How to Insulate Exposed Pipes and Save Heat

Why a short run of bare pipe costs more than you think

Have a look in your loft, your airing cupboard, or the cupboard where the boiler lives. Chances are you'll see a few metres of copper or plastic pipe, bright and bare, doing its best to warm the house in all the wrong places. A hot water pipe sitting at around 60°C in a loft that's just above freezing is effectively a radiator with no controls on it. It leaks heat all day, every day, and you pay for every bit of it twice: once to heat the water, and again because your boiler has to keep reheating what the pipe has thrown away.

The numbers are modest per metre but they add up quickly. A bare 15mm hot pipe loses somewhere in the region of 30 to 40 watts for every metre of length, and a 22mm pipe loses noticeably more. Wrap it properly and you can cut that loss by roughly three-quarters. Twenty metres of unlagged pipework in a cold loft is a bit like leaving a small electric heater switched on permanently. Then there's the winter side of it. Cold pipes in a draughty loft or a garage freeze, and a frozen pipe doesn't usually announce itself until the thaw, when it splits and starts soaking the ceiling below.

What you'll need before you start

• Foam tube lagging in the right bore — 15mm, 22mm and 28mm are the common sizes in UK homes. Pre-slit tubes are far easier to fit around existing pipework.
• Felt or foil-backed wrap for awkward shapes, tight corners and anything that foam tube won't sit on neatly.
• Lagging tape and cable ties to hold joints and runs in place. Proper lagging tape is worth the small extra cost; electrical tape goes brittle and peels.
• A sharp knife or sturdy scissors, a tape measure, and gloves. If you're working in a loft with mineral wool insulation, add a dust mask and long sleeves.
• Pre-formed elbow and tee pieces if you can find them in your pipe size, plus offcuts of tube for everything in between.

Choosing the right size and thickness

Measure the outside diameter of the pipe, not the bore of the fitting. Most tap and radiator pipework is 15mm, cylinder and boiler flow and return pipework is often 22mm, and mains supply can be 25mm or 28mm. If you're unsure, wrap a piece of string around the pipe, measure that length and divide by 3.14 for the diameter. The lagging bore needs to match: too loose and you leave an air channel that does very little; too tight and you'll split the foam trying to force it on.

Thickness matters more than most people realise. Thin 9mm wall foam is cheap and better than nothing, but it's a compromise. For hot water and heating pipes, go for at least 20mm wall thickness, and 25mm where the pipe is in a cold loft, a garage or anywhere outdoors. Current building standards for new heating pipework generally expect around 25mm, so matching that on an older system is a sensible target. Buy the thicker stuff once and forget about it.

Fitting it properly, step by step

• Cut each length slightly longer than the gap you're filling, then trim to fit. A snug butt joint is much better than a loose overlap that opens up.
• Slit the tube lengthways if it isn't pre-slit, open it gently and spring it over the pipe. Don't stretch it thin as you go.
• Push the sections together tightly and tape every joint. Joints are where most of the remaining heat escapes.
• Treat bends and tees with care. Cut short angled pieces of tube to follow the curve, or use pre-formed sections, and tape them firmly. A bare elbow undoes a lot of good work.
• Secure long runs with cable ties every half metre or so, particularly in a loft where pipes are easy to knock.
• Lag cold pipes too. It stops them sweating in summer and freezing in winter, and keeps the cold side properly cold.
• Leave valves and isolators accessible. A small cut-out so you can reach the handle is far better than a length of lagging you have to destroy in an emergency.
• Insulate the cold water tank on the top and sides, but never underneath it. A little warmth rising from the room below is what stops a loft tank freezing.
• Keep lagging away from boiler flues, immersion heater thermostats, hot electrical cables, frost stats and safety vent pipes. If in doubt, leave it exposed and ask.

The spots people always forget

The loft run between the cylinder and the shower is the classic one, but there are plenty more: pipework along an external kitchen or utility wall, the condensate pipe from a condensing boiler (a common cause of boiler lockouts on a cold morning), outdoor taps and the pipe feeding them, pipes in a detached garage or outbuilding, and the short runs inside cupboards on north-facing walls. The length of pipe between the boiler and the point where it disappears into the wall is often missed as well. If a pipe is on the cold side of your insulation, it needs lagging.

When it's worth getting a plumber involved

Most of this is genuinely a Saturday morning job. But there are times to hand it over: if a pipe has already frozen or is weeping, if you can't safely get into the loft or the space is cramped, if the pipework is boxed in, or if the work is near the boiler, a sealed system or anything gas-related. Family-run plumbing firms are usually happy to lag pipework while they're there for another job, which works out cheaper than a separate visit, and it's a sensible thing to ask about during an annual boiler service.

One last thing worth saying: lagging slows heat loss, it doesn't fix a struggling system. If your hot water runs lukewarm a few metres from the cylinder, or the boiler seems to run constantly, insulation will help but it won't solve it. Get the pipes wrapped, then get the rest checked.