[Equest-users] Slab extending through wall

[Kapil Upadhyaya]

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<http://www.kirksey.com>
o  713 426 7508  |  f  713 850 7308  |  kapilu at kirksey.com<mailto: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.
[cid:image003.png at 01CEDA3E.13EE1280]
[cid:image004.png at 01CEDA3E.13EE1280]

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<http://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<http://www.rdhbe.com/database/files/library/Balcony_and_Slab_Edge_Thermal_Bridges___1___R_values_and_Energy_Code_Canada__Sept_24_13.pdf>

Regards,
Bill

William Bishop, PE, BEMP, BEAP, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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P   Sustainability – the forest AND the trees. P



From: equest-users-bounces at lists.onebuilding.org<mailto: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<mailto: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<mailto:dgriffin at etcgrp.com>
www.etcgrp.com<http://www.etcgrp.com>

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Energy Engineering for a Sustainable Future
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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<mailto: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
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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<mailto:kcoleman at nexant.com>

P Please consider the environment before printing this email


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