Energy efficient walls are a key component to the envelope of a high performing building. Other parts of the envelope include the roof, windows and doors, and foundation. An energy efficient envelope is a thermal barrier that stops the flow of heat by air, vapor and radiation. The transfer of heat through a wall occurs by three primary mechanisms: thermal conduction, thermal convection, and thermal radiation. An energy efficient wall should be constructed to prevent the flow of heat by all three of these mechanisms.
Thermal Conduction - Transfer of Heat on the Molecular Level
Thermal conduction occurs when hot molecules spontaneously move towards and collide with cooler molecules. The collision causes the energy of the hot molecule to transfer to the cooler molecule. Thermal conduction occurs in a wall when there is warm air on one side of a wall and cooler air on the other side of the wall. The warm air always moves towards the cooler air. The more tightly packed (densely) the molecules are in a substance, the better the substance conducts heat and the faster the hot air will move towards the cooler air. The molecules in metals are tightly packed. Therefore, metal is a good conductor, and heat will quickly move through it. Wood is less dense than metal, so it is a less efficient conductor of heat. So, wood buildings lose less heat and stay warmer than metal buildings. Buildings intended for occupancy typically utilize low-density insulation in their design, which slows conduction through the envelope.The effective R-value1 of a building’s wall assembly is its resistance to conduction. Problems in a wall’s assembly, such as thermal bridging, increase the wall’s conductivity. An energy efficient wall assembly should have low conductivity and a high effective R-value.
Thermal Convection - Transfer of Heat by the Flow of Air
Thermal convection occurs when heat moves from the warmer side to the cooler side of a wall by the flow of liquid or air molecules. When occupants of a building feel a draft, they are experiencing thermal convection.Thermal convection is often the biggest source of energy loss in a building. In wood-frame buildings convection occurs through the framing connections, wall, floor, roof, and fenestration intersections. A tightly sealed building envelope with adequate air and moisture barriers can stop thermal convection and create a comfortable environment for the occupants.
Thermal Radiation - Transfer of Heat by Solar Energy
Thermal radiation transfers heat from a warm area to a cooler area by electromagnetic waves, which for buildings is mainly the sun’s rays. Thermal radiation between two surfaces does not require a medium, like air and water, to move through space. The two surfaces need only be in view of each other. All materials absorb, reflect and emit radiant energy. A material’s ability to radiate absorbed energy, or emit energy as thermal radiation, is called emissivity. A high emissivity material has low reflectivity and will quickly absorb energy that strikes it. The design of an energy efficient wall should incorporate materials with high reflectivity and low emissivity to limit the absorption of radiant heat.
Bautex™ Walls Stop Thermal Conduction, Convection, and Radiation
The Bautex Wall System delivers a high-level of continuous insulation and creates a tight energy efficient envelope. Continuous insulation2 in the construction of a wall’s assembly stops the flow of heat by conduction, convection, and radiation. Continuous insulation also prevents thermal bridging, which is a common issue with conventional construction like wood and metal framing. Conduction tests were done on Bautex Blocks in accordance with ASTM C 518 – 10. The Blocks have an R-value per inch: 1.84 and a whole wall R-value (with concrete) of R-14. Thus, Bautex Blocks stop thermal conduction. Also, The Bautex Air and Moisture Barrier reduces thermal convection and thermal radiation by preventing air, moisture, and ultraviolet waves from passing through the wall. The Bautex Wall Assembly is energy efficient and meets the thermal performance required of the IRC and IBC. Bautex Blocks are also easy to install, fire rated, noise reducing and storm-resistant. Bautex Blocks can be utilized in both commercial and residential buildings and are the ideal choice for energy efficient wall construction.
The building and construction industry use the R-value as a measure of thermal resistance or conductivity. The effective R-value includes all the materials used in its construction: the drywall, studs, insulation, plywood, siding, etc. The higher the R-value, the better the thermal performance the material or system.
Continuous insulation covers all parts of building other than the windows, skylights, doors, and building service systems. Continuous insulation helps create an insulated, airtight, and energy efficient building. The ASHRAE 90.1 and 2015 IECC require continuous insulation.