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50to50 Wiki > Wiki Pages > Thermal Bridging
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Thermal Bridging
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« Systems Tune-Up |
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Thermal Bridging |
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Thermal bridging occurs in building envelopes when relatively high thermal conductivity materials such as steel and concrete create pathways for heat loss that bypass thermal insulation. When these materials provide an uninterrupted “short circuit” between the interior and exterior of a building, the resulting impact on envelope R-value can be significant. This effect is most significant in cold climates during the winter when the indoor-outdoor temperature difference is greatest. Thermal bridging can result in localized cold spots on the interior of a wall assembly that are at risk for condensation. Since it involves the design and installation of the building envelope (including structural components), thermal bridging is best addressed in the design process with new buildings or gut rehabs.
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Contents
1.Definition
2. Use/Application
a. Established Techniques
b. Emerging Trends
3. Use an Integrated Approach
4. Resources
5. Associated Strategies
6. Case Studies |
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Definition
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Thermal bridging occurs in building envelopes when relatively high thermal conductivity materials such as steel and concrete create pathways for heat loss that bypass thermal insulation. When these materials provide an uninterrupted “short circuit” between the interior and exterior of a building, the resulting impact on envelope R-value can be significant. This effect is most significant in cold climates during the winter when the indoor-outdoor temperature difference is greatest.
Thermal bridging can result in localized cold spots on the interior of a wall assembly that are at risk for condensation. Since it involves the design and installation of the building envelope (including structural components), thermal bridging is best addressed in the design process with new buildings or gut rehabs.
According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1, a 3.5” steel stud wall system with R-13 fiberglass batt, ½” interior gypsum board, and ½”exterior gypsum board will only have a whole wall R-value of R-8. Even though the steel studs make up a relatively small fraction of the wall area (16” on-center), thermal bridging through these members greatly reduces the effectiveness of the fiberglass insulation. The images below illustrate computer simulations of the heat flow though this type of wall assembly with and without the steel studs using THERM modeling software. In addition to condensation, cold spots due to thermal bridging can result in particle deposition such as the “ghosting” depicted in the picture below.
Thermal bridging is also commonly found at the following locations:
- At the edge of un-insulated concrete floor planks that penetrate wall insulation (including slab on grade).
- In curtain walls at locations where walls are fastened to the building (see figure below).
- Throughout metal buildings.
Source: http://www.carlisle-syntec.com/documents/reslib/ConstructionSpecifier_ThermalBridging.pdf
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Use / Application
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The basic approach to minimizing thermal bridging is to design a wall system with a thermal envelope (insulation layer) that is continuous at all interfaces. This insulation layer must therefore completely cover either the inside or outside of concrete or steel building members. The images below illustrate continuous layers of insulation installed on the exterior of steel framing that acts as a “thermal break,” minimizing bridging though the studs. Note that while the 1” mineral wool addresses thermal bridging due to steel studs, this insulation is interrupted by the concrete plank and therefore does not address thermal bridging at the floor plank edge.
For more complex building assemblies, two-dimensional heat transfer modeling should be used to inform the envelope design process.
THERM is a 2-D finite element software program developed by Lawrence Berkeley National Laboratory (LBNL) specifically for use in evaluating the thermal performance of building assemblies. This software allows for the calculation of whole wall R-values and surface temperatures and can be used to import geometries from CAD. In addition, the software includes a built-in material library with the thermal properties of many common building components.
There are other strategies for preventing thermal bridging. For example, the use of Structural Insulated Panels (SIPS) reduces or eliminates thermal bridging because SIPs are the structural elements and there are no studs or braces to cause breaks in the insulative action. Unlike stick and batt construction, which can be subject to poorly installed insulation, the nature of SIPs is such that the structural and insulative elements are joined as one. There are no hidden gaps, because a solid layer of foam insulation is integral to panel construction.
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Established Techniques |
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Emerging Trends |
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Use an Integrated Approach |
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A new way of thinking must be adopted to meet the goal of reducing carbon emissions associated with buildings. Your solutions can begin by integrating four possible methods. None works alone, and they are not all relevant in considering every strategy. However, considering the following tactics is necessary:
- Reduce the overall energy use in your building
- Implementing strategies to minimize thermal bridging increases whole wall R-value which will reduce heating energy use.
- Specify energy efficient equipment and technologies
- Conventional insulation materials designed and specified in the appropriate manner can be used to address thermal bridging in buildings. Heat transfer modeling software is a powerful tool that can be used to inform this process.
- Use renewable strategies and purchase green power
- Implementing strategies to reduce thermal bridging minimizes energy waste which should be addressed before renewable energy systems are considered.
- Educate building owners, operators, and occupants
- Education of design professionals on the concept of whole wall R-value (versus simply “R-13” fiberglass batt) is critical
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Last modified at 2/27/2009 11:12 PM by jamie nace
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