Dear All,

I would like to know if and how you consider ASHRAE's definition of

conditioned space on page 13 of the 2004 version.

Specifically, I am interested to know if you abide by this definition when

listing the conditioned vs unconditioned area? The definition seems really

specific to me, explaining how the system serving a conditioned area must

have a certain capacity per square foot of building area served. It even

goes on to explain how for some spaces one must consider the sum of the

products of various U-factors and wall areas. To me, it does not seem like

one can eyeball all these comparisons and definitions and therefore one has

to go ahead and measure the capacities of systems and compare them with the

area they serve, as well as do computations for rooms that might be

indirectly conditioned.

The PRM report for LEED requires you to list unconditioned and conditioned

areas separately. Do any of you actually go through the calculations to find

out if each single space in your project complies with the definitions put

forward in the 90.1 document? It seems to me like more effort than what it's

worth. I don't recall ever using this classification of unconditioned vs

conditioned anywhere else in the LEED certification process. Especially, I

don't recall ever having to use this definition to do the PRM model itself

under Appendix G. Has anyone come across another place than the PRM report

where the efforts of classifying their spaces accoring to the 90.1

definition above has been rewarded? Please also let me know if any one of

you have ever experienced trouble getting your project LEED certified

because of lack of compliance with this definition.

Lastly, the definition explicitly states that "Crawlspaces, attics, and

parking garages with natural or mechanical ventilation are not considered

enclosed spaces." Do most of you usually leave out car parks from the PRM

area report? I am working on a building where about 60% of the floor area is

parking space, meaning the sum total in my Space Summary will be much less

than stated otherwise in the documents submitted to USGBC. I know that they

are looking for all the total areas to be pretty consistent, and that some

people experience problems when thes total areas don't match up. Has anyone

ever dealt with this issue? If so, how did you deal with it?

I appreciate any response you may have to any or all of my questions.

Sincerely yours,

*Ulrik Welle-Strand Horn *

Dear Ulrik,

Regarding "conditioned space":

"Conditioned space" seems pretty straightforward to me. If, for example,

the cooling system for a 10,000 sq.ft. space has 10 tons cooling capacity,

then the capacity is 12Btu/hr/ft2 (10tons x 12,000 Btu/hr/ton / 10,000 ft2)

This exceeds the ASHRAE threshold of 5 Btu/h/ft2, so it's "conditioned". If

the cooling system was only 4 tons, it would not be "conditioned" (only 4.8

Btu/hr/ft2).

The definition depends only indirectly on "U" values and wall areas. For

any given construction, you choose to provide an air conditioning system or

not. You can also choose to cool a lot or a little. Each choice affects

whether you end up with an air conditioning system greater than 5 Btu/hr/ft2

or not.

Heating works the same way, except the threshold varies by climate zone.

James V. Dirkes II, P.E., LEED AP

Dear Mr. James V. Dirkes II:

Could you please clarify what you mean by "The definition depends only

indirectly on "U" values and wall areas?" I am not designing the mechanical

system, I am only reviewing it. Therefore I can't chose which spaces to

condition. The building has many rooms that don't have any cooling coils,

nor do they have any cold air entering the rooms. These must surely be

indirectly conditioned spaces?

For these indirectly conditioned spaces, I am wondering if I need to perform

the U-factor and surface area calculations and comparisons? The spaces in

question surely are not ventilated with 3 ACH from conditioned spaces. As

there are quite a few of these spaces, I just wanted to know if it is really

common practice to perform these calculations?

Thank you for a quick reply.

Sincerely yours,

*Ulrik Welle-Strand Horn *

Now your question becomes more clear! If your principal concern is

"indirectly conditioned space", I'll say that:

*

I'm making some assumptions without seeing your floor plan, mostly

because I'm not sure why there would be a significant number of spaces which

have no ventilation, heating or cooling.

*

Any "non-conditioned" space within an otherwise conditioned area

will become cool or warm because of heat transfer from the adjoining spaces.

Effectively this means that the main heating / cooling plant serves this

"non-conditioned" area regardless of whether there is ductwork, fan coils,

etc. Therefore, if you want to properly predict the heating / cooling

energy, you must include the heat transfer to and from these

"non-conditioned" areas. I think that's all ASHRAE is trying to account for

in their definition.

*

The energy model, as a result, should include these

"non-conditioned" areas with appropriate definitions for the wall

construction and internal mass. The airflow and heating / cooling capacity

can be zero or perhaps the setpoints can be chosen such that no heating or

cooling will ever be required...

p.s., I don't know about "common practice" since I'm one consultant among

many and have never discussed it with others. If the "non-conditioned"

spaces are a very small portion of the overall area, perhaps they can be

ignored in the energy model. If not, they should be included. Regardless,

the USGBC reviewer will have the final say on the matter. I hope this

helps!

James V. Dirkes II, P.E., LEED AP