There isn’t a single energy saving measure in the house that pays for itself faster than this one. And yet in most crawl spaces you’ll still find bare copper pipes quietly heating the soil.
The answer straight away: you insulate central heating pipes in every unheated space, so the crawl space, the loft, the garage, the shed and the meter cupboard, on the flow pipe and the return pipe alike. Use PE foam pipe lagging with a wall thickness of at least 9 mm, and 13 mm or thicker where it can get properly cold. Per metre of lagged pipe you save roughly 3 m³ of gas a year, around 30 kWh, about 4 euros. The material costs 1 to 3 euros a metre.
Do the sums: your investment is usually back within a year, and often within a couple of months of the heating season. There’s no home battery, no solar panel and no heat pump that matches that.
And then the point almost every guide skips: the gain isn’t in the straight runs, those are easy. The gain is in the bends, tees, fittings and clips that everyone leaves bare. On a pipe run with a lot of bends, those uncovered bits can account for a substantial share of your heat loss, while together they might be ten percent of the length. Lagging properly, right through the bends, is the difference between a half job and a job that’s actually right.
In this article: which pipes you do and don’t lag (including the tricky business of drinking water), how to work out the right size, which material belongs where, the fire safety rule at the boiler, and how to survive doing this in a crawl space.
Why this pays off so well
A bare heating pipe is really just a long, thin radiator. It sits at 40 to 70 degrees and gives that heat off to the air around it. If it runs through your living room, no harm done, you wanted heat there anyway. If it runs through your crawl space, you’re heating the ground under your house.
Heating pipes in an unheated crawl space lose roughly a few hundred kilowatt hours a year, depending on the length, the diameter and how hard you run the heating. That’s heat you pay gas for and get nothing back from.
What you notice besides the saving:
- Faster warm-up. The radiator gets up to temperature quicker, because the water loses less heat on the way
- A warmer floor. Sounds contradictory, but in an insulated crawl space the pipes are no longer the reason your floor is sweating
- Less risk of freezing. Lagging slows cooling down considerably. In a hard frost that’s no guarantee, but it is a solid buffer
- Hot water at the tap sooner. If you lag the hot water pipe as well, there’s less cooled-off water sitting in the pipe
One thing up front, in the interest of honesty: if you have a well insulated crawl space with an insulated floor above it, some of that lost heat has ended up in the house anyway. The saving is then a bit smaller. In a crawl space with open air bricks and a cold, draughty ground, the saving is at its biggest.
Which pipes do you lag, and which not?
This is the question most articles brush past. The honest version:
| Pipe | Where | Lag it? |
|---|---|---|
| Heating flow and return | Crawl space | Yes, absolutely |
| Heating flow and return | Unheated loft | Yes |
| Heating flow and return | Garage, shed, outbuilding | Yes |
| Heating flow and return | Meter cupboard, plant room | Yes |
| Heating flow and return | Living room, kitchen, bedroom | No, that heat ends up in the room |
| Heating flow and return | Unheated spare room or hallway | Yes, if you genuinely don’t heat that room |
| Pipes to the underfloor heating manifold | Crawl space or cupboard | Yes, right up to the manifold |
| Underfloor heating loops in the floor | In the screed | No, that’s the whole point |
| Hot water pipe from the boiler to the taps | Unheated spaces | Yes, in an ordinary house |
| Cold water pipe | Alongside a hot heating pipe | Yes, but to keep it cold |
| Cold water pipe | Space at risk of freezing | Yes, against freezing |
| Boiler flue | Anywhere | No, never with PE foam |
The drinking water story, explained properly
A lot gets muddled online here. The core of it:
Cold water pipes. The water regulations require enough distance between drinking water pipes and heating pipes that the water doesn’t unintentionally warm up above 25 degrees or cool below 2 degrees. Legionella multiplies between roughly 25 and 50 degrees. So a cold water pipe running right alongside your hot heating pipe, going lukewarm as a result, is a genuine reason to insulate: not to hold heat in, but to keep the cold water pipe cold.
So the rule is: if a cold water pipe runs close to a heat source, lag it (or move it further away). Never put a cold water pipe and a heating pipe together inside one length of lagging, because then you’re doing exactly the opposite.
Hot water pipes in an ordinary house. In a normal domestic set-up with a combi boiler or a cylinder, lagging the hot water pipe is simply worth doing: less heat loss and hot water at the tap sooner.
The exception you’ll run into online. Communal drinking water systems (flats, care homes, hotels) fall under separate legionella rules, where certain sections of pipe are specifically meant to be able to cool down quickly. If you live in a flat with a communal hot water supply, get the plumber or the building’s management to take a look before you touch the water pipes. On your own heating pipes you can, in almost every case, simply crack on.
A practical safety rule for at home: got a pipe you’re not sure about, heating or drinking water? Feel it with the heating running, and again with the heating off. A heating pipe warms up and cools down again. A cold water pipe stays cold. Still not sure? Leave it alone and ask your plumber.
Choosing the right size: diameter and thickness
This is where people come home with the wrong bag.
Step 1: measure the outside diameter of your pipe
Not the “nominal size” or what you think it is. Pipe lagging is sold by internal diameter, which matches the outside diameter of your pipe.
| Pipe type | Common outside diameters |
|---|---|
| Copper (heating) | 12, 15, 18, 22, 28 mm |
| Copper (domestic water) | 12, 15, 22 mm |
| Multilayer composite or plastic | 16, 20, 25, 32 mm |
| Steel (old, threaded) | 21.3 / 26.9 / 33.7 mm |
Measuring without callipers: wrap a strip of paper tightly around the pipe, mark where it overlaps, measure that length and divide by 3.14. That’s your outside diameter. A circumference of 47 mm is a 15 mm pipe. A circumference of 69 mm is 22 mm.
In most houses the main runs are 22 mm and the branches to the radiators are 15 mm. Buy a few metres of each and you’ll usually be fine.
Step 2: choose the wall thickness
| Wall thickness | What it’s good for |
|---|---|
| 6 mm | Only when there genuinely isn’t room. Does little |
| 9 mm | A fine standard for heating pipes in the crawl space and meter cupboard |
| 13 mm | Better. The choice for spots at risk of freezing and unheated lofts |
| 19 to 20 mm | For long runs, cold spots, or when you want to do it properly |
| 25 mm and thicker | Mainly for large diameters and plant installations |
Rule of thumb: thicker lagging does deliver more, but with diminishing returns. The jump from 0 to 9 mm is by far the biggest. From 9 to 13 mm you still gain a decent amount. From 13 to 20 mm there’s a bit more, but the lagging gets bulky and stops fitting everywhere.
More important than thickness: that there’s something on every last bit of it, that all the seams are closed, and that the bends have been done. Thinner lagging that closes neatly everywhere works better than thick lagging with gaps in it.
Step 3: choose the material
| Material | Properties | What it’s good for | Price |
|---|---|---|---|
| PE foam (polyethylene) | Cheap, light, easy to cut, usually with a self-adhesive seam | Standard heating pipes in the crawl space, loft, meter cupboard | Low |
| Elastomeric or synthetic rubber (Armaflex type) | Vapour tight, flexible, higher temperature rating | Cold water pipes, chilled pipework, spots at risk of condensation, higher temperatures | Medium |
| Mineral wool with foil facing | Non-combustible, high temperature rating | At the boiler, close to hot components, industrial use | Higher |
| PIR sections | High insulation value in a small thickness | Where space is tight | Higher |
For nine out of ten jobs at home: PE foam. It’s cheap, it does the job, and you cut it with a bread knife.
Use elastomeric (rubber) wherever condensation can form, so around cold water pipes in a warm or damp space. PE foam isn’t fully vapour tight, and moisture can then condense inside the lagging against the cold pipe. That’s exactly what you don’t want.
White or grey? Purely cosmetic. White PE foam exists for spots on show (meter cupboard, plant room). The insulation value is identical.
Fire safety: the rule at the boiler
This appears in almost no DIY article, and it belongs in one.
PE foam is combustible. It melts and burns at high temperature. In a crawl space, or on a pipe running at 60 degrees, that’s no problem at all. Right next to a boiler it becomes a different story.
What you don’t do:
- Never put PE foam or any other plastic foam around a flue. On an older boiler that can head towards 90 degrees or higher, and higher still if something goes wrong. If you want to insulate a flue (against condensation in a cold loft, say), use mineral wool or another non-combustible material, and check first whether the boiler manufacturer allows it at all
- Not pressed tight against the hot parts of the boiler
- Not right alongside a wood burner, stove pipe or any other open heat source
What you do: at the boiler, keep a good few centimetres of clearance from the flue and from hot components, or switch to mineral wool with a foil facing at that point. Mineral wool and glass wool are non-combustible and comfortably handle hundreds of degrees.
Check the maximum temperature on the packaging too. PE foam for heating pipes is usually rated to around 95 degrees, which is plenty for normal domestic heating. If you have an older system set to 90 degrees, take a careful look at the specification.
The step-by-step
What you need:
- Pipe lagging in the right diameters and thicknesses
- A sharp bread knife, utility knife or a pair of insulation shears
- Insulation tape or aluminium tape (usually not needed with self-adhesive lagging, but always handy to have)
- Tape measure, pencil
- Work gloves and a good torch or head torch
- For the crawl space: overalls, knee pads and a dust mask
Steps:
-
Do a lap and measure. Walk the pipe runs, note the diameters and count the metres. Count bends and tees separately, because they need extra material. Add 10 to 20 percent to your total for offcuts
-
Clean and dry the pipes. Dust and cobwebs stop the self-adhesive seam from sticking. Wet is no good either. Give the pipe a quick wipe with a cloth
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Cut to length. Measure the run and cut the lagging square with a sharp knife. Better a fraction too long than too short: the lagging compresses slightly at the joint and everything then butts up properly
-
Slide it on or clip it round. If you can slide it over the end of the pipe, that’s the neatest result. In practice the pipe is usually fixed in place, so you use lagging with a lengthwise seam: open it up, lay it round the pipe, press it shut
-
Seal the lengthwise seam. If there’s a self-adhesive strip, pull the film off and press firmly along the whole length. If there isn’t one, tape the seam up with insulation tape. An open seam costs you a large part of the effect
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Close the butt joints. Where two lengths of lagging meet, they need to butt up tightly. Wrap a ring of tape round it. A one centimetre gap between two lengths is a hole for heat to pour out of
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Do the bends. This is the most important part. Two methods:
- Ready-made bend sections. Quickest and neatest, available per diameter
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Cutting mitres. Cut two pieces of lagging at 45 degrees and butt them together. For a 90 degree bend you need two 45 degree halves. Finish with tape afterwards
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Do the tees and fittings. Notch a piece of lagging so it drops over the tee, or use a ready-made tee section. Valves and fittings can be finished with separate insulation caps or a neat bit of bespoke cutting. Anything you leave bare keeps leaking heat
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Watch the pipe clips. Everywhere the pipe hangs off a joist or wall on a clip, the lagging is either squashed or missing. Two solutions: use a clip designed to take lagging (with a plastic or rubber insert), or notch the lagging so it runs right up against the clip and tape the opening shut
-
Check the lot. Walk the whole run back and look for gaps, open seams and bare patches. Fire the heating up and run your hand along the lagging: wherever you feel warmth, heat is escaping
Crawl space: how to survive it
Most of the gain is in the crawl space, and that’s also the most miserable place to work. Practical matters:
Check before you go in:
- Is there standing water? Sort that first. In a wet crawl space no insulation works properly, and it’s a nasty environment on top of that
- Are the air bricks clear? A crawl space has to be able to ventilate. Don’t block them with your insulation material
- Do you suspect asbestos? Old pipe insulation can contain asbestos, particularly the white, plaster-like or cardboard-like wrapping around old pipes. Don’t strip it off yourself. Have it tested and, if need be, removed by a licensed contractor
- Is there power and light? Take a good battery head torch, not a loose lamp on an extension lead trailing over damp ground
Kit: overalls, knee pads, work gloves, a dust mask and a head torch. Take your tools in a bucket or a bag you can push along ahead of you, and make sure someone in the house knows you’re down there.
Order of work: cut as many pieces to size as you can above ground, after measuring them up with a tape on a stick. Crawling back and forth every time you need to cut a piece costs you half an hour a go.
Rodents. In some crawl spaces mice or rats chew PE foam to build nests with. If you see droppings or gnawed material, deal with the rodent problem first, or choose a material with a tough outer jacket.
What does it cost and what does it return?
Material costs:
| Item | Price |
|---|---|
| PE foam pipe lagging 9 mm, per metre | approx. 1 to 2 euros |
| PE foam pipe lagging 13 mm, per metre | approx. 1.50 to 3 euros |
| Elastomeric (rubber) lagging, per metre | approx. 3 to 7 euros |
| Bend and tee sections, each | approx. 1 to 3 euros |
| Insulation tape (roll) | approx. 5 to 12 euros |
| Overalls (one-off) | approx. 15 to 40 euros |
For an average house with 10 to 25 metres of pipe to lag, you land on roughly 25 to 90 euros of material.
The saving: roughly 3 m³ of gas per metre a year. At 15 metres that’s 45 m³, at 25 metres around 75 m³. One cubic metre of gas is about 10 kWh, so your meter counts cubic metres while your bill works in kilowatt hours. At a gas price of 1.20 to 1.50 euros per m³ you’re talking about roughly 55 to 110 euros a year.
Payback: usually under a year, and often within a single heating season.
There’s no other energy saving measure in the house that manages that.
Time investment: allow half a day to a full day for an average crawl space, plus an hour for the meter cupboard and the loft. Most of the time goes on crawling back and forth, not on the lagging itself.
With a heat pump or low temperature heating
If you have a heat pump, or you run at low temperature (35 to 45 degrees flow, say), the temperature difference with the crawl space is smaller and so is the loss per metre. The saving per metre is therefore lower than the 3 m³ that applies with a gas boiler.
Lagging is still worth doing, for three reasons:
- Every kilowatt hour you lose in the crawl space has to be generated by the heat pump, which then runs longer and at lower efficiency
- At low temperature, every degree you lose on the way weighs relatively heavier, because you have less headroom to the room temperature you want
- If you cool in summer (with underfloor cooling, or an air-to-water heat pump that cools), cold water runs through those pipes and you get condensation on bare pipework. Then lagging isn’t a saving measure but a necessity, and you want vapour tight material like elastomeric
The eight mistakes you want to avoid
- Not sealing the lengthwise seam. Warm air creeps out through the slit and you get a fraction of the effect
- Gaps between two lengths. Tape every butt joint, always
- Skipping bends and tees. That’s where it loses a disproportionate amount of heat
- Buying the wrong diameter. Too big sits loose and insulates poorly, too small won’t go round
- PE foam at the flue. Non-combustible material, or keep your distance
- Cold and hot water pipe together in one length of lagging. Then your cold water pipe warms up, which is precisely what you don’t want
- Stuffing air bricks with offcuts. Your crawl space has to ventilate
- Stripping old pipe insulation yourself without knowing what it is. It could be asbestos, get it tested
What to pick up in the same afternoon
If you’re busy with small savings anyway, these are the jobs that fit in the same hour:
- Radiator reflector foil behind radiators on an outside wall. A few euros, and it bounces heat back into the room
- Draught excluder strips along doors and windows, and a draught brush under the front door
- The manifold of your underfloor heating, insulated with an insulation jacket or separate insulation caps
- The cylinder or thermal store, checked for a decent insulation jacket
- Bleeding the radiators: bleed the lot and bring the water pressure back up (usually between 1.5 and 2 bar with the system cold)
- Turning the flow temperature down to 60 degrees or lower, if your radiators can handle it. The boiler then condenses better
Checklist: insulating central heating pipes in short
- Walk the pipe runs in the crawl space, loft, garage, shed and meter cupboard
- Measure the outside diameter with a strip of paper: circumference divided by 3.14
- Choose 9 mm as standard, 13 mm or thicker where freezing is a risk
- Choose PE foam for heating, elastomeric for cold water and condensation risk
- Count the metres and add 10 to 20 percent for offcuts
- Clean and dry the pipes
- Cut to length, clip it round, seal the lengthwise seam
- Tape all the butt joints
- Finish every bend, tee and valve
- Solve the pipe clips with lagging-friendly clips or neat fitting
- Keep plastic foam away from the flue
- Fire the heating up and feel whether heat is still escaping anywhere
Frequently asked questions
How much do I save by insulating central heating pipes?
Per metre of lagged pipe you save roughly 3 m³ of gas a year, about 4 euros. In an average house with 10 to 25 metres of pipe in unheated spaces, that comes to roughly 55 to 110 euros a year. The exact saving depends on the pipe diameter, the flow temperature, how many hours you run the heating and how cold the space the pipes pass through is.
How quickly does pipe lagging pay for itself?
Usually within a year, and often within a single heating season. The material costs around 1 to 3 euros a metre and saves around 4 euros per metre per year. That makes pipe lagging the energy saving measure with the shortest payback time in the house, shorter than radiator foil, draught strips or anything else.
What thickness of pipe lagging should I get?
For heating pipes in a crawl space or meter cupboard, 9 mm PE foam is a fine standard. If the pipe runs through a space where it can freeze, such as an unheated loft or garage, go for 13 mm minimum. Thicker lagging adds a bit more, but with diminishing returns. More important than thickness is that the lagging is on every part of the pipe and that all seams and bends are neatly closed.
Do I need to lag the return pipe too?
Yes. The return is cooler than the flow, true, but in a crawl space at 8 degrees a return at 40 degrees is still a serious heat source. Always lag both pipes. Do only the flow and you’re leaving roughly half the available saving on the table.
Should I lag central heating pipes in the living room as well?
No. Heat released there simply warms the room you wanted warm. Only lag pipes in spaces you don’t heat: crawl space, unheated loft, garage, shed and meter cupboard. For a room you deliberately leave unheated, you do lag the pipes running through it.
Am I allowed to insulate water pipes?
Hot water pipes in a normal domestic system can be lagged without any issue, and it saves heat loss and waiting time at the tap. Cold water pipes you lag when they run close to a heat source, because they need to stay below 25 degrees to hold back legionella growth. Use vapour tight material like elastomeric there, and never put a cold water pipe and a heating pipe together inside one length of lagging. If you live in a flat with a communal hot water supply, check with the building’s management or the plumber first, because separate legionella rules apply there.
Can I get lagging around a pipe that’s fixed in place?
Yes. Most pipe lagging has a lengthwise seam already slit, often with a self-adhesive strip. You open the sleeve up, lay it round the pipe and press the seam shut. If there’s no seam, you cut one yourself along the length with a sharp bread knife or utility knife. Always seal the seam afterwards, otherwise it only half works.
Can I insulate my boiler flue as well?
Not with PE foam or any other plastic foam, that’s combustible and the flue gets too hot. If you want to insulate a flue, against condensation in a cold loft for example, use non-combustible material such as mineral wool with a foil facing, and check with the boiler manufacturer first whether that’s permitted for your type of boiler. If in doubt, have your heating engineer judge it.
Is there a grant for pipe insulation?
The big national schemes are aimed at major insulation work such as roof, floor, wall and glazing, not at a roll of pipe lagging. Pipe insulation can ride along when it’s part of a wider heating upgrade, and plenty of local councils and energy suppliers run their own vouchers or handouts for small energy saving measures, which pipe lagging regularly falls under. Check with your council or your local energy advice service. Given a payback time of under a year, though, there’s no need to wait for anything.
What if there’s old, crumbling insulation around my pipes?
Don’t remove it yourself before you know what it is. Old pipe insulation, especially the white, plaster-like or cardboard-like type around pipes in older houses, can contain asbestos. Have it surveyed and, if it does turn out to be asbestos, removed by a licensed contractor. Fitting new material over the top without disturbing the old layer is sometimes an option, but discuss that first.
Insulation, heating and building all hang together. Plug the small heat leaks and you save straight away, but the really big steps get made when you build and renovate. Planning an extension, a conversion or a garden room where the insulation and the services need to be right from day one? Then have a quick look at Fred’s DIY Plans. Explained plainly, do it yourself, exactly the way Fred would go about it.