The estimator who carries temporary heat in general conditions needs a fuel number before the enclosure goes up. Heating degree days turn the weather into that number, and they give the project manager a way to check it at every propane delivery. The method here combines a heat-loss formula from a University of Arizona engineering course with degree-day forecasts and propane prices from the U.S. Energy Information Administration (EIA).
Heating degree days
A heating degree day measures how far a day’s average temperature falls below 65°F. EIA takes the average as the day’s high plus its low, divided by two. In EIA’s own example, a high of 33°F and a low of 25°F average 29°F, which counts as 36 heating degree days. Adding the days gives a month or a season.
EIA forecasts degree days by Census region in its Winter Fuels Outlook. The 2026-27 edition, published October 6, assumes 4,802 heating degree days in the Midwest from November through March, 2% fewer than last winter. EIA also runs side cases 10% colder and 10% warmer, because weather is a significant source of uncertainty in its forecast.
From degree days to gallons
The course notes, by Gene Giacomelli of the university’s Controlled Environment Agriculture Center, size a greenhouse heater with one equation. Heat loss in Btu per hour equals U times the surface area times the temperature difference between inside and outside. U is the overall heat transfer coefficient, meaning the Btu per hour that one square foot of covering loses for each degree of difference.
The notes give U as 1.2 for a single layer of polyethylene and 0.8 for a double layer, both with air infiltration counted. A greenhouse is a poly enclosure with a heater in it, which makes it a close match for a building shell wrapped for winter work.
The notes keep two numbers apart. Heater size comes from the coldest expected outside temperature. Fuel use depends on the temperature difference at every hour of the season, and the degree-day procedure estimates it from monthly totals. To use it at any setpoint, convert the month’s degree days to an average temperature and measure the gap from your setpoint. At 50°F, that means subtracting 15 degree days for each day in the month.
- Multiply U by the enclosure’s surface area to get UA, the heat loss per degree.
- Multiply UA by 24 hours and by the month’s degree days at your setpoint to get Btu for the month.
- Divide by the fuel’s heat content times the heater’s efficiency to get gallons. EIA lists propane at 91,452 Btu per gallon.
- Multiply gallons by the delivered price.
A worked example
This example uses round numbers. A contractor wraps a building shell in 20,000 square feet of single-layer poly and holds 50°F inside from November through March, using EIA’s Midwest forecast. With U at 1.2, UA is 24,000 Btu per hour per degree. The example assumes 80% of the propane’s heat reaches the space and a price of $2.30 a gallon. EIA’s weekly survey put the Midwest residential propane average at $2.298 on November 2.
January burns 5,865 gallons, 29% of the season
Propane for a 20,000 sq. ft. single-poly enclosure held at 50°F, gallons per month. Example calculation.
| Month | EIA Midwest heating degree days (base 65F) | Days | Degree days at a 50F setpoint | Heat needed (million Btu) | Propane gallons | Propane cost at $2.30 |
|---|---|---|---|---|---|---|
| Nov | 726 | 30 | 276 | 159.0 | 2,173 | $4,998 |
| Dec | 1,062 | 31 | 597 | 343.9 | 4,700 | $10,810 |
| Jan | 1,210 | 31 | 745 | 429.1 | 5,865 | $13,490 |
| Feb | 997 | 28 | 577 | 332.4 | 4,543 | $10,448 |
| Mar | 806 | 31 | 341 | 196.4 | 2,685 | $6,175 |
The season needs about 1,461 million Btu, close to 20,000 gallons and $45,900 of propane. Each degree day at the setpoint burns 7.9 gallons, about $18. That rate is the figure to track on site.
Heater capacity comes from the sizing equation. With a design low of 0°F, the enclosure loses 24,000 times 50, or 1.2 million Btu per hour, and the heaters together have to deliver that output.
How weather moves the budget
EIA’s side cases shift degree days 10% in either direction. At a 50°F setpoint the fuel shifts further, because the 15-per-day subtraction stays fixed while the total changes.
| Case | Season degree days (base 65F) | Degree days at a 50F setpoint | Propane gallons | Propane cost at $2.30 | Change in fuel |
|---|---|---|---|---|---|
| EIA base case | 4,801 | 2,536 | 19,966 | $45,922 | 0.0% |
| 10% colder | 5,281 | 3,016 | 23,746 | $54,615 | 18.9% |
| 10% warmer | 4,321 | 2,056 | 16,186 | $37,228 | -18.9% |
A 10% colder winter adds about 3,780 gallons and $8,700 to the example, a 19% increase. An estimator can carry the colder case as contingency, or write temporary heat as an allowance tied to measured degree days.
Limits of the method
EIA weights its regional degree days by population, so the Midwest figure leans toward its cities. A job in northern Minnesota will see more degree days than the regional number. EIA points to the National Weather Service Climate Prediction Center for degree days by location, and the weather delay days post shows how to pull NOAA normals for a single station.
The monthly shortcut runs low in months that include days warmer than the setpoint, which matters most in November and March. The Arizona notes also leave out solar gain and assume the same temperature difference all day. The poly U values assume a closed enclosure. Open hoistways and doors left up for deliveries add air leakage that the coefficient does not cover.
Ventilation adds load as well. OSHA’s standard for temporary heating devices, 1926.154, requires enough fresh air to protect workers, with mechanical ventilation where natural supply is inadequate, and that air has to be heated. The same standard keeps heaters at least 10 feet from combustible tarps, canvas or similar coverings, which affects where heaters go inside a wrapped shell.
How to use it on your jobs
Track gallons per degree day. At each propane fill, log the gallons delivered and the degree days at your setpoint since the last fill, using the nearest weather station. Divide the first by the second. The example estimate is 7.9, so a job running at 11 is losing more heat than the estimate assumed, and the project manager can see the overrun by December.
After a winter or two, the measured rates by enclosure type become the basis for the next bid. The fuel price adjustment post shows how to tie a fuel clause to an EIA price series, and the wind chill post covers the crew side of the same cold days.
To start, pick one job that will be enclosed this winter. Measure the poly area from the enclosure plan, then run the four steps against EIA’s regional forecast and your supplier’s current propane quote.