Above Grade External Wall Area Weighted U-value Calculation

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Dear All,

I have submitted a project for review and received a comment requesting me
to include the concrete slabs between floors (Not drop beams) in the
External Wall' area weighted U-value calculation, the reviewer believes that
concrete slabs acts as a thermal bridge and should be insulated from the
outer side (The slab thickness), my understanding that concrete slab
shouldn't be regarded as a part of the occupied area and therefore shouldn't
be included in the area weighted U-value calculation.

I would appreciate if you can advise on the above query!

Thanks & Best Regards,

Sherif Farag

Sherif Farag's picture
Joined: 2012-12-25
Reputation: 0

What the reviewer said is technically correct, but not often done in
simple UA calculations because of the difficulty is calculating U-values
for such thermal bridging.
The effects can be substantial, especially for heavy-mass concrete
construction. I've seen in China there's a standard method to account
for thermal bridging in calculating wall U-values. Have not seen a
similar technique for the US, don't know about the situation in Europe.


Joe Huang's picture
Joined: 2011-09-30
Reputation: 406

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Mike Kennedy's picture
Joined: 2011-10-02
Reputation: 0

ISO has a standard methodology for accounting for thermal bridging as
described in ISO 10221 and it is a standard part of using the ISO 13790
methodology for heating/cooling calculations. ASHRAE has researched a
similar method which is described in ASHRAE Research Project 1365-RP
?Thermal Performance of Building Envelope Details for Mid- and High-Rise
Buildings? I don't know if this is going to lead to an ASHRAE standard or

That said, the ISO method is not a US standard and doesn't have ANSI or
ASHRAE equivalents, Further, I don't know of any specific code/energy
standard requirements to use the ISO standard.

Ralph T Muehleisen

Ralph Muehleisen3's picture
Joined: 2012-07-27
Reputation: 0

It isn't like the reviewer is making things up, I think it is pretty clear from the language of 90.1 Appendix G that you have to account for these exposed slab edges. Frankly 90.1-2007 is a little more forgiving, allowing for area-weighted U-factors for the entire wall, I believe previous versions of Appendix G (2004 for example) actually specifically required that you model uninsulated assemblies as their own discrete wall types. See excerpts below:

ASHARE 90.1-2007 Appendix G:
[cid:image002.png at 01CEBDBC.188EDC00]

ASHRAE 90.1-2004 Appendix G:
[cid:image006.jpg at 01CEBDBC.188EDC00]
We usually model the slab edges as separate walls (only about 1 foot high) and make the other walls "shorter" when you take out this area. If you don't do this in the wizard, you can set up a macro in the .inp to to it, but it takes a bit of work. Frankly if you already have the model set up without slab areas, it seems much easier to to do the weighted U-factor calculation.

Nathan Miller - PE, LEED(r)AP BD+C, CEM

Nathan Miller's picture
Joined: 2011-09-30
Reputation: 200

You might find the report by morrison hershfield useful for researching

*Jeremiah D. Crossett

Jeremiah Crossett2's picture
Joined: 2012-12-14
Reputation: 0

Jeremiah/ Nathan/ Joe/ Ralph/ and Mike;

Many thanks for your informative input, really appreciated;

Best Regards,

Sherif Farag

Sherif Farag's picture
Joined: 2012-12-25
Reputation: 0