What is thermal phase shift?
Thermal phase shift is the time required for heat to travel through a building layer — for example a roof or wall. A large phase shift means that midday solar heat only reaches the interior in the evening, keeping the indoor temperature more stable.
Why is phase shift important?
In winter, insulation protects against heat loss — the U-value is decisive. In summer, another property matters: how the building deals with the diurnal temperature swing. A building with good phase shift:
- Does not heat up quickly at noon — heat “waits” in the structure
- Releases heat at night — when it cools outside
- Needs less cooling — energy-efficient comfort
- Has a more stable indoor climate — fewer temperature spikes
Application
Phase shift becomes especially important for:
| Building element | Recommended phase shift |
|---|---|
| Pitched roof | at least 8 hours |
| Flat roof | at least 10 hours |
| South-facing wall | at least 6 hours |
| Bedroom wall | at least 8 hours |
Material comparison
Phase shift depends on the specific heat capacity and density of the insulation:
| Material | Density kg/m³ | Phase shift at 20 cm |
|---|---|---|
| EPS | 15–30 | 4–5 h |
| PIR / PUR | 30–40 | 4–5 h |
| Glass wool | 16–40 | 5–6 h |
| Stone wool | 60–150 | 7–8 h |
| Cellulose | 35–60 | 9–11 h |
| Wood fibre | 50–250 | 10–14 h |
| Straw | 90–110 | 11–13 h |
Combination with U-value
U-value (winter) and phase shift (summer) are both important. Natural and dense insulations typically give a high phase shift; lightweight plastic insulations offer good U-values at thin thicknesses but limited summer protection.
Optimal design
For good summer comfort, alongside insulation, consider:
- Internal mass — concrete or clay walls store heat
- External shading — prevents direct solar gain
- Night ventilation — cools the mass at night
- Reflective roof finish — less absorption
Example: heavy roof pack
A pitched roof for summer comfort:
- Tiles — top layer
- Breather membrane — protective
- Counter-battens and battens — air gap
- Wood-fibre boards 24 cm — high phase shift
- Vapour control layer — airtight
- Plasterboard — interior finish
Calculation
The engineering formula:
t ≈ d × √(ρ × c / 2λπω)
where:
– d = thickness (m)
– ρ = density (kg/m³)
– c = specific heat capacity (J/kg·K)
– λ = thermal conductivity (W/m·K)
– ω = angular frequency of the day
In practice, tables and software tools are used.
Related terms
- U-value
- Insulation
- Wood-fibre board
- Cellulose
- Climate-adaptive
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.