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Vapour barrier tape in a loft

Updated: October 2026

You are boarding a loft or lining a dormer, and the vapour barrier tape loft question is where the sheet sits. Tape the warm-side air and vapour control layer so the laps stay shut, and leave the ventilation path the roof was designed to keep. The Products Compared editorial team used Approved Document C for England, public summaries of BS 5250:2021, and the amazon.co.uk listings dated 16 October 2026. A qualifying purchase can earn a commission at no extra cost to you. Lap technique stays on the [how to tape a vapour barrier](/hub/site-barriers/vapour-barrier-tape/sub/vapour-barrier-tape-how-to-seal) page. The rolls are on the [vapour barrier tape comparison](/hub/site-barriers/vapour-barrier-tape).

You are boarding a loft or lining a dormer, and the vapour barrier tape loft question is where the sheet sits.

Our top picks

tesa tesa 60077, 50 mm

Best overall

tesa tesa 60077, 50 mm
8.2/10about £30.84
View offer
DuPont Tyvek AirGuard 1310V, 60 mm

Rank 2

DuPont Tyvek AirGuard 1310V, 60 mm
8.6/10about £28.00
View offer
Ealyder Ealyder polyethylene, 100 mm

Best value

Ealyder Ealyder polyethylene, 100 mm
6.8/10about £17.99
View offer
On this page
  1. 1.Where the sheet sits in a loft
  2. 2.What taping does not replace
  3. 3.Hatches, lights and pipes
  4. 4.When the loft is no longer a tape job
  5. 5.Tape the laps, leave the eaves

Where the sheet sits in a loft

In the usual UK cases cited from BS 5250, the vapour control layer is on the warm side of the insulation. In a loft room that is the room side of whatever is keeping you warm, whether the insulation is between the rafters of a dormer or above the ceiling of a cold roof. The tape sits on that sheet. It does not turn a breather membrane on the cold side into an AVCL because you taped it carefully. Approved Document C is England's guidance for Requirement C2. It defines a vapour control layer and does not name a roll. That is not a rule to add a sheet to every loft. Tape on the cold face seals the wrong plane.

What taping does not replace

Taping every lap does not let you close the eaves. A cold pitched roof keeps insulation at ceiling level and a ventilated void above it. The IKO summary of BS 5250 still expects that void to be ventilated in the cases it describes, even when the ceiling has a sealed AVCL. If you pack the eaves and tape across the ventilation slot, the tape did its job on the ceiling and the roof lost the air path it was using. If you cannot seal the sheet and keep that void ventilated, stop. The buying guide matches a roll to the sheet once the build-up is clear.
tesa tesa 60077, 50 mm

Testsieger

tesa tesa 60077, 50 mm
8.2/10about £30.84
View offer

Hatches, lights and pipes

The loft hatch is the hole people board around and forget. Treat it as a penetration you still have to open. The ceiling sheet should meet a seal on the frame or the lid so the layer is continuous when the hatch is shut. Tape that glues the lid shut is not that seal. Recessed lights are the stop, not a craft project. A ring of tape around a hot lamp is not a fire-rated hood. A moving soil pipe or a flue is not a collar job, and tape does not set flue clearance.

When the loft is no longer a tape job

Stop when spray foam is already between the rafters, and stop when you cannot point at the ventilation path. BS 5250 is not a certificate on these rolls. If the ceiling is the line for an air test, the designer's specification beats this article. When the job really is a simple internal lap, a 25 m specialist tape can run out on a dormer once pipes and the hatch are included. The longer polyethylene roll is the length option, with UK reviews that describe lift-off if the sheet flexes. Read that condition on the best vapour barrier tape page before you treat metres as quality.

Tape the laps, leave the eaves

If you can point at the warm face of the insulation, tape that sheet and leave the eaves path the roof was built with. If the roof is cold, a sealed ceiling does not replace the ventilated void. If spray foam is already in the rafters, or you cannot find the ventilation path, stop. Metres of tape do not change which roof you are lining.

Common questions

Does a loft room need a vapour control layer?
In the usual cases summarised from BS 5250, a warm-side AVCL with sealed laps is what that guidance still describes, including the GOV.UK note on warm pitched roofs. That is not a rule that every loft in every nation gets the same sheet. Approved Document C is England's document. If you cannot see how this roof manages moisture, ask before you add a layer that changes the drying path.
Which side of the insulation does the tape sit on?
On the warm side, with the sheet. In a loft room that is the room face of the insulation, not the felt side and not a breather membrane in the ventilation zone. Tape on the cold face does not fix condensation. If the insulation is too patchy for you to point at the warm face, sort that before you tape.
If I tape every lap, can I close the eaves?
No. Sealed laps on the ceiling do not replace the ventilated void a cold roof uses. Closing the eaves with insulation or tape can leave still air under the underlay. The IKO summary of BS 5250 keeps ventilation in the picture alongside the sealed sheet. If you cannot ventilate and seal, stop rather than buying a wider roll.
What is different about a cold roof and a warm roof?
A cold roof has the insulation at ceiling level and a ventilated space above it. A warm roof puts insulation in the slope. Both can still need a warm-side AVCL in the usual BS 5250 cases, and the ventilation or underlay detail is not the same. Tape does not convert one into the other. Know which roof you are lining before you close it.
Can I tape around recessed lights?
Do not use tape as the hood. A UK review mentioned covering spotlights while waiting for proper covers. A ring of tape is not a fire-rated hood and it does not make an open fitting safe in insulation. Fit the covers the lights require, or leave the fitting to an electrician, especially on a fire-rated ceiling.
What if spray foam is already between the rafters?
Stop. GOV.UK's note on spray foam in timber sloped roofs is there because the foam changes the roof. A new sheet and a roll of tape are not a retrofit to invent from this page. Get advice before you seal the ceiling. The same pause applies if you cannot find the ventilation path in a dormer someone else has altered.

Related pages

Vapour barrier tape vs foil tape

Vapour barrier tape

Best vapour barrier tape: top pick on these listings

Vapour barrier tape buying guide for sheet and width

How to tape a vapour barrier so the lap stays shut

Sources

Public UK sources from the research digest. Listing prices, stars, and review counts are the amazon.co.uk lines stored with these four ASINs. Page updated October 2026.

  1. Approved Document C — Requirement C2 and the vapour control layer (retrieved October 2026)
  2. BSI BS 5250:2021 — Management of moisture in buildings (retrieved October 2026)
  3. IKO note on BS 5250:2021 (retrieved October 2026)
  4. A. Proctor Group — installing air and vapour control layers (retrieved October 2026)
  5. Novia vapour control layers (retrieved October 2026)
  6. DuPont UK TDS, AirGuard tape 1310V (retrieved October 2026)
  7. tesa UK 60077 note (retrieved October 2026)
  8. Passivhaus Trust airtightness guide, June 2020 (retrieved October 2026)