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).

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. 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 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 that if thermal bridges at balconies are not taken care of, the balconies act as “cooling fins”; conducting the heat off the building and cooling the rooms adjacent to the balconies.

Thermal Image of Aqua Tower, Chicago, IL USA

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.

It has been demonstrated that the amount of losses by thermal bridges are 20% of the global thermal bridges. This means that the thermal bridges have a big importance on the energy costs, thermal comfort, salubrity and economic - environmental expenses. This is why the approved new regulation has much higher requirements and demands than the older one, which was much simpler for being the first thermal legislation.

Examples

Thermal bridge caused by floor

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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. Insulation applied to the interior surface of a wall is bridged by floor slabs and partitions; if these project on the exterior of the wall they form "fins," which provide a large surface exposure area for heat loss. The mullions of curtain walls have long been understood to act as thermal bridges within vision glazing systems. Building codes and other energy standards provide maximum allowable U-values for the whole assembly, accounting for the frame, the edge of the glass that has been derated by the frame and the center of glass performance. In most curtain wall buildings, however, this is only part of the installation. Areas between floors and sometimes across the façade are blanked off to create spandrel panels and these are insulated in a variety of ways. However, because the mullions are simply part of the system, few of us really consider the thermal impact the mullions can have circumventing the insulation.

• Metal ties in cavity walls are another type of thermal bridge commonly found in masonry construction. Serious problems may also occur at metal window frames and sash, and at metal curtain wall mullions, which either partially or completely bridge the wall and often present a fin exposed to the outside. Removing the metal and highly conductivity elements which penetrate the insulation layer will supply a better integrated performance for the entire building.

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.

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thermal bridge detail for masonry building

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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

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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. 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 significant thermal bridges. To combat this, a thermal break in the assembly, which is typically ¼ of an inch to one inch thick and made of a less conductive polyester reinforced nylon, has become a typical component in modern curtain walls. The thermal break is located between the face plate and the structural part of the mullion, the rail, in line with the glazing pocket. This creates a “cold” side for the portion of the frame in front of the glass, and a “warm” side with the structure on the backside. When insulation is added in a spandrel panel, it is most often added along the backside of the panel, between the innermost surfaces of the rails, and is often supported with a metal back pan. The insulation creates a “warm” side and “cool” side of mullion rail and completely disconnects the thermal barrier of the insulation from the thermal break in the frame. In our installations that used this detail, a 70% decrease in thermal performance could be observed.

Thermal Bridging in Construction

Thermal bridge classification

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The corner of the external wall and the junction between the wall and the roof both result in a geometric thermal bridge

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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

Repeating thermal bridges usually follow a regular pattern and are evenly distributed over an area of the thermal envelope. Typical examples include: 1 Ceiling joists in cold pitched roofs that are insulated at ceiling level. 2 Ground floor joists in an insulated suspended timber ground floor. 3 Timber studwork and I-beams in timber frame construction. 4 Mortar joints in an insulating block inner leaf. 5 Steel wall ties in masonry cavity external wall construction.

• Non-repeating thermal bridges - such as the bridging of a cavity wall by a single lintel

Non-repeating thermal bridges are intermittent and occur at a specific point in the construction. They are often caused by discontinuities in the thermal envelope. These discontinuities may be a result of the construction method used or may be due to changes in materials over the thermal envelope. They commonly occur around openings and other instances where materials of different thermal conductivities form part of the external envelope. Typical examples include: 1 Around windows, doors and rooflights. 2 Around loft hatches. 3 Where internal walls or floors penetrate the thermal envelope. 4 Where steel I-beams have been used to support timber roofs.

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

Geometrical thermal bridges, as the name suggests, are a result of the geometry (or shape) of the thermal envelope. They can be 2-dimensional (where 2 planes intersect) or 3-dimensional (where 3 or more planes intersect). The occurrence of geometric thermal bridging is likely to increase the more complex the building geometry. Typical examples include: 1 At the corner of an external wall. 2 At wall/roof junctions. 3 At wall/floor junctions. 4 Junctions between windows and doors and walls. 5 Junctions between adjacent walls.

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

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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.

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, 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.

See also

• Damp proofing
• Thermal break
• Building insulation
• R-value insulation
• Thermal insulation
• Thermal conductivity
• List of thermal conductivities
• Thermal resistance

External links

• Design Guide: Solutions to Prevent Thermal Bridging.
• Manufactured Structural Thermal Breaks.
• EU IEE SAVE Project ASIEPI: topic 'Thermal bridges' - An effective handling of thermal bridges in the EPBD context
• Passivhaus Institute: Thermal Bridges in construction - how to avoid them
• A bridge too far - ASHRAE Journal article on thermal bridging
• International Building Code, 2009: Interior Environment
• Online Energy2D simulation of thermal bridge (Java required)
• How to Avoid for Passive Houses
• What Defines Thermal Bridge Free Design

References

Notes