To reduce energy losses occurring in the attic , it is important to insulate them sufficiently. Depending on the type of attic (lost, converted or habitable), the recommended methods and insulation differ. For lost attic space, the choice between blowing insulation or laying insulation on the floor depends on the accessibility of the premises. In the case of convertible attics, the desire to keep the maximum living space and the available budget make it possible to opt for interior or exterior insulation.
The main characteristics of insulation for attics
These insulators are classified into 4 categories:
- natural: vegetable wool (flax, cotton, hemp, etc.), animal wool (sheep), duck feathers, cork, wood fibers, etc.
- synthetic: polystyrene, polyurethane …
- minerals: glass wool (made from silica), rock wool (based on basalt), perlite (siliceous volcanic rock)
- thin: multiple layers of aluminum and intermediate material (wadding, felt, etc.)
| Insulators | Soundproofing | Thermal insulation | Durability | Thermal conductivity (λ) | Types of attic |
| Vegetable wool | Low | Very good | Good | 0.035 to 0.042 | All |
| Animal wool or feathers | Low | Very good | Good | 0.033 to 0.042 | All |
| Cork | Good | Very good | Excellent | 0.034 to 0.049 | All |
| Wood fibers | Good | Good | Excellent | 0.037 to 0.049 | All |
| Perlite | Good | Good | Excellent | 0.050 to 0.060 | Lost |
| Mineral wools (rock or glass) | Very good | Very good | Average | 0.030 to 0.040 | All |
| Polystyrene and polyurethane | Average | Very good | Average | 0.022 to 0.035 | Habitable |
| Thin insulation | Low | Low | Average | 0.1 to 1 | All, combined with other insulation |
Thermal conductivity (λ, in W / mK) represents the insulating power of a material. The lower it is, the better the insulating capacities.
The strengths and weaknesses of thermal insulation methods
Whatever technique you choose (interior or exterior), the main benefit of attic insulation is to improve the thermal performance of the entire home, reducing energy losses and expenditure.
The thermal insulation inside (ITI), cheaper than the thermal insulation from the outside (ITE), is easier to implement. However, especially in the case of habitable attics, it reduces the living area. It involves emptying the attic during the work and redoing the existing decoration.
The ITE, particularly effective, makes it possible to maintain the height under the ceiling as the visible beams. She does not ask to empty the attic when they are fitted out. More complex and costly than ITI, it requires the house to remain without cover during the works.
To calculate the thickness of insulation to be used, we apply the formula e = λ * R with:
- e: thickness, in meters
- λ: thermal conductivity provided by the manufacturer (in W / mK)
- R: desired thermal resistance (in m2.K / W)
To qualify for certain government aid , thermal resistance must be above threshold values.
The price of insulation and the project budget to insulate its attic
Depending on the type of insulation, we see the following prices (in € / m2):
- vegetable wools: 12 to 25
- animal wool: 15 to 20
- cork: 10 to 30
- wood fiber: 15 to 20
- perlite: 20 to 45
- mineral wools: 3 to 5
- polystyrene, polyurethane: 10 to 20
- thin insulators: 5 to 10
The total budget for roof insulation depends on the surface to be insulated, the technique chosen and the company carrying out the work. The ITI is more economical (20 to 70 € / m2) than the ITE (90 to 130 € / m2).
Several financial aids promote the insulation of the attic:
- Energy Transition Tax Credit
- ANAH programs
- energy check
- energy bonus
- 1 € lost attic insulation device
- Premium Boost Insulation
Insulation techniques according to the type of attic
In the case of habitable or converted attics, the techniques are:
- from the outside: after removing the roof, interlocking insulating panels are placed on the framework or experienced self-supporting caissons are installed. In both cases, the internal face of the facings of the panels or boxes acts as a false ceiling for the attic. The third method, known as sarking, consists of placing successive layers from the rafters (battens, vapor barriers, insulating material, under-roof screen, counter battens and battens).
- from the inside: the panels or rolls of insulation are placed under the roof, possibly covered by a vapor barrier and then a facing.
In the case of lost attics, we can insulate:
- by blowing (recommended in the case of inaccessible attics): loose insulation (rock wool, glass wool, etc.) is sprayed into the attic.
- by laying rolls or insulating panels on the floor: after installing a vapor barrier, one or even two layers of insulation are available.
Some of the brands known for their insulation products include: Isover, Rockwool, 3M, Actis, PureOne, URSA, Knauf and ITR.
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