In the field of construction, thermal time delay refers to the time it takes for a thermal wave to travel through a wall or material. In other words, it refers to the time lag between the peak of external heat and the moment that heat reaches the interior of the building.
Understanding thermal lag and how to calculate it is crucial for selecting the right building materials and improving comfort in both winter and summer.
Definition of thermal phase shift:
Phase shift is a measure of time that indicates how long 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 phase shift depends on several factors, including the thickness and thermal conductivity of the materials that make up the wall. It can be calculated using the following formula:
Phase shift (in hours) = Total wall thickness (in meters) / (2 × √(thermal conductivity of the main material × density of the main material × specific heat of the main material))
This calculation provides an estimate of the thermal phase shift for a given material or wall.
Examples of thermal phase shift in insulation materials:
| Example of a wall | Phase Shift |
|---|---|
Insulated lost attic floor made of cellulose insulation
|
11 a.m. |
Roof rafters insulated from the inside with recycled cotton
|
7.6 hours |
Wood-frame wall with rigid wood-fiber exterior wall insulation and blown-in cellulose insulation e interior insulation
|
11.8 hours |
Interior lining with insulation made of hemp, grass, or a blend (flax, jute, hemp, etc.)
|
11.8 hours |
For details on sample configurations and their indicative data, see: Guide to Bio-based Materials and Summer Comfort (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.
The Role of Thermal Phase Shift in Winter
In winter, thanks to the high phase shift, heat takes longer to escape from the building, which leads to energy savings: the heating systems don't have to work as hard to maintain a stable temperature.
The Role of Thermal Phase Shift in the Summer
Conversely, in the summer, it takes longer for the heat to enter the building. This means the interior stays cool longer, and the air conditioning is used less.
During heat waves and periods of intense heat, a high phase shift allows for the reversal of heat flow through nighttime ventilation, when outdoor temperatures are cooler.
Thermal phase shift is a key factor to consider when designing or renovating a building. It directly affects the thermal comfort of occupants, particularly on hot summer days. A high thermal phase shift helps reduce indoor temperature fluctuations, thereby providing a more comfortable 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 thermal diffusivity of materials. Thermal diffusivity represents the rate at which heat spreads through a material. The lower the thermal diffusivity, the greater the phase shift will be. Thus, by choosing materials with low thermal diffusivity, you maximize the potential for thermal phase shift, which results in improved thermal comfort and a reduction in the impact of summer heat waves.
