----- Original Message -----
Sent: Wednesday, December 03, 2003 1:01
PM
Subject: [BLDG-SIM] Increased R value
Credit for Concrete and Themal Massing
My area of expertise is simulation of
mechanical systems, and I do not claim to be an expert in heavy mass
walls. However I'll stick my neck out and add my thoughts to this
discussion, and invite others to respond.
I agree with Curt Petersen. But, in
addition to being frequency dependent, my understanding is that
heavy-mass wall performance is also dependent on the average daily
temperature differential across the wall, the placement of mass vs.
insulation, and interior heat gains.
1) Mass vs. Resistance (vs. daily outdoor
temperature swing)
Assume a lightweight interior space, having no
internal loads, is to be maintained at 70F. If the average daily
outdoor temperature is also 70F, and the daily outdoor temperature is
swinging +/- 15F about the average, then a heavy construction will maintain
the interior temperature very close to 70F. In this situation, the
heavy mass wall will perform better than a resistance wall.
If instead the average daily temperature
is 30F, with the same daily swing, then the interior temperature will be
close to 30F. This is also true of a resistance wall. However,
the daily (not instantaneous) amount of heat required to maintain the
interior temperature at 70F is now a function of the resistance, not the
impedance, and the low-resistance heavy-mass wall will require
considerably more heat to maintain 70F.
In other words, for a heavy mass wall to get
credit for impedance, the outdoor temperature swing must overlap the desired
interior temperature. The closer the average outdoor temperature to
the indoor, the greater the effect of the impedance. This has
significant ramifications for different climates. For moderate climates or moderate seasons where the
outdoor temperature swing overlaps the desired interior temperature for the
majority of hours, heavy mass walls can work well. For more extreme
climates or seasons, it's hard to see how a
heavy mass wall can be of significant benefit.
I live in Sacramento, Calif where the average
lo/hi summer temperatures are 60F and 95F. Here, a mass wall can
work well. However, in winter, the average temperatures are
around 40F/60F, and I wouldn't be willing to trade a well-insulated house
for one built out of uninsulated concrete!
2) Placement of mass vs. insulation (vs. solar
gains)
The above is a little too simplistic because it
ignores the placement of mass vs. insulation, and solar
gains. Assume the sun is shining strongly on a wall. If the
insulation is to the outside of the mass, then the outer wall surface
temperature (sol-air temperature) will be hotter than if the mass is to the
outside. This is because the mass readily conducts heat inward,
whereas insulation doesn't. Since
re-radiation is proportional to the fourth power of absolute
temperature, placing the insulation to the outside will cause the wall to
instantaneously reject a greater portion of the solar gain. Placing
the mass to the outside allows the wall to capture more of the solar gain;
part re-readiates at night, but part conducts into the space. So which
is better? It depends on whether you are more concerned about winter
or summer performance, and how sunny those seasons are.
I recently participated in a study of a
refrigerated warehouse that demonstrated this effect. Holding all
factors constant except the placement of mass vs. insulation, mass to the
outside of the wall increased annual cooling loads because of the increased
capture of solar gains. (And by the way, this effect was captured in
DOE-2).
3) Effect of interior loads
High interiors loads and/or solar gains thru
windows, as well as when they occur bias all of the above.
Conclusion? For residential buildings, I
believe that a high-resistance shell with a moderate amount of interior
mass is the most cost-effective approach for most climates. While
overly simple, a key concept is the "time constant" of a house,
which is the product of resistance and capacitance. The interior
temperature decays to the exterior temperature as a function of the
time constant ( exp(1/RC) ). The greater the time constant, the
slower the decay rate. In general it is cheaper to achieve a given
time constant by adding insulation instead of thermal
mass.
I just built a two-story house and am currently
testing this theory. The interior mass consists of a
concrete slab with about 60% tile on the first floor, and 5/8"
sheetrock interior walls. The exterior walls are 2x6 studs with
damp-spray rock-wool insulation (R22 in a 5-1/2" cavity, with no
voids). The windows are fiberglass, low-e2, with overhangs on the
east, south, and west exposures; most windows facing north and south (~14%
of floor area). The attic has blown rock-wool. The roof is
metal over 2" fiberglass insulation, with a radiant barrier on the
underside. I opted for a metal roof instead of tile because we live in
earthquake country, and I wanted the roof to be as light as possible, while
still durable. The kitchen appliances are all electric, as the
house is well-sealed and I didn't want to worry about the NOx and CO
produced by a gas stove. All bathroom exhaust fans are on timers to
ensure moisture removal after bathing.
We moved in late last summer. During
September, the outdoor temperature range was typically 60F to
95F. The interior temperature stayed in the low-70's without any
air conditioning, and swung about 2-3F. We ran the whole house fan a
couple of nights just to see how cool we could get it, but didn't need
to. I don't know about winter performance yet, but so far we have been
heating the entire house (3500 sq.ft.) using two 20,000 Btuh, 80%
efficient, thermostatically-controlled gas fireplaces. Outdoor
temperatures have been as low as 35F, but we have not yet needed the
conventional forced-air gas furnaces to maintain 70F inside, and most of the
time the fireplaces are off.
This house hardly fits the classic definition
of a "passive solar" house, but it performs like one because of its high
time constant, achieved primarily through insulation rather than mass.
While the tile floors were great during the summer, I am waiting until March
to assess the comfort of these floors with colder ground temperatures (the
slab is not insulated). If anyone wants an update later on,
send me your address.
Comments?
----- Original Message -----
Sent: Tuesday, December 02, 2003
11:21 AM
Subject: [BLDG-SIM] Increased R value
Credit for Concrete and Themal Massing
Dave:
This concept
falls into the "cheater" category. What happens is that the proponents of
heavy construction elements show that the steady periodic behavior of a
heavy structure, at some well-selected frequency, is better than the
behavior at steady state. The electrical analogy would be to replace
resistance with capacitive impedance. Of course it depends on the
frequency, and it is not resistance, so the steady state (zero frequency)
result will be different.
A reasonable building simulation tool
properly accounts for both the capacitance and the resistance of a
building element. The "effective R" is bogus.
Curtis
Pedersen
Professor Emeritus of Mechanical Engineering
University of
Illinois
On Tuesday, December 2, 2003, at 11:26 AM, David
Stewart wrote:
Dear Folks
Has
anyone explored the increased R value credits for thermal massing for
example Dow T-Mass etc. and can one simply increase the R value to the
'effective R value while changing the mass in DOE. I would like
some more background on this credit with 3rd party validation
Thanks
Dave
Stewart
David
C. Stewart & Associates Inc.
16 Shawinigan Road
Dartmouth, NS
B2W 3A3
Website:
http://dcsa.ca
Tel:
902 462 8111
Fax 902 435 6646
<David C
Stewart MS P. Eng..vcf>
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