| Determining Insulation Thickness |
Spray Foam '06 |
1. Economic Thickness
It doesn't pay to keep adding insulation thickness because the cost of the insulation will be more than the energy saved. This is an example of the Law of Diminishing Returns.
Law of Diminishing Returns: The incremental return on an investment decreases with added investment.
The first half-inch of SPF gives you the most bang for your buck. At some point
it doesn't pay to add more insulation.
Insulation Efficiency Based on 1/2-inch Plywood = 0%
| Inches of 2 lb SPF | R-Value | Efficiency | Inches of .5 lb SPF |
| 0 |
0.64 |
0.00% |
0.64 |
| 0.5 |
3.99 |
84% |
1.04 |
| 1 |
7.34 |
91.30% |
1.91 |
| 1.5 |
10.69 |
94% |
2.79 |
| 2 |
14.04 |
95.40% |
3.66 |
| 2.5 |
17.39 |
96.30% |
4.54 |
| 3 |
20.74 |
96.90% |
5.41 |
| 3.5 |
24.09 |
97.30% |
6.28 |
| 4 |
27.44 |
97.70% |
7.16 |
| 4.5 |
30.79 |
97.90% |
8.03 |
| 5 |
34.14 |
98.10% |
8.91 |
| 5.5 |
37.49 |
98.30% |
9.78 |
| 6 |
40.84 |
98.40% |
10.66 |
Economic Thickness determines what SPF thickness cost the least for the customer by comparing future energy dollars to current insulation dollars. This involves complex financial calculations.
Example Calculation
| Project: |
Fire suppression water storage tank |
| Location: |
Fargo, ND (cold climate) |
| Size: |
20 ft high by 20 ft diameter |
| Temperature: |
Maintain water at 45° F or higher |
| Cost of Capital: |
3% per year |
| Fuel Inflation: |
10% per year |
| Heating: |
Steam heat from #2 fuel oil @ $2 per gallon |
| |
|
| Annual Cost to heat Tank: |
$15,800 per year |
| Present Value of 10 years: |
$210,00 |
| Cost of Insulation: |
First 1/2-inch + coating = $3,060;
$700 per add'l half inch. |
|
