Aluminium foil tape: which width for insulation, ducting and DPM?
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Aluminium foil tape is a self-adhesive foil tape used to seal joints in foil-faced insulation board, ducting, pipe lagging, vapour barriers and DPM. It keeps moisture and vapour out and it holds through heat. For most insulation board joints a 48 or 50mm tape is enough. Go to 72mm for ducting and membrane laps, and 96mm where the joint is wider or the surface is uneven. A 45m roll seals about 12 full 2400 x 1200mm boards. A single 50mm roll is £5.00 and trade boxes are £80.
Ready to order? For a small job, a single 50mm x 45m roll is £5.00. For a bigger run, the 48mm box of 24 is £80, which works out at £3.33 a roll.
What is aluminium foil tape used for?
Foil tape is a thin aluminium backing with a pressure-sensitive acrylic adhesive. Its job is to seal a joint so that air, vapour and moisture cannot pass through it. On site that usually means:
- Foil-faced insulation board. Taping the seams on PIR boards so the foil facing carries on across the joint.
- Ducting and vents. Sealing joints in HVAC and extract ductwork. You will see it sold as HVAC or duct foil tape.
- Pipe lagging. Closing the seams and ends of foil-faced lagging.
- DPM and vapour barriers. Sealing laps in polythene sheet. On a breather membrane, check first, because some makers want their own tape.
Our 50mm roll is rated from minus 20C to plus 120C. It conforms to awkward surfaces and is flame retardant.
Which width of foil tape do I need?
There is no fixed rule, and board makers do not all say the same thing. The principle is simple: the tape has to bridge the joint and leave enough clean bond on each side. This is how we would pick it across the counter:
| Width | Good for | How we sell it | Cost per roll |
|---|---|---|---|
| 50mm | Tight board joints, pipe lagging, small jobs | Single 45m roll, £5.00 | £5.00 |
| 48mm | The same jobs, for a whole house of board | Box of 24, £80 | £3.33 |
| 72mm | Ducting, DPM and vapour barrier laps | Box of 16, £80 | £5.00 |
| 96mm | Wider joints, uneven surfaces, patches and repairs | Box of 12, £80 | £6.67 |
If you are working to a board maker's or designer's spec, follow that. If in doubt, go wider: a wider tape costs a little more, but it gives you more margin on a joint that is not perfectly straight.
How much foil tape do I need?
A roll is 45m long, so it seals 45 metres of joint. Each 2400 x 1200mm insulation board adds about 3.6m of joint, because every board shares its edges with the boards next to it. That gives you:
- About 12 boards per roll, or about 34m² of board, with a bit left for waste.
- A box of 24 x 48mm rolls covers about 288 boards, or about 800m² of board.
For ducting, measure the joints: a round duct joint uses its circumference plus a small overlap, so a 150mm duct uses about 0.5m per joint. For membrane laps, a roll does 45m of lap.
Foil tape or cloth duct tape?
They are not the same job. Cloth duct tape is a general site tape for repairs, bundling and holding things in place. It tears by hand and it is cheap, but it is not a vapour barrier. Use foil tape wherever the joint has to stop vapour or moisture, or wherever it will see heat, such as ductwork, pipe lagging and foil-faced board. Taping PIR board with cloth tape breaks the foil layer you have paid for.
How do you apply foil tape so it stays stuck?
Nearly every foil tape failure comes down to the surface.
- Clean and dry. Wipe off dust, grease and damp first. Dust from cutting PIR is the usual culprit, so brush it off every joint.
- Butt the joint tight. Push the boards together with no gap, and fill any real gap with foam first. Tape seals a joint, it does not fill one.
- Centre the tape. Lay it straight down the middle of the joint so you have equal bond on both sides.
- Press it down hard. Pressure-sensitive adhesive needs pressure. Rub along the whole length with a cloth or a plastic squeegee, and push out any bubbles.
- Watch the cold. Adhesive grabs less well on a cold or frosty surface, so warm the rolls in the van and tape the boards when they are dry.
Common mistakes to avoid
- Taping over dust. It looks fine on the day and lifts a week later. Clean every joint.
- Using tape to fill a gap. A gap under the tape is still a gap. Fill it first.
- Not pressing it down. Laid on lightly, the adhesive never fully bonds. Rub it down.
- Using it on a radon or gas membrane without checking. Those systems have their own specified tapes. Follow the membrane spec.
- Assuming any foil tape meets a fire spec. If the job calls for a set fire rating, check the data for that exact tape.
The short version
Foil tape seals joints in foil-faced board, ducting, lagging and polythene laps. Use 48 or 50mm on tight board joints, 72mm for ducting and laps, and 96mm for wider joints and repairs. A 45m roll does about 12 boards. Keep the surface clean and dry, butt the joint tight and press the tape down hard. Buy a single 50mm roll for a small job, or a box of 48mm, 72mm or 96mm for a bigger run.
By Steve, Builders Hardware Direct. Steve runs the BHD trade counter in Gorslas and supplies tapes, membranes and fixings to builders and installers across South Wales. Not sure which width? Call the counter on 01269 267030 or order online for fast UK delivery.
