Roof Insulation Guide: UK Types, Costs and Building Regulations
25% of your heating bill escapes through the roof if your loft is poorly insulated. Topping up from 100mm to the current 270mm standard costs £300 to £600 professionally installed — and pays for itself within two to three winters. That makes it one of the highest-return home improvements available. This guide covers the four main insulation types, realistic 2026 costs, Building Regulations Part L requirements, government grant eligibility, and the practical mistakes that cause problems (especially spray foam and its impact on mortgage lending).
Types of Roof Insulation
The right insulation depends on whether you are insulating at ceiling level (cold roof) or rafter level (warm roof):
- Mineral wool rolls (glass or rock wool) - The most common loft insulation. Laid between and over the ceiling joists to a total depth of 270mm. Cheap, effective and easy to install. Brands include Knauf, Isover and Rockwool.
- PIR rigid boards (Celotex, Kingspan, Recticel) - Used between or below rafters in loft conversions and warm roof constructions. Higher insulation value per millimetre (better lambda value) than mineral wool, so you can achieve the required U-value in less depth.
- Spray foam insulation - Applied directly to the underside of the roof tiles or slates. Expands to fill gaps and bonds to surfaces. Controversial in the UK market due to concerns about moisture trapping and mortgage lender acceptance.
- Blown fibre or cellulose - Loose fill material blown into the loft space by machine. Useful for hard-to-access areas or lofts with irregular joist spacing. Settles over time, so needs to be installed deeper than the target thickness.
Costs for Roof Insulation
For a typical three-bedroom semi-detached house (roughly 40 to 50 square metres of loft floor):
- 270mm mineral wool (DIY) - £150 to £300 for materials. A competent DIYer can do this in a day.
- 270mm mineral wool (professional installation) - £300 to £600 including materials and labour.
- PIR boards between rafters - £1,500 to £3,000 installed. More expensive because of the material cost and the skilled labour needed to cut and fit boards precisely.
- Spray foam - £1,200 to £2,500 for a typical loft. Must be done by a specialist installer.
- Blown fibre - £400 to £800 installed. Quick to install (a few hours for a typical loft).
Government grants may be available through the ECO scheme or the Great British Insulation Scheme. Eligibility depends on your property type, energy rating and household income. Check with your energy supplier or local authority.
Building Regulations (Part L)
Building regulations set minimum insulation standards for roof work:
- New builds - Must achieve a roof U-value of 0.13 W/m2K or better. This typically requires 300mm or more of mineral wool equivalent.
- Loft conversions - The roof slope must achieve a U-value of 0.18 W/m2K. This usually means 100 to 120mm of PIR board between rafters plus additional insulation below.
- Re-roofing (over 25% of area) - If you are replacing more than a quarter of the roof covering, you must upgrade thermal performance "where technically, functionally and economically feasible." In practice, this usually means adding insulation at ceiling level if it is currently below standard.
- Loft insulation top-up - Adding insulation to an existing loft does not require building regulations notification if you are simply topping up mineral wool. However, the work should still meet the recommended 270mm minimum.
These are minimum standards. Going beyond them makes sense economically, especially given rising energy costs. The marginal cost of adding an extra 100mm of mineral wool is negligible compared to the long-term energy savings. If you are doing a loft conversion, insulation requirements are stricter — see our loft conversion guide for details.
Important Practical Considerations
A few things people often overlook when insulating their roof:
- Ventilation - Loft insulation at ceiling level creates a cold roof space above. Adequate ventilation (typically 10mm continuous gap at eaves and 5mm at ridge) prevents condensation forming on the underside of the roof covering. Do not block soffit vents with insulation.
- Pipes and tanks - Any water tanks or pipes in the loft space must be insulated separately, because the loft insulation below them means the loft is now colder. Frozen pipes in uninsulated lofts are a common winter problem.
- Electrical cables - Do not bury electrical cables under thick insulation without checking they are rated for it. Cables overheat when surrounded by insulation. An electrician can advise on whether your existing cables are suitable.
- Spray foam caution - Many mortgage lenders are cautious about properties with spray foam insulation because it can mask roof problems and is difficult to remove for inspection. If you plan to sell your property, spray foam can complicate the process. Check with your mortgage lender before having it installed. Also ensure your roof is in good condition first — our leak prevention guide covers what to check. If you are considering a full re-roof at the same time, see our roof replacement cost guide.
Frequently asked questions
Is 100mm of loft insulation enough?
No. 100mm was the standard decades ago but current building regulations recommend 270mm of mineral wool or equivalent. If you only have 100mm, topping up to 270mm is one of the cheapest and most effective energy improvements you can make. The additional material costs around £150 to £250 for an average loft.
Will loft insulation make my house warmer in summer too?
Yes. Insulation works both ways, keeping heat in during winter and reducing heat transfer from a hot roof space in summer. Rooms directly below the loft will be noticeably cooler on hot days with good insulation in place.
Can I lay new insulation on top of old?
Yes, this is standard practice. The first layer (usually 100mm) sits between the joists. Additional layers are laid across the top of the joists at right angles to avoid compressing the first layer and to cover the thermal bridge of the joists themselves.
Does spray foam insulation cause problems with mortgages?
It can. Some mortgage lenders are reluctant to lend on properties with spray foam applied directly to the roof structure, because it prevents inspection of the timbers and underlay, and can trap moisture. Always check with your lender first and use a BBA-certified installer if you proceed.