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Insulation: What Works

Spray foam vs Thinsulate vs Celotex. Science, not opinions.

Beginner⏱ 12 min read

Contents

1

Why Insulation Matters More Than You Think

A van is a metal box. Metal conducts heat 1000x faster than brick. Without insulation:

•Your van reaches 0°C inside on a UK winter night
•Condensation forms on every metal surface → rust
•Your diesel heater runs constantly → fuel bill
•You're essentially sleeping in a tent with walls

**The physics:** You're trying to slow heat transfer from inside (your body, heater) to outside (the cold). You also need to prevent warm, moist air from touching cold metal surfaces (condensation).

What R-value do you need?

•Walls: R-3 to R-5 (25-50mm of good insulation)
•Floor: R-2 to R-3 (you lose less heat through the floor)
•Ceiling: R-5 to R-7 (heat rises — insulate this best)
•Total budget: £200-500 for a full van
2

The Options (Honest Comparison)

Celotex/Kingspan (PIR rigid board) — £15-25 per sheet:

•R-value: 0.022 W/mK (excellent)
•Easy to cut and shape
•Great for flat panels (floor, ceiling)
•Doesn't absorb moisture
•Can't conform to curves without gaps
•**Best for: Floor and ceiling**

Thinsulate (SM600L) — £80-120 per roll:

•R-value: 0.039 W/mK (good)
•Flexible, conforms to curves and ribs
•Acts as both insulation AND vapour barrier (if you tape the seams)
•Lightweight
•More expensive per m²
•**Best for: Walls and cavities**

Spray foam (2-part kits) — £100-200 per kit:

•R-value: 0.028 W/mK (good)
•Fills every gap, no thermal bridges
•Excellent sound deadening
•Messy to apply (protect everything)
•Can trap moisture if done wrong
•Hard to remove if you change your mind
•**Best for: Awkward shapes, wheel arches, ribs**

Don't use:

•Fibreglass (absorbs moisture, itches like hell)
•Reflective bubble wrap (does almost nothing in a van)
•Expanding foam from a can (not the same as 2-part kits — shrinks, cracks, absorbs water)
3

The Vapour Barrier Problem

This is the most debated topic in van conversion. Here's the truth:

**The problem:** Warm, moist air from inside the van hits cold metal → condensation → rust. You MUST keep moist air away from bare metal.

Solution 1: Vapour barrier on the warm side (most common)

•Insulation against the metal
•Vapour barrier (plastic sheet, foil tape, or Thinsulate's built-in barrier) on the room-facing side
•This is what house builders do
•Works IF you seal every gap perfectly (near impossible in a van)

Solution 2: Breathable insulation (simpler)

•Use moisture-permeable insulation (sheep's wool, recycled denim)
•Allow airflow behind panels
•Accept some moisture and let it dry out
•Less perfect but more practical for DIY

Solution 3: Spray foam (best seal)

•Spray foam IS the vapour barrier if applied correctly
•No gaps, no seams, no tape to fail
•But it's permanent and hard to inspect behind

**What we recommend:** Thinsulate on walls (it's its own vapour barrier when seams are taped), Celotex on floor and ceiling, spray foam for wheel arches and awkward gaps.

4

Step-by-Step Installation

Floor insulation:

1. Clean the bare metal floor
2. Lay 25mm Celotex between the floor ribs (cut to fit snugly)
3. Fill any gaps with spray foam
4. Lay 12mm ply over the top (screw into existing floor fixings or use adhesive)
5. Your floor finish (vinyl/laminate) goes on top

Wall insulation:

1. Apply sound deadening to bare metal (Silentcoat or similar)
2. Push-cut Thinsulate into the wall cavities between ribs
3. Use spray adhesive (3M 90) to hold it in place
4. Tape all seams with foil tape (THIS is your vapour barrier)
5. Leave a small air gap between insulation and wall panels

Ceiling insulation:

1. Same as walls — Thinsulate or Celotex between ribs
2. The ceiling gets the most condensation, so insulate it well
3. If you have a MaxxFan, insulate around the opening carefully

**Pro tip:** Insulate BEFORE running any cables or pipes. It's 10x easier to work with bare ribs than to try and insulate around existing wiring.

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