Pipe lagging: insulating pipes and loft tanks against frost and heat loss

Lagging is insulation wrapped around a pipe. On cold water pipes in unheated spaces it slows down freezing. On hot water pipes it keeps heat in, so less is wasted between the cylinder or boiler and the tap. It is one of the cheapest winter jobs in a house, and most of it is well within reach of a householder.

What the regulations say, in general terms

In England and Wales, the Water Supply (Water Fittings) Regulations 1999, Schedule 2 say every water fitting shall be constructed and installed so as to "inhibit damage by freezing or any other cause". They also say every storage cistern shall be fitted with "thermal insulation to minimize freezing or undue warming".

The regulations do not give a single lagging thickness that suits every home, and we do not quote one here. A plumber or the insulation maker's own data sheet is the place to check what suits a particular pipe in a particular space. Scotland and Northern Ireland have their own water fittings rules, so check with your own supplier there.

Which pipes to lag

Start with the pipes most likely to freeze:

• pipes in the loft, especially the feed into the cold water cistern and the pipes coming out of it;
• pipes in an unheated garage, outbuilding, utility room or under-stairs cupboard against an outside wall;
• pipes under a suspended timber floor, where cold air moves through the void;
• the pipe to an outside tap, as it runs through and along the outside wall. See our guide to the frost-free outdoor tap for the tap itself.

Then the pipes where lagging saves heat:

• hot water pipes from the cylinder, especially the first metre or two;
• hot water pipes running through unheated spaces.

An older bare copper cylinder can take an insulating jacket too. See hot water cylinders and immersion heaters.

Heating pipes in rooms you heat do not usually need lagging. The heat they give off warms the room.

Fitting foam pipe lagging

Foam lagging is a tube with a slit along its length. It comes in sizes to fit common pipe diameters, so measure the pipe first (15mm and 22mm are the usual domestic copper and plastic sizes) and buy the size that matches.

1. Check the pipe is dry and not leaking. Lagging over a weeping joint hides the leak.
2. Open the slit and push the lagging over the pipe. On a self-sealing type, peel off the backing strip and press the edges together.
3. Cut the lagging at bends with a mitre, or with small V cuts on the inside of the bend, so it closes round the corner without gaps.
4. Butt each length tightly against the next. Tape the joins and any slit that will not stay closed.
5. Do not leave short bare sections at valves, joints or where the pipe goes through a wall or ceiling. Those are where pipes often freeze first. Leave the handles of stopcocks and isolation valves free so you can still turn them.

Foil wrap insulation is an alternative for awkward runs, bends and tight gaps. It is wrapped round the pipe in a spiral with an overlap.

The loft cistern

A cold water cistern in the loft needs protecting too.

• Use a cistern jacket or insulation made for the purpose, fitted round the sides and over the lid.
• Keep the lid on. It keeps out dirt and insects as well as cold.
• Insulate the pipes going in and out right up to the cistern.
• Do not insulate directly underneath the cistern. A common approach is to leave the loft insulation out from under it, so some warmth from the rooms below can rise and help stop it freezing.
• Leave access to the float valve and the servicing valves so you can still reach them.

If you have topped up your loft insulation, check that the pipes and cistern are now above it, not buried in the cold part of the loft on the wrong side of it.

When it is not a DIY job

• Pipes inside walls or under solid floors. You cannot lag them without opening up the structure. If one freezes regularly, ask a plumber about rerouting or protecting it.
• Pipes in a void you cannot safely reach, such as a low loft without boards, or under a floor where you would need to lift boards over cables. Do not stand on ceiling joists without boards or crawl over wiring.
• Pipes near a flue or a boiler. Do not wrap insulation round a boiler, its flue, its condensate pipe or any gas pipe. Anything to do with the boiler is a matter for its maker's instructions or a Gas Safe registered engineer. If a condensate pipe freezes, see frozen condensate pipe.
• Pipes that are already damaged or leaking. Fix the leak first. See finding a leak and the stopcock.

If a pipe does freeze, our guide to frozen and burst pipes covers what to do.

Pipe insulation at Wickes

When we checked Wickes on 13 November 2026, a search for pipe lagging did not show a standard foam tube lagging product page we could link to. These are the closest products we found.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Does pipe lagging stop pipes freezing completely?

No. Lagging slows down heat loss, so the water in the pipe takes much longer to freeze. In a long, severe cold spell in an unheated space, a lagged pipe can still freeze. Keeping some heat in the house and fixing draughts round the pipe both help.

Should I lag central heating pipes?

Only where they run through unheated spaces, such as a loft, garage or floor void. In heated rooms, the heat from the pipes is not wasted.

What size pipe lagging do I need?

Match the inside size of the lagging to the outside diameter of the pipe. Most domestic water pipes are 15mm or 22mm. Measure, or check the size printed on plastic pipe.