Why Depth Matters More Than Most People Think

Loft insulation works by trapping still air, and the more of it you have, the slower heat escapes through your roof. Depth is therefore the single biggest lever you can pull in a loft — far more important than the brand on the packaging or whether you choose rolls or slabs. In a typical uninsulated UK home, the roof accounts for roughly a quarter of all heat loss, which is why topping up an old 100mm layer is one of the most cost-effective improvements available.

That said, the returns are not linear. The first 100mm does the heavy lifting; going from 100mm to 270mm delivers a real but smaller improvement; pushing beyond 300mm in a typical pitched roof rarely justifies the expense. The goal is to reach the point where adding more wool saves so little that the money is better spent elsewhere — usually on the loft hatch, draughtproofing or wall insulation.

Measure What You Already Have

Before you buy anything, get up into the loft with a torch and a tape measure. You need four numbers: the depth of the joists, the spacing between them, the depth of any existing insulation, and the distance from the top of the joists to the underside of the roof covering.

• Joist depth: commonly 100mm in older homes, occasionally 75mm, and 150mm or more in newer builds.
• Joist spacing: usually 400mm or 600mm centres, which affects how rolls are laid.
• Existing insulation: check whether it is mineral wool, a rigid board, or loose fill, and whether it is damp or compressed.
• Headroom and access: a tight loft with a low ridge makes rigid boards easier to handle than bulky rolls.

If your loft already has 250mm or more of clean, dry, evenly laid insulation, you are close to the point of diminishing returns and should look at air sealing instead.

The Depths Worth Aiming For

Current UK guidance for a loft insulated at joist level is a minimum of 270mm of mineral wool, with many installers now specifying 300mm to allow for settling. This figure comes from the energy efficiency requirements in the Building Regulations, which set a target U-value of around 0.16 W/m²K for a renovated roof element. If you are insulating from scratch or your existing layer is under 100mm, aim to reach the full depth in one go.

Because most joists are only 100mm deep, reaching 270mm means two layers. The standard approach is to lay the first layer between the joists, filling the full depth, then cross-lay a second layer at right angles over the top of the joists. Cross-laying covers the timber itself, which otherwise acts as a thermal bridge, and it hides the gaps between the first layer.

Resist the temptation to squash thick insulation into a shallow space. Compressing mineral wool reduces the air it traps and lowers its performance — 200mm crammed into a 100mm gap behaves like 100mm.

Choosing a Material That Suits Your Joists

The right material depends on how much height you have and what you use the loft for.

• Mineral wool rolls: the cheapest option per square metre, easy to cut and lay yourself, and ideal for flat joist-level work.
• Sheep's wool and other natural fibres: pleasant to handle and good at managing moisture, but noticeably more expensive.
• Rigid PIR boards: roughly twice the insulation value per millimetre, so useful where height is tight, but they must be cut precisely and foamed into gaps to work well.
• Loose-fill or blown insulation: excellent for awkward shapes, trusses and uneven joist runs, though it is usually a professional job.

If you want to keep using the loft for storage, do not simply board over the joists and crush the insulation. Fit raised loft legs or a proper platform so the decking sits above a full-depth layer, leaving the insulation undisturbed underneath.

Ventilation, Cables and the Details People Miss

Good insulation without good ventilation causes damp. At the eaves, a continuous airflow must be maintained between the insulation and the underside of the roof covering. Use eaves vent trays or baffles, and never push wool into the gap above the wall plate — blocked eaves lead to condensation on the roof deck, damp timbers and, eventually, rot.

Services deserve equal care. Electrical cables buried under insulation cannot shed heat, so their current-carrying capacity drops sharply — in some cases by around half. Where possible, keep cables above the insulation or reroute them. Recessed downlights need fire hoods and clearance, and any pipework or cold water tanks should be lagged, with insulation wrapped around the sides and over the top of tanks but not beneath them if the loft floor is warm.

Finally, deal with the loft hatch. An ill-fitting, uninsulated hatch can leak more heat than a poorly insulated section of roof, so draughtproof the frame and fix an insulated cap or board to the back of the hatch itself.

Costs, Payback and Order of Work

For a typical 50m² loft, materials for a DIY mineral wool top-up often come to somewhere between £250 and £500, while professional installation generally lands in the £400–£800 range depending on access and existing insulation. Payback is commonly two to five years, and government-backed schemes occasionally cover part or all of the cost for eligible households, so it is worth checking before you commit.

If you are working to a budget, do things in this order: seal and insulate the loft hatch, clear and protect the eaves ventilation, sort out cables and downlights, then lay insulation to full depth. That sequence gives you the biggest improvement in comfort and bills for the least outlay — and leaves the loft ready for a future top-up if you ever decide you want one.