Greenhouse Heat Loss Calculations
J. Raymond Kessler, Jr.
The selection of the number of heaters and the capacity of the heaters for a greenhouse depends on estimating the expected heat loss from the structure. Heat exchange between a greenhouse interior and exterior is the sum of all sources of heat and the rate of heat loss. Sources of heat in a greenhouse include solar, heater, lighting, electric motor heat, and heat from specialized equipment such as carbon dioxide generators. Sources of heat loss from a greenhouse include conduction and convection of heat through the greenhouse glazing, heat loss by air exchange between the inside and outside air (usually infiltration through cracks in the structure), heat loss by evaporating water, and heat loss by thermal radiation.
Detached, Gableroof Greenhouse
Example: Glasscovered, all metal frame greenhouse, 30 feet wide and 100 feet long. Curtain wall constructed of 4inch concrete block, 2 feet tall. Glass wall above the curtain is 6 feet tall. The average wind velocity expected is 15 mph. A 60°F temperature difference is expected between the low outside (0°) and desired inside temperature (60°F). Actual wind speed and low outside temperature for your area can be obtained from the U.S. Weather Bureau.
1. Set up the table below.
2. In table 1, obtain the appropriate heatloss value for both gables. For a 30foot width, it is 18 mBTU/hr. Enter in the table below.
3. In table 1, find the appropriate heatloss value for both roofs. For a 30foot width and 100 foot length, it is 266 mBTU/hr. Enter in the table below.
4. Calculate the length of the glazed side wall perimeter, which is 100+30+100+30=260 feet. Find the heatloss for the glazed side wall, 6 feet tall and 260 feet perimeter in table 2. Because there is no entry for 260 feet, look up the value for 200 feet (95 mBTU/hr) and 60 feet (28 mBTU/hr) and add them together, it is 123 mBTU/hr. Enter in the table below.
5. Find the heatloss for the 4inch concrete block curtain wall, 2 feet tall and 260 feet perimeter in table 2. Because there is no entry for 260 feet, look up the value for 200 feet (32 mBTU/hr) and 60 feet (9 mBTU/hr) and add them together, it is 41 mBTU/hr. Enter in the table below.
6. Determine the climate factor (K) for your area. In the example, in table 3 at a 15 mph wind velocity and 60°F temperature difference, K=0.84. Enter in the table below.
7. Determine the glazing construction factor (C) for the type of greenhouse. In the example, in table 4 for a glasscovered, all metal frame greenhouse, C=1.08. Enter in the table below.
8. Determine the nontransparent curtainwall construction factor (CW) for the type of greenhouse. In the example, in table 5 for a 4inch concrete block curtain wall, CW=0.64. Enter in the table below.
9. Determine the heat loss for each portion of the greenhouse by multiplying the standard heat loss by the climate factor (K) and the construction factor (C or CW).
10. Add the 4 heat loss values together to arrive at the total heat loss per hour from the greenhouse. The heating system must deliver at least 391, 272 BTU/hr to maintain a minimum 60°F at an outside temperature of 0°F and wind velocity of 15 mph.
Greenhouse Component 
Standard heat loss (mBTU/hr) 
Climate factor (K) 
Construction factor ( C or CW) 
Heat loss (mBTH/hr) 

Gable 
18 
× 
0.84 
× 
1.08 
= 
16.330 
Roof 
266 
× 
0.84 
× 
1.08 
= 
241.315 
Wall (glazed) 
123 
× 
0.84 
× 
1.08 
= 
111.586 
Curtain wall 
41 
× 
0.84 
× 
0.64 
= 
22.042 




Total heat loss 
= 
391.273 
Detached, Quonsetroof Greenhouse
Determination of heat loss from a Quonsetstyle greenhouse requires some modifications because of the difference in shape. Quonsetstyle greenhouses may be covered with polyethylene film plastic, fiberglass reinforced panels or polycarbonate. A curtain wall is rarely used. Two components are considered in the heat loss calculations: 1) the two ends collectively, and 2) the roof covering that extends for the length of the greenhouse groundtoground.
Example: Metalframe, Quonset greenhouse that is 30 feet wide and 100 feet long covered with two layers of polyethylene, each with a covering width of 40 feet. A temperature difference of 60°F and an average wind velocity of 15 mph are expected.
1. Set up the table below.
2. In table 6, obtain the appropriate heatloss value for both ends. For a 40foot covering width, it is 40 mBTU/hr. Enter in the table below.
3. In table 6, obtain the appropriate heatloss value for the groundtoground roof covering. For a 40foot covering and 100 foot length, it is 316 mBTU/hr. Enter in the table below.
4. Determine the climate factor (K) for your area. In the example, in table 3 at a 15 mph wind velocity and 60°F temperature difference, K=0.84. Enter in the table below.
5. Determine the glazing construction factor (C) for the type of greenhouse. In the example, in table 4 for a double layer, metal frame greenhouse, C=0.70. Enter in the table below.
6. Determine the heat loss for each portion of the greenhouse by multiplying the standard heat loss by the climate factor (K) and the construction factor (C).
7. Add the 2 heat loss values together to arrive at the total heat loss per hour from the greenhouse. The heating system must deliver at least 209,328 BTU/hr to maintain a minimum 60°F at an outside temperature of 0°F and wind velocity of 15 mph.
Greenhouse Component 
Standard heat loss (mBTU/hr) 
Climate factor (K) 
Construction factor ( C) 
Heat loss (mBTH/hr) 

Both ends 
40 
× 
0.84 
× 
0.70 
= 
23.520 
Covering 
316 
× 
0.84 
× 
0.70 
= 
185.808 




Total heat loss 
= 
209.328 
GutterConnected Greenhouse
Heat loss calculations for gutterconnected greenhouses are simple continuations of the procedures for detached gable or Quonsetroof greenhouses. Three components are considered in the heat loss calculations: 1) the ends or gables, 2) the roof or covering, and 3) the walls. The wall height is the distance from the ground to the gutter, and the wall length is the perimeter of the greenhouse. The heatloss calculated for one roof or covering is multiplied by the number of bays. Then multiply the heatloss calculated for one end or gable by the number of bays.
Example: Gutterconnected, Quonsetstyle greenhouse with 4 bays each measuring 20 feet wide, 5 feet tall, and 100 feet long. The walls are 12 feet high, and the covering width is 24 feet. The greenhouse frame is metal, and the entire greenhouse is covered with doublelayer polyethylene. A temperature difference of 65°F and a wind velocity of 20 mph are expected.
1. Set up the table below.
2. Calculate the length of the side wall perimeter, which is 100+(20×4)+100+(20×4)=360 feet. Find the heatloss for the side wall, 12 feet tall and 360 feet perimeter in table 2. Because there is no entry for 360 feet, look up the value for 300 feet (288 mBTU/hr) and 60 feet (58 mBTU/hr) and add them together, it is 346 mBTU/hr. Enter in the table below.
3. Determine the heatloss from the 4 combined roofs. In table 6, obtain the appropriate heatloss value for the roof covering. For a 24foot covering and 100 foot length, it is 190 mBTU/hr. Multiply this by 4, it is 760 mBTU/hr. Enter in the table below.
4. Determine the heatloss from the 8 combined ends. In table 6, obtain the appropriate heatloss value for both ends of a bay. For a 24foot covering width, it is 15 mBTU/hr. Multiply this by 4, it is 60 mBTU/hr. Enter in the table below.
5. Determine the climate factor (K) for your area. In the example, in table 3 at a 20 mph wind velocity and 65°F temperature difference, K=0.96. Enter in the table below.
6. Determine the glazing construction factor (C) for the type of greenhouse. In the example, in table 4 for a double layer, metal frame greenhouse, C=0.70. Enter in the table below.
Greenhouse Component 
Standard heat loss (mBTU/hr) 
Climate factor (K) 
Construction factor ( C) 
Heat loss (mBTH/hr) 

Walls 
346 
× 
0.96 
× 
0.70 
= 
232.512 
Roof 
760 
× 
0.96 
× 
0.70 
= 
510.720 
Ends 
60 
× 
0.96 
× 
0.70 
= 
40.32 




Total heat loss 
= 
783.552 
Table 1. Standard heatloss values for gables and roofs of a gableroof greenhouse 

Greenhouse Length in feet 
Greenhouse width in feet 

16 
18 
20 
22 
24 
26 
28 
30 
32 
34 
36 
38 
40 
50 
60 

Gable loss (both) in mBTU/hr 

5 
6 
8 
10 
11 
13 
15 
18 
20 
23 
26 
29 
32 
50 
72 

Roof loss (both) in mBTU/hr 

5 
7 
8 
9 
10 
11 
12 
12 
13 
14 
15 
16 
17 
18 
22 
26 
10 
14 
16 
18 
19 
21 
23 
25 
27 
28 
30 
32 
34 
35 
45 
54 
20 
28 
32 
35 
39 
42 
46 
50 
53 
57 
60 
64 
67 
71 
88 
106 
30 
42 
48 
53 
58 
64 
69 
74 
80 
85 
90 
96 
101 
106 
133 
160 
40 
57 
64 
71 
78 
85 
92 
99 
106 
113 
120 
127 
135 
142 
177 
212 
50 
71 
80 
89 
97 
106 
115 
124 
133 
142 
151 
159 
168 
177 
222 
266 
60 
85 
96 
106 
117 
127 
138 
149 
159 
170 
181 
191 
202 
212 
265 
318 
70 
99 
112 
124 
136 
149 
161 
173 
186 
198 
211 
223 
235 
248 
310 
372 
80 
113 
127 
142 
156 
170 
184 
198 
212 
227 
241 
255 
269 
283 
354 
424 
90 
127 
143 
159 
175 
191 
207 
223 
239 
255 
271 
287 
303 
319 
398 
478 
100 
142 
159 
177 
195 
212 
230 
248 
266 
283 
301 
319 
336 
354 
443 
532 
200 
283 
319 
354 
390 
425 
460 
496 
531 
567 
602 
637 
673 
708 
885 
1,062 
300 
425 
478 
531 
584 
637 
690 
743 
797 
850 
903 
956 
1,009 
1,062 
1,328 
1,594 
400 
566 
637 
708 
779 
850 
920 
991 
1,062 
1,133 
1,204 
1,274 
1,345 
1,416 
1,770 
2,124 
500 
708 
797 
885 
974 
1,062 
1,150 
1,239 
1,328 
1,417 
1,505 
1,593 
1,682 
1,770 
2,213 
2,666 
Table 2. Standard heatloss values for greenhouse walls 

Wall length in feet 
Wall height in feet 

2 
4 
6 
8 
10 
12 
14 

Wall loss in mBTU/hr 

5 
1 
2 
2 
3 
4 
5 
6 
10 
2 
3 
5 
6 
8 
10 
11 
20 
3 
6 
9 
13 
16 
19 
22 
30 
5 
9 
14 
19 
24 
29 
34 
40 
6 
13 
19 
26 
32 
38 
45 
50 
8 
16 
24 
32 
40 
48 
56 
60 
9 
19 
28 
38 
47 
58 
67 
70 
11 
22 
33 
44 
55 
67 
78 
80 
13 
25 
38 
51 
63 
77 
90 
90 
14 
28 
43 
58 
71 
86 
101 
100 
16 
32 
47 
64 
79 
96 
112 
200 
32 
63 
95 
128 
158 
192 
224 
300 
47 
95 
142 
192 
237 
288 
336 
400 
63 
127 
190 
256 
316 
384 
448 
500 
79 
158 
237 
320 
395 
480 
560 
Table 3. Climate factors (K) for various average wind velocity and temperature conditions 

Inside to outside temperature difference (°F) 
Wind velocity in mph 

15 
20 
25 
30 
35 

30 
.41 
.43 
.46 
.48 
.50 
35 
.48 
.50 
.53 
.55 
.57 
40 
.55 
.57 
.60 
.62 
.64 
45 
.62 
.65 
.67 
.70 
.72 
50 
.69 
.72 
.74 
.77 
.80 
55 
.77 
.80 
.83 
.86 
.89 
60 
.84 
.88 
.91 
.94 
.98 
65 
.92 
.96 
.99 
1.03 
1.07 
70 
1.00 
1.04 
1.08 
1.12 
1.16 
75 
1.08 
1.12 
1.17 
1.21 
1.25 
80 
1.16 
1.21 
1.26 
1.30 
1.35 
85 
1.25 
1.30 
1.35 
1.40 
1.45 
90 
1.33 
1.38 
1.44 
1.49 
1.54 
Table 4. Greenhouse construction factors (C) for common types of greenhouses 

Type of greenhouse 
C 

All metal frame (tight glass house—20 or 24 inch glass width) 
1.08 

Wood and steel (tight glass house—16 or 20 inch glass width—metal gutters, vents, headers, etc.) 
1.05 

Wood house (glass with wood bars, gutters, vents, etc.—up to and including 20inch glass width 



Good tight house 
1.00 

Fairly tight house 
1.13 

Loose house 
1.25 
FRPcovered wood house 
.95 

FRPcovered metal house 
1.00 

Double glass house with 1inch air space 
.70 

Plastic covered metal house (single layer) 
1.00 

Plastic covered metal house (double layer) 
.70 
Table 5. Curtainwall construction factor (CW) for various types of nontransparent materials 

Type of material 
CW 
Glass 
1.13 
Asbestoscement 
1.15 
Concrete, 4inch 
.78 
Concrete, 8inch 
.48 
Concrete block, 4inch 
.64 
Concrete block, 4inch 
.51 
Table 6. Standard heatloss values for Quonsetstyle greenhouses for the combined ends and the entire covering along the length of the greenhouse 

Greenhouse Length in feet 
Covering width in feet 

18 
20 
22 
24 
26 
28 
30 
32 
34 
36 
38 
40 

End loss (both) in mBTU/hr 

8 
10 
12 
15 
17 
20 
23 
26 
29 
33 
36 
40 

Roof covering loss in mBTU/hr 

5 
7 
8 
9 
9 
10 
11 
12 
13 
13 
14 
15 
16 
10 
14 
16 
17 
19 
21 
22 
24 
25 
27 
28 
30 
32 
20 
28 
32 
35 
39 
41 
44 
47 
51 
54 
57 
60 
63 
30 
43 
47 
52 
57 
62 
66 
71 
76 
81 
85 
90 
95 
40 
57 
63 
70 
76 
82 
89 
95 
101 
103 
114 
120 
127 
50 
71 
79 
87 
95 
103 
111 
119 
127 
134 
142 
150 
158 
60 
85 
95 
104 
114 
123 
133 
142 
152 
161 
171 
180 
190 
70 
100 
111 
122 
133 
144 
155 
166 
177 
188 
199 
211 
222 
80 
114 
127 
139 
152 
164 
177 
190 
202 
215 
228 
240 
253 
90 
128 
142 
157 
171 
185 
199 
214 
228 
242 
256 
271 
285 
100 
142 
158 
174 
190 
206 
221 
237 
253 
269 
285 
301 
316 
200 
285 
316 
348 
380 
411 
443 
475 
506 
538 
570 
601 
633 
300 
427 
475 
522 
569 
617 
664 
712 
759 
807 
854 
902 
949 
400 
570 
633 
696 
759 
822 
886 
949 
1,012 
1,075 
1,139 
1,202 
1,265 
500 
712 
791 
870 
949 
1,028 
1,107 
1,187 
1,265 
1,345 
1,424 
1,503 
1,582 