Why the loft is the easiest place to start

If your rooms feel chilly even when the heating has been running for hours, there is a good chance you are warming the sky. In a typical uninsulated home, roughly a quarter of the heat you pay for escapes through the roof. The loft is usually the simplest place to fix that. The space is accessible, the work is not disruptive, and a competent DIYer can often finish the job over a weekend.

The current benchmark for a loft floor is a total insulation depth of about 270mm. Many UK homes still have the 100mm quilt that was laid decades ago, or nothing at all between the joists. That means there is a genuinely worthwhile improvement sitting directly above your ceiling, and you will notice the difference most in the rooms immediately below.

Check your joists before you buy anything

Loft insulation depth is dictated by what is already up there. Take a torch, a tape measure and a notepad into the loft and note three things.

• Joist depth. Ceiling joists in older homes are commonly 100mm deep, which is far less than the 270mm you are aiming for. You cannot squeeze 270mm of quilt into a 100mm gap without compressing it, and compressed insulation performs noticeably worse.
• Joist spacing. Look for 400mm or 600mm centres, measured from the centre of one joist to the centre of the next. This determines how many rolls you need, and it also tells you whether standard loft-leg stilts will line up if you plan to board the area afterwards.
• Existing insulation and its condition. If the old quilt is damp, filthy or flattened, it is worth removing rather than building on top of it.

Also check for anything that penetrates the ceiling — cables, pipework, downlights, a chimney breast — as these all affect how you lay the material.

Cross-lay to reach the full depth

The trick to hitting 270mm without squashing anything is to work in two layers. Fill the gaps between the joists with the first layer, bringing it level with the top of the timber. Then lay the second layer at right angles to the joists, running across them. This cross-lay pattern covers the joists themselves and breaks the thermal bridge that would otherwise let heat travel up through the timber and away.

Mineral wool is the traditional choice and remains excellent value. Rigid boards perform well where space is tight, and natural options such as sheep wool or wood fibre suit older, breathable roofs. Whatever you choose, keep the layers loose and full. Never compress insulation to make it fit, and never leave gaps or folds that let air move through.

Never block the eaves

This is the mistake that causes the most trouble. Roofs need to breathe. Air must be able to move in at the eaves and out at the ridge or high-level vents, carrying moisture away with it. If you push insulation right into the gap where the rafters meet the wall plate, you block that airflow, and the result is condensation on the underside of the roof covering, damp timbers and, eventually, rot.

Keep a clear gap of about 50mm between the top of the insulation and the underside of the roof covering at the eaves. If the space is tight, use eaves ventilation trays — rigid plastic baffles that hold the insulation back while still allowing air to pass. Check that the soffit vents along the outside edge are not painted over or blocked, and clear any debris that has collected around them.

Watch the cables, lights and tanks

A buried cable gets hotter than one in open air, so electrical cable ratings assume a certain amount of ventilation. Lighting circuits running through the loft are especially prone to overheating once covered. Where you can, lay cables over the top of the insulation rather than under it, and keep them out of the deepest layers.

• Recessed downlights. Leave the clearance the manufacturer specifies around and above each fitting, and fit insulation hoods or caps rated for the purpose if you need to cover them.
• Water tanks and pipework. Insulate the tank itself on the top and sides but not underneath, and leave the floor beneath it free of quilt so a little warmth can rise. Lag every pipe in the loft, because a properly insulated loft is a colder loft and unlagged pipes are more likely to freeze.
• Chimneys and flues. Never box a flue in with insulation. Leave a clear air gap around it.

Doing the job safely and what it costs

Wear gloves, a dust mask and long sleeves — mineral wool irritates skin and eyes. Work from boards laid across the joists rather than stepping directly onto them, and never put weight on the ceiling below. If you want to keep storage space, fit proprietary loft legs to raise boarding above the insulation, so you retain the full depth without squashing it.

For a typical three-bedroom semi-detached house, topping up from 100mm to 270mm is well within reach of a DIY budget, and the payback period is generally short. A well-insulated loft should also make the house feel more comfortable: warmer in winter, less prone to draughts, and cheaper to run. If you would rather not tackle it yourself, a local insulation installer can advise on depth, ventilation and any grant schemes currently available through energy suppliers.

Take an afternoon to look at what is above your head. It is one of the few home improvements that pays you back every winter.