Insulating a roof means four different jobs depending on where the insulation goes. At ceiling level in a cold loft, between and over the rafters in a warm pitched roof, above the deck on a flat roof, or wrapped around a room in the roof. They have different costs, different rules and different failure modes, and the most expensive mistakes come from treating them as one decision.
Cold loft: the lowest cost of roof insulation, and still the best value
Mineral or glass wool laid at ceiling level, between and then across the joists, is the least expensive insulation in a house and usually the largest single saving. The practical constraints are that the loft becomes colder, so pipes and tanks up there need lagging, and that boarding over the top without a raised platform squashes the insulation and undoes much of the benefit. Ventilation at the eaves has to stay clear.
Warm pitched roof insulation: when the loft is a room
If you want the roof space usable, the insulation line moves to the rafters. Done properly that usually means insulation between the rafters plus a continuous layer over or under them to deal with the cold bridge at every rafter, with attention to where moisture can condense inside the build up. Rafter depth is the usual constraint in an older house, and the honest answer is often that the rafters need battening out.
Flat roof insulation requirements: what the regulations expect
On a flat roof the modern default is a warm deck, with the insulation above the structural deck and below the waterproofing, keeping the deck warm and the dew point outside it. When you renovate a thermal element, Approved Document L sets an expectation for the improved performance, which is why a flat roof recover is the moment to get the insulation right rather than a separate project later.
Spray foam needs care, not enthusiasm
Spray foam applied to the underside of a pitched roof has caused real problems in the British housing market, because some installations sealed the roof void, trapped moisture against the timbers, and made the roof structure impossible to inspect. Several lenders and surveyors treat it as a defect, which can affect a sale or a remortgage. If you are considering it, get an independent survey of the roof structure and ventilation first and understand the position on resale.
Conservatory roof insulation cost, and whether a shed is worth insulating
Insulating a conservatory roof is usually part of replacing it, either with an insulated solid roof or an internal insulated lining, and the gain depends as much on the glazed walls as on the roof. A shed is a different calculation: insulation helps if the building is used and heated, and does very little for a store, so it is worth deciding what the building is for before spending on it.
Grants and who does the work
Government backed schemes for insulation come and go and the eligibility rules change, so check the current position rather than an old article. Where a scheme is involved it will normally require a registered installer and a recognised standard of work, and quality assurance bodies exist precisely because retrofit insulation done badly causes damp. Get the paperwork, whoever pays.
Questions people ask about roof insulation
How much loft insulation is enough?
Current guidance points at a substantially deeper layer than most older houses have, and topping up an existing thin layer is usually straightforward. Check the depth you have before buying anything, and keep the eaves ventilation path clear when you add to it.
Is spray foam insulation a problem when selling a house?
It can be. Some lenders and surveyors treat certain spray foam installations as a defect because they prevent inspection of the roof timbers and can trap moisture. Independent advice before installing, and documentation if it is already there, both help.
Can I insulate a flat roof from underneath?
It is possible and it is usually the worse option, because it leaves the deck cold and moves the condensation risk into the structure. Where the roof is being recovered anyway, insulating above the deck is the better answer.
Does insulating a room in the roof need approval?
Insulation work as part of a conversion or a renovation of a thermal element generally falls under the building regulations, and a loft conversion certainly does. Ask the contractor which route applies and who is certifying it.
Is fibreglass loft insulation still a reasonable choice?
For a cold loft it remains the standard and the cheapest effective option. Rigid boards earn their price where depth is limited, such as between rafters, and the two are often used together on the same house.