Insulate pipes to stop freezing
Updated: 23 October 2026
A cold week is forecast, the loft floor is already quilted, and the pipes that used to sit in warm air now sit in a space that behaves like outdoors. You want to insulate pipes to stop freezing before a burst or a dead boiler. Lagging slows that freeze. It does not switch the physics off, and a frozen condensate is a boiler shutdown, not a cold tap you can ignore until spring. The [pipe insulation comparison](/hub/diy-supplies/pipe-insulation) lists the tubes and wraps with their GBP prices. Water-company advice and the listings saved on 21 October 2026 were used for this page. We did not freeze a pipe in a chamber. A purchase can earn a commission at no extra cost to you.
A cold week is forecast, the loft floor is already quilted, and the pipes that used to sit in warm air now sit in a space that behaves like outdoors.
A cold week is forecast, the loft floor is already quilted, and the pipes that used to sit in warm air now sit in a space that behaves like outdoors. You want to insulate pipes to stop freezing before a burst or a dead boiler. Lagging slows that freeze. It does not switch the physics off, and a frozen condensate is a boiler shutdown, not a cold tap you can ignore until spring. The pipe insulation comparison lists the tubes and wraps with their GBP prices. Water-company advice and the listings saved on 21 October 2026 were used for this page. We did not freeze a pipe in a chamber. A purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Wall | Pipe size | Outdoor claim | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Climaflex Climaflex 22 mm, 13 mm wall, 10 m | 13 mm | 22 mm | Non-absorbent | 7.6 | about £11.05 | View offer |
| Alternative |  LavaTherm LavaTherm 28 mm self-seal, 2 m | 9 mm | 28 mm | One outside review | 7.2 | about £13.75 | View offer |
 K-FLEX K-Flex 15 mm self-seal, 2 m | 9 mm title | 15 mm bore | Not stated | 7.4 | about £8.99 | View offer |
On this page
Where a freeze starts
The pipes that freeze first are the ones that lost their borrowed heat. Severn Trent and WaterSafe both point at loft, garage, and outside runs. Once you lay insulation across the loft floor, the pipes above that quilt are no longer in the warm part of the house. Energy Saving Trust is explicit about the tank: insulate the top and sides of a cold-water tank, and do not put loft insulation under it. The little warmth from below is what keeps that cistern from freezing.
Affinity Water’s packet language matches the buying decision. The numbers 15 and 13 on a sleeve are pipe diameter and lagging thickness, not a promise. Tape the joints. A slit left open on a loft run is almost a bare pipe, because the air path is the freeze path. United Utilities adds waterproof lagging outdoors. A polyethylene bullet that says the foam does not absorb water is not the same as a jacket rated for sun and rain over several winters. Inside this set, a 9 mm wall is what K-Flex and LavaTherm print. Climaflex is the 13 mm wall, and it is sold for 22 mm pipe, not as a condensate kit. None of these listings is a test that a pipe will survive a stated number of degrees below zero. Thames Water, in December 2025, put typical internal lagging at around £1 a metre and published £330 as their average repair figure when a private pipe bursts. The £330 is their number for a repair, not a quote for your house. Which tube matches which copper is the pipe lagging buying guide.
What to do before the cold
Find the stop tap before you need it in the dark. Severn Trent treats the pipework past the external stop tap as the householder’s. If a pipe has already split, the useful action is to shut the water off, then call someone, not to add foam to a leak. For an outside tap, Affinity’s sequence is to isolate it and open the tap so the spur drains. Cover the tap as well as the pipe. A lagged pipe that ends in a bare bib tap still freezes at the fitting you use in summer.
If you are going away, WaterSafe’s winter advice is to leave the heating on low, around 14 °C, and to lag the pipes in unheated rooms. That is a thermostat setting, not a property of the foam. United Utilities says the same about leaving heating on low. An empty house with the heating off and the loft pipes bare is the case lagging is meant to slow, and it will not carry a fortnight of hard frost on its own if the seam is open and the elbow is bare. Fit what you can reach while the pipes are still liquid. The cutting sequence, including mitres and tape, is how to lag pipes. Do not insulate under the cold-water tank. Do not thaw a pipe you suspect is frozen with a naked flame. Severn Trent’s thaw is gentle heat. The listing to open when you know the diameter is on best pipe insulation, and it is still only lagging, not a guarantee.
A frozen condensate is not a cold tap
A combi that dies on the coldest morning is often the condensate, the small plastic pipe that takes condensate outside, blocked with ice. That is a different object from a 15 mm heating pipe in the loft. The Heating and Hotwater Industry Council’s guide prefers an internal discharge. Where the pipe must run outside, HHIC asks for at least a 30 mm internal diameter and at least 13 mm of waterproof, UV-resistant insulation, sealed, and carried to the tip. A 9 mm tube is below that 13 mm line. Climaflex’s 13 mm wall is the only kept tube at that thickness, and it is sold for 22 mm pipe. Do not call it an HHIC-compliant condensate jacket. Follow the boiler manual for the route.
Thaw from somewhere you can reach, with a hot-water bottle or warm water, starting where the pipe is open to the air. HHIC’s householder sheet is the same idea. Do not pour boiling water over the pipe. Do not use a kettle as a pressure washer on a plastic fitting. When the ice has gone, reset the boiler the way the manual describes. If it will not reset, or you cannot see the pipe, call a competent heating engineer. Rerouting a condensate indoors, and anything on the gas side or the flue, is not a lagging job.
What the foam cannot promise
The foam buys time. It does not promise a winter with no freeze. An open seam, a bare outside tap, a 3.8 mm foil wrap used as if it were 13 mm of foam, and a house left unheated for a long spell can still lose.
The Energy Saving Trust figure of about £6 a year, for heat lost from pipes in a typical gas-heated three-bedroom semi, is a running-cost note from January 2024. It is not evidence that a particular tube stops a burst. Lag the exposed pipe, close the seam, drain the outside tap, and treat a silent boiler as condensate until you know otherwise.
Common questions
Will foam lagging stop my pipes freezing?
It slows freezing. It does not make a freeze impossible. Severn Trent and WaterSafe still tell you to lag loft, garage, and outside pipes, and to tape the seam, because an open slit barely helps. A long cold spell, a bare elbow, or a house left with the heating off can still win. No listing in this set prints a freeze test with a temperature and a duration.
Which pipes freeze first after I insulate the loft?
The ones above the new quilt: loft runs that used to sit in warm air, plus garage pipes and anything on an outside wall. The cold-water tank is the exception to more insulation everywhere. Insulate its top and sides, and do not lay quilt underneath it. That underside is the warmth the cistern was using.
What do I do with an outside tap?
Isolate the spur and open the tap so it drains, which is Affinity Water’s advice, then lag the pipe and cover the tap itself. A foam tube that stops 100 mm short of the bib still leaves the fitting bare. If the tap has already split, use the stop tap and call someone. Foam does not mend a burst.
My boiler has stopped in the cold. Is that the condensate?
Often, on a combi, yes: the external condensate has frozen and the boiler has locked out. It is not the same job as a cold kitchen tap. Thaw the plastic pipe gently from the outlet, with warm water or a hot-water bottle, then reset. If you cannot reach it, or the reset fails, call a heating engineer. Do not pour boiling water.
Can I pour boiling water on a frozen pipe?
No. Use warm water or a hot-water bottle, and only where you can do that without soaking electrics or climbing somewhere unsafe. Severn Trent rules out a naked flame. Boiling water on cold plastic is how a condensate fitting cracks. If the freeze is inside a floor, do not open the floor to chase it with a kettle.
Is 9 mm enough on an outside condensate?
HHIC’s external guide asks for at least 13 mm of waterproof, UV-resistant insulation, sealed to the tip, on a pipe of at least 30 mm internal diameter, and it prefers an internal discharge in the first place. A 9 mm heating-pipe tube is below that 13 mm line. The 13 mm Climaflex wall in this set is sold for 22 mm pipe, not as a condensate kit. Use the boiler manual.
When do I call a plumber or a Gas Safe engineer?
Call when a pipe has burst, when you cannot reach the freeze safely, when the boiler will not reset after a gentle thaw, or when the condensate should be moved indoors. Gas, oil, and flue work are not part of fitting foam. A water company will treat private pipework past the external stop tap as yours, which is why the stop tap is worth finding before the cold arrives.
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)