How do I insulate pipes and a tank in a loft?

This is the classic unintended consequence of insulating a loft. The insulation works, the loft gets colder, and the first hard frost afterwards bursts a pipe that had survived for decades. The job is not difficult, but one part of it is genuinely counter-intuitive and gets done wrong constantly.

Safety

Step by step

1. Lag every pipe, not just the long straight runs.Freezing starts at bends, valves, tees and stop taps, because those are the fiddly places lagging gets left off. A pipe insulated for ninety per cent of its length can still freeze at the uninsulated tee.
2. Butt the lagging tightly and tape the joints.Gaps at the joins in the foam are cold spots. Cut mitres at bends so the insulation is continuous rather than leaving the elbow bare.
3. Insulate the sides and top of a cold water tank.A proper jacket, or boards cut to fit, all the way round and over the top. Include the lid, and make sure it still lifts.
4. Do not insulate the ceiling underneath the tank.This is the counter-intuitive part. Leaving that patch of ceiling uninsulated lets a little warmth from the room below reach the tank, which is what stops it freezing. Insulating under it removes the tank's only heat source.
5. Keep the tank covered but ventilated as designed.A lid keeps dust, insects and debris out of stored water. It should be the tank's own lid or a proper replacement, not an improvised sheet.
6. Insulate any expansion or feed pipes to the tank as well.These are small-bore and freeze first. The overflow or warning pipe running out through the wall is particularly exposed.
7. Check for pipes buried in the insulation.A pipe lying under loft quilt is insulated from the house's warmth rather than protected from the cold. Lift it clear and lag it, or insulate above it.
8. Look at the whole loft after insulating the ceiling.Anything up there that dislikes being cold — pipework, a tank, a boiler expansion vessel — is now in a colder space than before, and should be reviewed as a set.

Common mistakes

• Insulating the ceiling under the cold water tank. — It cuts off the small amount of warmth that keeps the tank above freezing, and the tank is the thing most likely to freeze and cause serious water damage.
• Lagging the straight runs only. — The unlagged valve or elbow becomes the coldest point on the pipe, so that is where it freezes and splits.

Questions people ask