Thermal phase shift is the time it takes heat to pass through a wall from the outside to the inside. In hemp-lime systems, this parameter is one of the main contributors to summer comfort because it delays and attenuates the inward transfer of heat generated by solar radiation during the day.
What is thermal phase shift?
While thermal transmittance describes heat flow under steady-state conditions, thermal phase shift measures the time delay with which the heat wave passes through a wall under real daily cyclic conditions, in accordance with dynamic calculation criteria also referenced in UNI EN ISO 13786 and in technical documentation published by ENEA on building energy efficiency. A wall with a long phase shift causes the external heat peak recorded during the hottest afternoon hours to reach the inside of the building many hours later, when outdoor temperatures have already fallen. This behaviour depends on the material’s mass, density and specific heat capacity, as well as on wall thickness.
Thermal phase-shift data for Biomattone® blocks
The Biomattone® block technical data sheet reports the thermal phase shift measured without plaster for each size in the range. As block thickness increases, the phase shift therefore increases significantly:
| Packaging | Thickness | Thermal phase shift | U-value (W/m²K) |
|---|---|---|---|
| BA20 | 20 cm | 11h 19′ | 0.21 |
| BA30 | 30 cm | 18h 14′ | 0.14 |
| BA40 | 40 cm | 25h 08′ | 0.11 |
| BA50 | 50 cm | 32h 04′ | 0.09 |
Biomattone® blocks also have a density of 310 kg/m³, thermal conductivity λ of 0.044 W/mK and specific heat capacity of 1,280 J/kgK. Combined with the selected thickness, these values determine the phase shift shown in the table.
Why thermal phase shift improves summer comfort
With a phase shift of more than 25 hours, as in the BA40 and BA50 sizes, heat absorbed by the façade during the day reaches the interior when it is already night-time and the outdoor temperature has fallen. This effect therefore reduces the heat peaks perceived indoors during the most critical daytime hours and limits the need for active cooling. A building system with a good thermal phase shift does not, however, remove the need for careful design of shading, ventilation and building orientation.
Phase shift and U-value: two parameters to consider together
Two walls with the same U-value can nevertheless behave very differently in summer if they have different mass and thermal phase shift. For this reason, the most suitable Biomattone® size should be selected together with the TecnoCanapa technical department, taking into account the local climate, façade exposure and the project’s overall performance requirements rather than winter U-value alone.
Frequently asked questions about thermal phase shift
What is the thermal phase shift of Biomattone® BA30?
According to the technical data sheet, BA30 (30 cm thick) has a thermal phase shift of 18 hours and 14 minutes without plaster.
Is a longer phase shift always better?
A long phase shift is beneficial for summer comfort, but the choice of thickness also depends on climate, orientation and the project’s structural requirements.
Does thermal phase shift replace winter thermal insulation?
No. Phase shift and U-value are complementary parameters: the first affects summer comfort, while the second affects winter heat loss.
For a complete overview of indoor-comfort parameters, see the guide to hygrothermal comfort in hemp-lime sustainable construction. To learn more about the available sizes, visit the Biomattone® page or request the complete technical data sheets.
