Should you add loft insulation when re-roofing?

In a cold roof, insulation lies on the ceiling joists and the loft above stays at outdoor temperature. The ceiling stops heat escaping, and the loft is ventilated to carry off moist air. It is the traditional arrangement in British houses, and it is the cheaper and simpler option when nobody uses the loft as a room. The insulation can be topped up in layers, running across the joists at right angles to the first, to build up the thickness.

In a warm roof, insulation is fitted along the slope, between or under the rafters, and the loft space sits inside the heated envelope. It suits a loft conversion or a house with tanks and pipework in the loft that must stay frost-free. It costs more and needs more care at the eaves, ridge and gable ends. Either way, the choice is much easier to make with the covering off, when the rafters and joists are open to view and nothing has to be lifted or cut through.

Approved Document L, the Building Regulations guidance on energy, links roof work to insulation in two ways. When a roof is renovated as a whole, Part L says the whole element should be brought to 0.16 W/m²K once the work covers more than 50% of its surface, or amounts to a major renovation, which it defines as more than 25% of the external envelope. Stripping a roof down to the rafters and rebuilding it counts as renovation. A wholly new roof structure faces the tighter 0.15 W/m²K in Table 4.2.

That sounds technical, so here is the practical reading. A full re-roof usually triggers a thermal upgrade, and the building control notification is handled for you where it applies. Part L itself accepts a lesser standard where headroom is limited, and it expects the insulation and any air space together to match the rafter depth. Our survey tells you which route your roof falls under before the quote.

Warm, damp air from kitchens, showers and breathing rises into the loft. If it meets cold timber and has nowhere to go, it condenses, and the water soaks battens, rafters and felt. Over some winters that leads to mould and rot. The standard BS 5250 deals with moisture in buildings, and Marley's guidance, which draws on it, shows how eaves ventilation of 10 mm along the eaves is the starting point for pitches above 15 degrees with non-breathable underlay, with additional high-level ventilation on steep or long roofs.

Insulation makes this matter more, not less. Packed over the eaves, it blocks the flow of air from the soffit, so a baffle or vent tray is fitted at each rafter bay to keep a clear channel. Our page on fascias and gutters during a re-roof explains how the eaves are detailed, including vented soffits. Never stuff the eaves to hit a target thickness. A slightly thinner layer with air moving above it does more good than a thick one that traps damp against the timbers, and the thickness can be made up elsewhere, for example by laying a second layer across the joists.

Foam sprayed under the slope is sometimes pitched as a quick fix. The Energy Saving Trust describes firms offering to cure a leaking roof by spraying foam beneath it while leaving the fault untouched, and says it does not recommend that. It adds that timbers may rot where the roof leaks or condensation forms, a risk it says is lower with open-cell foam.

The RICS consumer guide suggests asking your mortgage provider, then your home insurer, if the policy permits it, and says its presence can affect a surveyor's valuation opinion. Reports in November 2024 named several lenders that were declining applications for houses with spray foam, while others looked at each case individually. Foam also hides the condition of the timbers behind it, which is one reason a roofer wants to see them before and after a new covering. If foam is already there, our page on replacing roof timbers notes the extra inspection that follows.

Insulation, underlay, battens and tiles all sit in a stack, and a change to one affects the rest. A thicker insulation layer at rafter level may raise the roof line slightly or need deeper rafters. If you reuse tiles, the batten gauge and the finished profile must still suit them, which our page on reusing roof tiles explains. Solar panels fitted later need to be planned with the structure, too.

Tell us at the survey whether you plan a loft conversion, a boarded floor for storage or new recessed lights in the ceiling below, as each changes the answer. Downlights, extractor fans and cable runs all need sealing or protecting before insulation goes over them, and they are far easier to deal with while the ceiling is accessible from above. Photographs of the loft with the enquiry form help us look at the likeliest problems first. The written quote follows the survey, and our workmanship carries a 10-year guarantee in writing. The rest of the process is on the roof replacement page.