If you do only one thing to a draughty, expensive house, make it the loft. Warm air rises, and in a home with little or no insulation above the ceiling a great deal of the heat you have paid for drifts straight up and out through the roof. Of every improvement available, topping up the loft returns the most warmth and the most money for the least outlay and effort, which is why every sensible list of energy jobs starts here.
Why the loft comes first
Around a quarter of the heat lost from an uninsulated house escapes through the roof, more than through any single wall and far more than through the windows everyone frets about. That alone would make it worth doing, but the loft has a second advantage that the walls and windows do not: it is cheap, it needs no specialist for a standard accessible loft, and it is quick. A roll of mineral wool, a craft knife and an afternoon is genuinely all a typical job takes. High heat loss meeting low cost is exactly the combination that makes something the best value in the house, and the loft is the clearest example of it.
Depth is what counts
The single number that matters is depth, because insulation works by trapping still air and the more thickness you have, the more it traps. Current guidance points to roughly 270mm of mineral wool, which is deeper than people expect, about the height of a house brick stood on end plus a bit. A great many lofts fall well short of this. Some have nothing at all; many have a thin layer laid decades ago that has since settled, compressed and lost much of its effect. A quick test is to look across the loft: if the tops of the joists are poking out above the insulation, you have nowhere near enough, since the joists themselves are usually only around 100mm deep.
Topping a thin existing layer up to full depth is one of the most satisfying jobs going, because the difference is immediate. You lay the first layer between the joists and a second layer across them at right angles, which covers the joists themselves and stops them acting as cold bridges. Within a day or two of cold weather you notice the house holding its warmth for longer after the heating goes off, which is the whole point.
Doing it yourself
For an accessible loft this is firmly a job most people can do. Wear a mask, long sleeves and gloves, since mineral wool irritates skin and lungs, and work off boards laid across the joists rather than treading between them, where a foot can go through the ceiling below. Roll the first layer snugly between the joists without crushing it, then lay the second layer across the top. Cut it slightly oversize so it fills the space without gaps, as gaps are where the heat finds its way out. Take care around recessed downlights and any other heat source, leaving the clearance the fitting requires, and do not cover modern light fittings unless they are rated to be covered.
Boarding for storage without ruining it
The common mistake is to lay loft boards straight onto the joists for storage. Because full-depth insulation is far deeper than the joists, boarding directly on top squashes it flat, and crushed insulation loses much of its value, so you end up with a tidy storage deck and a cold house. The fix is raised loft legs, sometimes called boarding stilts, plastic or metal supports that lift the boards above the full depth of insulation and leave it uncompressed underneath. They cost a little more than boards alone and turn what would be a self-defeating job into one that keeps both the storage and the saving.
Ventilation and damp
A cold loft needs to breathe, and this is the detail that catches people out. The space above the insulation should stay ventilated through the gaps at the eaves, so that any moisture rising from the house below can escape rather than condensing on the cold roof timbers. Stuff insulation hard into the eaves and block that airflow and you risk trapping damp, which leads to mould and rot in the roof structure. Leave the eaves gaps clear, using eaves vents or baffles if needed to hold the insulation back from the very edge.
One more placement point: insulate the loft floor, above the heated rooms, not the underside of the roof, unless you are deliberately creating a warm loft room. And do not insulate beneath the cold-water tank if you have one, since you want a little of the warmth from below to keep reaching it so it does not freeze in a hard winter. Insulate around and over the sides of the tank instead, and fit it an insulating jacket of its own.
What it saves
For a house going from little or no loft insulation up to full depth, this is typically one of the largest single savings on the heating bill available from any one measure, and because the materials are cheap the cost is usually repaid within a small number of winters and then keeps saving for the life of the house. Topping up an already-decent layer saves less, since the first inches do most of the work, but going from nothing to 270mm is transformative. Pair it with sealing the draughts and turning down the boiler flow temperature and you have done the three cheapest, highest-value heating jobs there are before spending serious money on anything.