Cold roof vs warm roof: where should loft insulation go?

Cold roof: insulating the loft floor

This is what most people mean by loft insulation. Mineral wool goes between the joists and a second layer across them, to 270mm, and the roof space above stays cold and ventilated. The Energy Saving Trust (EST) describes it as the easiest loft to insulate if access is good and there is no damp, and as something many people can do themselves. It is also more efficient than insulating the roof, because you are not heating the air in the loft before it reaches the insulation.

The consequences of a cold loft are practical rather than serious: water tanks and pipes up there need lagging so they do not freeze, and storage needs a raised floor so the insulation is not squashed.

Warm roof: insulating the rafters

The alternative is fitting rigid insulation boards between and over the sloping rafters. The EST sets out the trade-off clearly:

The EST’s warning about the “other work” row is worth repeating: if the gable walls, party walls and chimneys in the loft are left uninsulated, heat bypasses the new rafter insulation and it underperforms.

Room-in-roof and loft conversions

Rooms with sloping ceilings can be insulated, which keeps heat in during winter and stops them overheating in summer. The EST says this is a job for trained installers, who often remove plasterboard to fit insulation between and over the rafters and on any vertical walls. Flat sections of ceiling can take ordinary rolls, and dormer windows need insulating and ventilating too.

For the funded schemes, room-in-roof is its own category. Ofgem’s ECO4 guidance lists roof insulation as flat roof, pitched roof and room-in-roof, and requires the roof to be insulated before poorly rated homes can receive heating measures. In Scotland, the Home Energy Scotland Grant and Loan offers up to £3,000 for flat-roof or room-in-roof insulation, twice the loft grant.

Flat roofs

A flat roof has no loft to lay insulation in. The EST says flat roofs need professional installation, and the work is done from above or below by a specialist. They are outside the simple cold-loft job that most “free loft insulation” offers describe.

Where spray foam fits

Spray foam is one way of insulating at rafter level, and the EST describes closed-cell foam sprayed under an old, poorly maintained roof as one of the highest-risk insulation options. If a warm roof is what you need, rigid boards fitted by a competent installer, with the roof checked and ventilation planned, are the conventional route. Read spray foam insulation problems before agreeing to foam.

Which should you choose?

If you do not use the loft as a room and can reach it, a cold roof is cheaper, more efficient and simpler, and it is what the schemes and the EST figures assume. If you need a warm roof space, or the loft is already a room, budget for specialist work and ask about room-in-roof funding first. Either way, the ventilation guide explains how to keep the roof timbers dry.