How to lag pipes

Based on practical UK plumbing advice. This walk-through draws on the popular "HOW TO INSULATE PIPES — Plumbing Tips" from PlumberParts, a well-established UK plumbing advice channel. Their demonstration of fitting the lagging neatly at junctions and bends is particularly worth watching before you start — that is where most people make a mess of it.

1. Know which pipes to lag and why

Pipes most at risk of freezing are those running through unheated spaces: lofts, garages, under suspended floors, and along outside walls. Cold water supply pipes and the feed to an outside tap are the usual suspects. If a pipe runs through a heated room, freezing is not a concern — but lagging it still reduces heat loss.

Hot water pipes and central heating flow and return pipes are worth lagging everywhere accessible. The heat they shed into uninsulated voids is wasted energy. It adds up month by month across a typical winter.

2. Choose the right foam lagging

Foam pipe lagging is sold in standard lengths — usually one metre — and comes in sizes to match common pipe diameters: 15 mm and 22 mm copper pipe are the most common in UK homes. Check the outside diameter of your pipe before buying. The wrong size will not sit snugly and gaps will let cold in.

Thicker-walled lagging gives better protection for pipes in very exposed spots, like a loft where the temperature can drop significantly below zero. Standard 9 mm wall thickness is fine for most internal locations.

3. Gather your tools and materials

You will need: the foam lagging itself, a sharp bread knife or hacksaw for cutting, self-adhesive pipe lagging tape or duct tape to seal joints, and a marker pen. That is genuinely it. No specialist tools required.

If the existing lagging is old, brittle, or has gaps, remove it completely before adding new. Old foam that has gone stiff and crumbly provides almost no insulation. Start fresh rather than patching.

4. Measure and cut the lagging to length

Hold each length of lagging against the pipe run and mark the cut point with a pen before cutting. Most foam lagging comes pre-slit along its length — simply press it open and snap it onto the pipe. If yours is not pre-slit, cut along the length first, then fit it.

Cut straight, square ends where lengths join. A slightly diagonal cut at a joint means a gap, and gaps are where the cold gets in. Take your time with the measuring — it is quick work once you are in a rhythm.

5. Fit straight runs neatly

Press the lagging firmly onto the pipe so the split faces up. Press the two edges together along the full length. Most modern lagging has a self-adhesive strip inside the slit — if yours does, peel the backing and press the edges firmly together. If not, tape the join at 300 mm intervals.

Butt each length tightly against the next. Do not leave gaps between sections — even a 10 mm gap over a pipe that runs two metres through a cold loft will be the spot where moisture condenses and frost gets a foothold.

6. Handle bends and elbows carefully

Bends are where most people give up and leave gaps. That said — it is actually straightforward once you know the trick. Make a series of small diagonal cuts on the inside of the curve so the lagging bends without buckling or pulling away from the pipe. Think of it like cutting darts into fabric to make it curve.

For 90-degree elbows, cut two 45-degree mitred pieces and join them at the bend. Hold them tightly together and tape the joint firmly. It will not look perfect, but it will be airtight, which is what matters.

7. Seal T-junctions and valves

Where a branch pipe comes off the main run, cut a neat hole in the lagging at the junction point and slip the branch lagging through it. Tape the join firmly on all sides. It takes a bit of fiddling to get right, but the result is a continuous insulation barrier with no cold spots.

Around gate valves and stopcocks, fit the lagging as close as you can and then wrap the valve itself in pipe lagging tape. You still want to be able to turn it off in an emergency — do not entomb the valve completely, just make sure the pipe either side of it is covered.

8. Check and tape every joint

Walk the whole run once you are done and press every joint firmly. Any that have sprung open should be retaped. Pay particular attention to anywhere the pipe goes through a joist or wall — the pressure of fitting through a tight hole can pull joints apart.

In a loft, take a final look at the cold water tank if there is one. It needs a proper insulating jacket, sold separately at most DIY shops. A lagged pipe feeding an uninsulated tank is half a job.

When to call a handyman

Call Richard if access is difficult — cramped lofts, low crawl spaces under floors, or pipes behind boxing that needs removing and refitting. Lagging itself is well within a confident DIYer's abilities; it is the access and the associated tidying up that makes it worth getting someone in for.