Thermal Bridging Explained

Even the best-insulated campervan has weak points where heat escapes. These are thermal bridges — areas where the insulation is thinner or where a solid conductive material bypasses the insulation entirely.

Understanding and eliminating thermal bridges is the difference between a van that stays warm and dry through a UK winter and one that drips with condensation every morning.

Why It Matters in a UK Climate

A British van spends its winters in one of three states: cold and dry, cold and wet, or (most often) cold and very wet with high humidity. That combination is the perfect storm for thermal-bridging problems. A bridge that only costs you a little warmth in a dry climate becomes a condensation and mould problem in the UK, because the cold surface it creates sits right at the dew point of the warm, moist air inside your van.

The payoff is significant. A van with its bridges broken holds its heat longer (your diesel heater runs fewer hours per night), stays dry (no condensation ring around the windows or cold patches on the ceiling), and keeps interior humidity down — which means no musty smell, no damp patches on the wallboard, and far less risk of mould and rot over the years. For a full-timer or a winter road-tripper, this is one of the highest-value "invisible" upgrades in the whole conversion.

What Is a Thermal Bridge?

A thermal bridge (also called a cold bridge) is a path of higher thermal conductivity that bypasses the insulation layer. Heat flows through this path more easily, creating a cold spot on the interior surface.

In a campervan, this looks like:

• Condensation on exposed metal ribs
• Frost forming on screw heads in winter
• Cold patches on the ceiling where the roof bars attach
• Damp wallboard at the van's ribs

The Physics

Heat always takes the path of least resistance. Your insulation has a thermal resistance of about R-0.7 per 25mm. The steel or aluminium van body has a thermal resistance of effectively zero — about R-0.0003 per mm.

If just 2% of your wall area is exposed metal (from ribs, screws, or framing), that 2% can account for 30-50% of the heat loss — and it creates condensation on that spot.

Where Thermal Bridges Occur in Vans

1. The Van's Metal Ribs

The van body is built from a metal frame with thin panels welded to it. The space between the ribs is easy to insulate — fill with foam board or wool. The ribs themselves are 15-20mm of solid steel, conducting heat directly from inside to outside.

Fix: Cover the ribs with at least 10mm of closed-cell foam (Armaflex) before fitting your main insulation. This 'thermal break' layer prevents the rib from conducting heat straight through.

2. Screws and Fixings

Every screw that penetrates through to the van's metal body is a tiny thermal bridge. With 50-100 screws in a typical conversion, they add up.

Fix: Use self-tapping screws with bonded washers (EPDM or neoprene) that provide some insulation. Alternatively, apply a dab of silicone sealant around each screw head before covering with wallboard.

3. Window Cutouts

The metal edge around a window opening is a thermal bridge to the outside. In winter, this edge gets cold and causes condensation around the window frame.

Fix:

• Use double-glazed windows (Dometic or Movera) — the frame design includes thermal breaks
• Apply 10mm closed-cell foam tape between the window frame and the van body
• Fit thermal blinds or window covers (Silver Screens or DIY Reflectix panels)

4. Roof Rails and Vent Openings

If you have roof bars, a roof fan, or a solar panel mount, each attachment point creates a thermal bridge through the roof insulation.

Fix:

• Use plastic mounting feet for roof bars where possible
• Apply a 5mm EPDM foam gasket under any roof-mounted equipment
• Fill gaps around roof fan installation with closed-cell foam before sealing with Sikaflex

5. Floor to Wall Joints

The floor-wall junction is a common area where insulation gaps form. Water and cold air pool at floor level.

Fix: Overlap your floor vapour barrier 10cm up the wall and tape it to the wall vapour barrier. Ensure insulation extends to the lowest point of the wall.

6. The Bulkhead (if retained)

If you keep the driver cab bulkhead, the metal behind the cab is a massive thermal bridge to the cold cab area.

Fix: Fit a removable insulated curtain between cab and living area. A 10mm foam-backed curtain with a batten for mounting reduces heat loss through the bulkhead significantly.

How to Detect Thermal Bridges

Thermal imaging camera: The best tool. A Flir One or Hikmicro phone attachment costs £200-300 and shows cold spots instantly. Check your van on a cold day after heating the interior for an hour.

The hand test: On a cold morning, run your hand over the interior walls. Any noticeably cold patches are thermal bridges.

Condensation mapping: After a cold night, check where condensation forms. The pattern reveals your worst bridges.

Solving Thermal Bridges: Practical Steps

Before Insulation

1. Rib covers: Cut 10mm Armaflex into strips 40mm wide and glue them over every exposed metal rib using spray adhesive
2. Screw isolators: Use plastic spacer washers (nylon or POM) under any screws that attach to the van body
3. Cavity fills: Use expanding foam (the low-expansion window and door type, not the high-expansion general purpose) to fill irregular cavities around window frames and door pillars

During Insulation

1. Continuous layer: Ensure your insulation is continuous — no gaps. Overlap insulation at corners
2. Vapour barrier: Must be continuous. Tape every seam. A gap in the vapour barrier creates a cold spot where moisture accumulates
3. Floor insulation: Use a full sheet of 25mm XPS under the floor ply, not strips between the floor ribs

After Building

1. Thermal curtains: Heavy curtains at the cab bulkhead and on exterior doors
2. Door insulation: Modify the rear barn doors with 10mm Armaflex on the inner panels, covered with fabric-faced plywood
3. Draught excluders: A simple foam tube at the bottom of sliding door and rear doors prevents cold air ingress

Real-World Example

In our LWB Transit build, we measured the following improvements from addressing thermal bridges:

Conclusion

Thermal bridging is often overlooked by first-time van converters, but it is responsible for most condensation problems in finished builds. The fixes are cheap (Armaflex tape costs £15, a roll of closed-cell foam is £20) but they need to be applied before the insulation and interior boards go in.

Take the time to cover metal ribs, seal gaps, and maintain a continuous vapour barrier. Your warm, dry van will thank you when the temperature drops below freezing. If you only do three things, do these: cover the ribs, seal the window reveals, and tape the vapour barrier seams — together they solve the vast majority of the damp and cold-spot problems in a UK build.

DIY Materials and Typical Costs

The beauty of breaking thermal bridges is that it is mostly labour and cheap materials, not expensive kit. Here is a realistic budget for a full-size conversion:

That is a small fraction of the cost of a full insulation job, and it is applied before the interior boards go in — so it is almost free to add at the right stage and painful to retrofit later.

A Quick-Start Checklist

Before you close up the walls, tick these off:

• Every exposed metal rib covered with at least 10mm closed-cell foam.
• Window and roof-light reveals sealed with a foam kerb and butyl tape.
• Fixings isolated with bonded or plastic spacer washers.
• Floor covered with a continuous XPS sheet, not strips between ribs.
• Vapour barrier continuous with every seam taped.
• Bulkhead / cab break insulated with a curtain or board.
• Hand-test on a cold morning: no cold, damp patches remain.

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