Almost everyone who arrives at an off-grid power shortfall arrives with the wrong diagnosis. The battery is too small. The array is undersized. The heater is underpowered. Buy more of something.
In heating and cooling — which is where the overwhelming majority of off-grid energy goes — that diagnosis is wrong most of the time. You are not short of watts. You are leaking them. A rig with a poor envelope is a bucket with holes in it, and the standard response is to buy a bigger tap.
This matters commercially, not just technically. Another 200Ah of lithium is a four-figure decision. Another 400W of array is a four-figure decision plus roof space you may not have. Closing the holes in the envelope is usually a two- or three-figure decision, and unlike the battery it keeps working for the life of the vehicle without degrading, and it improves comfort in both directions — winter and summer.
The arithmetic nobody runs before they buy batteries
Heat loss through an envelope is proportional to two things: the area of the envelope multiplied by its thermal conductance, and the temperature difference across it. That is the whole model. Everything else is detail.
The practical consequence is that heat loss scales linearly with the indoor–outdoor temperature gap. Holding 68°F inside when it is 48°F outside costs you half of what it costs to hold the same 68°F when it is 28°F outside. And it means that halving the conductance of the envelope halves the energy required to hold any given temperature — not “improves efficiency somewhat”, but halves it.
Put numbers on it. A 2kW diesel air heater produces roughly 6,800 BTU/h at full output. A 5kW unit produces roughly 17,000 BTU/h. If your build needs the 5kW unit running at a steady medium output to hold temperature at 20°F, and an envelope upgrade cuts your heat loss by a third, you are now inside the range where the 2kW unit holds the same temperature — with lower fuel burn, lower 12V draw from the combustion fan, and considerably less cycling.
The same logic runs in reverse in summer, where the stakes are higher because cooling is electrically expensive rather than fuel-expensive. Compressor duty cycle is a direct function of heat gain. Cut the gain and you cut the amp-hours. That is the single largest lever available on the question of whether air conditioning on battery is realistic for your build, and it is a lever most people never pull because insulation is invisible and a bigger battery is not.
R-value is a laboratory number, and your rig is not a laboratory
Here is where the published guidance starts to fall apart, and it is worth being blunt about it.
The nominal R-values quoted for common materials are broadly agreed:
| Material | Nominal R per inch | Practical notes |
|---|---|---|
| Polyisocyanurate board | ~6.0–6.5 | Highest on paper. Derates in cold — see below. Handles heat well. |
| XPS board | ~5.0 | Lower on paper, more stable across temperature. Does not compress, so it is the sensible floor material. |
| Closed-cell spray foam | ~6.0–7.0 | Seals and insulates in one pass. Effectively permanent; makes later repairs and rust inspection difficult. |
| 3M Thinsulate | ~3.3 | Fast to install, fills irregular cavities, tolerates moisture cycling. Needs roughly twice the thickness for the same R. |
| Sheep or hemp wool | ~3.3 | Similar profile to Thinsulate. Absorbs and releases moisture without losing much thermal performance. |
What is not agreed — and this is the important part — is what polyiso actually does when it gets cold. Published figures for the derating range from mild to severe. Some sources put it at roughly R5.6 per inch at 75°F falling to R5.0 at 15°F, which is trivial. Others put the effective figure at around R3.5 per inch at 25°F. Others still claim a collapse from R6.0 to R2.0 at 15°F. Those are not small discrepancies; they are the difference between polyiso being the obvious choice and polyiso being a poor one in exactly the conditions where insulation matters most.
We are not going to pretend to resolve that here, because the field data to resolve it does not appear to exist in public. What we will say is what follows from the disagreement: do not design a cold-weather build to polyiso’s nominal R-value. If you are regularly camping below freezing, either use XPS, which everyone agrees is thermally boring and stable, or specify polyiso with enough margin that a significant derate does not put you in trouble. Designing to the best-case published number for a material whose worst-case published number is a third of that is not a calculation, it is a hope.
Thermal bridging is where the R-value actually goes
You can install a perfect insulation layer and still end up with a cold build, because a vehicle shell is a lattice of steel or aluminium ribs, and metal conducts heat around your insulation with enthusiasm. Every rib, every frame member, every window flange, every screw penetrating from the interior to the shell is a bypass.
The commonly cited rule of thumb is that a 5% gap in coverage can cost you 25% or more of the effective performance. Treat that as illustrative rather than measured — it comes from builder experience rather than a controlled test — but the direction is not in dispute, and it is why a continuous layer over the ribs is worth more than a thicker layer between them. If you can only do one thing, cover the ribs. Insulating the cavities and leaving the structure bare is the most common expensive mistake in the category.
The windows deserve a paragraph of their own. Single-pane acrylic and glass are thermal holes so large that in many builds they dominate the loss calculation regardless of what you did with the walls. Insulated window covers are the highest return per dollar of anything in this article, and they cost less than a single lithium cell.
Condensation: the failure mode that ruins the vehicle, not just the night
Two adults sleeping in a small enclosed space put a meaningful volume of water vapour into the air overnight — roughly a pint to a litre each, before you account for cooking, wet gear or a kettle. That water has to go somewhere. In a warm, tight, poorly ventilated space it goes to the coldest surface available, which is whatever part of the shell your insulation failed to cover.
This is where insulation choice becomes a moisture-management choice rather than a thermal one. Rigid foam, properly sealed at every joint, is close to vapour-impermeable: water vapour does not reach the metal because it cannot get through. Fibre insulations — wool, Thinsulate — allow vapour to pass, on the theory that it will also dry back out when conditions change. At least one side-by-side test box comparison found that both fibre materials allowed vapour to penetrate and condense in the outer layers and against the shell, while sealed polyiso did not; the same test found the resulting condensation did not appear to degrade the fibre insulation’s thermal performance much.
Both approaches work. What does not work is the hybrid nobody plans and everybody builds: rigid foam with unsealed joints and gaps, which lets vapour reach the metal and then traps it there with no drying path. If you are going the foam route, seal the joints properly, or you have chosen the worst of both systems.
The consequence of getting this wrong is not condensation on the windows. It is corrosion behind panels you cannot inspect, mould in cavities you cannot reach, and a resale conversation you do not want to have. That is a line item on the five-year cost of ownership that never appears in anyone’s spreadsheet.
Ventilation is not the enemy of insulation
There is a persistent belief that ventilation undoes insulation — that every cubic foot of air you exchange is heat thrown away. It is true in a narrow arithmetic sense and wrong in every practical one.
The distinction that matters is between controlled and uncontrolled air exchange. Uncontrolled exchange is draughts: unsealed floor penetrations, door seals, cable grommets, a badly fitted roof vent. It happens continuously, you cannot regulate it, and it delivers no benefit. Controlled exchange is a roof fan or a cracked vent running when you need it, moving humid air out and dry air in, at a rate you choose.
The heat cost of controlled ventilation is real but small relative to what the moisture costs you if you skip it. A roof fan on a low setting draws single-digit watts. A build with unmanaged humidity costs you sheet metal.
The practical target is straightforward: seal every uncontrolled path you can find, then add deliberate ventilation you can switch on. In cold weather that means cracking a vent when cooking and running the fan briefly before sleeping. In hot weather it means using ventilation as your first cooling stage before the compressor ever starts, which is the same principle that governs where heat pumps stop being useful and a fuel-burning heater takes over.
How to find out whether the envelope is actually your problem
Before spending anything, spend an evening measuring. Three tests, none of which require equipment you cannot borrow.
The decay test. Heat the interior to a stable temperature on a cold, still night, then switch everything off and log the interior temperature every fifteen minutes for two hours alongside the outside temperature. What you are looking for is the rate of fall relative to the temperature gap. A rig that drops 15°F in ninety minutes against a 30°F gap is losing heat fast enough that no plausible battery bank will fix it. Repeat the test after any change and you have a before-and-after that is worth more than any manufacturer’s R-value claim.
The dew-point survey. On the morning after a cold night, before you ventilate, go around the interior with a hand and look for where the condensation is. It will collect at the coldest points, which are your thermal bridges. Window frames, the ribs behind wall panels, the roof vent flange, the wheel arch, and — in a surprising number of builds — a run of steel structure someone insulated around rather than over.
The infrared pass. A borrowed or rented thermal camera, used from outside on a cold night with the heat on, will show you the bridges as bright lines in about four minutes. It is the fastest diagnosis available and the one that most often changes people’s plans.
Where to spend, in order
- Insulated window covers. Cheapest, fastest, largest single effect in most builds. Reversible.
- Sealing uncontrolled air paths. Floor penetrations, wiring grommets, the gap behind the wheel arch trim. Costs a tube of sealant and an afternoon.
- A continuous layer over the ribs and frame. More valuable per inch than a thicker cavity fill.
- Cavity insulation. Worth doing, and the part everyone starts with, which is why so many builds end up with an insulated cavity and a bridged frame.
- Floor. XPS, because it does not compress under cabinetry. Do the rust inspection before you bury it.
- A controllable roof fan, if you do not already have one.
What we would not spend on
Reflective bubble foil sold on the strength of a headline R-value. Radiant barriers do genuinely useful work against direct solar gain when there is an air gap on the reflective side — that is real physics. What is not real is the R-value printed on the packaging, which assumes an installation geometry almost nobody achieves in a vehicle. Use it as a thermal break over a rib, or against the roof for solar gain, and value it accordingly. Do not use it as your insulation.
We would also be cautious about closed-cell spray foam in a vehicle shell. It performs well and seals beautifully. It also makes any future repair, rust remediation or wiring change into a demolition job, and it adds weight that is hard to estimate before you commit. In a fixed cabin it is an easy recommendation. In something that flexes and that you may want to modify or sell, it is a decision you make once.
The point
Every watt-hour you do not lose through the envelope is a watt-hour you never have to generate, store, carry or pay for. It is the only capacity upgrade that does not degrade, does not need charging and does not appear on your roof.
Fix the envelope first. Then size the system. Doing it the other way round is how people end up with 600Ah of lithium and a cold bedroom.