CHAPTER 13
Factors That Increase
Indoors Humidity
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he basic premise
implied by a discussion of outdoors leisure thermal comfort is that PMVSHADE
can be approximated as a fictitious enclosure for which all (fictional outdoor)
surfaces are at the same temperature as the ambient air temperature. To the
extent that such approximation may hold true for outdoors in the shade, such a
concept generally does not hold for the indoors environment. This is because
domestic indoor space especially during the summer normally functions as a
“heat sink” and as a “generator” of space heat and air moisture; these
conditions beyond a given threshold lending to physiological discomfort. The
following discussion applies to summer climate conditions.
The
Dwelling in Summer as a Heat Sink
In the traditional
home, resort cabin, recreational vehicle, or mobile home, the roof is likely to
be exposed to direct sunlight. Although walls also transmit considerable solar
heat, the roof accounts for most heat entry into the living space.
Figures 13-1 and 13-2 illustrate the daily
cooling load for flat-roofed dwellings with one or two inches of insulation.
Figure
13-1: Heat Entry Into Flat Sheet Steel Roof
Each of the two charts – sheet steel roof
and wood roof – relate to the following specifications*:
• Indoor air temperature constant and equals 78°F.
• Outdoor MAXTMP equals 95°F.
• Outdoor AVETMP equals 85°F.
• Outdoor MINTMP equals 74°F.
These outdoor temperatures are typical of
what we might expect for San Antonio, Texas, during July.
The charted hourly data in Figures 13-1 and
13-2 represent an indoor space that is maintained at a constant 78°F, implying
steady-state removal of heat.
Figure 13-1 illustrates the magnitude of
peak heat gain that occurs for a flat metal-roof. At the peak, with two-inches
of insulation, approximately ten BTU/hr/square-foot enters the interior
dwelling space from the roof alone.
Figure
13-2: Heat Entry Into Flat Wood Roof
With a one-hour period of time corresponding
to this peak heat entry and a ten-foot by ten-foot section of the ceiling (that
is, one-hundred square-feet), we have:
1000 BTU = 10
BTU/hr/square-foot x 1-hour x 100 square-foot
Thus, about one thousand BTUs enter the
ten-foot by ten-foot space during that single hour*.
For only a ten-foot by ten-foot section of ceiling, this heat entry is
(ideally) sufficient to raise the temperature of two hundred pounds of water*
(about the weight of a human) by about 5°F
(assuming perfect heat absorption). Actually, radiant energy is absorbed first
by the floor, walls and other objects within the space. Only later, after these
surfaces become warmer than the space air, will their heat become transferred
to the space air by convection.
The point is that heat entry through the
roof alone is more than sufficient to raise the temperature of the indoor space
to a level higher than that of the outdoor space. When additional heat entry
through walls and windows as well as energy transfer due to ventilation and air
infiltration is considered, it is easy to appreciate how thermal comfort can
vary between indoors and outdoors. In addition, heat and air moisture are also
generated within the occupied space, to be discussed next.
Indoor
Factors that Diminish
Thermal Comfort
In addition to solar
heat gain from the exterior of dwelling, occupants, lights and appliances also generate
interior heat. Moreover, air moisture from the occupants, cooking, washing,
etc., accumulates and contributes to a higher dew point temperature.
Total heat
gain from three sedentary adult occupants would be about 1000 BTU/hr†, an amount of heat gain about
equivalent to the peak solar radiation heat gain through the ten-foot by
ten-foot section of metal roof discussed earlier.
In summary,
thermal comfort of indoor space differs substantially from outdoor space,
because external solar radiation enters into the dwelling’s living space and
heat and moisture are generated from within. ■
