Climate conditions during monitoring period
The central Virginia weather patterns were not especially cooperative during the four week monitoring period. Weather profiles over previous decades led us to anticipate at least several daily intervals of low ambient temperatures in the high teens and low 20’s degF. During a normal winter season average diurnal temperatures frequently vary from a low of 30 degF to a high of 45 degF. However, perhaps due to the El Nino influenced atmospheric patterns, our region experienced an uncharacteristically warm winter climate, as did much of the northeast and middle Atlantic areas of the country. Throughout the monitoring interval the outside air temperatures recorded at the Aquatic Center revealed an average low-to-high temperature range of approximately 40 degF-to-upper 50’s degF.
Following a review of the four week outdoor temperature profile, Figure 1, the students selected four days that the logger data from the 14 HOBO’s would be simultaneously analyzed, using the lowest and highest outdoor temperatures on each day.
Water vapor pressure
Standing in the pool space, one readily senses the presence
of water vapor in the air; indeed, it "feels" humid. If the constant
evaporation of water from the large pool surface creates a relatively high
water vapor pressure compared to the water vapor pressure of the outdoor
air mass, water vapor movement will occur through the envelope materials.
The BoxCar Pro software conveniently computes the absolute humidity for
each set of db + rh values. Since the absolute humidity is a measure
of the amount of water (by weight) per unit volume of air, it is an indicator
of relative water vapor pressure. Table I shows
the absolute humidity data (measured in grams per cubic meter) and confirms,
in all cases, that a difference in water vapor pressure existed on each
side of the building envelope. (By plotting the db + rh values on
the psychometric chart, the actual water vapor pressure can also be determined
in inches of mercury).
Pool space envelope performance
East and South walls
A composite profile graph of the exterior dry-bulb as well as the interior dry-bulb and dew point temperatures is shown for the East wall (Figure 2) and South wall (Figure 3). These graphs indicate that the interior dew point temperature remained well below the surface dry-bulb temperature. Similar results were observed with all HOBO’s placed on the exterior walls. The actual recorded temperature and rh values for each HOBO on the four selected days are shown in Table II. The data confirms that the dew point temperatures are lower than the actual surface temperatures, therefore no condensation could occur on either the masonry or glass surfaces. It is obvious the perimeter air supply provides a sufficiently warm wall and window "wash" to preclude any formation of condensation.
The typical wall section diagram, Figure 4, lists the component wall materials and corresponding R-values and shows the steady state temperature gradient. When the lowest recorded outdoor temperature (27.6 degF) occurred at 3:00 am, the highest interior wall surface temperature was 84.4 degF with a DP temperature of 47.8 degF. Under these conditions the temp. gradient diagram reveals that dew point would appropriately occur within the rigid insulation on the "cold side" of the vapor barrier. The vapor retarder (as noted on the construction drawings) is a fibered emulsion damp-proofing material with a permeance of 0.5 metric perms. It also serves as an adhesive for selected types of rigid insulation. If the low perm vapor barrier were properly and uniformly applied, no degradation of the component materials should be realized.
Roof and Skylight
previously stated, the height of the roof and skylight surfaces were beyond
the vertical reach of the Genie Powerlift machine; as a result, no measurements
were taken at these surfaces. However, the loggers measuring the return
air were placed on the bottom chords of the roof trusses - approximately
5 to 6 feet below the roof surface. One may reasonably assume the roof
surface db temperatures to be two to three degrees higher than the recorded
return air temperatures noted in Table III.
Assuming the interior roof surface at 90 degF, the temperature gradient for the typical roof section is presented in Figure 5 along with R-values for the constituent materials. The dew point temperatures, in the low-50 to low-60 degF range, occur within the rigid insulation on the "cold" side of the vapor barrier - a membrane combination of rubberized asphalt and polyethylene with a maximum permeance of 0.05 metric perms.
The skylights are thermally insulated Kalwall structural roof panels
supported by an aluminum frame - with thermal break. The ‘frame/center
of glass’ U value is 0.22/0.14. In a manner similar to the opaque
roof, the ducted warm air "wash" maintains the interior surface temperature
at approximately 90 degF (or higher) well above the dew point of the adjacent
air mass, as evidenced by the lack of condensation drops onto the pool
Recorded data on each side of the wall separating the pool space from the fitness/exercise room, presented in Table IV, confirms a lower dew point temperature than the actual db temperature of the air mass adjacent to those surfaces. As the temperature gradient through the wall varies only 5 to 10 degrees, it is maintained well above dew point and thereby precludes the need for a vapor barrier.
Lastly, it was interesting to experience an environment where actual relative humidity values do not necessarily indicate a person’s perception of humid air. In this building the sensation of humid air occurred within a nominal 30% to 50% rh range (Figure 6), due to the high dry bulb air temperatures.
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