Campervan Insulation Thickness Guide for UK Builds

A campervan can feel surprisingly cold even after the walls have been covered and the furniture is fitted. That is usually because insulation was treated as a single material choice rather than a build-wide decision. This campervan insulation thickness guide sets out sensible thicknesses for UK van conversions, where damp weather, cold nights and limited interior space all need consideration.

Thickness matters, but it is not the only factor. Material type, fitting quality, gaps around ribs, thermal bridges and the order of your build have just as much effect on comfort. The aim is not to fill every void with the thickest product available. It is to use the available depth properly without making the van difficult to line, wire or live in.

Start with the space you actually have

Before ordering insulation, measure the usable cavity depth in each part of the vehicle. A van body is not flat. Roof ribs, wall strengthening members, window apertures and door mechanisms all limit what can be installed.

A typical panel van may offer 20mm to 50mm of practical space in some wall cavities, while the roof can often accept more before the lining begins to feel low. Floors are different again: every extra millimetre raises furniture, reduces standing room and can complicate step heights at the side and rear doors.

For most conversions, a mixed-thickness approach is better than using one thickness throughout. Prioritise the large exposed surfaces first - roof, main side panels and floor - then use thinner, flexible material in awkward recesses. This gives a better result than forcing thick boards into areas where they will sit proud, compress badly or leave gaps.

Campervan insulation thickness guide by area

Roof: 25mm to 50mm where headroom allows

The roof is one of the most worthwhile places to insulate well. Warm air rises, and a bare metal roof loses heat quickly on a cold evening. It also receives direct sun, so good roof insulation helps slow heat gain during warmer weather.

Around 25mm is a practical minimum for many lined campervans. If the roof void and your finished headroom allow it, 40mm to 50mm can be worthwhile, particularly in high-roof vehicles or vans intended for year-round use. Do not simply choose 50mm because it is available. Check the depth after battens, wiring runs and roof lining have been allowed for.

Where ribs interrupt the space, fit insulation neatly between them and avoid leaving large uninsulated metal sections. A thinner layer over the ribs can help reduce cold spots, provided it does not interfere with the lining system. Keep cable routes planned and accessible before closing the roof.

Main wall panels: 25mm to 40mm

The broad wall cavities behind furniture and lining panels are usually the next priority. Between 25mm and 40mm of suitable thermal insulation is a sensible target for most medium and large van conversions.

Use the full cavity only where the material can sit without being excessively compressed. Compressing fibrous insulation reduces the air held within it, while forcing rigid material against the outer skin can make fitting difficult and create rattles. A close, even fit is more useful than a nominally thicker layer with voids around it.

At areas where cabinets, bed frames or interior panels mount to the body, make sure your fixing strategy is established first. You may need exposed structural fixing points, timber battens or rivnut locations. Insulation should support the lining build, not prevent secure furniture installation later.

Doors, lower sections and tight cavities: 10mm to 25mm

Sliding doors, rear doors and narrow lower wall sections need thinner material. Door cards have limited clearance, and thick insulation can foul trim, window mechanisms, latches or the sliding-door action.

A 10mm to 20mm closed-cell product is often practical here. In deeper door cavities, up to 25mm may fit, but only after checking every moving part. Do not block factory drain holes. Doors collect moisture, and water needs a clear route out of the vehicle.

These areas can also benefit from sound-deadening treatment before thermal insulation is fitted. Sound deadening reduces panel resonance; thermal insulation slows heat transfer. They do different jobs, so one should not be substituted for the other.

Floor: 10mm to 25mm

Floor insulation is a balance between warmth and headroom. A well-insulated floor makes a noticeable difference when stepping into the van on a cold morning, but 40mm or 50mm of floor build-up is rarely practical in a standard roof-height vehicle.

For many builds, 10mm to 20mm of rigid, moisture-resistant insulation beneath a plywood floor is a workable choice. A 25mm floor can suit a high-roof van or a build with generous internal height, but account for the total finished stack: insulation, battens or adhesive, plywood, vinyl or other floor covering.

The floor material must tolerate point loads from furniture, appliances and occupants. Soft insulation can compress below a cabinet or seat base, leaving an uneven floor. Plan heavy fixed items and use a properly supported floor structure rather than relying on insulation alone.

Material choice changes the thickness decision

A thickness figure only makes sense alongside the insulation type. Flexible recycled-fibre products are useful for irregular cavities and can be packed around ribs and curved panels. Closed-cell foam products are useful where moisture resistance, thin profiles or firm support are needed. Rigid boards can work well on flatter floor and wall areas where a stable layer is required.

Avoid using expanding foam as a general cavity filler. It can make future repairs difficult, trap problems behind the lining and expand into places it should not reach. Loose household loft insulation is also unsuitable for most van applications because vibration, moisture and sagging create long-term issues.

Whatever material you choose, fit it without major gaps. A 25mm layer installed continuously can outperform a thicker layer with exposed metal around the edges. Tape joints where the product system requires it, and use suitable adhesive or mechanical retention so insulation does not fall behind panels over time.

Do not confuse vapour control with waterproofing

Campervans produce a lot of moisture in a small volume. Cooking, wet clothing, breathing and heating all raise humidity. When warm internal air meets cold metal, condensation can form behind lining panels.

Insulation slows that temperature change, but it does not remove moisture from the van. Ventilation remains essential. Roof vents, opening windows and controlled fresh-air paths help manage condensation far more effectively than sealing every surface without a plan.

Be cautious with generic online advice about adding a complete vapour barrier behind every campervan lining. Vans have complex shapes, numerous penetrations and metal skins that need to dry. A poorly sealed barrier can allow moisture behind it and make it harder to escape. Use compatible materials and follow the installation guidance for the insulation system you have selected.

Protect usable space where it matters

The best insulation thickness is not necessarily the maximum thickness. In a compact van, losing 30mm from each wall can noticeably narrow the bed area, galley or seating space. That matters particularly when building around a transverse bed or fitting standard-width furniture.

It can be sensible to use 40mm in the roof and large hidden wall cavities, then reduce to 20mm or 25mm near furniture faces and door openings. This protects the areas that lose the most heat while keeping the finished interior workable. High-roof vans and larger wheelbase vehicles can usually accommodate a more generous build-up than compact vans.

If you are fitting windows, complete the main insulation and lining plan before cutting or trimming around apertures. Window surrounds need clean edges, stable backing and enough space for finishing materials. Retrofitting thick insulation after windows and furniture are installed is slower and usually produces a less tidy result.

A practical installation order

Insulation is easier when it follows the right build sequence. First, inspect for leaks, corrosion, damaged seam sealer and blocked drains. Fix those issues before covering the metalwork. Next, apply sound-deadening material to the appropriate large panels, then install thermal insulation in cavities.

After that, run cables for lighting, solar, speakers, pumps and other electrical equipment. Photograph cable routes before the lining goes on. Install battens or fixing points, fit wall and roof linings, then move on to flooring and furniture. This order avoids pulling fresh insulation back out to route one missed cable or reinforce one cabinet fixing.

For a standard UK campervan build, 25mm to 40mm in the roof and main walls, 10mm to 25mm in confined cavities, and 10mm to 20mm in the floor is a solid starting point. Adjust it to your vehicle, intended season of use and available headroom, then fit every layer carefully. A tidy, continuous installation will do more for comfort than extra thickness in the wrong place.