Hi all,
First post!! Here goes. Regarding complience with section 5.5 of ashrae 90.1 using the prescriptive path. Although not stated directly does the following in section 5.2 apply? - "Prescriptive building envelope option, provided that 1. Vertical fenestration area does not exceed 40% gross wall area for each space conditioning category".
Thus, if a fabric alteration proposed (as part of the client scope) is the installation of full height glazing (exceeeding the 40%) does meeting the section 5.5 requirement then not apply? and if so, I presume this is taken on a space by space basis, so that spaces with less than a 40% ratio must comply?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
November 20, 2012 - 2:21 pm
Two related but separate issues. The 40% glazing applies to the building as a whole so I do not think it impacts a tenant improvement unless perhaps this change puts the whole building over 40%. On a whole building basis you would not meet the prescriptive requirements if the glazing is over 40%. You would then need to show compliance through the performance path in Section 11.
You would need to meet the prescriptive requirements for the new windows, so 5.5 does apply in either case.
In general floor-to-ceiling glass would certainly not be considered a positive green building strategy as it usually increases energy use and is detrimental to thermal comfort and good daylighting.
Graham Langton
Building Services EngineerPM-Group
1 thumbs up
November 21, 2012 - 7:23 am
Marcus,
Appriciate the reply. Just to confirm on which complience path to take - In my case as discribed, if the new glazing contained within the scope for is above 40% for a perticular facade, but the building (which is outside the scope and the LEED boundary) remains under 40%, the path is the prescriptive method? Otherwise, if this brought the entire building over 40%, the section 11 path would have to be taken?
In general, what elements dictate the use of section 11? Is it the case that for any reason, if any section of the relative prescriptive requirements can not be met, this dictates that the section 11 path must now be taken?
If that is the case an accurate energy model (complete, and containing elements likly not within the scope e.g. lighting gain, full fabic specifications) must be created. From a simulation perspective the creation of this full dynamic simulation model seems very onerous and potientially not relative to the scope of works.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
November 21, 2012 - 11:04 am
In general the two approaches are related to code compliance. The prescriptive approach is basically a checklist - meet all of the prescriptive requirements in each section X.5. One would use the performance approach (Section 11 model) if the project cannot demonstrate compliance with one or more of the prescriptive requirements. In order to be in compliance through the performance path the overall energy cost must be better than the baseline.
So yes if you cannot meet a single prescriptive requirement then the performance path must be followed.
You are certainly correct regarding difficulty level and that is one reason why designers strive to meet the prescriptive requirements instead of ignoring them.
It is not fully clear to me how the 40% prescriptive requirement is applied or enforced in a CI project in either path. This is typically not an issue since the envelope of the building is very rarely changed to the degree that you describe within a CI project scope.
The way that I would approach it - if the whole building is under 40% after the project implementation I would ignore the issue and follow the prescriptive path. If the whole building is already over 40% (or will be over 40% as a result of the project) I would follow the performance path or submit a LEED Interpretation seeking an exemption from doing so.