Everything about a loft's insulation comes down to one choice: do you put the insulation on the floor and leave the roof cold, or do you bring it up into the slope of the roof and make the loft warm? Both approaches work when they are built properly. Both fail in recognisable ways when they are not. This guide compares them side by side, in plain terms, covering how each one moves heat and moisture, which suits which house, and what to think about if you have inherited a warm roof that someone else installed.
Do not confuse this with the flat roof question, where the same words mean something different; that is covered in our guide to warm roof versus cold roof construction.
What a cold loft is
A cold loft is the traditional arrangement. Insulation, usually mineral wool quilt laid in two layers, lies between and over the ceiling joists, so the heat stays in the rooms below. The loft above is outside the insulated shell, and so it runs close to the temperature of the air outdoors. Cold air enters at the eaves through vents and leaves at the ridge or through high-level tile vents, passing across the underside of the tiles and sweeping out any moisture that has drifted up from the house.
The roof structure stays cold and dry, the timbers are easy to inspect, and the system is cheap to build or top up. The downsides are that the loft is not a usable room, pipes and tanks in the cold space need lagging against frost, and loft hatches and light fittings leak heat unless they are sealed. Thickness also matters. Current guidance for existing homes expects a good depth of mineral wool, and many older lofts have far less than that.
What a warm loft is
A warm loft moves the insulation up to the pitch of the roof. Boards, quilt or foam sit between and under the rafters, so the whole roof space becomes part of the heated building. Pipes and tanks stay frost free, the roof space is usable, and there is no need for a thick layer on the floor that makes storage awkward. It is the standard approach for loft conversions, where rooms need to be warm, and it can be used in an unconverted loft that someone wants to keep as a store.
The price of a warm roof is detail. A warm roof must be built so that moisture cannot condense inside the structure. Either there is an air gap above the insulation, with ventilation to carry the damp away, or the roof is built as an unventilated warm roof with a vapour control layer on the warm side and the right membrane above. Both are legitimate, but both need to be designed. That is why spray foam, which makes an unventilated warm roof with no design at all, causes so much concern.
Side-by-side comparison
| Question | Cold loft | Warm loft |
|---|---|---|
| Where is the insulation? | On the loft floor, between and over joists | In the roof slope, between and under rafters |
| Is the loft heated? | No, it runs close to outdoor temperature | Yes, it is inside the insulated shell |
| How does moisture leave? | Through eaves and ridge ventilation | Through an air gap, or is stopped at a vapour layer |
| Can the rafters be inspected? | Easily, with a torch | Often not, once boards or foam cover them |
| Best suited to | Unused lofts and low budgets | Loft rooms, and lofts with services |
| Main risk if done badly | Blocked eaves, cold bridges, frozen pipes | Condensation in the structure |
Which one suits which house
For a plain semi in Basingstoke with a loft that nobody uses, a cold loft with a good depth of insulation and clear ventilation is simple, cheap and easy to monitor. For a loft being turned into a bedroom or an office, a warm roof is the only sensible choice, and the work will need building control involvement, because the Building Regulations cover structure, escape and energy performance when a loft becomes a room. A protected stair and means of escape are matters for your building control body or architect.
Older houses with solid sarking boards need extra care. Boarded roofs behave differently from modern roofs with a membrane, and sealing them from the inside can leave no route for moisture to escape. In those houses, we generally lean towards a cold roof, or a warm roof designed with a ventilated gap above the insulation. A listed or conservation-area house calls for caution as well, since changes to the roof may need consent from the council's conservation officer.
Whichever you choose, the roof covering matters. A warm roof makes the underlay and battens the first line of defence for the timbers, so a tired underlay is a reason to re-cover at the same time, and a cold roof does not mind it as much. If you are thinking about re-roofing, the two jobs can share a scaffold.
Why spray foam is a different case
Spray foam is one method of making a warm roof, but it skips the design that a proper warm roof needs. The RICS consumer guide on foam explains that it converts a cold roof into a warm roof, and it warns that poor installation can cause uncontrolled condensation. Foam applied hard to the underlay removes any air gap, and it may bury timber that was not dry to begin with. Our guide to spray foam insulation problems goes through what this looks like in practice, and spray foam alternatives shows other ways of reaching a warm roof without those risks.
Moving from a foamed roof back to something else
Owners who find themselves with a foamed warm roof they do not want have two directions to go. One is to return to a cold loft: remove the foam, lay insulation on the floor, fit a breathable underlay and restore ventilation. The other is to rebuild a proper warm roof with boards and an air gap or vapour layer, which suits a loft that is being used. Both start with the same step, which is to find out how the timbers are. Our page on insulating after spray foam removal compares the routes, and the page on spray foam removal explains how the foam comes off.
Regulations and the energy angle
Insulating a loft floor is normally not notifiable on its own. A larger re-cover of the roof can be, and once the work affects enough of the roof, the insulation of the whole roof may need upgrading under Approved Document L, because the roof counts as a thermal element. If the new covering is heavier than the old by a set margin, that also needs approval, and the roof structure may need strengthening. Your building control body, whether the council or a registered building control approver, confirms what applies, and we handle that side where work is notifiable.
On energy, both arrangements can reach a good standard. A cold loft with a deep layer on the floor loses little heat through the ceiling, and a well-built warm roof does the same through the slope. What matters more than the label is the depth of insulation, the continuity at the edges, and how well air leaks are sealed. Gaps at the eaves, loose hatches and unsealed downlights undo a lot of good insulation in either design.
Building a cold loft well
A cold loft looks easy, and many are done poorly. The first layer of quilt goes between the joists, running right to the wall plate but stopping short of the eaves ventilation, so that a gap stays open for air to enter. A ventilation tray, a rigid plastic channel pushed into the eaves, holds the quilt back from the underlay. The second layer runs across the joists, which covers the timber and removes the thermal bridge where heat would otherwise escape through each joist. Pipework in the loft gets insulation of its own on top of the quilt, with the cold tank insulated on its sides and lid but left uncovered underneath so heat from the ceiling stops it freezing. Downlights need covers rated for insulation contact, and the hatch gets a draught seal and an insulated lid. Extractor ducts must run to the outside and not end in the loft.
Building a warm roof well
A warm roof demands more drawing before any work starts. In a ventilated design, a clear air gap is kept above the insulation, with a breathable underlay over the rafters and vents at the eaves and ridge. Rigid boards fit tightly between the rafters, with a further layer under them to cover the timber and stop the cold bridges. A vapour control layer goes on the warm side, with its joints taped and its edges sealed at the walls and around any penetrations. In an unventilated design, the layers are chosen so that condensation cannot form where the temperature drops, which usually means a proportion of the insulation sits above the rafters. Either way, someone has to calculate the build-up, and building control will want to see it. The detail at the eaves and ridge decides whether the whole thing works. Careless sealing there is where most warm roofs let moist air in.
Costs, and what to ask before you commit
We do not have a sourced price for loft insulation work, and quoting one would be guesswork, so use a local figure from a survey of your own loft. Instead, think about what makes the bill vary: floor area and roof area, the depth and type of insulation chosen, access to the loft, whether the roof is being re-covered at the same time, and whether the loft is being converted into a room. Ask any firm for a written specification that names the product, the thickness and the ventilation arrangement. If someone quotes without visiting, or quotes a single price for both approaches, that is a reason to ask more questions. Our fixed written quote follows the survey, and it sets out the layers in plain words.
Making the decision
Ask three questions. Will the loft be used as living space now or later? Is the roof covering in good order, or due for replacement? Do you have services or storage up there that need a frost-free space? If the answers are no, yes and no, a cold loft is usually the better value. If they are yes, no and yes, a warm roof is worth the extra cost, and the design detail should be written down before anyone starts. Start from the spray foam assessment and removal page if your loft already has foam, and we will look at what you have.
