Insulating a Garden Room, Shed or Van: The Breathable + Foil Approach

The golden rule: breathable outside, vapour barrier inside

Everything else in this guide follows from one principle. Moisture inside a heated space moves outwards towards the cold. Your job is to stop most of it leaving the room, and to give whatever does get through a way out.

So the layers, working from inside to outside, always run in this order:

• Internal lining — plywood, plasterboard or cladding
• Optional service void — battens for cables and sockets, which also gives the foil its air gap
• Vapour control layer — aluminium foil membrane, taped at every lap and edge
• Insulation — mineral wool or PIR between the frame members
• Structural frame and sheathing
• Breathable membrane — vapour permeable, water resistant
• Ventilated cavity (battens)
• External cladding or roof covering

Reverse the two membranes and you have built a moisture trap. The most common DIY failure in garden rooms is a polythene or foil sheet stapled to the outside of the frame under the cladding, which stops moisture escaping and holds it against the timber. Within a couple of winters the insulation is wet and the frame is going soft.

The second most common failure is a sealed box with no ventilation at all. Even a perfectly detailed build-up needs a way for moist air to leave the room — an openable window, a trickle vent, or an extract fan if there is a kettle, a shower or a lot of people in there.

Do I need a vapour barrier when insulating a shed or garden room?

If you are heating it, yes.

A garden office with a heater, a workshop you warm up on winter mornings, a gym, a studio — all of these create a warm side and a cold side, and therefore create a condensation risk within the build-up. A vapour control layer on the warm side is what prevents it.

An unheated, unlined store where you keep the mower is a different case. There is no warm side, so there is no interstitial condensation risk and no need for a VCL. Ventilation is all it needs.

The grey area is the shed that starts unheated and quietly becomes a workshop. If there is any chance you will heat and line it, build it properly the first time — retrofitting a vapour barrier means stripping the internal lining.

One point specific to garden rooms: they are small, well-sealed and often heavily occupied for their volume. One person working in a 3m × 4m office generates a lot of moisture relative to the air in the room. That combination is exactly why garden rooms suffer condensation problems out of proportion to their size, and why getting the membranes right matters more here than in a large house.

Wall, floor and roof build-ups

Choosing your materials

Three products do most of the work in a garden room or shed build.

Aluminium foil vapour control membrane for the warm side. It controls vapour, reflects radiant heat back into the room, and improves airtightness once taped. It is light, cuts with scissors or a Stanley knife, and is compatible with PIR boards, mineral wool and EPS. The 1m × 50m (50m²) roll covers a typical garden room; 10m² and 15m² rolls suit a shed or a van.

Breathable roofing membrane for the cold side, on both the walls behind the cladding and the roof beneath the covering. The 1m × 20m (20m²) roll at 120 g/m² is well sized for a shed or small garden room; the 1m × 50m (50m²) 100 g/m² roll suits a larger build. Look for BBA approval.

Reinforced aluminium foil tape, 48mm × 45m, for every lap, edge and penetration in the VCL. Buy more than you think you need. Running out of tape is how untaped laps happen, and an untaped VCL is barely a VCL.

For the insulation itself, mineral wool is cheapest per unit of performance and fine where you have depth in the frame. PIR gives the best performance per millimetre where depth is limited — which is often the case in a garden room built from 50 × 100mm studs, and always the case in a van.

Step-by-step: insulating the walls

• Fix the breathable membrane to the outside of the frame or sheathing, printed face outwards, lapping horizontal joints at least 100mm with the upper sheet over the lower.
• Batten over the membrane to create a ventilated cavity, then fix the external cladding to the battens. The cavity lets any water that gets past the cladding drain away and lets the membrane breathe.
• Fit the insulation between the studs from the inside, friction-fitted so it fills the bay completely without being compressed. Gaps and squashed sections are cold spots.
• Run the foil VCL across the inside face of the studs, reflective side towards the room, pulled taut but not stretched. Staple or tape it to the framing.
• Lap every joint by at least 100mm and tape it with reinforced foil tape, pressing the tape down along its full length.
• Seal the perimeter at floor, ceiling and around every window and door opening, so the membrane forms one continuous envelope rather than a set of separate panels.
• Batten out a 25–38mm service void on the room side of the VCL. Run all cables and socket boxes in this void so nothing pierces the membrane — and the gap gives the foil the air space it needs to reflect.
• Line with plywood or plasterboard, checking the whole membrane one last time before you cover it.

That service void is the single best detail in the whole build. It costs a few battens and 25mm of internal space, and it eliminates the most common cause of VCL failure — dozens of small unsealed holes for sockets and switches.

Insulating the roof and floor

Roof

The roof is where most of the heat goes and where condensation shows up first, so it deserves the most care.

On a pitched garden room roof, run the breathable membrane over the rafters beneath the battens and covering, insulate between the rafters, and fit the foil VCL across the underside of the rafters before lining. If you are using a non-breathable covering such as EPDM or torch-on felt over boarding, you need a ventilated gap between the insulation and the deck — at least 50mm, ventilated at both eaves — because moisture cannot escape upwards through the covering.

On a shed roof, a breathable membrane under the covering is a worthwhile upgrade on traditional shed felt. Shed felt typically lasts 5–10 years; a breathable membrane lasts considerably longer and manages moisture rather than trapping it.

Floor

A raised timber floor over a ventilated void loses more heat than people expect, and it is easy to insulate while the deck is off.

Insulate between the joists, fit the foil VCL above the insulation and below the deck, and protect the underside — a breathable membrane or a robust board keeps out ground moisture, draughts and rodents while letting the structure breathe. Keep the void beneath ventilated.

Van and campervan specifics

A van is the same physics in a harder container. Two things make it different from a garden room.

Thermal bridging. The steel ribs and panels of the vehicle conduct heat straight from inside to outside, and those bridges are cold surfaces where condensation forms first. Insulate over the ribs as well as between them wherever you have the depth, rather than only filling the cavities.

Moisture load. A van is a very small volume with people sleeping, cooking and often drying wet gear in it. The moisture generated per cubic metre of air is far higher than in a house. That makes both a properly taped vapour control layer and real ventilation essential — a roof vent or fan is not optional in a van you sleep in.

Foil membrane suits vans well: it is thin where every millimetre counts, light, cuts to shape around curves and ribs easily, and reflects radiant heat, which helps significantly with summer heat gain through a metal roof. Tape every joint and every penetration.

Shopping list

For a typical 3m × 4m garden room:

• Aluminium foil VCL, 1m × 50m (50m²) — walls, roof and floor
• Breathable roofing membrane, 1m × 50m (50m²) or 1m × 20m (20m²) depending on size — walls and roof
• Reinforced aluminium foil tape, 48mm × 45m — two rolls minimum
• Mineral wool or PIR insulation to suit your frame depth
• Treated battens, 25 × 50mm — cavity battens and service void

For a shed or van, the 15m² or 10m² foil VCL rolls and the 20m² breathable membrane roll will usually be enough, plus a roll of foil tape.

Everything ships on free next-working-day UK delivery with same-day dispatch before 1pm, and bulk pricing applies across the whole order — so buying the full build-up together costs less than piecing it out.

Frequently asked questions

Do I need a vapour barrier in a shed?

If you heat and line it, yes — a heated shed has a warm side and a cold side, and a vapour control layer on the warm side prevents condensation forming inside the walls. An unheated, unlined store does not need one.

What insulation is best for a garden room?

Mineral wool where you have depth in the frame, PIR where depth is limited. Whichever you choose, pair it with a foil vapour control layer on the inside and a breathable membrane on the outside.

Which side does the breathable membrane go in a shed?

The cold side — outside the frame, behind the cladding and beneath the roof covering, with the printed face outwards. The vapour barrier goes on the opposite side, inside the frame.

Can you use foil membrane in a van?

Yes, and it suits vans particularly well — thin, light, easy to cut around ribs and curves, and reflective, which helps a lot with summer heat gain through a metal roof. Tape every joint and penetration.

How do you stop condensation in a garden room?

Three things together: a taped vapour control layer on the warm side, a breathable membrane on the cold side, and real ventilation — an openable window, trickle vent or extract fan. Sealing the fabric without ventilating simply moves the condensation to your windows.

Products mentioned in this guide

• 50m² Aluminium Foil VCL 1m × 50m
• 15m²/10m² Aluminium Foil VCL
• 20m² Breathable Membrane 1m × 20m 120 GSM
• 50m² Breathable Roofing Felt 1m × 50m 100 GSM
• Reinforced Aluminium Foil Tape 48mm × 45m
• Treated Batten 25 × 50mm
• Insulation category

Related guides

• What Is a Vapour Control Layer (VCL)?
• Vapour Barrier vs Breathable Membrane
• Does Foil Insulation Actually Work?
• Roofing Felt Explained
• Breathable Roofing Membrane vs Felt