A thermal bridge, also called a cold bridge, heat bridge, or thermal bypass, is an area or component of an object which has higher thermal conductivity than the surrounding materials, [1] creating a path of least resistance for heat transfer. [2] Thermal bridges result in an overall reduction in thermal resistance of the object. The term is frequently discussed in the context of a building's thermal envelope where thermal bridges result in heat transfer into or out of conditioned space.
Thermal bridges in buildings may impact the amount of energy required to heat and cool a space, cause condensation (moisture) within the building envelope, [3] and result in thermal discomfort. In colder climates (such as the United Kingdom), thermal heat bridges can result in additional heat losses and require additional energy to mitigate.
There are strategies to reduce or prevent thermal bridging, such as limiting the number of building members that span from unconditioned to conditioned space and applying continuous insulation materials to create thermal breaks.
Heat transfer occurs through three mechanisms: convection, radiation, and conduction. [4] A thermal bridge is an example of heat transfer through conduction. The rate of heat transfer depends on the thermal conductivity of the material and the temperature difference experienced on either side of the thermal bridge. When a temperature difference is present, heat flow will follow the path of least resistance through the material with the highest thermal conductivity and lowest thermal resistance; this path is a thermal bridge. [5] Thermal bridging describes a situation in a building where there is a direct connection between the outside and inside through one or more elements that possess a higher thermal conductivity than the rest of the envelope of the building.
Surveying buildings for thermal bridges is performed using passive infrared thermography (IRT) according to the International Organization for Standardization (ISO). Infrared Thermography of buildings can allow thermal signatures that indicate heat leaks. IRT detects thermal abnormalities that are linked to the movement of fluids through building elements, highlighting the variations in the thermal properties of the materials that correspondingly cause a major change in temperature. The drop shadow effect, a situation in which the surrounding environment casts a shadow on the facade of the building, can lead to potential accuracy issues of measurements through inconsistent facade sun exposure. An alternative analysis method, Iterative Filtering (IF), can be used to solve this problem.
In all thermographic building inspections, the thermal image interpretation if performed by a human operator, involving a high level of subjectivity and expertise of the operator. Automated analysis approaches, such as Laser scanning technologies can provide thermal imaging on 3 dimensional CAD model surfaces and metric information to thermographic analyses. [6] Surface temperature data in 3D models can identify and measure thermal irregularities of thermal bridges and insulation leaks. Thermal imaging can also be acquired through the use of unmanned aerial vehicles (UAV), fusing thermal data from multiple cameras and platforms. The UAV uses an infrared camera to generate a thermal field image of recorded temperature values, where every pixel represents radiative energy emitted by the surface of the building. [7]
Frequently, thermal bridging is used in reference to a building’s thermal envelope, which is a layer of the building enclosure system that resists heat flow between the interior conditioned environment and the exterior unconditioned environment. Heat will transfer through a building’s thermal envelope at different rates depending on the materials present throughout the envelope. Heat transfer will be greater at thermal bridge locations than where insulation exists because there is less thermal resistance. [8] In the winter, when exterior temperature is typically lower than interior temperature, heat flows outward and will flow at greater rates through thermal bridges. At a thermal bridge location, the surface temperature on the inside of the building envelope will be lower than the surrounding area. In the summer, when the exterior temperature is typically higher than the interior temperature, heat flows inward, and at greater rates through thermal bridges. [9] This causes winter heat losses and summer heat gains for conditioned spaces in buildings. [10]
Despite insulation requirements specified by various national regulations, thermal bridging in a building's envelope remain a weak spot in the construction industry. Moreover, in many countries building design practices implement partial insulation measurements foreseen by regulations. [11] As a result, thermal losses are greater in practice that is anticipated during the design stage.
An assembly such as an exterior wall or insulated ceiling is generally classified by a U-factor, in W/m2·K, that reflects the overall rate of heat transfer per unit area for all the materials within an assembly, not just the insulation layer. Heat transfer via thermal bridges reduces the overall thermal resistance of an assembly, resulting in an increased U-factor. [12]
Thermal bridges can occur at several locations within a building envelope; most commonly, they occur at junctions between two or more building elements. Common locations include:
Structural elements remain a weak point in construction, commonly leading to thermal bridges that result in high heat loss and low surface temperatures in a room.
While thermal bridges exist in various types of building enclosures, masonry walls experience significantly increased U-factors caused by thermal bridges. Comparing thermal conductivities between different building materials allows for assessment of performance relative to other design options. Brick materials, which are usually used for facade enclosures, typically have higher thermal conductivities than timber, depending on the brick density and wood type. [15] Concrete, which may be used for floors and edge beams in masonry buildings are common thermal bridges, especially at the corners. Depending on the physical makeup of the concrete, the thermal conductivity can be greater than that of brick materials. [15] In addition to heat transfer, if the indoor environment is not adequately vented, thermal bridging may cause the brick material to absorb rainwater and humidity into the wall, which can result in mold growth and deterioration of building envelope material.
Similar to masonry walls, curtain walls can experience significantly increased U-factors due to thermal bridging. Curtain wall frames are often constructed with highly conductive aluminum, which has a typical thermal conductivity above 200 W/m·K. In comparison, wood framing members are typically between 0.68 and 1.25 W/m·K. [15] The aluminum frame for most curtain wall constructions extends from the exterior of the building through to the interior, creating thermal bridges. [16]
Thermal bridging can result in increased energy required to heat or cool a conditioned space due to winter heat loss and summer heat gain. At interior locations near thermal bridges, occupants may experience thermal discomfort due to the difference in temperature. [17] Additionally, when the temperature difference between indoor and outdoor space is large and there is warm and humid air indoors, such as the conditions experienced in the winter, there is a risk of condensation in the building envelope due to the cooler temperature on the interior surface at thermal bridge locations. [17] Condensation can ultimately result in mold growth with consequent poor indoor air quality and insulation degradation, reducing the insulation performance and causing insulation to perform inconsistently throughout the thermal envelope [18]
There are several methods that have been proven to reduce or eliminate thermal bridging depending on the cause, location, and the construction type. The objective of these methods is to either create a thermal break where a building component would span from exterior to interior otherwise, or to reduce the number of building components spanning from exterior to interior. These strategies include:
Due to their significant impacts on heat transfer, correctly modeling the impacts of thermal bridges is important to estimate overall energy use. Thermal bridges are characterized by multi-dimensional heat transfer, and therefore they cannot be adequately approximated by steady-state one-dimensional (1D) models of calculation typically used to estimate the thermal performance of buildings in most building energy simulation tools. [21] Steady state heat transfer models are based on simple heat flow where heat is driven by a temperature difference that does not fluctuate over time so that heat flow is always in one direction. This type of 1D model can substantially underestimate heat transfer through the envelope when thermal bridges are present, resulting in lower predicted building energy use. [22]
The currently available solutions are to enable two-dimensional (2D) and three-dimensional (3D) heat transfer capabilities in modeling software or, more commonly, to use a method that translates multi-dimensional heat transfer into an equivalent 1D component to use in building simulation software. This latter method can be accomplished through the equivalent wall method in which a complex dynamic assembly, such as a wall with a thermal bridge, is represented by a 1D multi-layered assembly that has equivalent thermal characteristics. [23]
A Trombe wall is a massive equator-facing wall that is painted a dark color in order to absorb thermal energy from incident sunlight and covered with a glass on the outside with an insulating air-gap between the wall and the glaze. A Trombe wall is a passive solar building design strategy that adopts the concept of indirect-gain, where sunlight first strikes a solar energy collection surface in contact with a thermal mass of air. The sunlight absorbed by the mass is converted to thermal energy (heat) and then transferred into the living space.
In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.
Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials.
In building design, thermal mass is a property of the matter of a building that requires a flow of heat in order for it to change temperature. In scientific writing the term "heat capacity" is preferred. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structural elements can be designed to work alongside a construction's lighter thermal resistance components to create energy efficient buildings.
The R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat, in the context of construction. R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions. The measure is therefore equally relevant for lowering energy bills for heating in the winter, for cooling in the summer, and for general comfort.
Straw-bale construction is a building method that uses bales of straw as structural elements, building insulation, or both. This construction method is commonly used in natural building or "brown" construction projects. Research has shown that straw-bale construction is a sustainable method for building, from the standpoint of both materials and energy needed for heating and cooling.
Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR) is a ventilation system that recovers energy by operating between two air sources at different temperatures. It is used to reduce the heating and cooling demands of buildings.
Insulating concrete form or insulated concrete form (ICF) is a system of formwork for reinforced concrete usually made with a rigid thermal insulation that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs. The forms are interlocking modular units that are dry-stacked and filled with concrete. The units lock together somewhat like Lego bricks and create a form for the structural walls or floors of a building. ICF construction has become commonplace for both low rise commercial and high performance residential construction as more stringent energy efficiency and natural disaster resistant building codes are adopted.
A vacuum insulated panel (VIP) is a form of thermal insulation consisting of a gas-tight enclosure surrounding a rigid core, from which the air has been evacuated. It is used in building construction, refrigeration units, and insulated shipping containers to provide better insulation performance than conventional insulation materials.
Superinsulation is an approach to building design, construction, and retrofitting that dramatically reduces heat loss by using much higher insulation levels and airtightness than average. Superinsulation is one of the ancestors of the passive house approach.
A building envelope or building enclosure is the physical separator between the conditioned and unconditioned environment of a building, including the resistance to air, water, heat, light, and noise transfer.
Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and convection. Use of underfloor heating dates back to the Neoglacial and Neolithic periods.
Building insulation is material used in a building to reduce the flow of thermal energy. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation. Often an insulation material will be chosen for its ability to perform several of these functions at once.
Hempcrete or hemplime is biocomposite material, a mixture of hemp hurds (shives) and lime, sand, or pozzolans, which is used as a material for construction and insulation. It is marketed under names like Hempcrete, Canobiote, Canosmose, Isochanvre and IsoHemp. Hempcrete is easier to work with than traditional lime mixes and acts as an insulator and moisture regulator. It lacks the brittleness of concrete and consequently does not need expansion joints.
In heat transfer, thermal engineering, and thermodynamics, thermal conductance and thermal resistance are fundamental concepts that describe the ability of materials or systems to conduct heat and the opposition they offer to the heat current. The ability to manipulate these properties allows engineers to control temperature gradient, prevent thermal shock, and maximize the efficiency of thermal systems. Furthermore, these principles find applications in a multitude of fields, including materials science, mechanical engineering, electronics, and energy management. Knowledge of these principles is crucial in various scientific, engineering, and everyday applications, from designing efficient temperature control, thermal insulation, and thermal management in industrial processes to optimizing the performance of electronic devices.
Insulating glass (IG) consists of two or more glass window panes separated by a space to reduce heat transfer across a part of the building envelope. A window with insulating glass is commonly known as double glazing or a double-paned window, triple glazing or a triple-paned window, or quadruple glazing or a quadruple-paned window, depending upon how many panes of glass are used in its construction.
Dynamic insulation is a form of insulation where cool outside air flowing through the thermal insulation in the envelope of a building will pick up heat from the insulation fibres. Buildings can be designed to exploit this to reduce the transmission heat loss (U-value) and to provide pre-warmed, draft free air to interior spaces. This is known as dynamic insulation since the U-value is no longer constant for a given wall or roof construction but varies with the speed of the air flowing through the insulation. Dynamic insulation is different from breathing walls. The positive aspects of dynamic insulation need to be weighed against the more conventional approach to building design which is to create an airtight envelope and provide appropriate ventilation using either natural ventilation or mechanical ventilation with heat recovery. The air-tight approach to building envelope design, unlike dynamic insulation, results in a building envelope that provides a consistent performance in terms of heat loss and risk of interstitial condensation that is independent of wind speed and direction. Under certain wind conditions a dynamically insulated building can have a higher heat transmission loss than an air-tight building with the same thickness of insulation. Often the air enters at about 15 °C.
Clothing insulation is the thermal insulation provided by clothing.
Sustainable refurbishment describes working on existing buildings to improve their environmental performance using sustainable methods and materials. A refurbishment or retrofit is defined as: "any work to a building over and above maintenance to change its capacity, function or performance' in other words, any intervention to adjust, reuse, or upgrade a building to suit new conditions or requirements". Refurbishment can be done to a part of a building, an entire building, or a campus. Sustainable refurbishment takes this a step further to modify the existing building to perform better in terms of its environmental impact and its occupants' environment.
Elżbieta Kossecka is a Polish physicist. She is a professor of technical sciences and a researcher at the Institute of Fundamental Technological Research, Polish Academy of Sciences.
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