How to Insulate Pipes: Lagging, Fitting and Frozen Pipe Tips

Wrapping your pipes in foam is one of the quickest home jobs there is, and it can save you from a frozen pipe or a flooded ceiling. Pipe insulation, usually called lagging, slows the loss of heat, so pipes in cold spots are less likely to freeze and hot pipes stay warmer for longer.

This guide is based on advice from SES Water, Cambridge Water, Homebuilding and Book a Builder UK. It covers which pipes to start with, how to fit lagging, and what to do if a pipe does freeze. Where the sources didn't give a detail, I've left it out rather than guess.

Why lagging pipes is worth doing

Homebuilding says lagging reduces heat loss between the boiler, your radiators and your taps, so the water stays hot longer and the system works more efficiently. It also says insulation can prevent pipes freezing or delay the point at which they do.

Water companies add a safety angle. SES Water says to fit lagging with no gaps at bends, valves or fittings, with extra attention to tanks and pipes in unheated places.

Which pipes to insulate first

• Pipes in lofts, garages and outbuildings. SES Water's winter advice puts tanks and pipes in unheated spaces at the top of the list.
• Pipes near the boiler and heating. Homebuilding's expert points to the pipes from the boiler, pipes leading to radiators and hot water supply pipes.
• The condensate pipe. Homebuilding says lagging it can help stop it freezing, which would otherwise stop the boiler.
• Cold water tanks. Book a Builder UK says to insulate tanks too, but keep the lids vented.

Choosing pipe insulation

Book a Builder UK says most UK homes have 15mm or 22mm pipework, and lagging should fit snugly, so measure the pipe diameter before buying. It also says plastic pipes lose heat faster than copper, so they need slightly thicker insulation. For most lofts it suggests a wall thickness of 15mm to 25mm.

It lists foam made from polyethylene, rubber (nitrile), phenolic foam and foil-faced types. Polyethylene is easy to fit, rubber is flexible for tight spaces, phenolic foam is rigid and harder to fit, and foil-faced lagging needs careful sealing.

How to insulate pipes step by step

1. Trace each run of pipe, including risers, tank feeds and the lengths near eaves.
2. Cut the lagging to length. Pre-cut short pieces for tight gaps between joists.
3. Open the slit and press the foam over the pipe. Butt the lengths tightly together, because Book a Builder UK warns that stretched foam can leave gaps.
4. Seal every joint and seam with insulation tape or foil tape so moist air can't get in.
5. Cut bends at 45 degrees. Book a Builder UK suggests using a mitre box for the angled cuts.
6. Wrap valves with offcuts, but keep them reachable for maintenance.
7. Run your hand along the pipe afterwards to check for bare sections, including behind timbers.

Lagging pipes in the loft: a warning

Loft insulation can make the pipes colder, because less warmth rises from the house. Book a Builder UK says to position pipes below the loft insulation where possible. If they sit above it, use much thicker lagging or have the pipes moved.

• Don't squash lagging. Crushed insulation works less well, so keep stored boxes off pipes.
• Don't block ventilation. Over-insulating can lead to condensation and damp.
• Take care when moving about. Don't step between joists, and make sure the loft is well lit.
• If you're adding more loft insulation, check where the pipes run first.

Book a Builder UK also notes that lagging slows heat loss but won't stop freezing in prolonged cold weather without some background heat, so a well-wrapped pipe isn't a guarantee.

Know where your stop tap is

SES Water says to find your internal stopcock, check you can turn it off, and know that it usually sits under the kitchen sink and closes clockwise. Fix dripping taps too, because even a small trickle can freeze a pipe. If you'd like to deal with one, my guide on how to fix a dripping tap has the steps.

What to do if an insulated pipe freezes anyway

Cambridge Water gives a short routine for a frozen pipe:

1. Shut off the incoming water at the stop tap.
2. Open the taps so water can escape as the pipe thaws.
3. Thaw the pipe slowly with hot water bottles or a hairdryer on a warm setting.

It says never to use a naked flame or blowtorch. If a pipe bursts, it says to turn off the water at the stop tap and switch off the boiler, switch off the electrics at the mains if water has reached them, open all the taps to drain the system and soak up the water with towels. Then call a WaterSafe recognised plumber.

When to get a professional involved

Book a Builder UK says that any change to boiler pipework may need a Gas Safe registered engineer. As a rule, wrapping exposed pipes in lofts and garages is a DIY job, but leave gas and boiler connections alone.

Other quick wins around the house

Lagging is one of those jobs that's easy to forget until the weather turns. While you're in the DIY frame of mind, the steps in how to fill nail holes in walls will tidy up the other end of the house.

Share your home project know-how

People searching for pipe advice often want the practical detail that only comes from someone who's done the job.

DIY writers can find home project sites that take articles from outside authors listed here.

Frequently asked questions

Which pipes should I insulate first?

Start with pipes in unheated places such as lofts and garages. Homebuilding also points to boiler pipes, radiator pipes and hot water supply pipes.

What size pipe insulation do I need?

Book a Builder UK says most UK homes have 15mm or 22mm pipework and the lagging should fit snugly, so measure the pipe diameter before you buy.

Do I need to tape the joints?

Yes. Book a Builder UK says to seal every joint with insulation or foil tape to limit condensation, and to leave no gaps.

Can I thaw a frozen pipe with a blowtorch?

No. Cambridge Water says never to use a naked flame or blowtorch. Use hot water bottles or a hairdryer on a warm setting instead.

Will lagging stop pipes freezing completely?

Not always. Homebuilding says it can prevent or delay freezing, and Book a Builder UK says it won't stop it in prolonged cold without background heat.