LEED Review Comment on U-Value Input Method

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We received the following comment on recent LEED review:

"The simulation input screenshots, provided in the EAc1 modeling narrative
report, indicate that the exterior wall and roof constructions were modeled
as QUICK surface type (UValue Input specification method), which does not
account for the timedelayed heat flow through the constructions as required
by Section G2.2.1(c). Revise the Proposed and Baseline models so the
exterior walls and roof surface types are modeled as DELAYED (Layer Input
specification method) with the thermal mass effects of the constructions
taken into consideration. In addition, provide a revised LVI report for
each model reflecting the changes."

Section G2.2.1(c) describes modeling software requirements, but I don't see
anywhere else in Appendix G that specifies that thermal mass effects have to
be included in the baseline model.

Previous review comments on other projects have led me to believe that
U-value input was the correct method to set up the baseline model.

If I revise the model to input each layer, what layers do I input?
90.1-2007 Appendix G states to use steel-framed walls, and the Tables
provide minimum R-value for insulation and overall assembly U-value. But it
does not appear to provide such other items as stud spacing, sheathing, or
even what material is on the outside of the building (for exterior walls).
Has anyone else had this type of comment before or are you using the layer
input method for baseline models? It seems that if I need to specify
layers, the resultant U-value should equal exactly the minimum U-value per
the 90.1 tables. That would lead me to believe that there might be
different combinations of layers that result in the same U-values but result
in different energy use in the baseline, and obviously I would want to have
the highest energy use for LEED purposes.

Michael Mantai's picture
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Michael,

My understanding has always been that delayed construction should be
used, though I can't find exact wording in Appendix G that requires it
other than G2.2.1(c). For other components/layers of steel-framed walls,
look to A3.3.1, and to Table A3.3 for assembly U-Factors for different
stud spacing. You should be pretty close to the required U-Factor if you
use the correct materials and thicknesses from A3.3. Yes, you may need
to tweak a layer or two to get the construction to match the U-Factor
exactly. As described in other posts, once you create these
constructions for the baseline, copy them for future models.

Regards,
Bill

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Michael, I'd advise that you email the project coordinator (or whatever GBCI calls the "head" of a project review team). Usually they will answer relatively quick and easy questions so that you don't have to risk improperly addressing a comment.

Ask them where in Appendix G it specifically requires the time delayed method be used.

James Hansen, PE, LEED AP

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Michael,

Agreed, appendix G does not specifically states that one needs to model delayed construction. However, going by the semantics used in Appendix G, one can conclude that delayed construction should be used. Consider the following:

Table G3.1-5 Building Envelope, under Baseline Building Performance, part (b) Opaque Assemblies: states that Opaque assemblies......shall confirm with the following common, lightweight assembly types and shall match the appropriate assembly U-factors.....

**The use of the term 'assemblies' suggests the need to model the whole assembly rather than only the U-factor**

To answer your other question, how do you know what comprises of the baseline opaque assembly, I'll suggest use Appendix A. For example, for steel framed walls, see section A3.3.1 General, you'll find the assembly layers that you can use to model the baseline above grade walls. Similarly, you can use respective sections for roof, floor, etc. to determine the baseline assembly layers.

If I remember correctly, somebody in the past has been kind enough to post the baseline assemblies that can be copied to the inp file (or imported into the inp file). Search the archives.

Furthermore, eQUEST has an extensive library of materials that one can use, which includes the thickens, specific heat and density of the material. You can create your own materials by using the ASHRAE Handbook of fundamentals, chapter 26 (2009).

Thanks.

Best regards,

Gaurav

Gaurav Mehta, LEED? AP BD+C

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FYI, Simulation General Requirements as laid out in 11.2 of the 90.1 User's Manual specifically call for the treatment of Thermal Mass Effects in the Minimum Modeling Capabilities. (see 11.1.2.3 and as already mentioned G2.2.1.c) and notes that "A building's ability to absorb and hold heat varies with its *type of construction* and with its system and ventilation characteristics. This affects the timing and magnitude of loads handled by the HVAC system. Simulation programs must be able to model these effects".

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But the Simulation General Requirements are for the simulation model itself
and it's capabilities, they do not address the simulation inputs.

I think this section of the code is what governs this issue:

Opaque Assemblies. Opaque assemblies used for new buildings or additions
shall conform with the following common, lightweight assembly types and
shall match the appropriate assembly maximum U-factors in Tables 5.5-1
through 5.5-8:

But I disagree with Guarav's interpretation for these reasons. The use of
the word assemblies might "suggest" the need to model the whole structure
but the use of "lightweight" in the sentence, and it's location after the
word *shall*, is the key. Those U-values in Tables 5.5-1 through 5.5-8 are
for lightweight construction. Lightweight construction is not delayed
construction. The Standard 90 committee even gave us a variety of wall types
to select from on those tables so that we would have an *appropriate
assembly maximum U-factor* to use.

Anyway, that's my interpretation.

Carol

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Another piece of the puzzle. From the 90.1 User's Manual, section on
Baseline Building Opaque Assemblies (p.G14 in 2004 ed.):

"The baseline building is assumed to be steel framed no matter what the
construction of the proposed building. If the proposed building has
thermal mass in the exterior construction and this is a benefit in a
particular climate, then the mass is credited in the building
performance rating method."

So delayed construction is the de facto method for modeling the proposed
envelope, and by extension, the baseline.

Bill

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I'll bite. What extends it to the baseline? I still see that it just says to
credit it to the proposed building. Wasn't this language created to guide
people to the fact that even if mass was added to a steel framed building it
still fell under the "steel framed" category and not the mass?

cmg750's picture
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Following the other comments on this, I am confused and worried too that if
they are requiring to simulate mass in the baseline, then how could we use
Mass constructions as "passive" design strategies and take credit for this
type of ECM?

Directly from what I was reading in the 2007 code: Table G3.1.5-Baseline
Building Enevelope
*Opaque Assemblies. Opaque assemblies used for new building or additions
shall conform with the following common, lightweight assembly types and
shall match the appropriate assembly maximum U-factors in Tables 5.5-1
through 5.5-8:*

Doesn't the reference to "lightweight" assemblies mean that you don't have
to account for thermal lags (massing)? This has always been my
interpretation. Therefore, when it comes to modeling the U-values for the
assemblies with the U-value method versus the layer method would be
acceptable for your baseline simulations. Where there is no requirement to
show any type of massing effects it shouldn't matter if you choose to use
the U-value input method or the layer-by-layer method.

But--it is important for the simulator to understand that when using eQuest
(I can't speak for other simulation tools); the input method has to be
matched in both the baseline and proposed. You can't choose U-value input
for the baseline and layer-by-layer for the proposed, you have to use the
"apples-to-apples" approach for both models.

It will be a big issue if GBCI mandates that we have to use only
layer-by-layer inputs for compliance where Appendix G is clearly stating
that there is no need to account for thermal lag in the baseline building as
it states "lightweight" construction. Any type of thermal lag
characteristics in lightweight construction are negligible to the
performance of such constructions as required by Appendix G baseline inputs.

pkg

Pasha Korber-Gonzalez's picture
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Bill---you had quoted "IF the proposed building has therma mass..."

What is your take on this if the proposed building doesn't have therma
mass? Then could the baseline use the U-value input method?

If they wanted to have massing for the baseline building all the time, I
don't think they should have used the word "if" in that statement.

pkg

Pasha Korber-Gonzalez's picture
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Like Pasha mentioned, if you use layer-by-layer method in the proposed,
you should use the same method in the baseline, unless you want to argue
that "lightweight" requires the use of the U-value construction method.
I don't see what advantage that serves, other than helping you avoid the
time of creating baseline envelope constructions. While "lightweight" is
not defined in 90.1, the baseline layer materials and thicknesses are
described in A3, so if you use the layer-by-layer method for both
baseline and proposed, and if there is a difference in the overall mass
of each wall construction, the modeling output will reflect that
difference. Both baseline and proposed constructions will have "mass",
and if the proposed construction is optimized, there will be energy
savings.

The eQUEST help menu item for "EXTERIOR-WALL and ROOF" states that
using LAYERS rather than U-VALUE can result in greater computational
time, but gives more accurate results. Computational time is at the
bottom of my eQUEST concerns. I have not compared modeling results of
LAYERS vs. U-VALUE. Delayed construction appears to be required by
Appendix G, is supposedly more accurate, and I don't see a good reason
not to use it.

Billzebub

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Bruce Easterbrook's picture
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I guess we'll have to agree to disagree.

Bilizebub: could you point out the section in LEED or Std 90 that says that
walls must both be layer by layer. Thanks.

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Carol,

As we noted earlier in this thread, we can't find an explicit
requirement that layer-by-layer be used. It is strongly implied for at
least the proposed in Appendix G, and it is good practice for several
reasons as Bruce describes below. From the 90.1 User's Manual - "The
general rule for the baseline building run is that all inputs must be
identical to the proposed design run, except for those features that are
allowed to differ." It seems logical to extend this general rule to
input methods as well as inputs. Would you accept the modeling results
if the proposed building was done in TRACE while the baseline was done
in eQUEST?

Bilbo

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I think some confusion stems from the definition of "lightweight". The
fundamentals book discusses light and heavy constructions in the radiant
time series, and defines a few examples in table 22, ch 30 (2005
handbook - NonRes Cooling and Heating Load Calcs, Radiant Time Series
Method). Light constructions are steel sidings, 2 inches of insulation,
an airspace, and gyp board. It also defines medium and heavy, with brick
and heavyweight concrete, respectively.

So when they say lightweight, I believe they're referring to something
similar. I don't believe "lightweight" is intended to mean "no-weight"
for the reasons Bruce described. It seems to me they're giving design
teams the opportunity to take advantage of a heavy exterior construction
if it reduces the peaks. They do ask that they conform with the
lightweight assemblies, which, in my opinion, just a U-value does not.

Eric

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While I agree that modeling the baseline with some level of light mass
should be done to get more accurate results, it's difficult to understand
why 90.1 would not specify a mass value to model. The definition of a
baseline is a minimum value for comparison. How can LEED reviewers judge
whether or not you're taking the appropriate credit for thermal mass when
the baseline building done by one modeler will have a different mass value
than one done by another modeler?

Values that are vague and undefined (process loads or lighting plug loads in
residential for example) are typically left the same between both models.
This allows for the factor to be accounted for, but provides no credit to
the proposed model. The same could be done for thermal mass, to account for
it in both models, but not provide credit. Without a defined baseline, I
don't see how one can justify whether or not they've modeled the correct
"lightweight" assembly mass value.

-Robby

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Hi Robby,

I'd encourage you to save and review the .inp file I shared on this
thread (open in notepad). Therein you'll find some commentary speaking
directly to this issue.

Appendix A describes the materials in these lightweight constructions,
and ASHRAE Fundamentals makes for a solid resource with regard to an
appropriate thermal mass for each layer. I have had reviewers challenge
my construction material properties, and it's very easy to respond when
you have a well-documented approach. I do not doubt others' baseline
construction masses are different due to different approaches and
references (or lack thereof), but provided the material properties used
are appropriate within reason, I would not be perturbed.

Even though 90.1 isn't easy to cite in this regard, I would consider it
reasonable for a reviewer to question/challenge how construction thermal
mass is handled (for LEED or otherwise). It's obviously a very
important variable with regard to modeled baseline/proposed performance,
and one of a variety of paths by which someone could "game the system"
if they were so inclined.

One could continue to take the approach of "make both match" with regard
to thermal mass, but (a) it's more work, and (b) you're probably missing
out on a performance gain in most cases, unless your proposed design is
an even lighter-weight construction (i.e. a tin shed?).

~Nick

NICK CATON, P.E.

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Robby,

I see your point. However, I'm curious how many wall assembly types you
can think of that meet the criteria they discuss in that section (and
whether their different themal delay properties would impact the
project):

* Lightweight (I assume this means no brick exteriors)

* Common

* Steel Framed

* R-13 + R-7.5ci (for instance, depending on climate zone)

Now, I'm not saying they couldn't be more explicit. You're absolutely
right that they could be. However, I could fairly easily justify steel
siding, 1.5 inches of polystyrene, steel framed wall with batt
insulation and a gyp board finish. Maybe small changes like vinyl siding
or an equivalent level of spray foam insulation would have marginally
different time delay properties, but I'm guessing they would be
negligible based on the information found in the chapter I previously
cited.

Eric

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cmg750's picture
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Well, perhaps I'm nitpicking, but I see two distinct statements here.

1. "shall conform with the following common, lightweight assembly
type"
AND

2. "shall match the appropriate assembly maximum U-factors in
Tables 5.5-1 through 5.5-8"

If a baseline model does not meet both requirements, it doesn't pass,
right?

A mass construction, to me, meets both requirements. Just because it has
mass does not mean that it doesn't match the assembly U-factors.

However, I feel that a U-value only wall meets #2, but doesn't meet #1.
My original point was that U-value only constructions don't conform with
lightweight assembly types because lightweight constructions, by
definition, have some mass (or else they'd be no-weight, right?). ASHRAE
has provided examples of "light constructions" in the Fundamentals book
as steel constructions without brick or concrete, so that tells me that
lightweight doesn't imply no mass.

But I'll confess that I'm only going on instinct and what I feel is the
spirit of the code as Bruce described. It is not terribly explicit, so I
understand your position and it could probably be argued both ways until
the cows come home. I'd just hate to be on the wrong end of it when the
final review came through...

Eric

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Bruce Easterbrook's picture
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Hi Bruce,

I enjoyed your response and support all your comments. I myself as a
modeler will always choose to use the U-value input method whenever I can
use this legitimately (i.e. something else isn't pushing me or mandating I
use layers method.)

Why- you ask? Pasha chooses this method for a couple of reasons:

- most of my models are already set with thier construction choices (with
or without massing), and therefore when I am building a compliance model (no
opportuntity for design analysis) then why take the extra time to input
layers-by-layers-by-layers when I know what the final construction choices
are. NOTE, none of these construction choices are ever mine....the
baseline is set by the minimum assembly U-value listed in the
B-tables...based on this given value, why should I waste my time building up
a layer-by-layer input....ASHRAE Standard 90.1 gives me the overall assembly
U-value that they want to see and therefore I am going to choose to use
thier data that they offered to me.
- The next reason I choose the U-value method is because in terms of
input time I like the one value input method. Now you might think this is
funny of me.....but it is also a part of my QC procedures...
- when I take-off my proposed design constructions (wall, roof,
etc.) I always open up my second laptop and open a new file in eQuest
where I take about 5-10 minutes and build-up the construction
layers in the
wizard so that I can double check what the output U-value is for the
assembly based on the DOE-2 calculations for the
layers-by-layers.... most
of the time I come soooooo very close to the layer-by-layer
U-value that was
calculated and so then I can choose to use a U-value method of input, and
not waste anymore time in my model with
layers-and-layers-and-layers....too
much extra information for me to visually manage; therefore I choose the
least tedious method of one input value = U-value method.

Just because I choose to use the U-value input method, doesn't automatically
imply that I took the lazy route, or didn't even consider or calculate the
layers-by-layers....THAT would be presupmtuous of the reviewer. That is
my "style" of modeling...I can't explain it nor should I have to justify it
to any reviewer why I choose that style of modeling...as long as I am
verifying that my output simulation results are indeed in the correct order
of magnitude and telling the "correct story" to my clients based on
engineering theory & calcs, then who is anyone else out there to tell me
that my style of modeling is wrong or inaccurate?

Now this brings me to the point of arguing the massing and sans
massing....IT DEPENDS!!!! If you have a project with massing that plays a
significant role in load shedding and such, then the simulator would be
ignorant to not use layers-by-layers...because as a trained engineer this
simulator should know if this does or doesn't make a difference in thier
analysis. ---The reviewer shouldnt have to worry their
pretty-little-heads about any of this because it is the responsibility of
the simulator to make sure that the simulation output is as close to
accurate as is possible with the nature of these programs we use.

However; if I choose (as an experience simulator and trained HVAC engineer)
that I don't want to include massing effects in my envelope constructions
because there is no opportunity or potential savings to be demonstrated from
the lack of such performance then who is the LEED reviewer to make that
engineering decision for me??? Again I would feel that this is very
PRESUMPTUOUS of them to blanket that (or any statement to me) especially if
they intend to "reform Pasha's style of modeling."

I've been able to gain the experience I have and submit successful LEED
models based on my professional education, training, testing, and
professional ethics in my work and projects. Based on this, it becomes my
sole right and my sole responsibility to the industry to make sure that
'Pasha's style of modelling' is in conformance with any standard or program
that wants to outline specific rules of engagement.

Until the point in time that any one authority states: *"you must use the
Layer-by-layer method for inputs in your simulation tool."* Then I am
allowed to reserve the right to use whatever input method that I deem is
best representation for my client's project and the modeling intent that
I've been asked to perform. *And as long as I have a sound engineering
foundation to stand on*, then my decision and approach will have a 99%
chance of being the most accurate for my client. The 1% outlier risk of
not falling into conformance rests on the "risk" of which LEED reviewer is
assigned to reviewing my model...

Hopefully it will be a reviewer that can discern the truth of a good
energy modeler in the work that I produced for them to reiview. If they do
pick on something that is allowed to be open for interpretation (as is this
case of massing vs no massing), then AS THE REVIEWERS THEY ARE WRONG for
trying to mandate something that thier organization has stated to be "open
for interrpretation."

IMHO---(take it with a grain of salt if you wish).....IF GBCI wants us to do
something as EAc1 energy modelers then they need to state it in Black &
White AND it has to be offered to all of us simulators at NO ADDITIONAL
COST----you can't make me comply with something but only provide me with the
compliance data if I pay for the information....that is being an Industry
Bully.

DEAR USGBC/GBCI--if you want me to do something specific for my energy
models then please publish it clearly and *at no cost *for all of us EAc1
energy modelers to follow. Only a true monopoly organization
*expects us*to follow thier rules and pay the cost to follow thier
rules at our own
expense. This type of "mandate" from any one organization makes them out
to be BULLIES to the little people like us.

Pasha

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I haven't seen where anyone in this discussion has actually compared
LAYERS to U-VALUE results for an ASHRAE "lightweight" construction, so I
did a quick experiment with one of my LEED project baselines. (It was
quick and easy - a lot less time than I spend on a typical forum post.)
Going from LAYERS to U-VALUE increased total building energy use in the
baseline by 1%. This is not insignificant, especially considering that
nearly half of the building energy is lighting, plug loads and DHW which
are not affected by the envelope. YMMV. [Beating dead horse now.]
Doesn't make sense to use LAYERS in the Proposed and U-VALUE in the
Baseline from a modeling standpoint and energy comparison, regardless of
the exact wording of ASHRAE, LEED or your favorite local compliance
path.

Bill

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I did this exact comparison a few days ago for a typical high rise condo project I worked on, and came up with a 2.1% increase in energy consumption going from layer to U-value. In a condo, the envelope has more of an impact on total annual energy consumption than an office (assuming that's what you compared Bill). So that makes sense. Also, considering that the point increments for EAc1 are as small as 2%, this small detail in model construction actually has the potential to change your # of LEED points fairly easily. In other words, someone needs to determine the answer to this question once and for all. I like the apples to apples comparison... if you have a fairly complicated fa?ade in your proposed building, and decide to calculate the U-value and use this simplified method in your model inputs, then you should be allowed to use the simplified U-value method in your baseline. Likewise, if you use layer construction in your proposed, it should be layer in the baseline. The only thing we should not be allowed to do is mix and match between baseline and proposed.

James Hansen, PE, LEED AP

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One other thing to consider, even if you're doing an apples-to-apples comparison, is your utility rate. If you're on a time of day rate, using a U-value construction type can shift the time the load hits the HVAC system. Although the energy usage may cancel out between proposed and baseline, your energy cost savings may be magnified or marginalized if the load is shifted to an improper portion of your rate structure.

Eric

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