[Equest-users] Slab extending through wall

[Daniel Knapp]

I think the key factor in comparing the case of a balcony and a slab edge flush with the exterior wall face is the effect of the balcony on the temperature of the concrete at the plane of the exterior wall. 

For the slab edge: the temperature of the slab edge depends on how quickly heat is transferred to the outdoor air, with a thin boundary layer of air slowing the transfer of heat between the concrete and the outdoor air. The temperature of the concrete will depend strongly on the rate heat transfer from the concrete to the outdoor air over a small surface area. 

For the balcony: the temperature of the concrete in the plane along the exterior wall surface will depend on the rate of heat transfer to the protruding balcony. The temperature of the balcony will depend on how quickly heat is transferred to the outdoor air over a much larger surface area than in the slab edge case. Since the heat transfer to the outdoor air from the balcony happens over a much larger surface area, it is possible for the temperature of the balcony to be closer to the temperature of the outdoor air than in the slab edge case in which case the balcony would lose heat faster. 

In determining which configuration leads to higher rates of heat loss, a lot depends on how the transfer between the concrete surface and the outdoor air is modelled. This strikes me as the item of greatest uncertainty in the Morrison Hershfield report, and the report does call this out on page 23: “Selecting heat transfer coefficients for accurate calculations can be a challenge for components where a larger percentage of the overall thermal resistance is the surface resistance. However, the thermal performance of insulated opaque building envelope components is the primary output of this project."

All the best,
Dan

—
Daniel Knapp, PhD, P Phys, LEED® AP O+M
danielk at arborus.ca

Arborus Consulting
Energy Strategies for the Built Environment
www.arborus.ca
76 Chamberlain Avenue 
Ottawa, ON, K1S 1V9 
Phone: (613) 234-7178 ext. 113
Fax: (613) 234-0740




On Nov 6, 2013, at 8:45 AM, Bruce Easterbrook <bruce5 at bellnet.ca> wrote:

> I would think the effect of the balconies would increase the thickness of the exterior laminar layer on the building.  The air flow is perpendicular to the fins.  In calm conditions the main driver is going to be stack effect.  The exposed edges are on a flat surface and would have a normal boundary layer.  Without a projection they would offer little resistance to the airflow from the stack effect.
> A 60% reduction in effective R-value for a building is quite a significant effect depending on where you are starting from.  A high ratio glass building is a low starting point.
> Bruce Easterbrook P.Eng.
> Abode Engineering
> 
> On 05/11/2013 05:25 PM, Robby Oylear wrote:
>> It surprises me that the report indicates a better R-value for the fin effect of a concrete balcony vs. the simplified exposed slab edge approach.  Seems to violate the basic teachings heat transfer 101.  I can't think of a logical explanation of how that could be the case and would chalk it up to intricacies of the modeling tool.  However I agree with Nathan that since the report doesn't show nearly any difference between the fin effect vs. the exposed slab edge that it's not necessarily worth the complication in modeling unless the balcony slabs are the focus of the study.
>> 
>> 
>> On Tue, Nov 5, 2013 at 2:00 PM, Nathan Miller <nathanm at rushingco.com> wrote:
>> Kapil,
>> 
>>  
>> No, I am only accounting for the “face” of the balcony, as if you sheared it off at the plane of the wall.
>> 
>>  
>> That is my point, the fancy thermal modeling in the report, which is supposed to account multi-dimensional fin-effects doesn’t come out with much different result than if you just take a simplified approach like a typical UA-trade-off calculation (one directional heat transfer, no fin effects accounted for). So by that line of reasoning, you would likely be OVER ESTIMATING heat loss if you throw in all three heat-exchanger surfaces that you mention in your methodology. I’d be interested if you compared your calculation results to the thermal modeling in the report if you feel like a little intellectual exercise.
>> 
>>  
>> Nathan Miller - PE, LEED®AP BD+C, CEM
>> 
>> Mechanical Engineer/Senior Energy Analyst
>> RUSHING | D 206-788-4577 |O 206-285-7100
>> 
>> www.rushingco.com
>> 
>>  
>> From: Kapil Upadhyaya [mailto:KapilU at kirksey.com] 
>> Sent: Tuesday, November 05, 2013 1:53 PM
>> To: Nathan Miller; Bishop, Bill; David Griffin; 'Coleman, Kevin'; equest-users at lists.onebuilding.org
>> Subject: RE: [Equest-users] Slab extending through wall
>> 
>>  
>> Nathan,
>> 
>> In the Balcony calculation, are you accounting for both the 'edges' and 'exposed area' of the balcony ?
>> 
>> In my opinion, a concrete balcony should be accounted for as a heat exchanger with 3 surfaces (perimeter edges, upper surface, lower surface); it looks like you are only accounting for the edges.
>> 
>>  
>> I had a 'heated' discussion about this with a vendor recently and the above is more or less a conclusion of that. Some of the structural thermal break products can be too expensive to justify based on energy savings, if area of balconies is not accounted for; the bigger your balcony, the better your payback from thermal breaks.
>> 
>>  
>> Best,
>> 
>>  
>> Kapil Upadhyaya, LEED AP
>> Associate
>> 
>> Kirksey | Architecture
>> 6909 Portwest Drive  |  Houston Texas 77024  |  www.kirksey.com 
>> o  713 426 7508  |  f  713 850 7308  |  kapilu at kirksey.com
>> 
>> 2012-2013 Firm of the Year 
>> Texas Society of Architects
>> 
>>  
>> From: Nathan Miller [mailto:nathanm at rushingco.com] 
>> Sent: Monday, November 04, 2013 5:27 PM
>> To: Bishop, Bill; David Griffin; 'Coleman, Kevin'; equest-users at lists.onebuilding.org
>> Subject: Re: [Equest-users] Slab extending through wall
>> 
>>  
>> To me the most interesting part of the report isn’t the conclusion that thermally broken balconies and slab edges give significantly better thermal performance (we all know that intuitively). Instead it gives a way of back checking what I would consider to be “standard practice” for calculating area-weighted U-factor for these same assemblies using default ASHRAE U-values to sophisticated thermal model results. Here are my thoughts, checking my calcs and comments appreciated.
>> 
>>  
>> Executive summary: It looks like our standard assembly performance tables and area-weighted U-factor calculations get us “close enough” to the results from this report that it doesn’t seem to suggest we should switch methodologies.
>> 
>>  
>> 
>> Comparison calculation for steel-framed walls (stud insulation + exterior rigid):
>> 
>> For a 16” OC metal stud wall, R-12 Batt cavity insulation and R-5 continuous rigid insulation,                         the report states that the Effective R-value for the wall + slab will be R-7.4. This is based on 8’8” floor-to-floor, and 8” slab.
>> 
>>  
>> Using ASHRAE 90.1-2007 Table A3.3 for steel framed walls, we would get an effective U-factor for the wall portion of U = 0.0785 (interpolating between the R-13 + R-11 batt values). If we use Table A3.1A for the exposed slab edge (assuming 8” normal weight solid concrete walls), we get a U-factor of U = 0.740 for the slab edge.
>> 
>>  
>> Doing an area weighted U-factor calc: Ueff = U1*A1+U2*A2/(A1+A2) or in this case, factoring out the Length of the wall section we use: U1*H1+U2*H2/(H1+H2) = (0.0785*8 + 0.740*8/12)/(8+8/12) = 0.1294 or an effective R-value of 7.73, which is pretty close to the R-7.4 claimed, a difference of about 4% in U-value. Maybe I’m cynical, but to me that seems close enough.
>> 
>>  
>> I only spot checked this assembly, so maybe there is more impact with different construction                         types.
>> 
>>  
>> Another interesting tid-bit: Apparently the much vaunted fin-effect is actually pretty minimal. If you look at the results of the exposed slab-edge condition vs the full balcony fin-effect, the balcony scenario actually shows slightly BETTER thermal performance. Odd. 
>> <Mail Attachment.png>
>> <Mail Attachment.png>
>> 
>>  
>> Nathan Miller - PE, LEED®AP BD+C, CEM
>> 
>> Mechanical Engineer/Senior Energy Analyst
>> RUSHING | D 206-788-4577 | O 206-285-7100
>> 
>> www.rushingco.com
>> 
>>  
>> From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bishop, Bill
>> Sent: Monday, November 04, 2013 1:42 PM
>> To: David Griffin; 'Coleman, Kevin'; equest-users at lists.onebuilding.org
>> Subject: Re: [Equest-users] Slab extending through wall
>> 
>>  
>> Here’s a timely report on effective R-values of assemblies with slab/balcony thermal bridges:
>> 
>> The Importance of Slab Edge and Balcony Thermal Bridges
>> 
>>  
>> Regards,
>> 
>> Bill
>> 
>>  
>> William Bishop, PE, BEMP, BEAP, LEED AP | Pathfinder Engineers & Architects LLP
>> 
>> Senior Energy Engineer
>> 
>> <Mail Attachment.jpeg>
>> 
>>  
>> 134 South Fitzhugh Street                 Rochester, NY 14608
>> 
>> T: (585) 325-6004 Ext. 114            F: (585) 325-6005
>> 
>> bbishop at pathfinder-ea.com           www.pathfinder-ea.com
>> 
>> P   Sustainability – the forest AND the trees. P      
>> 
>>  
>>  
>> From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of David Griffin
>> Sent: Monday, November 04, 2013 4:20 PM
>> To: 'Coleman, Kevin'; equest-users at lists.onebuilding.org
>> Subject: Re: [Equest-users] Slab extending through wall
>> 
>>  
>> Kevin,
>> 
>>  
>> I’m fairly sure eQUEST is not even going to closely approximate the heat loss/gain accurately unless you dial the loads in manually which means you have to calculate them by hand first. I suggest referring to ASHRAE-D-RP-1365 for guidance on estimating the loads. The section you are interested in starts on page 35.
>> 
>>  
>> Thanks,
>> 
>>  
>> 
>> David W. Griffin II
>> 
>> Energy Analyst
>> 
>> ETC Group, LLC
>> 
>> 801-278-1927 x 128
>> 
>> Cell 480-736-2945
>> 
>> dgriffin at etcgrp.com
>> 
>> www.etcgrp.com
>> 
>>  
>> <Mail Attachment.jpeg>
>> 
>> Energy Engineering for a Sustainable Future
>> 
>> 1997 South 1100 East, Salt Lake City, UT 84106-4471
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>> 
>>  
>> From: Coleman, Kevin [mailto:kcoleman at nexant.com] 
>> Sent: Monday, November 04, 2013 11:56 AM
>> To: equest-users at lists.onebuilding.org
>> Subject: [Equest-users] Slab extending through wall
>> 
>>  
>> I am evaluating a new 10-story hotel design that is proposing glass curtain walls and pre-tensioned concrete floors that project beyond the walls. On 3 sides the projections are minimal (6 inches). However, on one side, the projections extend about 3 ft.
>> 
>>  
>> For the small projections, I intent to use the following method recommended in the thread below. “In screen 4 of 25 of the DD shell wizard you can specify "Slab Penetrates Wall Plane" (check box).”
>> 
>>  
>> I am interested in any tips for modeling the thermal impacts of the larger projections. Thoughts? (I plan to use window shading for the light/shading impacts.)
>> 
>>  
>> I am hopeful that the analysis will help convince the design team to incorporate some insulation and thermal breaks!
>> 
>>  
>> Thanks in advance!
>> 
>> Kevin
>> 
>>  
>>  
>> Brian Fountain bfountain at greensim.com
>> 
>> Mon Dec 6 11:45:18 PST 2010
>> 
>> Previous message: [Equest-users] balcony
>> 
>> Next message: [Equest-users] Shading
>> 
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>> 
>>  
>> In screen 4 of 25 of the DD shell wizard you can specify "Slab
>> 
>> Penetrates Wall Plane" (check box).  If you do, eQUEST created a new
>> 
>> wall type of height equal to your slab thickness and composed of 1' of
>> 
>> concrete plus any slab edge insulation you specify on the same wiz
>> 
>> screen.   That provides a parallel path for the heat transfer from the
>> 
>> balcony or slab edge.  Not exactly 2D heat transfer modelling -- but at
>> 
>> least you aren't ignoring the effect of the slab edge or balcony.
>> 
>>  
>> Kevin Coleman, CEM, LEED AP  ■  Project Manager ■   Nexant, Inc.  ■ Demand Management
>> 
>> 1232 Fourier Drive, Suite 125 ■ Madison, WI 53717 ■  608.824.1230 ■  kcoleman at nexant.com
>> 
>>  
>> P Please consider the environment before printing this email
>> 
>>  
>>  
>> 
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