# Thermal bridge

Temperature distribution in a thermal bridge

A thermal bridge, also called a cold bridge, is a fundamental of heat transfer where a penetration of the insulation layer by a highly conductive 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, or thermal envelope).[1] Thermal bridging is created when materials that are poor thermal insulators come into contact, allowing heat to flow through the path of least thermal resistance created, although nearby layers of material separated by airspace allow little heat transfer.

[2] Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; the bridging itself needs to be treated carefully, rebuilt with a reduced cross-section or with materials that have better insulating properties, or with a section of material with low thermal conductivity installed between high conductivity components to retard the passage of heat through a wall or window assembly, called a thermal break. The thermograph photograph on the right shows an example of thermal bridge at balcony.[3]

Thermal Image of Aqua Tower, Chicago, IL USA

The impact of thermal bridges on energy use, thermal comfort and indoor air quality can be significant. For example, for recent Greek buildings, the majority of which are built in the last 20 years and partially insulated, because thermal bridges are not considered by the calculation procedure, research shows that the actual thermal losses in the cases of such buildings are by up to 35% higher than the initially estimated ones.[4]

## Concept

Thermal bridge at junction

Low-energy buildings use a thermal insulation layer that carefully enclosures the whole building without any missed area, which means “no holes” in any place that don’t have protecting insulation. If there is, heat transfers significantly through that hole, making the whole system of insulation defeated. 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, typically occurring in such regions as floor-wall connections and window installations, as well as mold growth in humid environments can also be effectively prevented by means of multi-dimensional evaluation during planning and detail design.

## Examples

Metal tie as thermal bridge in construction
• Concrete balconies that extend the floor slab through the building envelope are common examples of thermal bridging.
• In commercial construction, steel or concrete members incorporated in exterior wall or roof construction often form thermal bridges.[1]
• Metal ties in cavity walls are another type of thermal bridge commonly found in masonry construction.[1]

## Thermal Bridges and Building Enclosure Types

Thermal bridges take places commonly in reinforced concrete exterior walls surrounding seismic columns, ring beams, lintels doors, reinforced concrete or steel frame beams, columns, reinforced concrete or metal roof in a small side ribs, and metal curtain wall glazing where the metal frame and window extrude to outside.

While thermal bridges exist in various types of building enclosures, two types show significant reduced R-value caused by thermal bridges, masonry and curtain wall.

These exterior steel columns are heat sinks, drawing heat from the beams that penetrate from the conditioned space. Beams inside the building were dripping wet
Thermal image for mullion detail

### Masonry buildings

Concrete floors and edge beams, especially at the corners, are common thermal bridges. Physical heat conductivity for concrete material are 2~4 times higher than ordinary brick materials, which are usually used for facade enclosures. Meanwhile, if the indoor environment is not vented enough, the brick material will slowly absorb rainwater and also humidity into the wall. When the temperature difference between indoor and outdoor space is large, the indoor warm and humid air could condense in this area. Insulation layer’s R-value will be reduced significantly by humidity, thus the thermal performance for insulation wall is not evenly distributed.[5] The more humidity, the worse thermal performance. This uneven distribution finally cause thermal bridge for large area, sometimes the whole enclosure system.Because of multiple factors such as orientation, daylight, ventilation, temperature and human factors, the distribution of thermal bridge could also be uneven for a single building, which accounts for the phenomena that sometimes while some units are severely affected by thermal distribution together with mildew, some units are slightly affected.

### Curtain wall

Because curtain wall frames are made of highly conductive aluminum, which is about four times more conductive than steel, and typically go from the exterior of the building through to the interior, they are thermal bridges.[6]

## Thermal Bridging in Construction

### Thermal bridge classification

The corner of the external wall and the junction between the wall and the roof both result in a geometric thermal bridge
Use of top hat lintels results in a number of non-repeating thermal bridges at the window head

The building enclosure system provides a separation of interior physical environment from the exterior physical environment. While HVAC system are based on air tightness, control of diffusive vapor transfer, and thermal efficiency, there are three major systems that require careful-designed continuity: the air barrier system, the water barrier system and the thermal barrier system. In architectural detailed design and construction, scenarios providing chances penetrating this continuous layer are listed as below:

• Repeating thermal bridges - where bridges occur following a regular pattern, such that made by wall ties penetrating a cavity wall
• Non-repeating thermal bridges - such as the bridging of a cavity wall by a single lintel
• Geometrical thermal bridges - placed at the junction of two planes, such as at the corner of a wall

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

### Strategies and methods to reduce thermal bridges in practical construction

Thermal break in mullion

Multiple situations and interfaces could provide thermal bridges, these are classified as: roof-to-wall; steel stud construction, window-to-wall, wall-to-balcony slab; wall-to-wall; and sunshade-to-wall.[6]

While various methods are applied to each of these situations, several principles are followed methodologically:

• A continuous thermal barrier is needed in the building enclosure; the location of this barrier for most buildings should be outboard of highly conductive materials.
• Reduction and elimination of potential and actual thermal bridges are needed.
• Lapping of insulation where direct continuity is not possible can mitigate thermal bridges.
• Window-to-wall interfaces create additional challenges that need to be carefully reviewed for energy considerations and condensation risk due to positioning of the fenestration within the rest of the assembly.
• Reducing and limiting thermal bridging in buildings will typically reduce energy needs for the building.

## Current one-dimensional analysis and challenge

One-dimensional analysis is based on simple, steady state, 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. .

While most energy calculating software could only supply 1D heat flow analysis,[7] actually it is a 3D problem to understand thermal bridges, where there's the challenge for technical modeling software. Currently better methods and modeling software are still under research.