----- 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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