In the field of construction, thermal phase shift refers to the time it takes for a thermal wave to pass through a wall or material. In other words, it refers to the time lag between the peak of outdoor heat and the moment that heat reaches the interior of the building. Understanding phase shift and how it is calculated is crucial for choosing the right building materials and improving comfort in both winter and summer.
Definition of thermal lag:
Phase shift is a temporal measurement that indicates the time it takes for heat to pass through a wall. The denser and thicker a wall is, and the higher the thermal capacity of the materials it is made of, the longer the phase shift will be. It is expressed in hours.
Calculate the thermal phase shift:
Thermal lag depends on several factors, including the thickness and thermal conductivity of the materials making up the wall. It can be calculated using the following formula:
Phase shift (in hours) = Total wall thickness (in meters) / (2 x √(thermal conductivity of the main material x density of the main material x specific heat of the main material))
This calculation provides an estimate of the thermal phase shift for a given wall. It is important to note that this varies depending on the characteristics of the materials used.
Thermal phase shift of materials:
Here are some examples of thermal phase shift for different materials and construction systems:
An insulated attic with cellulose insulation has a thermal phase shift of 11 hours.
An insulated roof with recycled cotton has a thermal lag of 7.6 hours.
A wood-frame wall with rigid wood fiber exterior insulation and cellulose insulation has a thermal phase shift of 11.8 hours.
An interior lining with insulation made of hemp, grass, or a mixture (flax, jute, hemp, etc.) has a thermal phase shift of 11.8 hours.
To view details of configuration examples and their indicative data: Association of Bio-based Construction Manufacturers.
Interpreting phase shift data:
The higher the phase shift of a material or wall, the longer it takes for heat to pass through it. In winter, it takes longer for heat to escape from the building, which saves energy (as heating systems are used less to maintain a stable temperature). Conversely, in summer, it takes longer for heat to enter the building. This means that the interior stays cool for longer. During heat waves and periods of high temperatures, a high phase shift allows heat flows to be reversed through nighttime ventilation, when outside temperatures are cooler.
Thermal phase shift is an essential criterion to consider when designing and renovating a building. It directly affects the thermal comfort of occupants, especially during hot summer days. A high thermal phase shift reduces temperature fluctuations indoors, providing a more pleasant environment.
By choosing building materials with good thermal phase shift, such as wood-frame walls with bio-based insulation or attics with cellulose insulation , you can minimize the heat entering the building during the summer months. This reduces the need for energy-intensive air conditioning systems, contributing to energy savings and a smaller environmental footprint.
It is important to note that phase shift is closely related to the thermal diffusivity of materials. Diffusivity represents the speed at which heat propagates through a material. The lower the diffusivity, the higher the phase shift will be. Therefore, by choosing materials with low thermal diffusivity, you maximize the potential for thermal phase shift, which translates into improved thermal comfort and a reduction in the effect of summer heat peaks.
