Floor Insulation for Cold Floors: Complete Guide
A cold floor is one of the most common complaints in DIY campervan conversions. Heat rises, so the floor is always the coldest surface in your van. Without proper insulation, it creates a thermal bridge to the cold ground below, leading to condensation on your floorboards and a general chill that your heater struggles to overcome.
Why Floors Get Cold
The floor of your van is a metal panel sitting 30-60cm above the ground. In winter, the ground temperature in the UK drops to 2-8°C. The metal floor conducts this cold directly into your living space. The large surface area (6-10m² in a typical van) means a significant amount of heat is lost through the floor even if the rest of the van is well-insulated.
The physics: The floor is exposed to cold air flowing underneath the van while driving. Metal ribs underneath act as cooling fins that accelerate heat transfer. Without insulation between the interior and the metal floor, you are essentially sleeping on a radiator that is mounted to the cold ground.
Best Floor Insulation Materials
XPS Foam Board (Best Overall)
XPS (extruded polystyrene) is the standard choice for van floor insulation:
- Thermal conductivity: 0.028-0.032 W/mK (best of common materials)
- Compression strength: 200-500 kPa (excellent for flooring)
- Moisture resistance: Closed-cell, does not absorb water
- Cost: £5-8 per m² for 25mm
Available at B&Q, Wickes, Screwfix as Celotex, Kingspan, or generic XPS.
Thickness recommendation: 25mm minimum for van floors. 50mm if you have the headroom to spare.
PIR Foam Board (Premium)
PIR (polyisocyanurate) has slightly better thermal performance than XPS:
- Conductivity: 0.022-0.026 W/mK
- Compression strength: 150-250 kPa (slightly less than XPS)
- Cost: £8-12 per m² for 25mm
Brands: Celotex TB4000, Kingspan Thermaroof. Slightly more expensive but better R-value per mm.
Closed-Cell Foam (Best for Low Profile)
If you have limited floor-to-ceiling height and cannot afford 25mm + plywood:
- 10mm Armaflex or similar: £8-12 per m²
- Compression resistance: Moderate (softens under point loads)
- Best for: Micro-campers or van conversions where every mm of headroom counts
Installation Step by Step
Step 1: Prepare the Metal Floor
- Remove the factory plywood floor (if fitted)
- Wire brush any rust spots
- Treat rust with Kurust and allow to dry
- Vacuum all dust and debris
- Wipe down with panel wipe
Step 2: Fit a Vapour Barrier
This is the most important step. The floor vapour barrier prevents ground moisture from condensing on the cold metal and rotting your wooden subfloor.
Method:
- Lay 500-gauge polythene sheet over the entire metal floor
- Tape all seams with aluminium foil tape
- Bring the sheet 10cm up the side walls
- Tape to the wall vapour barrier (creates a sealed tub)
Step 3: Lay the Insulation
- Cut XPS/PIR boards to fit between the floor ribs
- For flat-floor vans (Transit, Sprinter): lay full sheets, cut around wheel arch intrusions
- Fill gaps around edges with low-expansion foam
If your van has pronounced floor ribs (common on Relay/Boxer/Ducato), fill the channels between ribs with 12mm foam before laying the main 25mm board.
Step 4: Add a Second Vapour Barrier (Optional but Recommended)
Lay a second polythene sheet over the insulation, taped to the wall barrier. This prevents any moisture that condenses on the wall from running down behind the floor insulation.
Step 5: Install the Subfloor
- 12mm or 18mm exterior-grade plywood (WBP or marine ply)
- Screw through into the van floor using self-tapping screws
- Countersink screw heads and fill with wood filler
- Apply a moisture-resistant sealant to the edges
Thickness Calculator
| Van Type | Recommended Floor Insulation | Notes |
|---|---|---|
| Micro-camper (Caddy, Berlingo) | 10mm closed-cell foam | Headroom is critical |
| Medium van (Transit Custom, Trafic) | 25mm XPS | Good balance of comfort and space |
| Large van (Sprinter, Relay) | 25-50mm XPS/PIR | 50mm if you are over 6ft and can afford the height loss |
| Pop-top conversion | 25mm XPS | Roof space compensates for floor height |
Common Mistakes
Skipping the vapour barrier: The floor will stay dry for a year, then gradually develop damp patches. By year three, the plywood will be rotting from the bottom up.
Using sheep's wool or fibre insulation: These compress underfoot, losing their R-value. Wool also absorbs moisture from beneath the van and stays wet.
Not sealing the plywood: Unsealed plywood edges wick moisture. Paint or seal all cut edges with exterior-grade wood sealant.
Tightly screwing through insulation: Over-tightening crushes the foam board, creating thin spots. Use countersunk washers.
Products List
| Item | Recommendation | Price |
|---|---|---|
| XPS foam board 25mm | Screwfix XPS | £18 per sheet (2400x1200mm) |
| Vapour barrier | Visqueen 500g polythene | £15 for 20m |
| Aluminium tape | HVAC foil tape | £8 per roll |
| Plywood subfloor | B&Q 12mm WBP ply | £35 per sheet |
| Low-expansion foam | Soudal window foam | £8 per can |
Conclusion
Floor insulation is not the place to cut corners in a van build. A properly insulated floor costs £80-150 and takes a day to fit. It makes the difference between a van that is comfortable at 0°C and one that requires constant heater running to keep your feet warm.
Our recommendation: 25mm XPS + vapour barrier above and below + 12mm exterior ply. Skip the underfloor insulation (difficult to install, traps moisture against the metal).
