How to lag pipes
Updated: 23 October 2026
The pipe is lagged along the straight and bare at the elbow, which is where the heat and the frost both find a way through. This is how to lag pipes you can already reach, with a knife and tape, once the heating is off and the bore matches the copper. It is the cutting sequence, not a ranking of brands. The [pipe insulation comparison](/hub/diy-supplies/pipe-insulation) is where the tubes, wraps, and prices sit. Fitting notes come from UK merchant guides and the review notes on the listings saved on 21 October 2026. We did not fit these products in a test house. A purchase can earn a commission at no extra cost to you.
The pipe is lagged along the straight and bare at the elbow, which is where the heat and the frost both find a way through.
The pipe is lagged along the straight and bare at the elbow, which is where the heat and the frost both find a way through. This is how to lag pipes you can already reach, with a knife and tape, once the heating is off and the bore matches the copper. It is the cutting sequence, not a ranking of brands. The pipe insulation comparison is where the tubes, wraps, and prices sit. Fitting notes come from UK merchant guides and the review notes on the listings saved on 21 October 2026. We did not fit these products in a test house. A purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Form | Length | Closure | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Climaflex Climaflex 22 mm, 13 mm wall, 10 m | Slit PE tube | 10 × 1 m | Slit only | 7.6 | about £11.05 | View offer |
| Alternative |  Fabian Clarke London Fabian Clarke foil wrap, 7.5 m | Foil wrap | 7.5 m × 80 mm | Tape extra | 7.0 | about £6.89 | View offer |
 K-FLEX K-Flex 15 mm self-seal, 2 m | Self-seal tube | 2 m | Self-seal | 7.4 | about £8.99 | View offer |
Before you cut
Turn the heating and the hot water off before you handle a pipe that has been hot. That is the Energy Saving Trust’s order, and it is the one that keeps a slit of foam off a pipe you cannot hold. Cold pipe, then measure, then cut. If you are only lagging cold mains in a garage, you still want the surface dry. Wet foam and a wet seam are how a self-seal strip gives up, which is already a complaint on one of the tubes in this set.
Check the hole against the outside of the pipe before you commit a 2 m length. UK copper is 15 mm, 22 mm, or 28 mm outside. A 15 mm tube will not pass a 22 mm pipe, and forcing it splits the wall or leaves the seam permanently open. The size choice, including why a 16 mm or 25 mm hole is a different product, is the pipe lagging buying guide. Have a sharp knife, a cloth, and tape on the boards before you climb. Climaflex in this set has no adhesive strip. K-Flex and LavaTherm claim one. Buy Insulation Online’s fitting guide says not to stretch the tube. A stretched wall is thinner than the wall you paid for. Do not start if the only way to the pipe is to open a floor, if water is already leaking, or if the job is a gas pipe, an oil line, or a flue. A condensate that should be moved indoors is the same kind of stop, and it is described on insulate pipes to stop freezing.
Straight pipes
Open the slit with your fingers, not by tearing the wall down to nothing. Seat the tube on the pipe so the foam touches copper all the way round, then close the seam along the whole length. On a self-seal tube, peel the release strip a short section at a time and press the joint shut. Do not peel 2 m of backing and then try to land it in one go. UK reviews of the K-Flex strip include missing tape and adhesive that would not stay stuck, so a turn of foil tape over the seam is reasonable even when the listing said self-seal. On the Climaflex slit there is nothing to peel. The seam is closed only if you tape it.
Which way the slit faces is argued in public and in the reviews, and this page will not pretend they agree. The Sandwich Handyman’s guide says to press the split facing up. A Climaflex review says to put the split at the bottom. The point both are reaching for is a seam that stays shut. An upward split can shed a drip on a cold pipe. A downward split can sit in the shadow of the pipe. Either fails if the cut is gaping. Tape the seam, and tape the butt where two lengths meet. The Climaflex pieces are 1 m. The K-Flex and LavaTherm tubes are 2 m. Cut square ends. A sloping cut at a straight joint leaves a diamond of bare copper. Space extra tape about every 300 mm on a plain slit if the foam is springy. Do not pull the tube lengthways to make a short piece reach.
Bends, tees, and tape
A bend left bare is the mistake that makes the straight work pointless. For a sharp 90 degree elbow, cut two ends at 45 degrees in a mitre box, or against a set square if you do not own a box, and butt the faces so the foam turns the corner. Plumbing Superstore’s cutting guide describes that 45 degree mitre, a wedge taken out for a tee, and shallower angles for a gentle bend. Tape the mitre after the faces meet. B&Q’s water-pipe guide uses the same idea: mitre, butt, tape, and put a small extra piece over a stopcock handle so the valve is not the cold spot.
A tee needs a notch, not a crushed tube. Cut a wedge from the run so the branch can leave, then tape the three seams. If the fitting is a reducer or a tap body that no tube will pass, stop forcing foam and use a wrap. The Fabian Clarke foil in this set is 3.8 mm and lists no adhesive, and reviewers add aluminium tape. That shape, and when it is the wrong substitute for a tube, is foam pipe lagging or foil wrap. Overlap the wrap and tape the end so it cannot unwind.
When to stop and call someone
Stop and call a plumber or a heating engineer when the pipe is inside a floor you would have to open, when a condensate should discharge indoors and you are only wrapping the outside run, or when water is already coming out. Do not describe this sequence as gas work or flue work.
Fitting the foam neatly does not guarantee the pipe will not freeze. Lagging slows the loss of heat. A long cold spell, an open seam, and a bare elbow still win. If the worry is a burst rather than a tidy mitre, the freeze page is the limit of what tape can do. A neat mitre does not thaw a frozen pipe and does not reset a boiler.
Common questions
Do I need tools to lag a pipe?
You need a sharp knife, tape, and a way to measure. A mitre box helps on a 45 degree elbow and is not essential if you can mark the angle. Self-seal tubes still deserve tape, because the strip on the K-Flex listing fails for some buyers. Climaflex has no strip at all. You do not need a heat gun, and you should not use a naked flame to shape the foam.
Which way should the slit face?
Guides disagree, so do not treat one direction as a rule. One UK handyman guide says facing up. A Climaflex reviewer says at the bottom. Close the seam either way, and tape it if it will not stay shut. An open slit, whichever way it points, is a gap in the insulation you have just fitted.
How do I get lagging round a bend?
Cut the foam, do not stretch it around the elbow and hope. A sharp 90 degree bend takes two 45 degree faces butted and taped. A gentle curve takes shallower cuts. A tee takes a wedge so the branch can leave. Leaving the bend bare undoes the straight run on either side.
Do I tape a self-seal tube as well?
Tape it if the strip is missing, if it will not stay stuck, or if the butt joints between lengths can open. The K-Flex reviews include both a strip that closes and a strip that fails. Tape on a sound self-seal seam is a belt to the braces. Tape on a plain slit, such as the Climaflex pack, is the closure itself.
Can I lag a pipe that is already hot?
Turn heating and hot water off first, and wait until you can hold the pipe. That is the Energy Saving Trust’s instruction for hot pipes. Foam fitted onto a hot surface is harder to seat, and a self-seal adhesive is being asked to cure in the wrong conditions. Cold mains can be lagged once the pipe is dry.
What about a tee or a stopcock?
Notch a tee with a wedge and tape the three seams. Cover a stopcock with a short extra piece so the handle is not left as bare metal, which is the B&Q approach. Do not crush a whole tube over the valve and call the lump a seal. If you cannot reach the fitting without opening the floor, stop.
When should I stop and call someone?
Stop if the pipe is buried in a floor, if water is already leaking, or if the work is gas, oil, or a flue. Stop if a boiler condensate ought to be rerouted indoors and you are only adding foam outside. A neat mitre does not thaw a frozen pipe and does not reset a boiler. Those jobs belong to someone who can work on that system.
Related pages
Sources
Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 21 October 2026. Updated 23 October 2026.
- Approved Document L, Volume 1 (2021, 2023 amendments) (retrieved 23 October 2026)
- BSI, BS 5422:2023 (retrieved 23 October 2026)
- Rockwool guide to BS 5422:2023 (retrieved 23 October 2026)
- Energy Saving Trust: insulating tanks and radiators (retrieved 23 October 2026)
- Thames Water winter tips, December 2025 (retrieved 23 October 2026)
- WaterSafe winter advice (retrieved 23 October 2026)
- HHIC condensate guide (retrieved 23 October 2026)
- B&Q: how to insulate water pipes (retrieved 23 October 2026)