When the old tiles come off and the rafters stand open to the sky, the roof is as accessible as it will ever be. Fitting insulation at that point is straightforward, because nothing needs to be dismantled to reach it. That is the practical argument. The regulatory argument is that the roof is classed as a thermal element of the building, so when a large part of it is renewed, the rules expect the insulation to be brought up to a modern standard. Many owners are surprised by this, and some roofers skip over it. This page explains when the requirement applies, which insulation methods suit which roofs, and how to avoid the main hazard, which is damp from poorly ventilated timbers.
When re-roofing brings an insulation upgrade into scope
The Planning Portal sets out the test for England. A repair or re-cover that touches less than a quarter of the whole building envelope and under half of the roof can go ahead without a building control application. Go past either figure and an application is required, and the roof's insulation, all of it and not just the new part, may then have to be improved to meet the energy rules in Approved Document L. The official wording is on the Planning Portal.
In plain terms, small repairs and patch jobs sit below the line, and a full strip and re-cover of a roof sits above it. Since heat is lost through the roof, a renewal on that scale should include improving its insulation. Replacing a flat roof membrane, as opposed to laying a new layer over the old, is also generally treated as triggering the same upgrade, although local interpretations vary and your building control body will tell you what applies. Where the job is notifiable, the application and inspections are ours to arrange, and our guide to roof replacement building regulations covers the wider picture.
What insulation does, and what it can go wrong
Insulation slows heat flowing out through the roof. In a typical house a good share of the warmth from the heating escapes upward, so a poorly insulated roof is expensive to live under, and a well-insulated one makes the rooms below more stable in both winter and summer. The standard way to compare products is the U-value, a measure of how much heat passes through a layer; the lower the number, the better the performance, and your building control body will say what target applies.
The risk lies in what insulation does to the roof structure. Heat that used to leak into the roof space kept the timbers a little warmer and drier. Insulation cuts that supply, so the timbers sit colder, and any warm moist air from the house that reaches them can condense. A roof with insulation therefore needs two things: an air and vapour barrier below to stop moisture reaching the cold side, and ventilation or a design that keeps the structure dry. This is the single most important idea in the whole subject. Our guide to investigating damp and condensation in roofs shows what happens when it is ignored.
Pitched roofs: cold roof insulation at ceiling level
The most common arrangement is the cold roof. Insulation lies between and over the ceiling joists, and the loft above it stays cold and ventilated. When re-roofing, we can top up the insulation with the roof open, fit proper baffles at the eaves so that insulation does not block the ventilation gap, and seal gaps and penetrations in the ceiling below. It is straightforward, the materials are cheap and the timbers stay in the cold, ventilated zone where they are designed to live. The downside is that the loft space is no longer warm enough to use for living, and storage requires raised boards above the insulation.
Ventilation is the key detail. Air needs to enter at the eaves, pass over the insulation and leave at or near the ridge. Where a roof has no ridge vent, we can add tile vents or fit vented eaves trays and soffit vents. A new underlay, if it is a modern breathable type, also helps moisture pass out of the roof space. Our guide to inspecting your loft shows what to look for after the work to check it is performing.
Pitched roofs: warm roof insulation above the rafters
Where the loft space is used, or is to be used, the insulation goes in the roof itself, and the choice is between above, between and below the rafters. Fitting rigid insulation boards over the rafters, under the battens, creates a warm roof: the whole structure sits inside the insulated envelope, which keeps the timbers warm and dry and leaves the rafters visible if you want them. It needs the roof to be stripped, which is exactly what a re-roof involves, and it raises the roof height slightly because of the board thickness. This affects the eaves and verge details, the flashings and the junctions with neighbouring roofs, and needs longer fixings through the boards. The covering has to be chosen and fixed for the extra depth.
Insulation between the rafters is another route. It keeps the roof height unchanged but is limited by the depth of the rafters, so achieving a modern standard often requires an additional layer of board below or above. If the roof is insulated between the rafters, the design must either leave a clear ventilated gap above the insulation or use a breathable membrane system that has been designed for an unventilated build-up. This is a point at which workmanship matters, because gaps and compressed insulation reduce performance and can let moisture in.
Spray foam: a different case
Foam sprayed onto the underside of the roof sheathes the rafters and felt in a single skin, which changes how the whole roof behaves. It has been widely sold, and it has caused problems. Consumer guidance from RICS, dated March 2023, warns that poor installation can lead to uncontrolled condensation, that a rafter may seem dry where you can see it while the face behind the foam stays wet, and that the foam can make the roof hard to inspect. A number of lenders have turned down applications on houses that have it, though their positions vary and change. If your roof already has spray foam, the usual fix is removal and a ventilation check, and our spray foam insulation pages explain the options. We would not recommend adding it as part of a re-roof.
Flat roofs: warm deck is the usual answer
On a flat roof the standard method is the warm roof, also called warm deck. Rigid insulation boards go over the deck, under the waterproof membrane, with a vapour control layer beneath to stop warm moist air reaching the cold deck. The deck and timbers stay on the warm side of the insulation, so condensation is much less likely. The boards also help create the falls, since tapered insulation can be used to slope the roof towards the outlets. A cold-deck flat roof, with insulation between the joists and a ventilated gap above, is possible but harder to detail well and easier to get wrong, so we tend to avoid it. Our companion pages on flat roofing in Basingstoke explain how the membranes fit above.
Materials in brief
Mineral wool, in rolls or slabs, is the usual choice for ceiling-level insulation and between rafters. Rigid boards, such as PIR (polyisocyanurate), give high performance for their thickness and suit warm roofs and flat decks. Wood fibre boards offer a natural, breathable option with good summer performance. Each has its own fire, moisture and cost behaviour, and the building control body will want to see that the product and the thickness meet the target. We specify the product to suit the roof, not the other way round.
Detailing that decides the result
A well-chosen insulation can be undone by poor detail. Gaps at the eaves, where insulation stops short of the wall plate, create cold bridges. Recessed downlights cut holes through the ceiling and leak warm air upward. An unsealed loft hatch acts like a chimney for moisture. Services such as pipes and cables that pass through the ceiling need sealing. Chimney breasts need their own treatment. And at the junction between walls and roof, insulation needs to meet wall insulation so that there is no break in the layer. These are the places where the job is won or lost, and where an inspection after the work is worth the effort.
Planning the insulation before the scaffold goes up
Because the insulation affects the roof's height, its detailing and its price, decide on it at the quote stage and not halfway through the job. Tell the roofer whether the loft is likely to be converted one day, because a warm roof or insulation between rafters keeps that option open, whereas a deep layer of quilt at ceiling level closes it off. Mention any plans for solar panels or a roof window, which both interact with the build-up. Ask what the finished eaves and verge will look like if boards are added, as the fascia line may need adjusting to suit.
It is also worth checking the ceiling below before the roof is touched. If plasterboard is stained or sagging, or if there are old downlights, it is easier to deal with them while the roof is being worked on. Good access to the loft makes everything simpler, and a new insulated hatch with a seal costs little. A job done as a sequence, ceiling sealed, insulation laid, ventilation confirmed, covering fixed, leaves nothing hidden that will need to be revisited.
Cost, planning and getting it signed off
Adding insulation during a re-roof adds to the materials bill, but the labour is small because the roof is open. The saving on heating is ongoing. Your quote should show the insulation as a line, with the product and thickness named, so you can compare it fairly with others; see our guide to how to compare roof replacement quotes. If the work is notifiable, the building control body inspects and issues a completion certificate, and our answer on whether you get a certificate for a new roof says what to expect. Raising the roof slightly with warm roof boards may need a planning check in some cases, so put the question to your council's planning team when the property is listed or sits in a conservation area.
We cover this in the survey: what is up there now, what the rules mean for your roof, which method suits it and what each option adds to the price. Where the work is notifiable, we handle the paperwork. Details of the service are on our roof replacement in Basingstoke page; for a survey, call 01256 213956 or use WhatsApp.
