The thing nobody says out loud
Pipe insulation is not a force field. It is a thermal resistance, and all it does is slow the rate at which heat moves out of the water and into the cold air around the pipe. If the pipe has a supply of heat — because it runs through a room that is heated, or through a void that borrows warmth from one — lagging keeps it above freezing indefinitely. If the pipe has no heat reaching it at all, lagging only extends the time before the water in it reaches the temperature of the space around it.
That distinction explains almost every "but I lagged it" call we take in January. The lagging was fine. The pipe had nothing to insulate.
It also explains why lagging pays for itself despite that limitation. Most London cold snaps are not four days of hard frost; they are a cold night, a cold morning and then a thaw. Buying twelve hours is usually buying the whole event.
Where the lagging on your pipes is missing
Walk a lagged run in any loft and you will find the same five gaps, in the same order of frequency.
- Elbows and tees. Foam sleeve comes in straight lengths, so the corners get skipped. A bare elbow in an otherwise insulated run is a cold bridge, and ice starts there.
- Valves, stop taps and tank connectors. Skipped because they are fiddly, and they are usually solid brass, which conducts heat away far faster than the copper either side.
- Pipe clips and the points where the pipe crosses a joist. The clip holds the pipe against cold timber and the sleeve is cut back to clear it.
- The last few inches. Where the pipe disappears into a wall, under a floor or through the eaves, the insulation stops short of the opening — which is exactly where cold air is being drawn in.
- The overflow and the tank itself. The tank gets a jacket and the overflow pipe running out through the eaves gets nothing, even though it is the coldest pipe in the loft.
The fix is not more insulation, it is continuous insulation. Mitre the foam around bends rather than butting it to the elbow, tape the joints, and wrap valves and fittings in self-adhesive insulating tape where a sleeve will not fit.
Fitting it so it actually works
- Match the bore to the pipe. Foam sleeve is sold by internal diameter — 15mm and 22mm cover most domestic pipework. A 22mm sleeve slipped over a 15mm pipe leaves an air gap that convects, and it will not stay put.
- Close the split. Almost all foam lagging has a longitudinal slit so it can be pushed onto an existing pipe. Tape it shut along its length. An open seam facing upwards is a slot for cold air, and in a draughty loft that is most of the benefit gone.
- Butt the lengths tight and tape every joint. Two centimetres of bare copper between sleeves is a heat sink.
- Do not compress it. Insulation works by trapping still air. Foam squashed flat under a clip or jammed between a pipe and a joist has no insulating value at that point.
- Dry the pipe and inspect it first. Lagging over a joint that is already weeping hides a slow leak and holds the water against the metal, which accelerates corrosion. If you find green or white crusting on a joint, deal with that before you cover it.
Thames Water's winter guidance puts foam lagging at around £1 a metre, which makes the whole of a typical house's vulnerable pipework a small materials cost. The limiting factor is care, not budget.
Outside, ordinary foam is worse than nothing
This is the most expensive mistake on the page. Standard indoor foam lagging absorbs water. Put it on an external run and it soaks in the first wet week, and a saturated sleeve is not insulation — it is a jacket of water held in permanent contact with the pipe, which then freezes as a solid mass around it.
Anything outside needs waterproof, weatherproof insulation. Baxi specify waterproof and weatherproof lagging for external condensate pipes; Worcester Bosch call for weather-resistant insulation, boxed in if necessary; Viessmann specify waterproof lagging. The same logic applies to any external water pipe, an outside tap tail, or a run across the back of an extension. WaterSafe's winter advice is simply to insulate outside taps and pipes in unheated areas with foam insulation, and outdoors that foam has to be the closed-cell weatherproof kind.
What lagging cannot save on its own
| Situation | Why insulation alone does not fix it |
|---|
| A cold water tank in a well-insulated loft | The tank holds a large volume with no heat input. Insulation under the tank rather than over it is what keeps it alive, because it lets warmth from the room below reach the tank base. Lagging the tank while burying its underside in quilt is the worst of both. |
| A long run through a genuinely unheated garage or vault | Nothing is warming it. Insulation delays the freeze, and in a multi-day cold spell the delay runs out. This is a re-routing or trace heating job. |
| The buried supply pipe under a path or drive | You cannot insulate what you cannot reach, and depth is what protects it. If it freezes, it freezes. |
| A boiler condensate pipe on an exposed wall | Insulation is part of the manufacturer specification, but so are diameter, length and fall. See why your condensate pipe keeps freezing. |
| Any pipe in a property with the heating switched off | Lagging slows a fall it cannot stop. Background heat is the other half of the answer — WaterSafe advise at least 14°C overnight and when you are away. |
Trace heating, and where it earns its place
Electric trace heating is a cable run along the pipe under the insulation that warms it when the temperature drops towards freezing. It is the correct answer for a single unavoidable cold run — an exposed condensate pipe, a pipe crossing an open vault, a run in a garage that cannot be moved. Baxi list fitting a trace heater among the options for a condensate pipe that keeps freezing.
Two conditions. It goes under the insulation, not over it, or it heats the loft instead of the pipe. And it needs a supply that stays on, which rules it out as the protection for a property you are leaving empty with the consumer unit off.
The two jobs worth paying for
If the same pipe freezes every winter despite good insulation, the pipe is in the wrong place and insulation is not the answer. Two pieces of work fix it permanently, and both are ordinary plumbing rather than anything exotic.
- Re-route the run out of the cold zone. Bringing a pipe down out of a loft and running it within the heated envelope, or re-routing a run away from an external wall, removes the problem rather than managing it. That is pipework repair and replacement, and October is a much better month for it than January.
- Rearrange the loft insulation. Under the tank, over the ceiling, and pipes lagged separately rather than buried in quilt.
When not to call us
All of the above is materials and an afternoon, or a general plumber for half a day. It is not leak detection and we would be the wrong people to ask.
What is ours: a pipe that has already split behind the lagging, or a slow leak you suspect is running inside an insulated run and cannot see. Foam sleeve is very good at hiding a weep, and by the time it shows at a ceiling the water has usually travelled. We pressure-test to confirm the loss, then trace it with thermal imaging and tracer gas so one board comes up rather than a floor — see how a leak inside a wall is located. £150 per hour, agreed before we attend, and no fee for the looking if we do not find it.
