meta_description: "Armaflex vs Celotex insulation for campervan conversions in the UK. Thermal performance, condensation management, cost, installation ease, and which suits UK damp conditions." author: "Van Life UK Team" read_time: "13 min" "
The insulation you choose for your van conversion determines how warm the van is in winter, how cool it stays in summer, and — most critically for UK conditions — how well it manages condensation. Armaflex and Celotex are the two most common choices for UK self-builds. They work on completely different principles and suit different build styles.
This guide compares them across every metric that matters for UK van life.
How They Work
Armaflex is a closed-cell nitrile rubber foam insulation, originally designed for HVAC pipe insulation. It comes in flexible sheets or rolls, typically 6mm, 10mm, or 19mm thick. The structure is a mass of tiny sealed air pockets that resist heat transfer. It is waterproof, vapour-proof, and does not absorb moisture.
Celotex (PIR) is a rigid polyisocyanurate foam board, faced with aluminium foil on both sides. It comes in rigid sheets, typically 25mm to 100mm thick. It has a high thermal performance per millimetre of thickness. The rigid board is cut to fit between van ribs and taped at the seams.
Thermal Performance
| Property | Armaflex (19mm) | Celotex (25mm) | Celotex (50mm) |
|---|---|---|---|
| R-value (m²K/W) | 0.55 | 1.30 | 2.60 |
| Thermal conductivity (W/mK) | 0.036 | 0.022 | 0.022 |
| Thickness needed for 1.0 R-value | 36mm | 22mm | 22mm |
| Effective in a standard Transit Custom rib depth (40mm) | Two layers of 19mm (38mm) = R 1.1 | 25mm + 19mm air gap = R 1.3 | Not possible (too thick) |
Celotex is the better insulator per millimetre. A single 25mm layer of Celotex between the van ribs (standard depth for a Ford Transit is 35–40mm) provides more thermal resistance than a 19mm Armaflex layer.
However, the thermal performance difference is academic for most van conversions. A well-sealed van with any continuous insulation layer — Armaflex or Celotex — is warm enough for UK winter van life with a 2kW diesel heater. The practical difference between R 1.1 and R 1.3 in the walls is about 1.5°C at the extremes. Not nothing, but not the deciding factor.
Condensation Management
This is where the two materials diverge significantly.
Armaflex is a vapour barrier. Closed-cell foam does not absorb or transmit moisture. If you seal the joints between Armaflex sheets with aluminium tape or contact adhesive, the entire insulation layer becomes a vapour-proof envelope. Condensation forms on the outer (van body) side of the insulation and runs down to the van floor where it drains. No moisture reaches the inner surface.
Celotex has aluminium foil facing that acts as a vapour barrier, but the seams between boards are weak points. If the foil tape fails (and it does over time as the van flexes), moisture reaches the PIR core. PIR foam absorbs water — not like a sponge, but the closed cells hold moisture against the structure. Wet PIR loses its thermal performance and can cause rust on the van body.
The critical difference: Armaflex is inherently tolerant of installation gaps because the material itself is the vapour barrier. Celotex relies entirely on the tape sealing every seam. In a moving, flexing van, tape fails.
For UK van builds (high humidity, temperature swings, constant vibration): Armaflex is the safer choice for condensation management.
Installation
| Factor | Armaflex | Celotex |
|---|---|---|
| Cutting | Scissors or utility knife, clean cut | Fine-tooth saw or knife, generates dust |
| Fitting curves | Flexible, conforms to curved panels | Cannot bend, must be cut and pieced |
| Filling irregular gaps | Scraps can be packed in | Must cut precise shapes |
| Adhesive | Contact adhesive (spray can) + aluminium tape | Foam board adhesive + foil tape |
| Time for a standard van | 6–10 hours | 8–14 hours |
| Skill required | Low (forgiving of mistakes) | Medium (need precise cutting) |
| Mess | Low (no dust) | Moderate (PIR dust is irritating) |
| Layering | Must overlap or tape seams carefully | Sheets edge-to-edge, tape over seams |
Armaflex is easier to install, especially for a first-time converter. The flexibility means you can push it into awkward spaces behind wheel arches, around windows, and into the van roof curves. Celotex requires more precise cutting and the rigid boards do not conform to curved surfaces.
Cost Comparison (UK, 2026)
| Material | Standard size | Cost per m² | Coverage per van (Transit LWB) | Total cost |
|---|---|---|---|---|
| Armaflex 10mm | 1.5m × 10m roll (15m²) | £10–£14 | ~25m² (walls + roof + doors) | £250–£350 |
| Armaflex 19mm | 1.5m × 10m roll (15m²) | £16–£22 | ~25m² | £400–£550 |
| Celotex 25mm | 2.4m × 1.2m sheets (2.88m² per sheet) | £7–£10 | ~25m² (∼9 sheets) | £63–£90 |
| Celotex 50mm | 2.4m × 1.2m sheets (2.88m² per sheet) | £12–£16 | ~25m² | £108–£144 |
Add £30–£50 for tape, adhesive, and incidental materials for either system.
Celotex is significantly cheaper — roughly one-third the cost of Armaflex for equivalent coverage. The price gap is the main reason Celotex is popular in budget builds.
Which Should You Choose?
Choose Armaflex if:
- You are building in the UK and condensation is your primary concern
- You are a first-time converter (the forgiving installation is worth the money)
- Your van has complex curved panels (roof, wheel arches, rear door recesses)
- You plan to keep the van for 5+ years and want the insulation to outlast the build
- Your van is a full-time residence (occupancy generates more moisture)
Choose Celotex if:
- You are building on a tight budget and the cost saving is meaningful
- Your van has simple, flat, rectangular panels (box vans, larger panel vans)
- You are confident in your tape sealing technique
- You are willing to inspect and re-tape seams annually
- You are building a weekend campervan with limited occupancy
The Compromise: Hybrid Approach
Many self-builders use a hybrid: Armaflex on the van roof (curved, high condensation risk), Celotex on the wall panels (flat, easier to seal), and Armaflex in the door recesses.
The roof is the highest condensation risk area because warm, moist air rises. Armaflex on the roof gives the best condensation protection. Celotex on the walls saves money where the condensation risk is lower.
Installation Tips for Either Material
Armaflex:
- Clean the van panels thoroughly before applying adhesive. Any grease or dirt prevents bonding and creates air gaps.
- Use 3M Super 77 or a similar spray contact adhesive. Spray both the foam and the van panel, wait 60 seconds, press together. The initial bond is strong enough to hold while you work.
- Tape all seams with Armaflex-specific aluminium tape (not duct tape, not gaffer tape). The Armaflex tape has a pressure-sensitive adhesive designed for foam.
- Overlap sheets by 2–3cm at seams and compress lightly. The material does not need to be perfectly flush — compression improves the seal.
Celotex:
- Cut with a fine-tooth saw or a utility knife with a fresh blade. Score the foil facing, snap the board, cut through the remaining foil.
- Wear a mask when cutting. PIR dust is an irritant and the glass-fibre reinforcement is unpleasant to breathe.
- Fit the boards snugly between van ribs. Gaps of more than 3mm should be filled with expanding foam (Soudal, Everbuild) designed for window and door frames — not standard PU foam which absorbs moisture.
- Tape every seam with 75mm or 100mm aluminium foil tape. Press firmly with a plastic spreader. Re-tape any sections that lift during the van build.
Related Reading
- Hettich Hinges & Runners for Van Conversions
- Peugeot Boxer Width Benefits for Van Conversions
- Remi Pop Top Roofs: Are They Worth It?
- Condensation Management in a Campervan
