What is thermal insulation?
Thermal insulation means limiting the flow of heat through a construction, such as a floor, wall or roof, using materials that are poor conductors of heat. Good insulation keeps heat in during winter and out during summer. It saves energy, improves comfort and reduces the risk of condensation and mould on the inside.
Key concepts
- Lambda (λ): the thermal conductivity of a material in W/(m·K). The lower the value, the better it insulates.
- R-value: the thermal resistance of a layer or a whole construction in m²·K/W, depending on λ and thickness. Higher is better. In the Netherlands, the terms Rd (for a product) and Rc (for the complete construction) are used.
- U-value: the common English measure of how much heat passes through a construction, in W/(m²·K). Lower is better.
Minimum insulation requirements for new buildings are set by national building regulations and differ per country.
Common materials
- Mineral wool (glass wool, stone wool): vapour-open, non-combustible, good sound absorption.
- Foam plastics (EPS, XPS, PIR, phenolic): thin for the same insulating value, available in pressure-resistant grades.
- Bio-based (wood fibre, flax, hemp, cellulose): vapour-open and with more heat storage capacity.
Where are the gains?
A rule of thumb for an existing home: start with the roof (heat rises), then the cavity wall and the floor, and at the same time deal with glazing and airtightness. Insulating without sealing gaps delivers less than you expect.
Common mistakes
- Thermal bridges: interruptions in the insulation (at lintels, floor edges, window frame junctions) cause heat loss and cold spots with a risk of mould.
- Wrong vapour control: a vapour control layer belongs on the warm side; the cold side must be able to release moisture.
- Gaps and open joints: boards must fit tightly; open seams let air and heat through.
- Forgetting ventilation: a better insulated, more airtight home needs good ventilation.
Related terms
- Rc value
- Insulation value (Rd-value)
- Thermal bridge
- Vapour control layer
- Airtightness
- Mineral wool
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.