Loft insulation: is topping up worth it?
If your loft was done before about 2003 it is probably at 100mm. Going to 270mm is the cheapest useful thing most houses can do, and the second half costs a fraction of the first.
How much you have now
Put your head through the hatch and look at the joists. Ceiling joists are typically around 100mm deep. If the insulation is level with the top of them, you have roughly 100mm. If it disappears well above them, you are closer to current standards.
100mm was the building regulation standard for decades. The recommendation now is 270mm of mineral wool.
What the difference is worth
Insulating a loft that has nothing in it to the full 270mm saves more than any other single measure of its price, and it is the cheapest work on this site. Topping up from 100mm saves less, because the first 100mm has already done most of the work — but it is also much cheaper to do.
Why the top-up still makes sense. The first 100mm does most of the work — that is simply how insulation behaves. But laying the second layer is cheap, because the access, the labour and the visit are the same. You are paying mostly for material.
The three things that go wrong
1. The eaves get blocked
Insulation pushed hard into the eaves stops the roof void breathing. The result is condensation on the underside of the roof, and eventually damp timbers. There should be a clear airway at the edge, and it is the single most common defect we find in lofts that were done cheaply.
2. The hatch and the pipes are left
An uninsulated loft hatch undoes a surprising amount of a well-insulated loft, and pipes and tanks that used to sit in a relatively warm loft are now sitting in a cold one. They need lagging at the same time, or you have swapped a heat loss problem for a freezing problem.
3. It gets laid in one direction
The first layer sits between the joists. The second goes across them, at right angles, so it covers the timber as well. Joists conduct heat straight through a layer that only sits between them.
When you should not do it
- If the loft is a room, or is going to be. The insulation goes at the roof slope instead, which is a different and more expensive job.
- If there is already damp up there. Find out why first. Insulating over a moisture problem hides it and makes it worse.
- If you store things up there and would compress it. Insulation only works because of the air trapped in it. Flattened insulation is not insulation. Boarding needs raising above the new depth.
VAT and funding
Insulation is zero-rated for VAT until 31 March 2027, so the figure on the quotation is the figure you pay.
It is also the measure that funded schemes are built around. Whether you qualify depends on the household and the property rather than on us, and it is worth checking before you pay for it privately.
Does it have to come first?
For a heat pump, no — not for the grant. The Boiler Upgrade Scheme dropped its loft and cavity insulation condition in 2024. But a better insulated house needs a smaller heat pump, which costs less to buy and less to run for its whole life, so the order still matters even though the rule has gone.
Sources. VAT and Boiler Upgrade Scheme positions as published by the UK government at the date shown above.