Thermal bridge
Encyclopedia
A thermal bridge, also called a cold bridge, is a fundamental of heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 where a penetration of the insulation layer by a highly conductive or noninsulating material takes place in the separation between the interior (or conditioned space) and exterior environments of a building assembly (also known as the building enclosure, building envelope
Building envelope
The building envelope is the physical separator between the interior and the exterior environments of a building. Another emerging term is "Building Enclosure". It serves as the outer shell to help maintain the indoor environment and facilitate its climate control...

, or thermal envelope).

Thermal bridging is created when materials that are poor thermal insulators
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

 come into contact, allowing heat to flow through the path of least thermal resistance (R-value
R-value
R-value can refer to:* Properties of materials:**R-value , the efficiency of insulation of a house**R-value , stability of soils and aggregates for pavement construction...

 or a materials effectiveness in resisting the conduction of heat) created, although nearby layers of material separated by airspace allow little heat transfer.

Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; the bridging has to be eliminated, rebuilt with a reduced cross-section or with materials that have better insulating properties, or with a section of material with low thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 installed between metal components to retard the passage of heat through a wall or window assembly, called a thermal break
Thermal break
A thermal break or thermal barrier is an element of low thermal conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials....

.

Concept

Several properties of materials affect heat transfer. Examples include thermal conductivities, specific heats, material densities, fluid velocities, fluid viscosities, and surface reflectance and emittance characteristics. Thermal bridges are characterized by multi-dimensional heat transfer, and therefore they cannot be adequately approximated by the one-dimensional models of calculation typically used in norms and standards for the thermal performance of buildings (U-values). Surface moisture due to condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

, typically occurring in such regions as floor-wall connections and window installations, as well as mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...

 growth in humid environments can also be effectively prevented by means of multi-dimensional evaluation during planning and detail design.
  • Concrete
    Concrete
    Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

     balconies that extend the floor slab through the building envelope are a common example of thermal bridging.
  • In commercial construction, steel or concrete members incorporated in exterior wall or roof construction often form thermal bridges.
  • Metal ties in cavity walls are another type of thermal bridge commonly found in masonry construction.

Thermal Bridging in Construction

Classification:
  • Repeating thermal bridges - where bridges occur following a regular pattern, such that made by wall ties penetrating a cavity wall
    Cavity wall
    Cavity walls consist of two 'skins' separated by a hollow space . The skins are commonly masonry such as brick or concrete block. Masonry is an absorbent material, and therefore will slowly draw rainwater or even humidity into the wall. The cavity serves as a way to drain this water back out...

  • Non-repeating thermal bridges - such as the bridging of a cavity wall
    Cavity wall
    Cavity walls consist of two 'skins' separated by a hollow space . The skins are commonly masonry such as brick or concrete block. Masonry is an absorbent material, and therefore will slowly draw rainwater or even humidity into the wall. The cavity serves as a way to drain this water back out...

     by a single lintel
  • Geometrical thermal bridges - placed at the junction of two planes, such as at the corner of a wall

Thermal bridge concept and interior insulation

Interior insulation is used for preventing heat as well as cold from escaping or entering the building envelope, individual rooms, as well as pipes and various appliances. Heat loss of a building is prevented primarily by exterior insulation in the walls because they cover the most surface area of any building. There are many different types of materials which can be used for insulation with different characteristics. The different spaces for insulation in a building define different uses for it.

Insulation requirements relating to thermal bridging

There are many different materials that are used for insulation, and new ones are often being created as the need for energy efficiency, sustainable design, and cheaper costs are currently what are driving new innovation. Currently, the types of insulation that are being used are:
  • Fiberglass or rock wool insulation,
  • Insulating glass or polystyrene
    Polystyrene
    Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

     rigid board insulation, formed in place polyurethane insulation,
  • Cellulose/perlite/vermiculite loose fill, and insulated pre-cast concrete insulation.


Each type of insulating material is used most effectively in various parts of buildings including interior walls, exterior envelopes, flooring, roofing, and basements. All insulating materials have a specific R-value
R-value
R-value can refer to:* Properties of materials:**R-value , the efficiency of insulation of a house**R-value , stability of soils and aggregates for pavement construction...

, which determines their thermal resistance. A good insulating material resists the conduction of heat. The higher the thermal resistance of a part of the building envelope is, the slower the heat loss. The larger the difference between the temperature inside and outside the building, the faster the building gains or losses heat. Designing a building’s walls, roofs, and floors for the maximum amount of thermal resistance results in optimal body comfort and energy conservation.

One-dimensional analysis

One-dimensional analysis is based on simple, steady state
Steady state
A system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...

, flow of heat, which means that heat is driven by a temperature differences that does not fluctuate so that heat flow is always in one direction. The product (kA) of thermal conductivity (k) and cross sectional area (A) of the heat flow path can be used in evaluating heat flow.

External links

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