity of the vapour barrier at the corners and joints
The ventilation under the roof covering is the most often forgotten point: an air gap of 2 to 3 cm between the insulation and the roof allows the water vapour to be evacuated and protects the roof structure. Without this ventilation, the humidity condenses in the insulation, which loses its performance and can make the timbers rot. The vapour barrier, fitted on the interior side, itself limits the amount of vapour that reaches the insulation.
Budget and renovation grants
The cost of materials for insulating unused loft space remains modest: allow about 3 to 6 euros per square metre for glass wool in rolls, and a little more for cellulose wadding or wood fibre. Loft insulation is among the works eligible for energy renovation grants in France, provided you call in a certified professional. Find out before you start: these grants can cover a significant part of the job's cost.
The mistakes to avoid
- Compressing the wool: a crushed wool loses almost all its performance
- Forgetting the vapour barrier: the humidity condenses in the insulation and rots the roof structure
- Blocking the ventilation space under the roof covering, essential for evacuating the vapour
- Insulating without checking the watertightness of the roof: the first rain ruins the job
- Covering unprotected cables or spotlights: risk of overheating and fire
Insulating your loft is a job within reach of a motivated DIYer, which quickly pays for itself on the heating bill. With 30 cm of well-laid wool, preserved ventilation and a correctly placed vapour barrier, your home gains comfort in winter and in summer, and its energy value improves on a lasting basis, a non-negligible asset for resale.