Internal mass modeling guidance in E+?

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Dear colleagues,

I'm looking into furniture/internal mass modeling in energy models
particularly within the environment of EnergyPlus but also not limited to

I have two questions I guess:

1, Are there any modeling guide or reference building report that may give
recommendations/instructions on how to represent furniture/internal mass
inside a building if there is no much detailed information? I guess all I'm
asking is are there any published data-set, or default values in terms of
material property and area to represent furniture like what could be easily
found about schedules and occupancy density?

COMNET modeling guide has only one paragraph about it and it mentioned "The
interior thermal mass and modeling assumptions in the baseline building
shall be the same as the proposed design."
Currently what I have is from Building America House Simulation Protocols:
"The internal mass of furniture and contents shall be equal to 8 lb/ft2 of
conditioned floor space. For solar distribution purposes, lightweight
furniture covering 40% of the floor area shall be assumed."

2, DOE reference commercial buildings have internal mass objects in the
idf. file, where does the information come from?

Appreciate your input!


Qinpeng Wang

Qinpeng Wang's picture
Joined: 2012-03-05
Reputation: 0

Although we would like to have all building inputs to be well-documented, this is still
far in the future, especially for those inputs having to do with lifestyle and behavior.
On internal mass, the proverbial story is to call a moving company and ask how much stuff
do they haul in a typical move. I actually had a researcher do that 20 years ago, and she
came up with a number of 5.5 lbs/ft2 for a residential building. I see that the Building
America has upped that to 8 lbs/ft2. I have no grounds to either believe or disbelieve that.

I've heard even less discussion on the internal mass of commercial buildings. As the
original developer of the DOE Reference Buildings, I can say that all those inputs for
internal conditions (mass, shading, occupancy schedules, etc.) are based on "engineering
judgment" (at least in my original work)

It's not particularly difficult to survey an appropriate number of houses/buildings and
come up with something more defensible, but sad to say, nobody wants to support that
because it's not regarded as serious research. About 20 years ago, a colleague at FSEC
and I came up with a Work Statement for ASHRAE TC 4.7 to survey a collection of homes and
develop prototypical operating conditions. It got approved but just at the verge of going
out for bid, ASHRAE Research decided to withdraw it.

Joe Huang

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

I believe that E+ and predecessors take/took what ever was described
as "internal mass" and parsed it into an equivalent "partition" of
some "equivalent" area. This can lead to some counter intuitive
results. Most might expect that "internal" or "thermal" mass is going
to moderate energy transfer ("store"). But suppose I describe a
banquet hall full of folding mdf or plywood tables (light weight).
Compared to an empty room, the area available to absorb radiation and
convect it to the room is increased by easily 1/3. All of a sudden the
room is "fast" and energy gains are translated into cooling loads much
more quickly.

So let's agree with Joe that your question can be answered by "walking
around." In looking around my office I find that one partition is
covered with book cases and that the other side of the partition is
also covered with books. If I thought this was an important feature
that needed to be modeled (which I don't and which isn't) I would not
describe any internal equivalent mass, I would simply describe a
partition made up of books-air space-gyp board wall-air space-books.

But why do I think this is irrelevant additional work? I can weigh
some books to determine their density. We think books are heavy but
imagine putting an equivalent volume of concrete in your backpack.
Then they are not so "thermally massey". Also the books never receive
direct sunlight and the temperature in my office varies little because
it is in my heavily insulated, air tight home. Since the guzinta
equals the guzouta over time and since the guzinta and guzouta, even
for this massive double book lined partition, are near zero because
the materials are at constant temperature, more or less, the mass of
the books is not very important to the room energy balance.

I don't know what thermal mass is anyway. The term must have evolved
partly from passive solar designs, where energy storing materials
exposed to the sun are important, and from the brick trade
association who left a confusing legacy of claims that a masonry house
had a higher R-value because of the "thermal mass." Mind you I think
bricks are great, I have some myself. However, there is nothing in the
definition of R-value that is labeled "brick."

My apologies Dr. Wang. I expect that what I have just said is more or
less useless as an answer to you questions. However, a productive
guide to a first approach might be found in:

Consider a Spherical Cow: A Course in Environmental Problem Solving, John Harte

It is easier to bound a problem than to defend anybody's guess at the
mean. The correct answer always seems to be "It Depends."

Doug Hittle

Doug Hittle's picture
Joined: 2012-11-15
Reputation: 0

There's also an issue here about whether or not there's any modeling of
internal partitions or fixtures included in your model. My personal
experience in modeling residential models is that they always lag more than
you expect, so I've regularly responded by jacking up the internal mass. I
certainly can fit metered HVAC data a lot better when I add mass beyond
what the BABM suggests. I've guessed that's because my partitions don't
quite match the amount of mass we're interacting with.

I get what Doug is saying that there isn't much temperature change
occurring in his bookshelf, but when you have literally several tons of
exposed mass, even a degree or two of temperature change is quite a bit of
heat. Those granite counters currently in fashion do something for the
building mass.

What's funny about this is that the conventional rule of thumb is that
residential construction is "light" and has little impact from mass, while
commercial buildings are "heavy" and have lots of mass impacts. My personal
experience in studying this is that mass is extremely important in
residential construction because they "float" a lot of the time and also
have higher solar gains than commercial spaces.

One possible area of fruitful research I've thought about is to try and get
some utility direct load control data and look at the "response time" of
residences in terms of how long it took to have the temperature rise a
certain amount when the HVAC system was constrained. You'd at least get
some good data about how responsive the houses are supposed to be and alter
what you can in your models to get them to better match real life (whether
or not you actually figured out the root cause of the problem in the

Justin Spencer's picture
Joined: 2011-10-02
Reputation: 1

I've had the same experience when comparing simulation results to measured indoor
temperatures for a typical house in California. The actual house temperatures were
substantially more flattened and lagged compared to what the simulation showed. It may be
that getting the right number on the amount of internal mass might not necessarily result
in a house model that responds similar to a real building. Then you have to decide
whether to "tune" the model to be physically correct or thermally correct!

Joe Huang

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

Perhaps mass could be related to occupancy quite easily. After all every
person needs a few square feet of desk, possibly some filing.

There is probably quite a simple arithmetic formula that holds true for
most situations, the arguments of which might be:
? occupant density
? desk space per occupant
? material thickness
? extra shelving required?

Ignore anything with less mass.

If you come up with an excel formula, I'd like a copy ;o)

Sent from my Android device. Please excuse typos, etc.

Chris Yates2's picture
Joined: 2011-10-02
Reputation: 0

Hi Folks:

This has to do with modeling in general.

I have a Window Film manufacturer with a product called Enerlogic VEP35 window film. It is apparently applied to the inside of a window.

The manufacturer claims that this product lowers the U-Value of single pane glass (1/4" or 6 mm) from 1.109 to 0.6. And with no air space between the film and the glass. "Revolutionary new technology"

Has anyone ever run into this product before? Why does this claim sound "too good to be true" ??


John Aulbach

John Aulbach's picture
Joined: 2011-09-30
Reputation: 1

Hi John,

As I understand the topic (at a baser level than others on these lists), R-values / U-values are a kind of flawed metric in and of themselves when discussing window films. Films can perform magical feats when it comes to cutting solar loads (perhaps turning your building into a 2-way mirror in the process), but quantifying that result into a U-value is a loopy path at best, as no thin/transparent coating prevents heat transfer by conduction in the same sense typical insulation does.

Regardless of manufacturer claims, I would be averse to modeling the effects of such films as a "u-factor factor" if charged with determining ROI. Also keep in mind solar loads aren't always a bad thing if we're talking annual energy bills.

See also:


Here's a (for cost?) paper that appears to be evaluating this exact sort of issue utilizing eQuest/WINDOW. Abstract/highlights there are intriguing. Possibly includes procedure you could draw from? I recognize Rongxin from the mailing lists here - perhaps he could fill you in further?



Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805


Here's a link to a video on their website:

It sounds like a good product, but bad physics (or at least bad explanation of the physics invoved). Low emissivity really affects radiative heat transfer much more than it does conductive transfer. They need to get this tested by an independent accredited third party if they want to be taken seriously.

Vikram Sami, LEED AP BD+C

Sami, Vikram's picture
Joined: 2011-09-30
Reputation: -1

Looks like they have NFRC ratings

Vikram Sami, LEED AP BD+C

Sami, Vikram's picture
Joined: 2011-09-30
Reputation: -1

Update on Window Film Product:

I DID find the product listed in the NFRC Database. But, as you can see, it only lists one U-Value for each product. Not a Delta U. So I think the product is rated for Shading Coefficient and VT at that particular U-Value.

I also have an email into their Product rating director to confirm my suspicions.

Come, Watson, Come, The game is afoot..


From:bldg-sim-bounces at [mailto:bldg-sim-bounces at] On Behalf Of John Aulbach
Sent: Tuesday, January 15, 2013 3:09 PM
To: bldg-sim at
Subject: [Bldg-sim] Window Film Product Claims
Hi Folks:
This has to do
with modeling in general.
I have a Window
Film manufacturer with a product called Enerlogic VEP35 window film. It is
apparently applied to the inside of a window.
The manufacturer
claims that this product lowers the U-Value of single pane glass (1/4" or
6 mm) from 1.109 to 0.6. And with no air space between the film and the glass.
"Revolutionary new technology"
Has anyone ever
run into this product before? Why does this claim sound "too good to be
true" ??
John Aulbach

John Aulbach's picture
Joined: 2011-09-30
Reputation: 1

NFRC ratings and the Window software historically use composite boundary conditions (convective and LW radiative). I think they have them split now in the latest version of Window 6/7 though one would have to check the manuals etc.

Historically, an internal film coefficient of hi=8.3 W/m2 oK (sorry but I am Canadian, Australian) or so was assumed which when you do the back of the envelope is about half convective and half LW radiative. So a lowE coat on the inner facing surface of a window (has to be exposed to air/gas to improve U-Value) would lower the radiative heat transfer coefficient leaving you with 4 or so for convection.

Its the first time I have seen a lowE coating on a film that can be exposed to atmosphere. I could be wrong but the Southwall XIR films use spectrally selective coatings or filters within the film raher than on the surface of the film. We are used to seeing it on glass as a hard or pyrolytic coat on Pilkington k-glass - I am sitting next to a piece in our home study that definitely lowers the U-Value with no air gap ? . The truly spectrally selective coatings are soft and prone to oxidation if exposed to atmosphere thus why we see them in sealed air cavities.

So if Enerlogic have managed to coat a film that can be retrofitted onto a piece of glass then they have something. I would be checking what warranty there is on the performance as it could be prone to oxidation or have UV stability issues (a historic problem with older generation retrofit films).


hamnmegs at's picture
Joined: 2011-10-02
Reputation: 0

I like some of Doug's points around bounding an assumption that is uncertain rather than arguing the exact assumption which if you are really lucky may be close to reality.

Books are probably a reasonably good insulator as they are cellulose and would have fine air gaps around them and possibly between the pages that act as an insulant so it gets back to the comment on 'fast' mass. I think the CIBSE methods define something called thermal admittance as the ratio of conductivity to specific heat capacity. What that means is a material can be heavy, but if it has a low conductivity or maybe finished with an insulative layer (think rubber backed carpet on concrete) then the ability for a material to absorb and release heat is diminished - the mass is no longer in close thermal contact.

I had the pleasure (or displeasure in the early stages) of diagnosing and rectifying a thermal comfort problem years ago where a new direct stick carpet was applied to a ground level slab that sat above a naturally ventilated carpark in a location that had an ambient winter design condition of 3C. The slab was not fully insulated below so became quite cool in places where insulation was missing. By our calcs (Therm and IES calibrated with an IR gun) the absence of a backing on the carpet resulted in the floor surface temperature being 1-2C cooler. This was material from a radiant temperature asymmetry point of view but also in cooling low level air that resulted in an unacceptable dry bulb gradient. We made the insulation continuous and the problem was solved but it was a real lesson in close coupling (fast) of thermal mass versus something that has less of an impact on the loads and thus temperatures.

So it does depend and you have to use engineering judgement, it isn't just the weight, it is how closely coupled (fast or slow ?) the mass is to the space.


hamnmegs at's picture
Joined: 2011-10-02
Reputation: 0

Thanks for this I've been really looking for this, thank you people I really appreciate it, I've been researching about my thesis and I guess the posts here are helpful. Cheers!. - Larry Starr Sarasota

larry2s's picture
Joined: 2013-09-20
Reputation: 0