270mm is the current UK recommended minimum depth. Most homes built before 2000 have significantly less than this.
UK Building Regulations (Approved Document L) recommend a minimum of 270mm of mineral wool loft insulation for new installations. This has been the recommendation since around 2002 — properties built or insulated before then are very likely to be underinsulated.
Common scenarios we find during surveys:
Quick check: Look into your loft hatch. If the insulation doesn't reach above the joists (typically 100mm), a top-up is definitely recommended. If it's level with the joists, you still likely need an additional layer on top.
A simple visual check tells you a lot. If you can see the tops of the joists, you're almost certainly underinsulated.
Push a ruler into the insulation next to a joist. Note the total depth. 270mm is around 10.5 inches — roughly the depth of two standard loft insulation rolls laid on top of each other.
270mm of insulation is installed in two layers:
100mm laid between the joists in the direction they run. This fills the joist depth and forms the base layer.
170mm laid at right angles across the joists. This covers the thermal bridge created by the joists themselves and brings the total to 270mm.
Important: Do not compress the insulation. Mineral wool insulation works by trapping air in its fibres. Compressing it reduces its effectiveness. Ensure both layers are laid flat and uncompressed.
The standard and most cost-effective option for accessible lofts. Easy to install, widely available, good thermal performance. Recommended for most properties.
Better for irregular loft spaces, awkward shapes or covering existing insulation. Can be machine-blown to achieve consistent depth. Suitable for both DIY and professional installation.
Used where the loft space needs to be usable (boarding over) or where standard rolls aren't appropriate. More expensive but provides a solid surface. Also used for sloped roof sections.
In most cases, yes. If your existing insulation is dry, undamaged and not contaminated, a top-up layer is perfectly acceptable and is the most cost-effective approach.
Removal of old insulation is needed if:
We inspect existing insulation during the survey and advise whether topping up or full replacement is appropriate.
Asbestos note: If your property was built before 1980 and has loose grey fibre-like insulation in the loft, do not disturb it. Contact us or an asbestos specialist before any work is carried out. Mineral wool (modern) is white/yellow and very fibrous; asbestos loose fill looks different and older.
We'll check your existing insulation, confirm the right depth and provide a fixed quote. Covering North East England, Cumbria and the Scottish Borders.
