How much loft insulation do I need UK?

UK building regulations recommend a minimum of 270mm of loft insulation, but 300-400mm provides better thermal performance. The first layer (typically 100mm) goes between the joists, with additional layers laid perpendicular on top to avoid thermal bridging.

Can I put new loft insulation over old?

Yes, you can add new insulation on top of existing material as long as it is dry and in good condition. The new layer should be laid at right angles to the first layer to minimize heat loss through the joists. Check for any damp, pest damage, or mould before adding more insulation.

How many rolls of loft insulation do I need?

A typical 100mm loft roll covers approximately 8-9 square metres. For a 50 square metre loft requiring 270mm total depth, you would need around 6 rolls for a 100mm layer. Use our calculator to get an accurate estimate based on your specific loft dimensions.

What is the best type of loft insulation?

Mineral wool (glass fibre or rock wool) rolls are the most cost-effective and common choice. PIR boards (Celotex/Kingspan) offer better thermal performance per thickness but cost more. Sheep wool and recycled options are available for eco-conscious homeowners. All types meet building regulations when installed correctly.

Should loft insulation go under or over joists?

Both. The first layer (typically 100mm) should fit snugly between the joists. Additional layers should be laid across the joists at 90 degrees to prevent thermal bridging. Never compress insulation as this reduces its effectiveness.

Do I need to remove old loft insulation?

Only remove old insulation if it is damp, mouldy, contaminated, or compressed flat. If the existing insulation is in good condition, you can simply add more on top. This is more cost-effective and less disruptive than complete replacement.

Tools

Account

Plaster & Plasterboard

Plastics and Drainage

Roofing

Timber

Loft Insulation Calculator

Loading calculator...