Heat rises, and in an uninsulated pitched-roof house the loft is where a large share of it escapes. Topping up or laying new insulation between and over the ceiling joists is one of the most common DIY energy-saving jobs in the UK because it needs no specialist tools and the materials are cheap. It is also a job that is easy to do badly: blocked eaves, buried cables and squashed insulation all cause problems that only show up later. This guide covers how much to lay, what to lay it with, how to protect ventilation, and how to judge whether the money is worth spending in your particular house.
How much insulation is enough?
The figure you will see quoted almost everywhere is 270 mm of mineral wool laid across the whole loft floor. That number is not magic: it is simply the depth of ordinary glass or rock mineral wool that gets a typical ceiling close to the U-value that building regulations currently look for in a roof (around 0.16 W/m²K for a renovated roof element in England, under Approved Document L). Different materials have different thermal conductivity, so the depth needed differs: sheep's wool, wood fibre and PIR boards each reach the same U-value at a different thickness. Always read the manufacturer's data sheet rather than copying a depth from a different product.
If you already have some insulation, measure it. Many 1980s and 1990s houses have 100 mm, which is a perfectly good base. Topping it up to 270 mm gives a real but smaller gain than insulating from nothing, because the first 100 mm does most of the work. Energy Saving Trust figures reported in 2026 put the saving from topping up from 120 mm to 270 mm at only around £17 a year for a typical semi, against roughly £140 to £360 a year when insulating a bare loft, depending on house type. Treat those as indicative: they depend on fuel prices and on how your home is heated.
Scotland has stricter expectations than England and Wales in its technical standards, and Northern Ireland has its own regulations. If you are in either, check the current handbook or technical booklet rather than assuming 270 mm is the target.
Cold loft or warm loft?
The standard approach is a cold loft: insulation goes on the ceiling below, and the roof space above stays cold and ventilated. This is cheap and effective, but it makes the loft unusable for anything other than light storage unless you raise the boarding. A warm roof puts insulation at the rafters, keeping the loft space within the heated envelope. It suits conversions and is a bigger, more expensive job, often involving the roof covering. Everything below is about the cold-loft route, which is what most DIYers do.
Choosing the material
| Material | Form | Good for | Watch out for |
|---|---|---|---|
| Glass or rock mineral wool | Rolls and slabs | Cheapest per square metre; easy to cut | Itchy; wear gloves, long sleeves, a mask and eye protection; do not compress it |
| Sheep's wool | Rolls | Pleasant to handle; handles moisture well | Costs several times more than mineral wool |
| Recycled-plastic or cellulose fibre | Rolls or blown | Lower irritation; good coverage in awkward spaces | Blown cellulose is usually a job for a contractor |
| PIR or phenolic boards | Rigid boards | High performance per millimetre; useful where depth is limited or over boarding | Needs careful cutting and sealing at edges |
The traditional layered method is to lay a first layer between the joists (joists in older houses are often 100 mm deep) and a second layer across them at right angles. The cross layer covers the joists, which are thermal bridges: timber conducts heat far better than insulation, so a single layer that stops flush with the joist tops leaves strips of heat loss across the ceiling.
Before you lay anything: the checks that matter
- Look for damp and leaks. Stained rafters, damp sarking felt or daylight through slates need fixing first. Insulation hides leaks; it does not cure them.
- Locate the electrics. Old rubber or lead-sheathed cable should be inspected by an electrician before it is covered. Modern PVC cables are generally fine, but a heavy covering of insulation over cables that carry high currents can make them run warm. Where a cable carries a heavy load, such as a cooker or shower feed, get an electrician's advice before burying it. Lay cables on top of the first layer rather than under it where practical and avoid wrapping them in insulation.
- Check recessed downlights. Standard halogen downlights get hot and should not be covered. If you want to insulate over them, use fire-rated, insulation-contact (IC-rated) hoods or replace them with IC-rated LED fittings. Never just pile insulation around a hot fitting.
- Locate flues, extract fans and water tanks. Extractor ducts need insulating in their own right (to stop condensation dripping back into the fan), and flues need clearance as specified by the appliance maker.
- Identify asbestos risk. Houses built before 2000 can contain asbestos in textured coatings, old insulation (loose-fill products sometimes found in loft spaces of some pre-1980s properties) or cement products. If you find suspicious material, do not disturb it; have it tested and removed by a licensed professional where required.
Keep the eaves open
The single most common error is pushing insulation hard into the eaves, blocking the airflow from soffit vents. A roof needs ventilation to carry away moisture that rises from the house below. Block it and the sarking felt and timbers get damp, mould appears and, over years, timber can decay. Use plastic eaves ventilation trays (sometimes called eaves vents or baffles) wedged between the rafters at the eaves, so insulation can run to the wall plate while a clear air gap of around 25 mm remains above it. Most manufacturers recommend keeping a continuous air path from the soffit into the loft space.
Step-by-step: laying a typical roll
- Clear the loft floor and sweep or vacuum dust. Fit a few boards across the joists for a working platform; never step between joists, because the ceiling plasterboard will not carry you.
- Fit eaves ventilation trays at each rafter bay around the edge.
- Check the joist spacing. Standard rolls are made to suit 400 mm and 600 mm centres, and the roll width listed on the pack will say which.
- Unroll the first layer between the joists and cut it with a long, sharp serrated knife against a straight edge. Butt lengths tightly; gaps are cold bridges. Do not squash or compress it: compressed wool loses much of its insulating power because the trapped air is what insulates.
- Lay the second layer across the joists at 90 degrees, again with tight joints. If you need to board part of the loft for storage, fit raised boarding on stilts or legs (which hold the boards above the insulation) rather than laying boards on top of squashed material.
- Leave the cold water tank and the pipes in the roof space on the cold side: insulate the tank sides and top, but leave a gap of insulation-free ceiling underneath the tank so that some heat from below keeps it from freezing. Insulate the pipes themselves.
- Insulate the loft hatch with a rigid board stuck to the top and fit draught-strip around the frame. A hatch that leaks warm moist air is a common cause of localised mould in the loft.
Cost and payback
A DIY job on a bare three-bedroom semi uses roughly 40–50 m² of loft floor. Taking 270 mm of rolled mineral wool as three layers of 90 mm, you need three times the floor area in rolls, so 45 m² of loft means about 135 m² of rolled material. Retail packs cover between about 5 and 10 m² each depending on thickness, so budget for a few hundred pounds in materials, plus eaves vents, tape and protective gear. Professional quotes for a standard semi typically land in the hundreds of pounds up to about a thousand, which is why the Energy Saving Trust's published installed costs (roughly £650 for a mid-terrace up to roughly £1,100 for a detached house, on its mid-2026 figures) are a sensible benchmark. These are the trust's estimates; get quotes for your own property.
An example, with made-up round numbers to show the arithmetic: if materials and sundries cost £400 and the first-year saving is £190 (the Energy Saving Trust figure quoted for a semi), simple payback is £400 / £190 ≈ 2.1 years. If you pay a contractor £750 for the same saving, payback is £750 / £190 ≈ 3.9 years. Your own number depends on how much loft insulation you have now, your fuel and whether gas, oil or electricity heats the home. The loft insulation depth, U-value and payback calculator lets you put in your own depth and costs.
VAT, grants and finance
In Great Britain, the installation of energy-saving materials such as insulation is currently zero-rated for VAT under a temporary relief that runs to 31 March 2027, after which the reduced 5% rate is due to return. That applies to installations by VAT-registered contractors, so a quote now may be cheaper than one next April. Grants change often: schemes such as ECO4 and the Warm Homes programmes are aimed at lower-income and vulnerable households and are channelled through suppliers or local authorities, and Scotland, Wales and Northern Ireland run their own schemes. Check gov.uk or your local authority for what you could qualify for before paying for a standard job yourself.
Do you need building regulations approval?
In England, laying new insulation between and over the joists does not normally need a building regulations application when it is simply being added, though re-covering or renovating the roof itself can bring Approved Document L into play. If you are converting the loft, forming rooms, or replacing the roof covering, the regulations apply. Our guide to which projects need building regulations approval sets out the common cases. Always check with your local building control team if in any doubt.
Common mistakes
- Leaving gaps between rolls or around the hatch.
- Covering the eaves vents.
- Storing heavy boxes on compressed insulation.
- Insulating the underside of the cold water tank.
- Burying old cabling or hot downlights.
- Choosing a thinner but cheaper product without checking its conductivity rating.
After insulating, the loft will be colder in winter and water pipes more vulnerable to freezing, so lag them. If the walls or windows are the real weak point, insulating the loft will not save as much as the headline suggests, and you may get better value from draught-proofing first. Combine the loft work with a view of the whole heating system: if you are considering a heat pump, a well-insulated home lets it run at lower flow temperatures, as discussed in our heat pump versus gas boiler comparison.
This is general information, not professional advice. Anyone unsure about wiring, asbestos or roof structure should consult a qualified professional.