2015 NECB Question, Intersecting Floor and Wall Area

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Good morning,

For you Canadians, I have a question about how to model the area where a
floor slab intersects the exterior wall.

Specifically, Section of the 2015 NECB says that "wall
assemblies shall be considered to include all related structural framing
and perimeter areas of intersecting interior walls *but shall not include
the perimeter areas where floor or roof slabs interrupt the wall's
construction." *

There is a reference to Note A- which provides an illustration
to depict the extent of the surface that is considered as a wall at the
intersection with a floor. In this note, it also says "This Code may have
different requirements for the perimeter area of floors."

Since I haven't seen any such requirements (how anyone else), I'm not sure
how to treat this area.

When I have a building in which the insulating layer in the exterior wall
assembly is intersected by the floor slab (thus breaking the insulation
plane), I typically model my Proposed building with a separate assembly for
the slab edge and assign a low R-value (reflective of a wall with no

My question then is, in this case, what would people model for the
Reference model in terms of R-value at the slab edge (where the floor
intersects the wall)?

As mentioned, I have not seen anything in NECB that specifically deals with
the treatment of the slab edge (other than at grade level). Would the
Reference model carry the same R-value as the Proposed model?

Thank you in advance for your input and thoughts.


Chris Hadlock's picture
Joined: 2012-01-23
Reputation: 0

Hi Chris,

Not a Canadian but this situation sounds very similar to what I also
frequently encounter when I have to model above-grade wall floor edges for
the baseline and proposed models. I couldn't find any mention of how to
model the baseline floor edges in ASHRAE 90.1-2016. For the baseline model
floor edges, I typically use the baseline above-grade wall
constructions/U-values for the floor edges and model the proposed floor
edges without any insulation as you have done. You can also try looking at
Energy Star's Multifamily High Rise Simulation Guidelines Version 1.0,
December 2017 Example 6-2, which supports your idea of modeling your
proposed floor edges without any insulation.

Kind regards,

On Thu, Apr 25, 2019 at 12:02 PM Chris Hadlock via Equest-users <

*Calvin Chu, EIT*
Energy Modeler
646-780-5536 | cchu at brightpower.com

11 Hanover Square, 21st Floor
New York, NY 10005

cchu's picture
Joined: 2018-07-12
Reputation: 0

The NECB 2011 and 2015 assume that the Reference case envelope is perfect and that there are no penetrations of the insulation plane. For the Proposed case, both standards give you the 2% opt out clause ? if the total penetrated area is less than/equal to the maximum of 2%, it can be ignored. If the area is greater than 2%, you need to calculate the effect of these penetrated areas on the overall wall R-value. The NECB 2017 mandates calculation of linear transition and point transition thermal bridges but still maintains that the Reference case envelope is perfect ? unlike the BC Hydro assumption that applies an insulation derating factor to the Reference case effective insulation.

I did a calculation of the effect of balconies on a typical MURB with 5% total balcony slab edge are total wall area. The Shock balcony thermal break system would take over 20 years to payback ? Toronto climate much longer in Vancouver. That is ignoring thermal comfort issues, potential condensation issues, etc.

Christopher R Jones, P.Eng.
T+ 1 416-644-0252

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Jones, Christopher2's picture
Joined: 2017-10-12
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