Van and RV Insulation by the Numbers: R-Values, Heat Load, and What It Means for Your AC, Heater and Battery (2026)

Quick answer

Conduction heat flow through a van’s shell is area × temperature difference ÷ R-value, and on our 330 ft² reference van that arithmetic puts a bare metal shell at roughly 10,040 BTU/h of loss on a 40°F night with 70°F inside, while one inch of R-6.5 foam on the walls, roof and floor cuts it to an estimated 2,270–3,810 BTU/h. The 30 ft² of glass is 9% of the surface but, once the walls are insulated, 27–62% of the loss. Every figure is conduction only, arithmetic from labelled assumptions; sun, people and air leaks come on top. This is the insulation guide our Class A post promised in July.

300 ft² of opaque shell, 30 ft² of glass (§3). Ranges run from nominal to bridged (half the insulation’s R-value with no air-film credit, DOE’s metal-framing worst case). No sun, occupants or air leakage. Arithmetic, not measurement.

1. The one formula, and a worked line

Heat flow by conduction, in BTU per hour, equals area in square feet × the indoor–outdoor temperature difference in °F ÷ total R-value, so one square foot of R-6.5 board with 30°F across it passes about 4.6 BTU/h. Lawrence Berkeley National Laboratory writes the same law as Q/A = U × (To − Ti), with U the reciprocal of total resistance; the Department of Energy notes that the R-values of layers add.

The worked line, for the moderate build at a 30°F difference:

• Opaque shell: 300 ft² × 30°F ÷ R-7.35 = 1,224 BTU/h
• Glass: 30 ft² × 30°F × U 1.16 = 1,044 BTU/h
• Total: about 2,270 BTU/h, estimated

Two things follow. Heat flow doubles when the temperature difference doubles, and halving it takes doubling R, so each extra inch saves less than the one before. Our air conditioner cooling ratings, and the Glacier Pro heat pump’s heating rating (above about 36°F only), are printed in BTU; they are hourly rates, comparable with the BTU/h figures here.

2. R-values per inch, from government sources

DOE rates closed-cell spray foam and polyiso board at R-6.5 per inch, XPS at R-5, EPS at R-3.8–4.4, and fiberglass batts and open-cell foam at about R-3.6–3.7; a single-pane window is roughly R-0.9.

The window rows are building windows sold 1993–2001, not vehicle glass, which may differ. Two cautions from DOE’s insulation fact sheet matter in a van: compressed insulation “will not give you its full rated R-value”, and reflective products, foil-faced bubble included, work against radiant heat, depend on the direction of heat flow and need an adjacent air space, so we assign them no contact R-value. We found no government per-inch figure for polyester or wool; use a manufacturer’s tested figure and label it as such.

3. The reference van and its three builds

Our reference van has 300 ft² of opaque shell and 30 ft² of glass, and the three builds put roughly R-1, R-3.25–7.35 and R-6.5–13.85 on the shell; at a 30°F difference that is an estimated 10,040, 2,270–3,810 and 1,690–2,430 BTU/h of conduction loss. Every input is a labelled assumption:

• Shell, 300 ft²: roof about 70, floor about 70, walls, doors and bulkhead about 160, our illustrative split for a high-roof cargo van. Glass, 30 ft², single pane unless stated.
• Air films, R-0.85: read from LBNL’s single-pane figure (R ≈ 0.86), since a single pane is mostly its two surface films.
• Bare shell, R-1: sheet metal plus films, rounded up, which flatters the bare van slightly. Not a government figure.
• Moderate: 1 in of polyiso or closed-cell foam, R-6.5 + R-0.85 = R-7.35 nominal. Thick: 2 in, R-13 + R-0.85 = R-13.85, where the cavity is deep enough.
• Bridged: through metal framing, DOE says “sometimes the insulated metal wall’s overall R-value can be as low as half the insulation’s R-value.” Van ribs are metal, so each range’s low end uses R-3.25 and R-6.5.
• Same build on every surface, no air leakage, 70°F inside in winter.

The first inch does most of the work: bare to moderate cuts conduction loss by 62–77%, and the second inch cuts a further 25–36%. And glass grows in share as the walls improve: at a 30°F difference it is 10% of the bare shell’s loss, 27–46% of the moderate build’s and 43–62% of the thick build’s. Double-pane building-window values halve the glass line, from 1,044 to 531 BTU/h.

4. Winter: the result against our heat pump and diesel heater

At 40°F outside and 70°F inside, the insulated builds lose an estimated 1,180–3,810 BTU/h by conduction, under the 8,500 BTU heating rating of the Glacier Pro 12V heat pump, which stops working effectively below about 36°F. At 20°F outside only the diesel heater applies: the bare shell’s ~16,740 BTU/h is about 98% of our 5 kW heater’s rated power, roughly 17,060 BTU/h by unit conversion, leaving almost no margin for air leaks.

A rating is not delivered capacity. A heat pump’s effectiveness depends on outdoor conditions, and our spec publishes no output-versus-temperature curve; it also says the unit is not for freezing winter survival. Air leakage adds to every loss figure. So, on paper, the insulated builds’ conduction loss on a 40°F night sits under the heat pump’s 8,500 BTU rating and the bare shell’s does not; neither result promises that a unit holds 70°F.

The diesel heater’s figures are page-listed: 5 kW (5,000 W) rated power, -40°F to 104°F operating range. The 17,060 BTU/h is a unit conversion of that rated power figure, 5,000 W ÷ 0.2930711 W per BTU/h (NIST’s factor), not a measured output. Its electrical draw is not published, so we do not quantify it. Which heat source fits which nights, and the heating battery math, belong to our propane-free heating post. A warm envelope helps the house bank too, as our LiFePO4-in-the-cold post explains.

5. Summer: conduction gain against 10,000 BTU

At 95°F outside and 75°F inside, conduction brings an estimated 6,700 BTU/h into the bare shell, 1,510–2,540 BTU/h into the moderate build and 1,130–1,620 BTU/h into the thick build, against the Summit 2 12V’s 10,000 BTU cooling rating.

The bare shell uses about two-thirds of that rating on conduction alone. Sun on the roof and through the glass, people, cooking and air leaks all add, and in direct sun they may well be the larger part; we have no sourced solar-gain figure for a van, so we leave it unquantified. Insulation shrinks the one load you can calculate and leaves capacity for the ones you cannot.

Our suitability guidance says extensive glass, weak insulation and several occupants can leave one unit providing only local cooling in a large vehicle, and no length or area cutoff guarantees otherwise. Hot-weather tactics beyond insulation, such as shade and pre-cooling, are in the desert post. Matching capacity to the rig is the BTU sizing guide’s job.

6. What it does for the battery

Insulation does not change the Summit 2 12V’s rated draw in each mode, Turbo 58A, Eco 29A and Sleep 21A, but a smaller heat load gives the variable-speed DC compressor a better chance of holding the setpoint in a lower-draw mode more of the time, and that is where amp-hours can be saved.

We publish no per-mode cooling capacity, so we cannot say which mode a given van needs, and we do not convert BTU/h into amps: the spec publishes no efficiency curve, and draw depends on setpoint, weather and hours run. Turbo’s 58A is a peak figure, not a continuous one. The Glacier Pro 12V heat pump, above its ~36°F floor only, follows the same logic: its heating draw is variable up to 55A, adjusts to ambient temperature and setpoint, and is often lower once the target is held. How per-mode draws add up over a night and a year is in the all-night post and the year-in-amp-hours ledger.

7. Where to spend first

If the shell is still bare metal, the first inch everywhere is the whole story, a 62–77% cut in conduction loss on our reference van; once it is insulated, the arithmetic ranks glass first, and sun exposure puts the roof second.

1. Glass. Per square foot, single pane (U 1.16) passes about 8.5 times what the moderate wall passes and 16 times the thick wall, both at their nominal ratings.
2. The roof. About 70 of our 300 ft² of opaque shell, so with the same build everywhere it carries about 23% of the opaque conduction loss before any sun is counted. DOE notes reflective products are most effective against downward heat flow when they face an air space, which makes the roof, against summer sun, their best surface.
3. The ribs and the leaks. Covering the metal framing, not just the bays between it, narrows the gap between our nominal and bridged figures. Air sealing is a separate job: insulation reduces air movement only within the space it occupies, DOE says.
4. A second inch. A further 25–36% on our van, after the three above.

The practical how, from window covers and roof surface to parking and ventilation, is in our extreme-heat guide.

FAQ

What R-value do I need for a camper van? We know of no government standard for vans to cite. On our 330 ft² reference van, one inch of R-6.5 foam (R-7.35 with air films) cuts conduction loss by an estimated 62–77% from bare metal, and a second inch cuts a further 25–36%. Use the R your cavity depth allows, then spend on glass and air sealing.

What is the best insulation for a van? By DOE’s figures, closed-cell spray foam and polyiso board lead at R-6.5 per inch, then XPS at R-5, EPS at R-3.8–4.4, and fiberglass batts and open-cell foam at about R-3.6–3.7. The choice also depends on cavity depth, compression (batts lose rating when squeezed) and moisture (open-cell foam passes vapor more easily).

Does insulation help an RV air conditioner? Yes. On our reference van at 95°F outside and 75°F inside, estimated conduction gain falls from about 6,700 BTU/h bare to 1,130–2,540 BTU/h with one or two inches of foam, against a Summit 2 12V’s 10,000 BTU rating. A smaller load gives the unit a better chance of holding the setpoint in a lower-draw mode more of the time, though sun and air leaks still count. We publish no per-mode cooling capacity, so this is not a promise of a particular mode or draw.

Can an 8,500 BTU heat pump heat an insulated van? Above about 36°F, on paper: at 40°F outside and 70°F inside our insulated builds lose an estimated 1,180–3,810 BTU/h by conduction, under the Glacier Pro 12V’s 8,500 BTU heating rating. Below about 36°F it stops working effectively; that is diesel territory. Effectiveness depends on outdoor conditions and air leaks add load, so this is a capacity comparison, not a promise.

How much heat do RV windows lose? By Lawrence Berkeley National Laboratory’s building-window values, single-pane glass passes about 1.16 BTU/h per square foot per °F and double-pane about 0.59. On our 30 ft² of glass at a 30°F difference that is about 1,044 and 531 BTU/h; with single pane, glass is 43–62% of the thick build’s loss. Vehicle glass may differ.

Related reading

• How to Cool a Large RV or Class A With 12V (and 110V) Air Conditioning
• How to Keep Your RV Cool in Extreme Heat (2026)
• How to Size BTUs for Your RV (2026)
• Can You Heat an RV Without Propane? (2026)
• LiFePO4 in the Cold (2026)
• Full-Time RV Living With a 12V AC: A Year in Amp-Hours (2026)
• Can You Run a 12V RV AC All Night on Battery? (2026)

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