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How to insulate a shed with polystyrene

Updated: 23 October 2026

You want a warmer summerhouse, and white polystyrene sheets look cheap enough to line the walls this weekend. Insulate a shed with polystyrene only if the sheet is bare EPS that fits the frame, the warm side can lose moisture the right way, and mains kit is not buried in the foam. The [polystyrene sheets comparison](/hub/diy-supplies/polystyrene-sheets) includes three Triplast EPS packs and craft sheets that will not do this job. This page is the lining, and where it stops. The Products Compared editorial team used those listings, the shared review notes, and public UK guidance, on 22 October 2026. We did not measure a shed. A qualifying purchase can earn a commission at no extra cost to you.

You want a warmer summerhouse, and white polystyrene sheets look cheap enough to line the walls this weekend.

Our top picks

Triplast Triplast 12-pack EPS 70, 25 mm

Best overall

Triplast Triplast 12-pack EPS 70, 25 mm
8.6/10about £47.79
View offer
Triplast Triplast 10-pack EPS 70, 10 mm

Best value

Triplast Triplast 10-pack EPS 70, 10 mm
8.6/10about £16.89
View offer
Triplast Triplast 25-pack EPS 70, 25 mm

Alternative

Triplast Triplast 25-pack EPS 70, 25 mm
8.6/10about £37.49
View offer
On this page
  1. 1.What people use the sheets for
  2. 2.Thickness and the frame
  3. 3.Moisture and mould
  4. 4.Fire, electrics, and where to stop

What people use the sheets for

Reviewers in the shared Triplast pool, which is not a test of one size, mention garden houses, summerhouses and sheds that felt warmer. They also call the sheet fragile, messy to cut, and not like dense blue or pink foam. That is a lining for a timber box, not a U-value and not a sound figure. You are in the right family only when the pack is bare EPS. The A3 board is 5 mm with a paper face. Safeprint is a 16.5 cm print sheet. Neither belongs on a stud. Which pack suits you is on the best polystyrene sheets page. Foam over mould hides the stain and leaves the damp. Dry the surface first.

Thickness and the frame

Ageas uses 50 mm of insulation in a 75 mm shed stud, with a 25 mm gap, as an example, and says to go thinner if the frame is shallower. Measure the stud. If the timber is 38 mm, a 50 mm board will not sit inside it. If the timber is 75 mm and you only have 25 mm, you have half the example. The 12-pack is 25 mm. The cheap Triplast pack is 10 mm. Neither is 50 mm. Two layers of 25 mm can approach the example if the stud is deep enough and you stagger the joints. A 10 mm sheet used as a whole wall is a draught liner. For foam in the wet, the EPS vs XPS page is the fork, because bead board takes up more water. A 600 x 400 mm sheet needs more cuts than a 1200 x 600 mm sheet, and each gap is a cold stripe. Solvent glue melts the face. Do not turn the Triplast lambda range, 0.038–0.030 W/mK, into a U-value.
Triplast Triplast 12-pack EPS 70, 25 mm

Testsieger

Triplast Triplast 12-pack EPS 70, 25 mm
8.6/10about £47.79
View offer

Moisture and mould

A heated shed drives vapour from the warm room towards the cold cladding. Ultimate Insulation Supplies says a heated shed wants a vapour control layer on the warm side. ShedBuilder puts that layer on the warm inside face and a breather on the cold outside, and says sealing both faces traps moisture. Tape the laps, and leave the cladding able to breathe. Do not stick boards over black mould and call the wall insulated. Dry the frame, fit the sheet, then the warm-side layer. The knife method is on the how to cut polystyrene sheets page.

Fire, electrics, and where to stop

A 2017 ministerial answer says unmodified polystyrene ceiling tiles do not comply with Part B and should not be used in new buildings. The Triplast bullet still mentions tiles and states no fire class. Do not stick these sheets up as a ceiling finish. Ageas tells householders to avoid polystyrene where a shed has electrics. Keep foam away from sockets, heaters and lamps. If a spur is already on the wall, an electrician should say what can stay covered. Regulation 7, on class A2-s1, d0 or A1, applies to external walls of relevant buildings. It does not ban every garden shed. Stop when the shed is fed from the house, when you are lining a roof that carries a room, or when the studs are already wet. For a dry, unheated store, 25 mm of EPS and a warm-side layer is a lining.

Common questions

Will these amazon sheets warm a summerhouse?
They can take the chill off a timber shed if you fit bare EPS to the depth of the frame and you close the joints. Reviewers in the shared pool describe summerhouses and sheds. They do not publish a temperature rise. The craft boards in the same comparison are the wrong product. Expect a lining, not a heated room.
Is 25 mm the same as the 50 mm people mention?
No. Ageas uses 50 mm in a 75 mm stud as an example, and says to go thinner only when the frame is shallower. The large Triplast pack is 25 mm. The cheap pack is 10 mm. Measure the stud before you decide that one layer is enough.
Do you put a plastic sheet on the inside or the outside?
On a heated shed, put the vapour control layer on the warm inside face, and leave the cold outside able to breathe. Sealing both faces traps moisture in the timber. Tape the laps. A shed is not a full house build-up, and the warm-side rule is still the one to follow.
Can polystyrene stop mould on a wall?
No. One reviewer fitted sheets in a wardrobe over mould. That covers the mark and leaves the damp behind the foam. Dry and clean the surface first. If the studs are wet, find the leak before you line anything. The sheet is not a mould treatment.
Is it all right behind a socket or a heater?
Ageas tells householders not to use polystyrene in a shed that has electrics. Keep the sheets clear of heaters, lamps and chargers. If a socket is already on the wall, ask an electrician before you cover that bay. Do not bury a spur in bead foam because the listing mentions insulation.
Can you stick the sheets up as ceiling tiles?
Do not. A ministerial answer in 2017 says unmodified polystyrene ceiling tiles do not comply with Part B and should not be used in new buildings. The Triplast bullet mentions tiles and gives no fire class. Use the sheet inside a frame if the rest of this page fits, not as a ceiling finish.
Will it soundproof the shed?
The listings do not give a decibel figure. A thin EPS sheet may dull a light rattle. It will not turn a shed beside a road into a quiet room. If sound is the real job, this pack is the wrong buy.

Related pages

Polystyrene sheets buying guide

Polystyrene sheets

Best polystyrene sheets for lining, mount or print

EPS vs XPS for damp, a floor load or a carved edge

How to cut polystyrene sheets with a knife or wire

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Updated 23 October 2026.

  1. British Plastics Federation, EPS and XPS (retrieved 23 October 2026)
  2. BSI, BS EN 13163 (retrieved 23 October 2026)
  3. Building Regulations 2010, regulation 7 (retrieved 23 October 2026)
  4. GOV.UK simpler recycling (retrieved 23 October 2026)
  5. Which? insulation boards (retrieved 23 October 2026)
  6. Polyfoam XPS, compressive strength (retrieved 23 October 2026)
  7. HSE PPS1, hot-wire cutting (retrieved 23 October 2026)
  8. Stylite expanded polystyrene datasheet (retrieved 23 October 2026)