Welcome to the first post of RomeroSostenible. It was created with the aim of gradually explaining all the aspects that influence the energy consumption of buildings. Energy efficiency is not an abstract concept: it directly depends on how the buildings we live and work in are designed, built, and used.
I want to begin this journey with a fundamental topic that will serve as a common thread for the following posts: heat transfer. Understanding how heat moves through materials and air is the starting point for correctly evaluating construction solutions and, later on, questioning some of the false promises that often surround certain products presented as “miraculous” in the world of insulation.
In buildings, thermal insulation is installed to reduce the movement of heat (heat flow) through the envelope, both in winter and summer. A common mistake is to think that in winter insulation “prevents cold from coming in” or that in summer it “prevents heat from entering.” What it really does is slow down the movement of heat from the warmer side to the cooler side, regardless of the season.
The Three Mechanisms of Heat Transfer
Heat always moves from a warmer area to a cooler one, and it does so through three main mechanisms: conduction, convection, and radiation.
Conduction: this is the easiest mechanism to visualize. It occurs when heat is transmitted directly through a solid material. If you place a metal spoon in a cup of hot coffee, you’ll soon notice that the other end gets warm. The same happens in buildings with materials such as concrete, brick, or steel: they are conductors that allow heat to “travel” from one side to the other. Thermal insulation works precisely because it provides high resistance to the flow of heat by conduction.
Convection: in this case, heat is transferred through the movement of a fluid, such as air or water. This phenomenon occurs near the building envelope, both on the outside and inside surfaces. That is why, in a construction element, two phenomena are combined: conduction through the solid materials that make it up and convection in the thin layers of air that are in contact with its exterior and interior surfaces.
Radiation: this is the transfer of heat in the form of electromagnetic waves. It does not require a medium to propagate—that’s why we receive heat from the Sun despite it being 150 million kilometers away. All bodies emit heat by radiation, including humans. That’s why, in emergency situations, a person is covered with a reflective sheet (silver or gold) to prevent heat loss through radiation—the typical scene where an accident victim is wrapped in a thermal blanket. In buildings, radiation is relevant in exposed roofs and façades, as well as in the heat exchange between interior surfaces at different temperatures.
The Thermal Resistance of Building Elements
To evaluate how a building envelope resists heat transfer, its total thermal resistance (R) is calculated. This is obtained by summing the individual thermal resistances of each material layer, which are determined by dividing the layer’s thickness by its thermal conductivity (λ), and then adding the values of the interior and exterior film coefficients, or surface thermal resistances. It is important to clarify here: the resistance of each layer in the envelope refers to heat flow by conduction, while the interior and exterior surface resistances represent the thin air layers next to the envelope, that is, convection.
It is important to note that the thermal conductivity values of insulating materials must be referenced to a mean temperature of 10 °C. If not, the UNE-EN ISO 10456 standard establishes a correction method that adjusts the λ value according to temperature. This is something that is often overlooked and can lead to misleading comparisons between products. The standard also applies other corrections, such as those related to the presence of moisture, in order to obtain the design thermal conductivity value.
The thermal conductivity values of materials other than thermal insulators must be obtained from official handbooks, such as the Building Elements Catalogue of the Spanish Technical Building Code (CTE).
On the other hand, the values of surface thermal resistances can be obtained from the supporting document DA DB-HE/1 of the Spanish Technical Building Code (CTE).
Practical example of thermal resistance calculation
To keep this from being just theory, let’s look at a simple example:
Double-leaf ceramic façade wall with an air cavity and insulation:
- Interior plaster coating (1,5 cm) – λ = 0,57 W/m·K
- Inner double-hollow brick wall (11 cm) – λ = 0,49 W/m·K
- Mineral wool thermal insulation (6 cm) – λ = 0,037 W/m·K
- Outer half-brick wall (11 cm) – λ = 0,81 W/m·K
- Exterior mortar render (1,5 cm) – λ = 1,00 W/m·K
Convection – Surface resistances according to DA DB-HE/1:
- Insite: Rsi = 0,13 m²K/W
- Outside: Rse = 0,04 m²K/W
Conduction – Calculation of the thermal resistances of each layer (R = e/λ):
- Plaster coating: 0,015 / 0,57 = 0,026 m²K/W
- Inner double-hollow brick wall: 0,11 / 0,49 = 0,224 m²K/W
- Mineral wool: 0,06 / 0,037 = 1,622 m²K/W
- Outer half-brick wall: 0,11 / 0,81 = 0,136 m²K/W
- Mortar render: 0,015 / 1,00 = 0,015 m²K/W
R thermal total = Rsi + ΣRcapas + Rse = 0,13 + (0,026 + 0,224 + 1,622 + 0,136 + 0,015) + 0,04 = 2,19 m²K/W
This result tells us that the wall offers a total thermal resistance of 2.19 m²·K/W to heat flow, a value that can be converted into thermal transmittance (U) using the formula U = 1/R, yielding U ≈ 0.46 W/m²·K.
Remember: higher thermal resistance values mean greater reduction of heat flow and, therefore, lower heating energy demand. The effect on cooling energy demand, however, may involve an increase depending on the climate zone and the building’s internal loads (related to building use—a residential building is not the same as an office building).
- High thermal transmittance values (U-value) – lower thermal resistance.
- Low thermal transmittance values (U-value) – higher thermal resistance.
The previous example is a simple case of a vertical wall in contact with the outdoor environment. For other types of situations (walls in contact with the ground, with uninhabited spaces, etc.), the calculation is different, and we will explain it in future posts on this blog. We will also cover the effect of air cavities in building envelopes.