Christian,
I agree that it is important for designers to understand the effects of thermal bridging at transitions in typical construction details. But convincing developers to spend the extra time and construction cost to reduce these thermal bridges is difficult considering the payback periods range from 30-40 years depending on the project and location.
[cid:image003.png at 01D09C46.E75BA0D0]
Christopher Jones, P.Eng.
Senior Engineer
WSP Canada Inc.
2300 Yonge Street, Suite 2300
Toronto, ON M4P 1E4
T +1 416-644-4226
F +1 416-487-9766
C +1 416-697-0056
www.wspgroup.com
A final comment on the original question of modeling thermal bridges in e-QUEST (or any
other whole-building energy simulation program). ASHRAE TC 4.7 had sponsored work on
this topic 14 years ago (2000-2001), "MODELING TWO- AND THREE- DIMENSIONAL HEAT TRANSFER
THROUGH COMPOSITE WALL AND ROOF ASSEMBLIES IN TRANSIENT ENERGY SIMULATION PROGRAMS
(1145-TRP)". The work was done by Enermodal in association with Oak Ridge National
Laboratory and the Polish Academy of Sciences. The solution was pretty much what I had
mentioned previously, i.e., use a 2-D conduction program (HEATING-7) to calculate response
factors, although the project went one step further to derive a fictitious 3-layer 1-D
wall section with the same dynamic thermal properties. For more information, please see
the full report available from ORNL
athttp://web.ornl.gov/sci/roofs+walls/research/detailed_papers/whole_bldg/ .
Although the project was successful technically, it seemed to have been ignored by the
simulation community, and even in many ways by TC 4.7 itself! To me,the lesson is that no
matter how good is the widget, there always has to be a tech transfer and dissemination
effort lest the widget just sits on the shelf. As a reaction to this thread, I am
currently bringing up the subject within TC4.7 whether we need to do a small follow-on
project to do a guideline on modeling 2-D/3-D heat transfer along with a computer tool
that makes it no more difficult (well... maybe a little more) to do than standard 1-D heat
transfer.
If there's anyone on this bulletin board who thinks such a project would be useful,
please let me know as I make my case to TC 4.7.
Joe
(currently chair of TC 4.7)
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
Joe,
If people are concerned about modeling for LEED baselines, there's no need
to understand bridging's impact because the 90.1 tables already account for
the bridging.
If, however, we want to prepare models that are true to the construction
reality, we need to have an understanding of how heat transfers in real
constructions.
Up until a few short years ago, that understanding had completely escaped
me. We'll just say that for 35 of 40 years of continuous involvement in
commercial building heating and cooling systems, I didn't know that my
U-values calculations were routinely off by 15-20%.
I bet I'm not alone!
Joe,
Thanks for the link to that TC4.7 report at ORNL! It looks like it has a lot of good information. I?m guessing the sample DOE2.1 code could be used as the basis for wall and roof assembly simulation in software compatible with DOE-2 envelope simulation methodology.
My main question is how do we use this data in the simplest , broadest and most consistent way? Do the research findings imply using different assembly U-factors than what is published in 90.1, Appendix A? Jim mentions that the tables already account for thermal bridging, but the Report was published in 2001, and I see no obvious differences between the Appendix A table values in 90.1-1999 and subsequent versions. The Report also looked at thermal capacitance as the second property needed for describing equivalent 1-D wall assemblies, but Appendix A only lists HC (Heat Capacity) values for mass walls and concrete layers.
I haven?t reviewed the whole report to see if it provides insight on modeling common thermal bridges in contemporary assemblies ? Z-girts, clips, angle irons etc. The tech transfer and dissemination that you mention should include incorporation into the standard requirements for modeling assemblies in 90.1, so that we are all still using similar baselines and modeling like-for-like.
Regards,
~Bill
William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
[cid:image002.jpg at 01D15E67.37063090] [cid:image004.jpg at 01D15E67.37063090]
134 South Fitzhugh Street Rochester, NY 14608
T: (585) 698-1956 F: (585) 325-6005
bbishop at pathfinder-ea.com www.pathfinder-ea.com
[http://png-5.findicons.com/files/icons/977/rrze/720/globe.png]Carbon Fee and Dividend - simple, effective, and market-based.
My understanding from the report and what Joe is saying is that we should not use simply the overall U-value of a construction when modeling the envelope otherwise the transient response factors are neglected.
The 90.1 construction overall U-values includes the thermal bridging of the typical framing but ignores the thermal bridging of the transitions (floor slabs, corners, wall-window transition, etc.). In their New Construction Program, BC Hydro mandates that all these thermal bridges be accounted for in both the proposed and baseline cases. For more information see:
https://www.bchydro.com/content/dam/BCHydro/customer-portal/documents/power-smart/builders-developers/base-building-u-values-03102015.pdf
Their webpage provides an Excel workbook for calculating the thermal perfomance including all interface details and also provides a working example. See the Resources section on this page:
https://www.bchydro.com/powersmart/business/programs/new-construction.html#thermal
[cid:image003.png at 01D09C46.E75BA0D0]
Christopher Jones, P.Eng.
Senior Engineer
WSP Canada Inc.
2300 Yonge Street, Suite 2300
Toronto, ON M4P 1E4
T +1 416-644-4226
F +1 416-487-9766
C +1 416-697-0056
www.wspgroup.com
One more thought on this topic.
The EE4 modeling program allows you to account for the effects of thermal bridging in envelope constructions. One defines the insulation layer along with the material and percentage of framing. The program then calculates the weighted average conductance, density, and specific heat of the combined layer and defines it as a MATERIAL in the DOE2 input file. It further defines the layers of the construction including this newly created material. I use this functionality to create the material and then copy the definition into the eQuest file I am working on. Below is a table of suggested framing percentages for different constructions.
[cid:image006.png at 01D15E70.EB820590]
[cid:image003.png at 01D09C46.E75BA0D0]
Christopher Jones, P.Eng.
Senior Engineer
WSP Canada Inc.
2300 Yonge Street, Suite 2300
Toronto, ON M4P 1E4
T +1 416-644-4226
F +1 416-487-9766
C +1 416-697-0056
www.wspgroup.com
When using the Building Envelope Thermal Bridging Guide, the recommendation is to build up the primary assembly in layers as you always would so your energy modeling program can still calculate relatively accurate response factors. Add additional heat loss by accounting for interface details and subsequently adjust your overall assembly U-value by reducing the thickness or increase the conductivity of the insulation layer. This way, you get the most accurate U-value and you keep the response factors for the effects of mass, at least as they apply to the majority of the assembly.
This method is easy to implement and would not require additional expertise in 2D and 3D thermal modeling. The benefit of the thermal bridging guide is that the data is catalogued so the thermal modeling need not be recreated for every project. There are also other sources of linear transmittance (though not all applicable to North American construction), including Passive House details, ISO 14683, etc.
Christian