How to Insulate Pipes – DIY Guide
Insulate pipes by measuring each run, cutting foam lagging tube to length with a sharp knife, and pressing it over the pipe so the self-adhesive seam closes fully. Pay particular attention to pipes in unheated spaces — lofts, garages, and under floors — where freezing risk is highest.
- Sharp craft knife or scissors
- Tape measure
- Marker pen
- Mitre block
- Pipe lagging adhesive tape
- Torch or head torch
- Safety gloves
- Foam pipe lagging tube (correct bore size)
- Self-adhesive pipe lagging tape
- Cable ties or lagging clips
- Waterproof pipe insulation tape
- Elbow and tee lagging sections
- Expanding foam filler (for gaps around pipes through walls)
Step-by-Step Guide
Locate and map all exposed pipe runs
Work through every unheated space in the property — loft, garage, under-floor void, and any external walls — and identify all copper or plastic pipes that lack insulation. Note the pipe diameter (typically 15 mm or 22 mm for domestic supply and central heating) so you buy lagging with the correct bore. If you are unsure whether a pipe run includes your central heating flow, check our plumbing complete guide before starting.
Buy the correct lagging and cut it to length
Foam lagging tube is sold in standard bore sizes — 15 mm and 22 mm are the most common — and in 1 m lengths. Lay a length against the pipe run, mark with a marker pen, and cut cleanly with a sharp craft knife. For straight runs this is a simple square cut. Use a mitre block to cut 45-degree angles at elbows so the lagging meets neatly without gaps. Avoid tearing or crushing the foam, which reduces its insulating value.
Fit lagging along straight pipe runs
Most foam lagging has a pre-cut slit along one side. Open the slit, press it over the pipe, and close the self-adhesive seam firmly. Run your thumb along the full length to ensure the seam is fully bonded — any gap is a cold bridge. On longer runs, butt each new length of lagging end to end and seal the joint with self-adhesive pipe lagging tape. This is especially important on pipes in the loft where cold air can pass through even small gaps. For older pipes showing signs of existing issues, check whether you also need to fix a leaking pipe joint before fitting lagging over it.
Insulate bends, elbows, and tee junctions
Straight lagging does not naturally turn corners. Cut two 45-degree mitred sections from your foam lagging and butt them together at each elbow, sealing with self-adhesive tape. For tee junctions, cut a notch out of one length so it wraps around the branch pipe. Proprietary moulded elbow and tee sections are available for 15 mm and 22 mm pipe if you prefer a neater finish. Any exposed metal at a joint is a potential freeze point, so take time to achieve full coverage.
Secure and seal in difficult access areas
In loft spaces, use cable ties or lagging clips every 500 mm to keep the lagging from slipping off runs that change direction or pass through joists. Where pipes penetrate external walls, fill the gap around the pipe with expanding foam filler after the lagging is in place — this blocks cold draughts that undermine even well-lagged pipes. If a pipe run is close to a boiler or hot water cylinder, check whether it is already warm enough not to need lagging, as over-insulating flow pipes near the boiler can marginally reduce efficiency. Cold-water supply pipes in unheated spaces — particularly in lofts — should always be fully lagged. If you are having persistent pressure or flow issues, it is worth checking how to fix low water pressure separately.
Check your work and test before winter
Walk every lagged run and look for uncovered sections, open seams, or gaps at joints. Press any lifting seams back down and add tape where needed. If the weather allows, run the heating system up to temperature and check that all lagged pipes feel warm on the outside — if a section stays cold, a valve or fitting may be restricting flow rather than the insulation being at fault. For any pipe that has already experienced a freeze, refer to our guide on how to fix a frozen pipe before relying solely on new lagging to prevent a repeat.
Frequently Asked Questions
What thickness of pipe lagging do I need?
For domestic cold-water supply pipes in unheated spaces, a wall thickness of at least 13 mm is recommended. For hot-water and central heating pipes where you want to retain heat, 25 mm foam lagging is more effective and is worth the small extra cost.
Do I need to insulate hot-water pipes as well as cold?
Can I insulate pipes myself or do I need a plumber?
Fitting foam pipe lagging to existing pipes is a straightforward DIY task requiring no specialist skills or tools. You do not need to turn off the water supply. Only call a professional if you need to move, extend, or alter pipe runs, or if any pipework is leaking.
How do I insulate pipes in a loft that are hard to reach?
Lay a board across the joists to give yourself a safe working platform — never step directly on the joists or plasterboard ceiling. Use a head torch, work systematically from one side to the other, and take measurements before cutting so you can prepare sections at floor level rather than cutting in an awkward position.
Will insulating pipes prevent them from bursting in very cold weather?
Foam lagging significantly reduces the risk of pipes freezing but is not a complete guarantee in prolonged sub-zero temperatures. The Energy Saving Trust recommends setting your heating to come on at a low temperature (around 10–15 °C) during very cold spells even when the property is unoccupied, as lagging combined with background heat is the most reliable protection.
When lagging pipes in a loft, also lag the cold-water storage tank itself using an insulation jacket — but leave the underside of the tank unlagged so rising warmth from below can help prevent it from freezing. Lagging the tank lid and all four sides while blocking the bottom is a common DIY error that actually increases freeze risk.
Sources
- Energy Saving Trust — Insulating your hot water tank, pipes and radiators — energysavingtrust.org.uk
- HSE — Cold stress and working in cold environments — hse.gov.uk
- Which? — How to insulate pipes and prevent them from freezing — which.co.uk
Safety Notice: Electrical and plumbing work can be dangerous if done incorrectly. In the UK, certain electrical work must comply with Part P of the Building Regulations and certain plumbing work with Part G. If in doubt, consult a qualified electrician (NICEIC/NAPIT registered) or plumber (CIPHE/WaterSafe registered). This guide is for general information only — it is not a substitute for professional advice.
