Fitting a rooftop or under bunk air conditioner is one of the more achievable big lap upgrades, but it is also one where a small mistake becomes an expensive one. This guide walks you through deciding on the type, checking your roof, cutting the aperture, sealing it properly, wiring it, and knowing exactly when to hand the job to a professional.
Two things must be right before you start. Your roof has to carry the load, both structurally and within your payload, since a rooftop unit adds 30 to 45kg up high and changes how the van sits and tows. And any 240V wiring is licensed electrical work in Australia. Doing it yourself is illegal and voids your insurance. That part goes to a licensed electrician, full stop.
Rooftop or Under Bunk: Decide First
Before you touch a tool, settle on the type of unit, because the whole install differs. A rooftop unit sits on the roof over the existing hatch aperture and blows cool air down through a ceiling plenum. It is the most common choice, keeps floor and cupboard space free, and cools evenly. The trade off is weight up high, extra wind height, and a hole in your roof.
An under bunk unit lives inside the van, usually in the front bed base or a large cupboard, and ducts cool air through vents. It keeps weight low and the roofline clean, which suits off road vans and anyone watching wind drag. The cost is the storage space it swallows and a more involved ducting run.
For most big lappers a rooftop reverse cycle unit is the straightforward pick. If you are unsure which unit suits your van, sort that out before going further.
Check the Roof Structure and Aperture
This is the step people skip and later regret. Get up on the roof and work out what you are dealing with before committing to a cut.
Most Australian vans come with a 360mm by 360mm vent aperture already framed into the roof, which happens to be the standard footprint that rooftop air conditioners are built to sit over. If you have an existing roof hatch of that size, your job is mostly a swap: remove the hatch and drop the air conditioner into the same hole. Measure it and confirm it is genuinely 360mm by 360mm, since some older or imported vans differ.
Check the roof thickness too. Air conditioners are rated for a roof panel range, commonly around 25mm to 65mm, and the mounting bolts need to clamp correctly within that. Then check the structure around the aperture. The roof needs framing or timber around the opening to carry the load and stop the panel flexing. A bare foam sandwich panel with no reinforcement is not enough on its own, and you may need to add a support frame inside.
Before you commit, weigh the payload. Add the unit’s mass to your loaded van and confirm you stay under your ATM and axle limits. A rooftop unit also raises your centre of gravity, so a van that towed happily can feel different afterwards.
Make or Use the Cut Out
If you already have a 360mm by 360mm hatch, remove it. Undo the internal fasteners, break the old sealant, lift the hatch clear, and scrape every trace of old butyl and silicone off the roof until you have a clean, flat surface. No cutting required, which is the ideal scenario.
If you are making a fresh cut out, mark a 360mm by 360mm square exactly where the unit will sit, checking it clears internal framing, wiring and gas lines below. Drill a pilot hole in each corner, then cut the square with a jigsaw fitted with a fine blade, going slowly through the aluminium skin and foam. Support the offcut so it does not drop and tear the ceiling lining as it comes free.
Measure the aperture twice and dry fit the unit before any sealant goes near it. Seal the raw cut edges with a coat of sealant so the exposed foam core cannot soak up moisture down the track.
Seal With Butyl Tape
The seal is what stands between you and a leaking roof, so take your time. Rooftop air conditioners seal against the roof with a thick butyl gasket, not silicone. Butyl stays flexible, moves with the van, and does not go brittle in the sun the way silicone can.
Lay a continuous strip of butyl tape around the aperture on the underside of the unit’s mounting flange, following the maker’s pattern with no gaps at the corners. Lower the unit squarely onto the clean roof so the butyl lands evenly all the way around. Then tighten the internal clamp bolts gradually and evenly, working around the unit rather than fully tightening one corner first. As you tighten, the butyl squeezes out to a visible bead around the base, which tells you the seal has taken. Do not over tighten, since crushing the gasket too far is as bad as leaving it loose.
Fit the unit on a warm day. Butyl is stiff and reluctant when it is cold, and it spreads into a proper seal far more willingly once the roof has warmed in the sun.
Wiring: 240V Versus 12V
Air conditioners in caravans come in two power types, and the distinction decides who can legally do this step. The common rooftop reverse cycle units are 240V, running off mains power at a powered site or from an inverter and battery bank. A smaller group of 12V units run straight off your battery system and suit off grid life, though they draw heavily and need serious battery and solar behind them.
For a 240V unit, the wiring is licensed electrical work and must be done by a licensed electrician. That covers the connection to the van’s 240V circuit, the isolation, and the compliance certificate. There is no DIY exemption for this in Australia, no matter how confident you are.
For a 12V unit, the DC wiring is more like a standard 12V accessory install, but the loads are large. You need correctly sized cable, the right fusing, and a battery system genuinely capable of the draw. If you are running any air conditioner off grid, the power system is the make or break part.
Fit the Internal Plenum and Controls
With the unit sealed on the roof and the wiring handled, move inside. The internal plenum, also called the ceiling assembly, is the part you see and touch from within the van. It clips or bolts up to the base of the rooftop unit through the aperture and carries the air vents, the filter, and usually the controls.
Line the plenum up with the unit above, connect the control wiring or plug as the maker directs, and fix it firmly to the roof so it does not rattle down the road. Set the adjustable vents so cool air spreads through the van rather than blasting one spot. If your unit uses a wall thermostat or remote instead of controls on the plenum, mount that at a sensible height, out of direct airflow so it reads the room correctly.
Test the Unit
With everything fitted and the electrician’s sign off done on a 240V unit, run it properly before you call the job finished.
Power up and run cooling first. You should feel cold air within a couple of minutes and hear the compressor settle into a steady note. Run reverse cycle heating next to confirm both modes work. Cycle through the fan speeds and check the thermostat actually cuts the compressor when it reaches the set temperature. While it runs, get back on the roof and look around the base for any sign the seal is not right, then check inside that the plenum is not vibrating loose.
Test the seal with a hose. Gently run water over and around the unit for a few minutes while someone watches the ceiling inside for any drip or damp patch. Far better to find a leak in the driveway than in the wet on a remote road.
When to Hand It to a Professional
Plenty of handy owners fit their own air conditioner and do a clean job of it. But be honest about where your skills stop, because the failure modes here are a leaking roof or a dangerous wiring fault, neither of which is cheap to fix.
Hand it over if any of these apply. The 240V connection is not optional: that goes to a licensed electrician every time. If you are cutting a fresh aperture into an unreinforced roof, or you are unsure the structure will carry the load, get a caravan repairer to assess and frame it. If your roof is an unusual thickness or your van is out of warranty risk, a professional fit protects you. And if the idea of cutting a 360mm square into your roof makes your stomach turn, that instinct is worth listening to. A professional rooftop install typically runs a few hundred dollars on top of the unit, which is cheap insurance against a leak over your bed.
- Decide rooftop or under bunk first, since the whole install differs. Rooftop over the standard 360mm by 360mm aperture is the common choice.
- Check roof thickness, structure and your payload before cutting anything.
- An existing 360mm hatch is a straight swap; a fresh cut out needs careful marking and a fine jigsaw blade.
- Seal with butyl tape, not silicone, and tighten the clamp bolts evenly until the gasket beads.
- 240V wiring is licensed work for an electrician. Only 12V DC wiring is DIY, and it needs a serious battery system behind it.
- Test cooling, heating and the thermostat, then hose test the seal before you trust it.
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