Van Insulation and Power Draw: How R-Value Cuts Your Heating Bill

Insulation is not an electrical component, which is why it gets left out of power planning — and it is the single cheapest way to reduce a van’s energy consumption. A well-insulated van needs its heater running perhaps a third as long as a poorly insulated one, and heater runtime is the largest controllable load in a UK winter. Eighty pounds of PIR board saves more energy over a season than several hundred pounds of extra battery, and unlike the battery it keeps saving every year for the life of the van.

Why This Belongs in a Power Guide

Heat leaves a van at a rate set by three things: the temperature difference between inside and outside, the surface area, and how well that surface resists heat flow. You cannot change the first two. Insulation is the only variable you control.

An uninsulated panel van on a 2°C January night loses heat so fast that a 2kW diesel heater runs almost continuously to hold 18°C inside. The same van with 25mm of PIR board on walls, roof and floor holds that temperature with the heater cycling perhaps a third of the time.

The diesel itself is cheap. The electrical consequence is not: a diesel heater draws 25–35W while running and 90–120W during each glow-plug start. A heater cycling on and off all night through a badly insulated van performs several times as many starts as one that runs longer, gentler burns — and it is the starts that cost you. Realistic overnight heater consumption ranges from about 150Wh in a well-insulated van to 400Wh or more in a poor one.

Two hundred and fifty watt-hours a night, across a four-month winter, is roughly 30kWh — comparable to what a 400W solar array generates across the whole of that period. Insulation is a generation source in everything but name.

R-Value in Plain Terms

R-value measures resistance to heat flow: higher is better, and it scales with thickness. Materials are usually quoted per 25mm.

• PIR / polyisocyanurate board (Celotex, Kingspan): R-6 to R-6.5 per 25mm. The highest practical value per millimetre, rigid, closed-cell so it does not absorb water.
• XPS foam board: R-5 per 25mm. Slightly less efficient, cheaper, easier to cut.
• Sheep’s wool: R-3.5 to R-4 per 25mm. Breathable, manages moisture well, more forgiving in irregular cavities.
• Recycled plastic batt (Thinsulate and similar): R-3.2 to R-5.2 per 25mm. Flexible, excellent in curved cavities and ribs, does not hold water.
• Closed-cell spray foam: R-6 to R-7 per 25mm. Highest value and seals everything, but permanent, messy, and hides corrosion you can no longer inspect.
• Bubble foil: R-1 to R-1.5 total. Sold heavily to van converters and close to useless as bulk insulation.

In a van, thickness is the constraint — every millimetre inward is interior space you lose. That is why PIR dominates: it delivers the most resistance per millimetre of thickness sacrificed. 25mm of PIR is worth roughly 45mm of sheep’s wool.

Diminishing returns arrive quickly. Going from nothing to 25mm cuts heat loss by roughly 75%. Going from 25mm to 50mm cuts the remainder by half again — worthwhile on the roof, rarely worth the lost width on the walls.

Thermal Bridging: Where Most Van Insulation Fails

A thermal bridge is a path where heat bypasses the insulation through something conductive. In a steel van the ribs, frame members and floor pan are all thermal bridges, and they are welded to the outer skin.

Insulate between the ribs only and you have created an efficient heat-loss grid: cold steel runs from the outside surface straight through to your interior panelling. In practice this can cost a third of the benefit you paid for, and it shows up as condensation forming in stripes exactly where the ribs are.

The fix is a thermal break. After filling the cavities, run a continuous layer of thin insulation across the ribs before the wall panels go on — 10mm of foam board or closed-cell foam is enough to interrupt the path. It costs perhaps £40 for a van and it is the difference between insulation that performs as calculated and insulation that performs at two thirds.

The same applies to the floor. A plywood floor laid straight onto the steel pan conducts heat into the ground all night. 18–25mm of foam board under the ply is one of the highest-return insulation decisions in the build, because cold floors also make an interior feel colder than the air temperature suggests.

Where the Heat Actually Escapes

Insulation budgets should follow heat loss, not surface area. In a typical panel van:

• Roof: largest single loss. Warm air rises, and the roof is a large flat area with nothing above it. First priority, and worth going thicker here than anywhere else.
• Cab and windscreen: often the second largest, and frequently ignored entirely. Glass has almost no thermal resistance. An insulated cab divider curtain is cheap, removable, and shrinks the heated volume dramatically.
• Floor: significant, and the one people most often skip because it costs headroom.
• Walls: substantial in total but spread over many small cavities, so slower to fill and easier to get wrong around ribs.
• Windows and rooflights: per square metre the worst performers in the vehicle. Thermal blinds or covers pay back fast.
• Air leaks: gaps around the side door seal, cable entries and vents. Not strictly insulation, but a draught defeats good insulation entirely.

Insulate in that order and the money goes where it works. Insulating walls beautifully while leaving the cab open to the living space is the most common way to spend a lot and gain little.

Condensation: The Problem Insulation Creates

Two people sleeping in a van produce roughly a litre of water vapour overnight, plus whatever cooking adds. That vapour condenses wherever it meets a surface below the dew point — and in an insulated van, the coldest surface is the steel hidden behind your beautiful new panelling.

This is how vans rust from the inside out, and it is caused by insulating without thinking about moisture.

Two workable approaches:

Seal completely. Closed-cell insulation (PIR, XPS, spray foam) bonded so no air can reach the steel behind it. No air contact means no condensation on the panel. This demands genuinely complete coverage with taped joints; a gap anywhere becomes the one cold spot every bit of moisture in the van finds.

Let it breathe. Sheep’s wool or recycled plastic batt, which tolerate and buffer moisture, combined with active ventilation. Moisture reaches the steel but never dwells, because airflow removes it.

What fails is the middle ground: partial closed-cell coverage with cold gaps between. Pick one philosophy and follow it.

Either way, ventilation is not optional. A powered roof vent running on low overnight draws 0.5–3W — a rounding error against a heater at 30W — and removes the moisture before it can condense. It is the same fan that does the summer work in the van cooling guide, which is why it is the best-value single item in a van build.

Real Numbers: What Insulation Saves

Overnight heater consumption on a 2°C night, holding roughly 18°C inside:

• Uninsulated van: heater running near-continuously, 350–450Wh electrical, plus 0.5–0.7 litres of diesel
• Partial insulation, thermal bridges unaddressed: 250–320Wh, 0.35–0.5 litres
• 25mm PIR throughout with thermal break: 150–200Wh, 0.2–0.3 litres
• 50mm roof, 25mm walls, insulated floor, cab curtain: 110–160Wh, 0.15–0.25 litres

The gap between the worst and best cases is around 250Wh a night. Across 120 winter nights that is 30kWh of electricity and roughly 40 litres of diesel — call it £60 of fuel and, more importantly, 30kWh you do not have to generate in the months when generating anything is hardest.

Set against a full insulation job at £250–450 in materials, it repays in a single winter and continues every year after.

Three Insulation Specifications

Budget — £120–200

25mm PIR on roof and walls between the ribs, 18mm foam board under the floor ply, no thermal break, cab curtain from an old blanket. Gets you most of the way for very little, and the missing thermal break is the obvious later upgrade.

Standard — £250–400

50mm PIR roof, 25mm PIR walls, 25mm floor, 10mm continuous thermal break over the ribs, taped joints throughout, recycled batt stuffed into ribs and cavities the board cannot reach, insulated cab divider. This is the specification that produces the 150Wh nights and it is what most builds should target.

Full-time winter — £450–700

As above plus thermal window covers, insulated rooflight blinds, floor upgraded to 25mm PIR with an air gap, and every cable entry and seal foamed. Diminishing returns are real at this level, but for a van lived in through January they are still returns.

How This Changes the Power System

The saving compounds through the rest of the build. Cutting overnight heater draw from 400Wh to 150Wh takes 250Wh a night off the winter budget — which is roughly what a 400W solar array delivers on a December day.

Put differently: insulating properly is equivalent to doubling your winter solar generation, for a quarter of the cost and with no roof space used.

It changes what you need to buy. A van needing 1,350Wh a night in winter wants 5,000Wh of storage to run comfortably between charges. The same van insulated to 1,100Wh manages on 3,000Wh — a difference of roughly £900 in lithium, against £300 of insulation. Whatever storage you end up with, whether a wired bank or an integrated unit like a Bluetti Elite 300 or the larger Elite 400, insulation decides how far it goes.

Which is why insulation belongs in the power plan, and why it should be settled before you size the battery rather than after. The full seasonal calculation is in the year-round van power guide.

Four Mistakes That Waste the Effort

Bubble foil as primary insulation. R-1 against PIR’s R-6. It has a role as a radiant barrier with an air gap; as bulk insulation it is close to nothing.

Insulating between ribs and stopping there. The ribs conduct straight through. A 10mm continuous layer over them recovers a third of the loss for about £40.

Leaving the cab open. The largest uninsulated area in the vehicle, sitting right next to the living space. A curtain is the cheapest big win available.

Sealing the van without ventilation. A litre of water vapour a night has to go somewhere. Without a vent it goes into the steel behind your panels.

Verdict: Insulate Before You Buy Batteries

Fit 50mm PIR on the roof, 25mm on the walls and floor, a continuous 10mm thermal break across the ribs, and an insulated cab divider. Roughly £250–400 in materials and a weekend of work.

That cuts winter heater consumption by 200–250Wh a night, which is worth more than a second solar panel and costs less than a quarter of the battery capacity it saves you buying. Do it before you specify the electrical system, because every figure in that specification depends on it.