Van Heat Loss Calculator: Size Your Heater and Insulation

Enter the insulated area of your van, the insulation and its thickness, and inside and outside temperatures. The calculator gives heat loss through the insulation in Btu per hour and watts, the share of a heater's output it takes, and fuel for the night. The defaults are example inputs; the heater defaults are Webasto's Air Top 2000 STC.

How much heat does your van lose?

Insulated area (walls, roof, floor)

sq ft

Insulation

Insulation thickness

Inside temperature

°F

Outside temperature

°F

Heater output at full power

Heater fuel use at full power

L/h

Hours of heat

Heat loss

2,571 Btu/h

754 watts, total R-5.6

Share of heater output

38%

At full power the heater cycles down

Fuel for the period

0.90 L

0.24 US gallons

Temperature difference

48 °F

Heat flow through insulation = area x temperature difference / R (the conduction formula), with R = R per inch x thickness; R-values add up (DOE). R per inch is the low end of each DOE range. This counts conduction through the insulated area only: metal ribs, windows, doors and air leaks add more loss, so treat the result as a floor. The heater defaults are Webasto's Air Top 2000 STC (2.0 kW, 0.24 L/h at full output); the fuel line assumes fuel use scales with output.

Method

R-value per inch is the low end of each range in DOE's insulation guide (for example polyiso R-5.6, XPS R-5.0, sheep's wool R-3.5, fiberglass batts R-2.9). DOE notes R-values are cumulative, so R = R per inch x thickness.
Heat loss = area x temperature difference / R, the conduction relation HyperPhysics gives (heat flow = conductivity x area x temperature difference / thickness), with R standing for thickness over conductivity.
Watts = Btu per hour / 3,412 x 1,000, using the EIA's 3,412 Btu per kWh.
Heater share = watts / heater output; fuel = share x full-power fuel rate x hours, assuming fuel scales with output.

Worked examples

Our arithmetic; heater 2.0 kW at 0.24 L/h (Webasto)
Inputs (300 sq ft, 68 F inside)	Heat loss	Watts	Share of 2 kW	Fuel, 10 h
1 in polyiso (R-5.6), 20 F outside	2,571 Btu/h	754	38%	0.90 L
2 in XPS (R-10), 20 F outside	1,440 Btu/h	422	21%	0.51 L
1 in fiberglass (R-2.9), 20 F outside	4,966 Btu/h	1,455	73%	1.75 L
1 in polyiso (R-5.6), 0 F outside	3,643 Btu/h	1,068	53%	1.28 L

What it leaves out

Only conduction through the insulated area is counted. Bare metal ribs, windows, doors and air leaks add more loss, so treat the result as a floor and leave headroom. DOE's guide also notes that reflective foil needs an air space to work.

Using the result

If the share is near 100 percent, add insulation before a bigger heater: best van insulation. Heaters: best diesel heaters.

Frequently asked questions

How do you calculate heat loss in a van?

Heat loss in Btu per hour = insulated area x temperature difference / total R-value. The calculator does it and converts to watts.

What size heater does a van need?

Enough to cover the heat loss with room to spare. In the examples, 300 sq ft at R-5.6 and 48 F of difference loses about 754 W, well within a 2 kW heater.

What R-value is van insulation?

DOE lists polyiso at R-5.6 to R-8 per inch, XPS at R-5, sheep's wool batts at R-3.5 and fiberglass batts at R-2.9 to R-3.8. Use the maker's figure where you have one.

Why is my van colder than the calculator says?

Windows, bare metal ribs and air leaks lose heat the calculator does not count. Cover windows and seal gaps.

How much diesel does a heater use overnight?

In the first example, about 0.9 L over 10 hours at 38 percent of a 2 kW heater's output, assuming fuel use scales with output.

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