Quantifying the Impact of Thermal Bridging and Mitigating Heat Loss in Building Envelopes
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Thermal bridging can greatly impact the thermal performance of building envelopes. This presentation discusses research from ASHRAE RP-1365 that quantified thermal bridging in common construction details using 3D modeling. It found that accounting for thermal bridges can decrease a wall's effective R-value by over 30%. The presentation also showed that improving details like slab edges and balcony connections through methods like insulation and thermal breaks provided significant energy savings compared to simply adding clear wall insulation. Overall, the research demonstrates the importance of considering thermal bridging when assessing building envelope performance and codes.
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Quantifying the Impact of Thermal Bridging and Mitigating Heat Loss in Building Envelopes
- 1. Building Envelope Performance â Quantifying and Mitigating the Impact of Thermal Bridging November 18, 2014
- 2. 2 Presentation Overview Thermal Bridging 101 Data â Where & How Findings & Applications 1 2 3
- 4. Effective Thermal Resistance What is a Thermal Bridge? ⢠Highly conductive material that by-passes insulation layer ⢠Areas of high heat transfer ⢠Can greatly affect the thermal performance of assemblies
- 5. Existing Sources of Information 5 R-8.5 R-13.5
- 6. Parallel Path Heat flow 6 Utotal ⢠Area weighted average of un-insulated assemblies ⢠Does not tell the whole story
- 7. Thermal Bridging ⢠Parallel path doesnât tell the whole story ⢠Many thermal bridges donât abide by âareasâ ie: shelf angle ⢠Lateral heat flow can greatly affect the thermal performance of assemblies
- 8. Addressing Lateral Heat Flow 8
- 9. Lateral Heat Flow 9 Parallel Path í â 11.5 With Lateral Heat Flow í â 9.8 í 20 íííí?
- 10. Overall Heat Loss oQ Q slab Q Additional heat loss due to the slab
- 11. Overall Heat Loss ď ď˝Qslab / L The linear transmittance represents the additional heat flow because of the slab, but with area set to zero
- 12. The Conceptual Leap Types of Transmittances Point ďŁ Linear ď Clear Field o U psi chi
- 13. Overall Heat Loss Total Heat Loss Heat Loss Due To Anomalies Heat Loss Due To Clear Field = + Q ďT ď˝ ď U ď A ďŤ ďď¨ďď LďŠďŤ ďď¨ďŁ ďŠ o / ( )
- 14. Data â Where & How?
- 15. ASHRAE 1365-RP 2011 Goals and Objectives of the Project 15 ⢠Calculate thermal performance data for common building envelope details for mid- and high-rise construction ⢠Develop procedures and a catalogue that will allow designers quick and straightforward access to information
- 16. ASHRAE 1365-RP Calibrated 3D Modeling Software 16 ⢠Heat transfer software by Siemens PLM Software, FEMAP & Nx ⢠Model and techniques calibrated and validated against measured and analytical solutions ⢠ISO Standards for glazing ⢠Guarded hot box test measurements, 29 in total
- 17. ASHRAE 1365-RP Details Catalogue 17 ⢠40 building assemblies and details ⢠Focus on opaque assemblies, but also includes some glazing transitions ⢠Details not already addressed in ASHRAE publications ⢠Highest priority on details with thermal bridges in 3D
- 18. Providing Results ASHRAE Data Sheets 18
- 19. ASHRAE Data Sheets 19 Providing Results
- 20. T ď T surface outside i T T inside outside T ď ď˝ ASHRAE Data Sheets surface i inside outside outside T ď˝T (T ďT )ďŤT 20 Providing Results
- 21. BETBG 21 Building Envelope Thermal Bridging Guide Analysis, Applications, & Insights
- 22. Funding Partners Private Clients ⢠Structural thermal breaks manufacturer ⢠EIFS ⢠Insulated Metal Panel ⢠Cladding attachments ⢠Vacuum insulated panels (VIP) in insulated glazed units for glazing spandrel sections
- 23. More Data & Connect the Dots 23 Whole Building Energy Analysis Construction Cost Analysis Thermal Performance Cost Benefit Analysis
- 24. BETBG Layout ⢠Introduction ⢠Part 1 Building Envelope Thermal Analysis 24 (BETA) Guide ⢠Part 2 Energy and Cost Analysis ⢠Part 3 Significance, Insights, and Next Steps ⢠Appendix A Material Data Catalogue ⢠Appendix B Thermal Data Catalogue ⢠Appendix C Energy Modeling Analysis and Results ⢠Appendix D Construction Costs ⢠Appendix E Cost Benefit Analysis
- 25. Organization of Details 25
- 26. Appendix A & B 26
- 29. Accounting for Details How much extra heat loss can details add? ⢠Standard 90.1-2004 Prescriptive Requirements for Zone 5 ⢠Mass Wall, U-0.090 or R-11.4 ci ⢠Steel-Framed Wall, U-0.064 or R-13 + R-7.5 ci Mass wall with R-11 insulation inboard; U-0.074 Steel stud with R-10 exterior insulation and horizontal girts at 24âo.c and R-12 in the stud cavity; U-0.061 29
- 30. Accounting for Details Typical Building 30 ⢠10 floors ⢠20% glazing ⢠Standard details Mass Concrete Wall o Exposed concrete slab o Un-insulated concrete parapet o Punched window in concrete opening o Steel-Framed Wall o Exterior insulated structural steel floor intersection o Insulated steel stud parapet o Punched window in steel stud opening with perimeter flashing
- 31. Accounting for Details 31 Transmittance Type Mass Concrete Wall Exterior Insulated Steel Stud Heat Loss (BTU/hr oF) % of Total Heat Loss (BTU/hr oF) % of Total Clear Wall 118 52 % 98 67 % Slab 92 40% 24 17 % Parapet 9 4% 4 3 % Window transition 8 4% 19 13 % Total 227 100 % 145 100 %
- 32. Accounting for Details 32 Performance Metric Mass Concrete Wall Exterior Insulated Steel Stud ASHRAE Prescriptive Requirements Overall Performance ASHRAE Prescriptive Requirements Overall Performance U (Btu/hrft2oF) 0.09 0.14 0.064 0.091 âEffectiveâ R (hr ft2 oF/BTU) R-11 R-7 R-15.6 R-11 % Difference 44 % 35%
- 33. 33 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 R-3.9 R-4.5 R-5.2 R-5.0 R-5.3 R-10.2 R-14.3 R-16.7 Contribution of Thermal Performance of Wall Assembly to Energy Use(GJ/m2 of Floor Area) Clear Wall Only Including Poor Details Including Efficient Details Additional building energy use based on thermal performance of the building wall assembly for varying amounts of nominal exterior insulation for a mid-rise MURB in Edmonton (overall assembly thermal resistance in ft2¡ºF¡h/Btu also given) U0.26 U0.10
- 35. CLADDING ATTACHMENTS Vertical Z-Girts Horizontal Z-Girts Mixed Z-Girts Intermittent Z-Girts 36
- 36. Clip Systems 37
- 37. Effect of Thermal bridging in 3D 38 ASHRAE 90.1 2010
- 38. Proprietary Systems Thermal vs Structural Performance 39
- 39. Slab Edge Interfaces 45 â â
- 40. Concrete Walls SI (W/mâK) IP (BTU/hrâftoF) ď 0.81 0.47 46 Think about it! An R10 wall would have a transmittance of 0.1 BTU/hrâft2oF. One linear foot of this detail is the same as 4.7 ft2 of R10 wall (or 7.3 ft2 of R15.6 wall)
- 41. Slab Edges â Balcony SI (W/mâK) IP (BTU/hrâftoF) ď 0.59 0.34
- 42. Slab Edges â Shelf Angle SI (W/mâK) IP (BTU/hrâftoF) ď 0.47 0.27
- 43. Slab Edges â Shelf Angle SI (W/mâK) IP (BTU/hrâftoF) ď 0.31 0.18
- 44. Slab Edges â Exterior Insulated SI (W/mâK) IP (BTU/hrâftoF) ď 0.16 0.09 50
- 45. Slab Edges â Balcony SI (W/mâK) IP (BTU/hrâftoF) ď 0.21 0.12
- 46. Thermal break (image courtesy of Halfen) Structural Thermal Breaks Structural thermal break (image courtesy of Fabreeka) Structural thermal break (image courtesy of Schock) Balcony connection (image courtesy of Lenton)
- 47. Curtain Wall ⢠Glazing area is major determinant of overall heat loss ⢠U value of opaque spandrel closer to âglazingâ values ⢠Improvements can and are being made⌠57
- 48. Glazing Spandrel Areas Curtain Wall Comparison 58 No Spray Foam Spray Foam
- 49. Glazing Spandrel Areas 3.4 4.2 4.8 5.0 7.4 8.2 8.8 9.1 10 9 8 7 6 5 4 3 2 1 0 0 5 10 15 20 25 30 Spandrel Section R Value Back Pan Insulation Detail 22 (Air in Stud Cavity) Detail 23 (Spray Foam in Stud Cavity) 59
- 50. Glazing Spandrel Areas No Spray Foam Spray Foam 60
- 52. Vacuum Insulated Panels 62
- 53. Vacuum Insulated Panels 63
- 54. Glazing Spandrel Areas 3.4 4.2 4.8 5.0 7.4 8.2 8.8 9.1 10 9 8 7 6 5 4 3 2 1 0 0 5 10 15 20 25 30 Spandrel Section R Value Back Pan Insulation Detail 22 (Air in Stud Cavity) Detail 23 (Spray Foam in Stud Cavity) 64 5 40
- 55. Placement of Insulation 65
- 56. Curtain Wall System Traditional Captured 66 Stick Built Structurally Glazed Unitized High Performance Captured Stick Built
- 57. Traditional Spandrel Insulation Stick Built Curtain Wall
- 58. Vacuum Insulated Spandrel Stick Built Curtain Wall
- 59. Major thermal break at verticals 69
- 60. High Performance Curtain Wall
- 61. Vacuum Insulated Spandrel Unitized Curtain Wall
- 65. How to Improve? 77 Better Glass +Better Thermal Break +More Insulation? Vision Opaque U-0.4, R-2.5 U-0.26, R-3.8 U-0.27, R-3.7 U-0.25, R-4.0
- 66. How to Improve? 78 Add R-12 Spray Foam? Vision Opaque U-0.4, R-2.5 U-0.4, R-2.5 U-0.27, R-3.7 U-0.23, R-4.4
- 67. How to Improve? 79 Better Deflection Header? Vision Opaque U-0.21, R-4.7 U-0.21, R-4.7 U-0.21, R-4.8 U-0.14, R-7.2
- 68. Further Improvements? + Bigger thermal break at deflection header + VIP insulation (R-40) aligned with thermal 80 break + Insulation outboard framing using clips and rails to support cladding (hybrid)
- 69. How to Improve? 36 inch high spandrel 81
- 70. Full Height Spandrels 82 Standard U-0.17, R-5.8 + R-12 SPF U-0.10, R-9.9
- 71. How to Improve? 83 Standard U-0.17, R-5.8 U-0.13, R-7.9 + more insulation + large thermal break U-0.11, R-9.4 + more insulation + large thermal break + deflection header
- 72. How to Improve? 84 Standard U-0.17, R-5.8 U-0.08, R-12.5 + more insulation + large thermal break + R-18 SPF U-0.06, R-16.0 + more insulation + large thermal break +R-18 SPF + deflection header
- 73. How to Improve? 85
- 74. Energy and Cost Analysis Cost Benefit Analysis ⢠The Impact of Interface Details ⢠Thermal Bridging Avoidance ⢠The Effectiveness of Adding More Insulation ⢠Ranking of Opaque Thermal Performance 88
- 76. Cost Benefit Analysis ⢠8 Archetype Buildings ⢠2 Glazing Ratios per Archetype ⢠3 Climate Zones ⢠10-20 assembly / detail scenarios each ⢠Over 500 discrete examples for energy and cost analysis ⢠Great place for practice⌠90
- 78. Weâre Not Building What We Think 92 ASHRAE Zone 5 Mass Wall Requirement Non-Residential Residential
- 79. Energy Curves 93
- 80. Payback and ROI ⢠Current envelope payback is flawed ⢠Starting R-value is unrealistically high ⢠Actual R-values lower, more savings ⢠Adding insulation not cost effective if 94 details not improved ⢠Thermal performance is not always driving the cost of the envelope
- 81. Multifamily High Rise Example 95
- 82. Multifamily High Rise Example ⢠âExpensiveâ options can look attractive when compared to 96 the cost effectiveness of adding insulation ⢠The cost to upgrade to thermally broken balconies and parapets for the high-rise MURB with 40% glazing may require two to three times the cost of increasing effective wall assembly R-value from R-15.6 to R-20, but ⢠Seven times more energy savings ⢠Better details AND adding insulation translates to the most energy savings and the best payback period
- 83. Commercial Building Example ⢠Curtain Wall and Split Insulated Steel Stud ⢠What is ROI on high performance options? ⢠Triple Glazing? VIP? 97
- 84. ROI 98 155 150 145 140 135 130 125 Baseline More Insulation Triple Glazing AIM with Double Glazing AIM with Triple Glazing AIM with Triple Glazing and Improved Stud Wall Annual Energy (ekWh/m2) 54 yrs 59 yrs 38 yrs Simple Payback U = 0.064 BTU/hr ft2 oF (0.36 W/m2K) per ASHRAE 90.1- 2010 - 51 yrs 18 yrs
- 85. Commercial Building Example ⢠10 stories, 100,000 sq ft ⢠~$50 million dollar project ⢠Chicago climate ⢠ASHRAE 90.1-2010 99
- 86. The Role of Energy Codes and Standards ⢠Industry needs a level playing field ⢠Requiring that thermal bridging at interface details be considered will be the catalyst for market transformation ⢠Incentivize effective solutions ⢠The guide can be leverage to help lead the way to constructive changes ⢠Changes to code are on the way 100
- 87. Next Steps ⢠Making the data in the guide dynamic ⢠Analysis has been ongoing, method of maintenance⌠âthereâs an app for thatâ ⢠Push authorities to adapt code requirements to include more clear approach on opaque envelope ⢠Make informed, data-driven decisions on your next project! 101
- 88. Questions
Notas do Editor
- Weâve come a long way from thinking our walls were nominal insulation to accounting for repetitive thermal bridging⌠to now accounting for details
- Not everything can be defined by areas
- Since the areas were sometimes so hard to find, one way, that we have found incredibly useful to avoid areas, is to use the method of linear transmittance. It may see complex at first, but at its heart it is incredibly simple Lets take a steel stud wall with a balcony slab as an example. Say you have a steel stud assembly and you want to know the affect of a slab going through it. Well, first we have the clear wall structure, which has an amount of heat loss Then you take that same wall structure with a slab running through it, which has a larger amount of heat loss. (show a wall, with and without a slab)The heat loss from the full assembly with the slab, subtract the heat loss from just the clear wall gives the heat loss due o the slab. We give this amount of heat loss due to the thermal anomaly the symbol phi. This is the linear transmittance.
- Mark Lawton This is assigned as a line of heat loss across the face of the clear wall. Then all you need to calculate the assembly U-value is have the clear wall U-value and add in the linear transmittance. Since the linear transmittance is a line, it has one dimension, a length. Therefore, in this case, you need to know how wide your wall is. Since the U-value is in m2, you have to divide by the total area of the wall. So now it doesnât matter how large your wall is, that linear transmittance stays the same. So if you have multiple slabs, you can just keep adding them up to the clear wall U-value.
- So there are three types of categories of these transmittances The clear field value, which are anomalies that occur so often that they can be lumped in with the clear wall value (girt attachments etc) Linear transmittances, which are anomalies that occur straight across the face of a wall, like a corner, or a slab, or a parapet Point transmittances are ones that occur at only small spots on a wall, for example a beam All of these can be used to calculate the overall U-value of the assembly
- So for the overall heat loss from a wall is just the contribution from all the major thermal bridges and the clear field. This approach allows the heat loss from the anomalies like slabs to be separated from the heat loss of the surrounding clear field.
- The purpose of this project was to model a catalogue of details to find the effects of thermal bridging These would be a series of general common details that can be found across north America Using this catalogue, we then developed a procedure that could be used by designers to easily incorporate the data to increase the accuracy and impact of their designs At the same time we modeled specific details to address certain questions that were prevalent in the industry
- We used the 3D software called NX from Siemens Just to brag a bit, this software is also used in designing space shuttles One of its modes, which we use, calculates the heat loss using finite element analysis Mech and some civil people may know what that is, but essentially it breaks down everything into very tiny blocks and calculates all the properties for each of those tiny elements (like the temperature and heat loss)
- The catalogue of details are found in two stages. The first is the description, including the dimensions and material properties for each assembly. The second is the thermal information, which includes the thermal images, clear field U-values and the linear transmittance when applicable. Additionally, there are temperature indices for condensation risk considerations.
- Several assemblies were modeled for varying amounts of exterior insulation. The nominal heat resistance R1D shows what the assembly would be without any thermal bridging. The Ro and Uo values are the clear field transmittance values with clear field thermal bridging included. R and U are the transmittance values when it includes the major thermal bridges (linear and point transmittances) for the assembly dimensions give in the description. Finally there is the psi value, which allows the R and U values to be calculated for any size wall.
- As stated previously, the data sheets also contain temperature indices. These show where some potential areas of concern regarding condensation. These were kept non-dimensionalized so that they can be used for any temperature difference. A value of 1 indicates the interior temperature, while 0 is the exterior temperature. For a given temperature difference, the actual temperature of the area of concern can be calculated using the temperature Note that these temperatures are meant to be used as general guidance. There are many other factors that contribute to condensation.
- -include âreducingâ, âthermal bridgingâ
- Clearly Delineated Parts Part 1 is a stand alone guide on thermal analysis Part 2 is focussed on methodology Part 3 is about market transformation Each part has its own table of contents and introduction The Appendices could be packaged with any of the Parts
- For clear field assemblies, the 3D modeling allows the thermal values for those assemblies with consistent thermal bridges (steel studs, z-girts) to be calculated more accurately compared to 2D. Now lets look at the effects of the large linear transmittances on the overall assembly values. Take two basic assemblies, a concrete mass wall with continuous insulation, and a steel stud insulation with split insulation and horizontal z-girts. From ASHRAE 90.1-2004, the prescriptive are shown above. Both these clear field assemblies meet the requirements. In fact, the mass wall exceeds the requirement by R-2.5, while the steel stud assembly only exceeds by ž of an R-value. Now lets see what happens when we include the effects of slabs and other details.
- For a 10 floor building and 20% glazing ratio, each assembly type has their own details Using the equations shown before, the amount of heat loss can be calculated
- This shows the relative contribution of each of the details to the overall heat loss The bottom shows the overall wall U and R values when the details are taken into account
- This shows the relative contribution of each of the details to the overall heat loss The bottom shows the overall wall U and R values when the details are taken into account
- This chart shows energy usage based on varying amounts of exterior insulation The higher the y bar, higher the energy usage contribution from the building envelope The x bar is increasing amounts of insulation Note that the performance of a wall with no exterior insulation with efficient details is almost identical to a wall with R25 with poor details (you get the same energy performance â but which one is more cost effective?) Note that efficient details installed with assemblies with R15 and R25 yield similar total values. Initial drop between no exterior and R5 big but subsequent drops smaller. This means that at some point increasing thermal performance will yield dismissing returns for energy savings. Big savings at lower Rvalues.
- But how do you deal with this in 2D
- This graph shows the effective R-values of the previous steel stud assemblies for varying amount of exterior insulation, along with the minimum requirements from ASHRAE 90.1-2010 and NECB 2011 The straight light blue line here shows what ASHRAE says the value of the wall would be with continuous exterior insulation. As you can see, with cladding attachments, the effective assembly U-values fall short of that to varying degrees. This indicates that it may be very difficult to reach prescriptive requirements in most climate zones with solely exterior insulation.
- 40 psf
- This is common detail for high rise residential construction Un-insulated slabs cut through wall This is a poor performing detail with psi of 0.34 How could you improve this detail?
- Shelf angles are relatively thin but are made of highly conductive steelâŚShould we ignore these? The results suggest not. This value is just on the edge between average and poor Definitely room for improvement here
- - Spacing the shelf angle out and away from the concrete on knife edges improves and lowers the linear transmittance. With a relatively low incremental cost in this detail, the thermal transmittance at the detail can be significantly lower at an acceptable level - This detail is now good⌠probably sufficient so you donât need to worry about,,, very little cost difference
- In the previous example, the exterior steel stud assembly had a slab edge that was insulated by the exterior insulation. There is still some effect of the I-beam and horizontal girt connection, however the linear transmittance value psi is much lower compared to the previous cases. These 4 examples of slab edges give an impression of the range of transmittances that can be expected from typical slab edge details.
- Consider installation of thermal break at slab Cost premium to detail, but significant return in efficiency Now you are done worrying about this location
- There is a lot of innovative products now available which can be used to cleverly reduce the thermal bridge effect of connections and penetrations through the insulation layer on commercial and domestic construction projects. Whilst these products do not in the main completely eliminate the thermal bridge effect, they reduce it to an acceptable level. In the not too distant future, these products will become as commonplace on building sites as wall ties, insulation batts and window flashing materials. The products have been developed to help reduce the effect of thermal bridges, they just have to be incorporated more into our new-build and retrofit strategies.
- Building materials that create significantly adverse thermal bridges include low resistivity products such as steel and concrete. Wood, plastic and foam, on the other hand, can also be used in structural design and yet have a much higher resistivity. It is important to broaden the pallet of materials used at critical junctions and connection points. In so doing, achieving a building with reduced thermal bridges will be much easier. These materials are widely available, easy to use and inexpensive. Note: Under certain extreme loads these products may not be structurally possible. Your structural engineer will need to calculate the loads to find the best product suitable or to assess whether a thermal break is possible.
- The elements depicted above are typically used at grade, including foam glass as well as autoclaved aerated concrete (AAC).
- Some of the details in the project were used to answer a few questions that are greatly debated in the industry. One of which was âWhat happens when you sprayfoam behind the backpan of a curtain wall spandrel panel?â This first image is a spandrel section without sprayfoam.
- Here you can see that adding an R-11 spray foam only, on average, adds about an R-4 to the assembly This is understandable since there is just so much aluminum in a curtain wall system where heat can bypass the backpan and sprayfoam. The next question designers have to consider is, with that much spray foam and only getting back R-4, is it financially worth it? In some cases, in order to meet code requirements, they may have to go that route.
- It was found that the temperature index of glass and frame was reduced by 10% and 15% respectively by adding the spray foam to the back-pan. Adding sprayfoam made the frame and glass slightly colder. This increases the risk of condensation, depending on the indoor RH and temperature difference.
- Here you can see that adding an R-11 spray foam only, on average, adds about an R-4 to the assembly This is understandable since there is just so much aluminum in a curtain wall system where heat can bypass the backpan and sprayfoam. The next question designers have to consider is, with that much spray foam and only getting back R-4, is it financially worth it? In some cases, in order to meet code requirements, they may have to go that route.
- It was found that the temperature index of glass and frame was reduced by 10% and 15% respectively by adding the spray foam to the back-pan. Adding sprayfoam made the frame and glass slightly colder. This increases the risk of condensation, depending on the indoor RH and temperature difference.
- Better glass doesnât help
- 8 archetypes, 2 glazing ratios, 3 climates
- Last scenario changes split insulated to exterior with clips⌠way cheaper, pays for AIM and TG.