Ohio State University Extension Bulletin

Research and Reviews: Dairy 2001

Special Circular 182-01

Environmental Conditions within Tunnel-Ventilated and Naturally Ventilated Dairy Freestall Facilities

Results and Discussion

Temperature-humidity indices

The average THI for each of the barns peaked between 70 and 75 during both July and August (70 may be considered a threshold for heat stress). Trends in average hourly THI values were very similar to those for dry-bulb temperature. Figure 3 illustrates the trends in the indices for one of the tunnel-ventilated barns. Indoor THI is higher than outdoor during the evening hours, but the THI values are nearly equal during the heat of the day, representing very good air exchange. While average outdoor black-globe humidity index (BGHI) values peaked at about 82 during August afternoons, indoor BGHI showed little effect of radiant heating, peaking at about 73 _ the same as THI. During the evening, both indoor and outdoor BGHI values were usually slightly lower than their corresponding THI values, showing that some radiant cooling was taking place.
Similar trends in THI and BGHI existed in the other barns. In the study barns, the numbers of hours in which indoor THI and BGHI values exceeded 70 were essentially the same. Additionally, radiant cooling during the evening hours offset any small amount of interior radiant heating occurring during the day. Therefore, further discussion focuses on THI rather than BGHI.
The small advantage that the tunnel-ventilated barns exhibited in terms of temperature moderation was repeated for THI. Table 2 shows that each of the tunnel-ventilated barns provided a slightly cooler interior environment than its naturally ventilated counterpart when outdoor conditions were potentially stressful (THI greater than 70). The average advantages in terms of reduced differentials between indoor and outdoor environments were -0.8°F and -0.4 for air temperature and THI, respectively. These differences were statistically significant.

Table 2. Barn environments for conditions when ambient THI ≥ 70 during summer 2000.¹

Farm Applicable
# of hours Temperature differential,
T_i - T_o (°F) THI differential,
THI_i - THI_o

Mean^a STD Mean^a STD

NV-NY 265 1.60 1.04 0.98 0.67

TV-NY 247 0.72 1.66 0.75 0.99

NV-OHC 478 1.04 1.17 0.73 0.69

TV-OHC 555 0.29 1.17 0.07 0.70

NV-OHW 642 1.37 1.08 0.82 0.70

TV-OHW 692 0.72 1.35 0.51 0.88

¹NV = naturally ventilated, NY = New York, TV = tunnel ventilated, OHC = central Ohio, OHW = western Ohio, THI = temperature-humidity index, and STD = standard deviation.
^aThe means for each barn pair differ at P < 0.01.

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Farm	Applicable # of hours	Temperature differential, T_i - T_o (°F)		THI differential, THI_i - THI_o
Farm	Applicable # of hours	Mean^a	STD	Mean^a	STD
NV-NY	265	1.60	1.04	0.98	0.67
TV-NY	247	0.72	1.66	0.75	0.99
NV-OHC	478	1.04	1.17	0.73	0.69
TV-OHC	555	0.29	1.17	0.07	0.70
NV-OHW	642	1.37	1.08	0.82	0.70
TV-OHW	692	0.72	1.35	0.51	0.88
¹NV = naturally ventilated, NY = New York, TV = tunnel ventilated, OHC = central Ohio, OHW = western Ohio, THI = temperature-humidity index, and STD = standard deviation. ^aThe means for each barn pair differ at P < 0.01.