The insulation technique used (indoor, outdoor), the type of wall (walls, roof and attic, floors), the constraints of the work and the budget will determine the characteristics of the insulator to be chosen.
These construction choices will determine the processing of the insulator for each zone (bulk, rolls, plates, panels, flakes or aggregates, construction material, etc.) in order to facilitate its implementation and maximise its effectiveness.
When insulating from the inside, you can combine different types of insulators. However, it is necessary to ensure that the combination of the insulation retained has always a resistance to the diffusion of water vapour that decreases from the inside to the outside of the wall. Water vapour must never be blocked in the building or insulation itself.
λ (W/m.K – Watt per meter Kelvin): Thermal conductivity of a material
- Low thermal conductivity materials are the best insulators. This value should be as low as possible. The best insulators have a coefficient λ = 0.007 W/m.K (while conventional insulators such as glass wool laine or expanded polystyrene λ = 0.04 W/m.K).
- The heat flow transmitted through the material depends on its thermal conductivity, thickness, surface and the difference in temperature between indoor and outdoor.
R (m².K/W): Thermal resistance of the insulators (applies to a single material)
- This corresponds to the “insulating ability” of a material of a given thickness.
- R = e/λ = thickness / thermal conductivity. An expanded polystyrene panel (λ = 0.032 W/m.K) of 10 cm will have an insulation ability of R = 0.1/0.032= 3.12 m².K/W.
- That’s the figure indicated on the packaging of the insulators. Higher the R is, the more efficient the wall is in terms of insulation. >> See the table of minimal regulatory values (later in this article).
U (W/m².K): Surface thermal transmission coefficient (applies to one multi-layer wall)
- It’s the amount of heat passing through one m2 of a heterogeneous (multi-layer) wall with a difference of 1°C between indoor and outdoor.
- Coefficient which defines the overall performance of the wall. This coefficient is the opposite of the thermal resistance (U=1/Rtotal_wall). Lower the U is, the more insulating the wall is.
- This coefficient is essential as it allows the calculation software to determine the actual losses (Watt) of a room. The overall thermal resistance of the wall (heterogeneous) is calculated, that is the sum of the resistances of each material composing the wall (ex.: drywall + glass wool + concrete + surface coating). For example Rtotal =0.24 m².K/W. For a difference of 15°C between the 2 sides of one wall (inside-outside) of 20m² we can calculate the loss. Loss (in W) = U (in W/m².K) x S (m²) x Δ (K) = (1/0.24) x 20 x 15 = 1250W. Therefore, a heater (or a heating system) capable of delivering that power is needed in the case of an outside temperature of 5°C and a desired room temperature of 20°C.
D (x 10-7 m²/s): Thermal diffusivity of an insulating material
- Characterises the capacity of a material to transfer more or less quickly the heat (from one wall to another). It reflects the inertia of the wall transmission (heat diffusion rate inside the material). The purpose is to slow the penetration of the external heat flow during the summer, and to limit the heat loss during the winter. The wood fibre has the lowest diffusivity, followed by the wood wool, OSB, aerated concrete. The higher the compression of the material (ex: rock wool 40kg/m³) the more the diffusivity is reduced.
E (J/m².s.°C): Specific thermal effusivity of an insulating material
- Characterises the rate at which the surface temperature of the material changes, i.e. the material’s capacity to exchange the energy more or less quickly with its environment. It plays the role of an internal atmosphere regulator.
- In the summer, in the event of large variations of heat, the concrete is the material (high effusivity) that best limits the heat input. It plays the role of a “thermal sponge” by limiting the heat input by day and by releasing the heat by night.
- In winter, the glass, hemp or sheep wools have a low effusivity, reflecting the fact that they are rapidly warmed at the surface (which helps to increase the temperature of the walls and thus to improve the apparent air temperature).
The control of thermal inertia reflects a combination of previous physical properties.
The technical properties – non thermal – to be taken into account are:
- general water behaviour (degree of water absorption, dimension variation, performance degradation, behaviour in case of over-moisture, …),
- water vapour permeability: water vapour diffusion resistance coefficient (μ), or resistance factor of one material layer Sd = μ x d (m),
- air flow resistance (R=Pa.s/m2),
- mass density ρ (in kg/m3) to determine the inertia,
- volume heat capacity S (in kJ/m³.°C),
- stability and maintenance of performance over time,
- acoustic efficiency,
- burning behaviour,
- price and complexity of implementation.
Environmental properties of the insulators (environmental impacts):
- production energy = grey energy, transport, carbon balance,
- ability to be recycled,
- ability to be renewed,
- environmental and human toxicity, gas emissions, etc.
Regardless of the thermal, acoustic and technical performance, materials with the best eco-balance possible (natural, consuming low energy in the process of their manufacture-transport, renewable) should be used.