Use interior dimensions or exterior dimensions?

[Matt Sharpe]

For,
Cond. Area
Foundations (knowing some stone walls will be 2 ft. thick)
Volume

[Gamaliel Lodge]

Outside dimensions are the standard for energy modeling. Here is a technical reference: http://www.nrel.gov/docs/fy06osti/38600.pdf (See below for corrections)

[Chris West]

Hi Matt and Gamaliel,

Gamaliel, I took your advice and read up in the document you attached. I don’t know what you are talking about though. What Matt was asking, I think, is what measurements we need to take to enter the Conditioned Area for Optimiser as well as the Volume for the infiltration. The Foundations is another question but an important one if we want these things to calibrate right (which is a problem).

The document says that the Gross Conditioned Floor Area of a building is defined as: ” Total interior floor area of a building’s conditioned spaces, measured from the inside surface of the exterior walls or from the interior surface of walls of adjoining buildings. The areas of interior walls, columns, and pillars are included in this measurement.”

This means your advice to use outside dimensions is wrong according to nrel. If Optimiser is using the Gross Conditioned Floor Area as the definition but is calculating to the Gross Building Floor Area (defined as the exterior dimensions) then we have a problem of semantics and your program should NOT say “Enter Conditioned Area here” but say, “Enter Gross Building Floor Area” and give a definition.

Likewise, the NREL document states that the Building Conditioned Volume is defined as the
Volume inside the building envelope of the conditioned spaces. This metric can be calculated as
the volume of the building if every space is conditioned or on a floor-by-floor basis. For spaces
with vertical walls and horizontal ceilings and floors, this is calculated as the Gross Conditioned
Floor Area times the height from the top surface of the finished floor to the top surface of the
finished floor separating levels of the building or to the inside surface of the roof for the top floor.

This is the Infiltration Volume that we need to include when we are calculating the ACH50 from the CFM50 number we get from our Blower Door Test.

Could you take a look at my comments and let me know if I am on the right track here.

Does Optimiser use the Gross Conditioned Floor Area calculation for Conditioned Floor Area in its calculations or does it use the Gross Building Floor Area?

Again, for Volume, Does Optimiser use the Wall Heights multiplied with the Gross Conditioned Floor Area for its infiltration volume calculation or does it use the Gross Conditioned Floor Area multiplied by the ceiling heights (plus extra calculations for non-rectangular room spaces like vaulted ceilings, etc.)?

Thank you.

Chris West
Caleb Contracting, LLC
Cambridge, Vt

[Gamaliel Lodge]

I assumed that this NREL document would be consistent with the RESENET HERS Standard that we used in development of our program, but it appears that on the subject of Conditioned Floor Area it is not. I’m a little puzzled by this. The HERS Standard references ANSI Z765-1996 for measurement of Conditioned Floor Area and the ANSI standard specifies exterior finished surfaces as the boundaries.

I also have the impression that most publicly available sources for conditioned area of buildings (e.g. assessors records) have been calculated to exterior dimensions.

The most important dimensions for model accuracy are the surface areas of exterior envelope components both standards agree that these should be calculated using exterior dimensions. We use the conditioned floor area to make initial estimates of other exterior surface areas, so it is most convenient for it to also be calculated with exterior dimensions.

The blower door CFM50 number is what is used in the infiltration calculations, so volume is not a critical dimension. The default Volume calculation in OptiMiser is essentially floor area multiplied by wall height plus conditioned crawl space volume. We also attempt to also estimate the volume of the joist area between conditioned floors. We do not attempt to estimate the impact of vaulted ceilings. This is basically impossible without a detailed geometric layout.

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