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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?
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