Cold Roof vs Warm Roof
There are two fundamentally different approaches to insulating a roof:
Cold roof means insulation at ceiling level — between and over the ceiling joists, with the loft space above remaining unheated. This is the standard approach for houses where the loft is just used for storage, and it's by far the cheaper option. Current Building Regs require 270mm of mineral wool or equivalent (typically 100mm between the joists plus 170mm cross-laid on top), giving a U-value of 0.16 W/m²K. Cost is roughly £15-25 per m² of ceiling area for materials and fitting.
Warm roof means insulation at rafter level — between and under the rafters — with the loft space effectively brought inside the thermal envelope. This is what you need if you're converting the loft into habitable space, or if you have a room-in-roof that's currently cold. Typical build-up is 100-150mm PIR board (Celotex, Kingspan) between the rafters plus 50mm underneath, giving a U-value of 0.18 W/m²K. Cost is roughly £60-90 per m² of slope, because you're working in a confined space with more expensive materials.
Retrofit Work to Existing Properties
For a typical three-bed Sheffield semi with 100mm of old mineral wool in the loft, upgrading to current standards means topping up another 170mm of cross-laid fresh mineral wool. Takes a day, costs £500-900 depending on loft size and access, pays back in about four years at current gas prices.
If there's no existing insulation at all — the occasional older property, or where previous insulation has become damp-damaged and been removed — full from-scratch installation is £800-1,400 for a typical property.
For properties with a room-in-roof that's currently freezing in winter, we can often retrofit warm-roof insulation from outside, as part of a re-roof — much easier than trying to work from inside. If you're contemplating a re-roof anyway, this is the single best opportunity to fix the thermal problem.
Ventilation — The Catch
Every time we install or top up loft insulation, we have to check ventilation. Adding insulation makes the roof void colder (because less heat escapes upwards), which massively increases condensation risk unless airflow is adequate. A lot of insulation cowboy firms just pile 270mm of mineral wool in on top of 1970s insulation and walk away — six months later the owners are calling someone out about mould and drips.
We always check eaves ventilation (often blocked off by pushed-in insulation) and install proprietary eaves vent trays if needed to maintain airflow. If the roof has no meaningful ventilation at all, we either add tile vents or recommend a combined insulation + dry ridge + eaves vent package.
Other Insulation Types
Sheep's wool — Thermafleece, natural sheep wool batts. Similar thermal performance to mineral wool, better hygroscopic properties (handles moisture better), more expensive. Good for traditional buildings where breathable construction matters.
Wood fibre — Steico, Pavatherm. Rigid insulation for between rafters in warm-roof applications. Breathable, carbon-negative, more expensive than PIR but suits heritage and eco-conscious builds.
Spray foam — open-cell or closed-cell PU foam. We do not recommend spray foam for most domestic applications. It's effective at sealing air leakage but causes real problems for mortgage lenders (many surveyors now flag it as defect), traps moisture against timber, and can't be removed without complete destruction of roof structure. The current RICS guidance is to avoid it in residential.
