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Thermal Bridging Literature Review

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Literature Summaries

Thermal bridges at the joints between walls and window frames

Erich Cziesielski, Proceedings of Windows in Building Design and Maintenance (1984)

- Looked at changing position of window and the addition of external insulation.
- The major concern is the occurance of a temperature drop at the mounting point between the window frame and outer wall.
- Mentions three methods to detect the occurence of thermal bridges in wall assemblies (hot-box testing, thermography, and analytical computation. In this instance the analytical method is utilized.
- The addition of insulation between the window and the wall reduces the occurrence of condensation.
- Found that windows should be mounted toward the inside or center of the opening.
- Showed that walls without external insulation can have damaging condensation issues.
- Provides evidence that internal insulation is not as effective as external insulation.
- Provides evidence that electric heating of walls using thin wires reduces the thermal bridge effect

Envelope the Steel!

James A. D’Aloisio1, 2, ASCE Structures Congress (2010)
1 Klepper, Hahn & Hyatt
2 SEI Sustainability Committee

- Introduces the basics of building envelopes, heat transfer, and thermal bridges for structural engineers
- Presents limitations and possible materials for thermally effecient design alternatives - Stresses that thermal bridges should be a consideration of structural engineers as it applies to building performance and structural durability
- Provides conceptual solutions to structural related thermal bridges, which include Cold-formed steel studs, roof overhangs, steel lintels, relieving angles, parimeter columns and spandrel beams, roof edges, exposed steel columns, balconies/canopies, and brick ties
- Ends with a mention that the Intergovenmental panel on Climate Change has stated that improved building performance is one of the best ways to reduce manmade negative environmental impacts.

Steel Framing and Building Envelopes

James A. D’Aloisio1, 2, Modern Steel Construction (January 2010)
1 Klepper, Hahn & Hyatt
2 SEI Sustainability Committee

- Begins by mentioning that the consideration of thermal bridges is new to structural engineers, as many assumed that the heat transfer along connections has little effect due to small area of contact.
- Provides a brief example for why small areas bridging the thermal plane have large effects on the heat transfer.
- Lays out a few methods to address energy efficiency in buildings (Thermal Infrared imaging, thermal modeling software, data from building).
- Emphasizes that the detailing of energy efficient connections falls on the structural engineer, and for safety and liability reasons does not the fall to the enclosure consultant or architect.
- One statement that stood out was that increasing the insulation, increases the structural attachment, which increases the thermal transfer.
- States that new energy efficient connection details need to be developed and provides examples that have been implimented in completed projects by Klepper, Hahn & Hyatt.

Structuring Sustainable Thermal Breaks: Opportunities and Liabilities

Russ Miller-Johnson1, ASCE Structures Congress (2010)
1 Engineering Ventures, PC

- Describes the importance of using alternatives to carbon steel at connection details in order to provide a thermal break.
- Provides examples of structural design issues when incorporating thermal breaks at connection details.
- Discusses the liability of using unconventional conection assemblies and materials, which are not typically covered in standards of practice and loss prevention criteria.
- Provides a list of other issues.
- Points out that the incorporation of these details may lead to a lengthy approval process, which may involve testing in addition to several other requirements.
-The paper ends on a brighter note by explaining that the incorporation of successful energy efficient connection details can offer beneficial energy conscience solutions.

Thermal Inefficiencies in Building Enclosures – Causes of Moisture Related Performance Problems

Paul E. Totten1 and M. Pazera1, ASCE Forensics Engineering Congress (2009)
1 Simpson Gumpertz & Heger Inc.

- Discusses the importance of limiting moisture related problems due to the thermal bridges, as well as unwanted air flows and leaks.
- Describes the reasoning for why these undesirable deficiencies should be prevented or remediated.
- Gives a general description of heat transfer mechanisms (conduction, convection, and radiation) and dew point temperature.
- Summarizes the methods to investigate, analyze, and diagnose moisture related issues (visual inspection, tracer smoke test, infrared thermography, and computational thermal analysis).
- Provides two case studies: The first highlights a condensation problem due to unintentional air infiltration and the second illistrates the occurrence of condensation due to thermal bridging.

The Effects of Thermal Bridging at Interface Conditions

Paul E. Totten1, Sean M. O'Brien1, Marcin Pazera1, Building Enclosure Science and Technology (BEST 1) conference (2008)
1 Simpson Gumpertz & Heger Inc.

- Discusses typical interfaces (roof-to-wall, stud framing, window-to-wall, wall-to-balcony, wall-to-wall, and sunshade-to-wall) where thermal bridges can occur, and offers suggestions on preventing or reducing their effects.
- Begins by describing thermal bridging, thermal breaks, and heat transfer.
- At the roof-to-wall interface ensure continuity of the thermal plane, especially for parpets.
- In stud framing construction provide proper amount of exterior insulation. In some instances a combination of cavity insulation and exterior insolation may be needed.
- Provides an example to illustrate that the addition of windows is detrimental to the clear-wall R-value. At window-to-wall interfaces the window thermal breaks should be in-line with the insulation plane.
- Cantilevered balconies act as heat fins. Solutions include the isolation of the balcony from the main structure or the use of proprietary thermal breaks.
- At wall-to-wall interfaces changes in material properties can lead to noticable thermal effects. In some instances lapping of insulation layers may be required.
- For sunshade-to-wall interfaces insulation should be lapped at the anchor and sunshades should not be connected directly to the structure.
- At the end figures are provided showing the temperature distributions for a few of the strateges discussed.

Thermal Bridging Solutions: Minimizing Structural Steel’s Impact on Building Envelope Energy Transfer

SEI/AISC Thermal Steel Bridging Task Committee, Supplement to Modern Steel Construction (March 2012)

- Begins with stating that, "Efforts to reduce energy use has been through improvement of mechanical, electrical, and glazing systems, and not structural thermal bridges.
- highlights that there is a misconception within the structural engineering community that energy efficiency is not the role of structural engineers.
- Includes a brief description of heat transfer through building envelopes (mechanisms, R-values and U-factors, Series vs. parallel flow paths)
- Describes two common methods used to quantify heat loss (Thermal infrared imaging, and building energy modeling).
- Discusses that thermal bridging is not exclusively delt with in codes, such as IECC, IGCC, ASHRAE 90.1, and ASHRAE 189.1, but are being evaluated.
- Mentions alternative materials to carbon steel, which are stainless steel (Conductivity is three times less), and Fiber reinforced plastics (FRP).
- States that codes and loss prevention criteria do not address the use of FRP. Also, AISC Specification for Structural Steel Buildings (ANSI/AISC 360-10) does not address non-steel assemblies.
- Provides descriptions for several conceptual solutions for various details and their feasibility (rooftop grillage posts, roof edge angle, shelf angle, steel lintel, roof canopy.
- Modeled a generic building for various climate zones using TRACE 700.
- Analysis of assemblies used THERM software.
- Follows with explaing the assumptions, limitations, alternatives, chalanges, and ended with the lessons learned.

New Materials and Concepts to Reduce Energy Losses Through Structural Thermal Bridges

R.P. Tye1, J.P. Silvers1, D.L. Brownell1, SE. Smith1, ASHRAE/DOC/BTEC Thermal Performance of the Exterior Envelopes of Buildings III, ASHRAE SP 49 (1986)
1 Dynatech R/D Company, Research comissioned by US DOE and Oak Ridge National Labratory'

- Discribes several reasons for the occurence of thermal bridges (high thermal conductivity of structural elements, connections, and enclosure penetrations, as well as geometric effects, and faulty installation).
- Mentions that high strength materials, which often times are high in density, have high thermal conductivities.
- Performed an intensive literature review of over 400 sources including topics in thermography, energy audits, in-situ measurements, and condensation.
- Based on the literature review the paper states that thermal bridges can have up to a 20% reduction in wall resistivity.
- Discusses solutions for reducing the thermal bridge effect by reducing contact area or material properties, or adding thermal breaks.
- Gives an example where channging from carbon steel ties to stainless steel can reduce the thermal conductivity by 1/3.
- Provides comments from interviews with architects, engineers, contractors, construction officials, manufacturers, and researchers on their familiarity with thermal bridges.
- Looks at new solutions for reducing thermal bridges using specific materials (insulation, insulated panels, masonry, mortar, wood, FRP fasteners, fiber reinforced composites, and honeycomb composites).
- Concludes by stating that the awareness of thermal bridging must be spread and that the current (1986) solutions are available but not well implimented.

Dynamic Evaluation of Thermal Bridges in a Typical Office Building

Douglas M. Burch1, John E. Seem2, George N. Walton1, Betty A. Licitra1, ASHRAE Transactions: research (1992)
1 US National Institute of Standards and Technology
2 Johnson Controls Inc., Milwaukee, WI

- Typical programs used for HVAC design, which use a one dimensional conduction transfer function (CTF), do not account for thermal bridges.
- The Inclusion of thermal bridges increases the heat transfer by 10-21%.
- Discusses the determination of CTF coefficients, accounting for thermal bridges.
- Performed a numerical steady-state and transient analysis for a common office building.
- Accounted for five thermal bridges: Window frame/wall interface, steel studs, floor/wall interface, floor slab penetrations, and ceiling fasteners
- Under steady-state conditions the increase in building heat transfer coefficient for the thermal bridges were 24%, 3.6%, 2.9%, 2.7%, and 0.1%, respectfully.
- The transient analysis uses the CTF method. The discussion outlines the method to calculate the heat transfer function coefficient.
- Results of the CTF equation are shown to be comparable to a finite difference solution.
- Discusses the method to incorporate CTF coefficients in computer programs.

Analysis of the influence of installation thermal bridges on windows performance: The case of clay block walls

Francesca Cappelletti1, Andrea Gasparella2, Piercarlo Romagnoni1, Paolo Baggio3, Energy and Buildings, 43(2011)
1 University IUAV of Venezia, Italy
2 Free University of Bolzano, Italy
3 University of Trento, Italy

- States that few studies have looked at the effect of window frame connections.
- States that energy performance is dependent on component materials, conection details, and installation.
- Looks at the influence of the window frame position and insulation position on the thermal properties of the window-wall interface.
- Modeling of window-wall system was done using THERM 5.2.
- The center of glass thermal transmittance was calculated using WINDOW 5.
- Thermal bridges are accounted for using the linear thermal transmittance (ψ) method as outlined in ISO 10211.
- Calculation of the frame thermal transmittance (Uf) and window thermal transmittance was done using ISO 10077-2 Annex C.
- The ψ value and Uf are dependent on the thermal coupling coeffcient (L2D). - Determines the heat loss due to shutter boxes, which treated as frames.
- Frame cavities are calculated fas unventilated and slightly ventilated based on ISO 10077-2.
- Provides a great figure of the the various wall-window assemblies and their corresponding L2D and ψ values.
- Compares the calculated ψ values with ψ values for similar configurations in ISO 14683 - Concludes that ther is a reduction in L2D and ψ with external insulation of the jambs and sills and increase in L2D and ψ as frame moves toward the interior.
- The 2-D transmittance (U2D) was calculated using the contribution of the ψ values for the head, sill and jamb, and the 1-D transmittance (Uw).
- The percent difference from 1-D to 2-D was calculated and ploted based on changes in the window height-to width ratio, and thickness.
- States the these plots could be helpful for designers.

The Significance of Bolts in the Thermal Performance of Curtain-Wall Frames for Glazed Facades

Brent Griffith, Elizabeth Finlayson, Mehrangiz Yazdanian, and Dariush Arasteh , ASHRAE Winter Meeting(1998)

- Studied the effect of bolt type (Nylon, stainless steel, and Carbon steel).
- Performed testing by applying a temperature differential on a curtain wall configurations (change in bolt spacings) and captured temerature variations using IR thermography on the warm side.
- Nylon bolts are used as the base case as they are assumed to be similar to a no bolt configuration, however the article notes that this type of bolt is unuseable in practice.
- Tested two senarios in order to eliminate convection and radiation effects: glazing cavities of the test specimens were filled with rigid insulation and glazing was replaced by rigid insulation.
- Modeled and performed sensitivity studies and using THERM 2.0 and compares to U-value results from the use of the parallel path method and isotherm planes method as reported in the ASHRAE handbook.
- Determined that bolt spacings less than 230 mm (9 in) can result in increased heat loss.
- Smaller bolts lead to less heat transfer.
- Isotherm planes method over predicts the effect of bolts on the U-value (conservative) and the parallel path method under predicts the the effect of bolts on the U-value

Development of Limits for the Linear Thermal Transmittance of Thermal Bridges in Buildings

A. Janssens, E. Van Londersele, B. Vandermarcke, S. Roels, P. Standaert, P. Wouters, 10th Thermal Performance of the Exterior Envelopes of Whole Buildings Conference (2007)

- Outlines one method that can be used to develop limits on the linear thermal transmittance (ψ-value) at junctions, which include the effects of geometry.
- Defines the linear thermal transmittance - Discusses the factors that affect the ψ-value: Continuity and thickness of insulation, U-value of building materials, and geometry of detail considered.
- Junction detail selection was based on 23 different details found in five typical masonry building types.
- Provides analysis based on the thermal quality of junction details: No consideration of thermal effects, Contiuity of insulation, and thermal bridge preventive construction.
- Discusses the contribution of thermal bridges based on the thermal quality of the junction. The contribution to heat transfer in thermal bridges can be reduce from 13%-17% to 1%-4% of the total heat transfer at building junctions by using appropriate detailing.
- Mentions current limits on ψ-value: EN 1999 states that ψ-value should be less than 0.1 W/m·K and Wouters et. al. (2003) states that ψ-values of >0.5 W/m·K are significant, 0.25 - 0.5 W/m·K are important, and 0.1 - 0.25 W/m·K are moderate.
- The proposed limits: Junctions at exterior corners (ψe < 0.00 W/mK), Junctions at interior corners (ψe < 0.15 W/mK), Balconies (ψe < 0.10 W/mK), Window junctions (ψe < 0.10 W/mK), and Structural connections (ψe = 0.05 W/mK).
- Proposes three options for compliance: Direct analysis using numerical methods, Indirect analysis sets a default on the ψ-value, and/or the designer/contrator may use approved construction details.

Thermal Performance of Three-Dimensional Building Envelope Assemblies and details for improving the Accuracy of Whole Building Performance Simulation

Christian Cianfrone1, Neil Norris1, Patrick Roppel1, and Medgar Marceau1, Fifth National Conference of IBPSA-USA (2012)
1 Morrison Hershfield

- Mentions that owners/operators want lower energy use and energy codes are becoming more stringent.
- States that in North America accounting for thermal bridges is to adress the heat transfer through studs, but other common thermal bridges are ignored.
- States that codes do not fully account for thermal bridges, based on the belief that the small areas have little affect on the heat transfer, the analysis is too intensive, and lack of required thermal information for details.
- Goes on to adress these beliefs by mentioning that a buildings heat flow is higher than typically assumed, the reduction of highly conductive thermal bridges may have more of an effect then increasing the insulation thickness, and implimenting solutions to thermal bridges has been made simple due to the availability of typical thermal bridge avoidance details.
- Shows a simple method to account for thermal bridges using ASHRAE RP-1365.
- ASHRAE RP-1365 and the method of linear transmittance is proposed as an alternative approach to the area weighted method used in in common practice, which can be difficult when looking at connections and/or using ASHRAE PR-1145.
- ASHRAE RP-1365 presents the thermal performance of 40 details.
- Describes the method of linear thermal transmittance, which simply subtracts the thermal bridge free detail performance from the actual detail performance to determine the contribution due to thermal bridges.
- Presents the method to calculate thermal transmittance for linear and point type thermal bridge details
- Discuses the ASHRAE RP-1365, its use in practice. Showed that when poor construction design/practices are used the heat transmittance may be reduced by up to 75% using the method of linear thermal transmittance.
- Described the method of incorporating thermal bridges in whole building energy simulation. - Look at a three typical buildings from US DOE commercial reference buildings in four cities in different hygrothermal regions of the United States.
- Heating climates (cold regions): Showed that the space conditioning requirments increases with colder temperatures, decreased floor space, and increased glazing ratio. Higher R-value decreases heating requirements (exponential decay). By not accounting for thermal bridges the heat loss may be up to 84%.
- Cooling climates (warm and hot regions): Showed that the space conditioning requirments increases with warmer climates, incresed floor space, and increased glazing ratio. Higher R-value decreases cooling requirements (exponetial decay). Cooling shows similar results to heating but not as dramatic.
- States that increasing the insulation thickness becomes less effective if Thermal bridges are not accounted for. There is a point at which increasing the R-value has little effect as shown in the results from the results from the whole building energy simulation.

New Developments in Mitigation of Thermal Bridges Generated by Light Gage Steel Framing Components

Peter Engelmann1, Bryan Urban1, Jan Kosny1, 9th Nordic Symposium on Building Physics (2011)
1Fraunhofer Center for Sustainable Energy Systems

- Looks at the effects of insulation type and thickness on the preventing thermal bridges in light gage steel framing.
- Begins by mentioning some advantages of steel stud over wood stud construction (e.g. higher dimensional stability, non-flamable, insect and mold resistant).
- Then goes on to discuss that steel has a much higher conductivity, which may be why it has not had much success in the thermally conscience residential market. On the other hand steel stud construction has been sucessful in the commercial industry, which has less stringent thermal performance requirements.
- Mentions that the framing factor can be taken as a measure of the thermal bridging in a walls.
- States that methods of improving the thermal performance include: reducing the the stud contact area (Raised corners and dimples in the flange), the use of furring strips between walls and exterior sheathing, and reducing the web area or replacing the web with a less conductive material. - Performed a comparison of using HEATING 7.3, which only looks at thermal bridging through studs. Surface temperatures were 70 °F interior and 20 °F exterior.
- Compares walls with spacers, exterior foam insulation, EIFS, interior aerogel insulation, exterior vacuum insulation.
- Results show that the U-value of the models increase as follows: vacuum insulation panels, EIFS, Aerogel, Foam sheathing, spacer.
- Concludes that U-value decreases with increased insulation thickness.
- Concludes that Aerrogel and vacuum insulation panels will show comparable results when space is limited or exterior insulation is not possible.
- Concludes that Aerrogel and vacuum insulation panels are very effective but are not economically feasible.