Insulate a Loft — Cost Calculator
Lay mineral wool insulation between and across loft joists to meet the recommended 270 mm total depth, reducing heat loss through the ceiling.
Your Project
Quality tier
Price Comparison
| Item | Retailer | Qty | Unit | Line Total |
|---|---|---|---|---|
| Insulation — Between Joists (100 mm) | ||||
| Loft Roll 100 mm1 roll (8.30 m²) | Homebase | 4 | £11.99 | £47.96 |
| Insulation — Across Joists (170 mm) | ||||
| Loft Roll 170 mm1 roll (4.80 m²) | Homebase | 7 | £12.50 | £87.50 |
| Sundries | ||||
| Dust Mask (FFP2)3 masks | Screwfix | 1 | £7.99 | £7.99 |
| Total | £143.45 | |||
| Cheapest per item across Homebase, Screwfix | ||||
DIY vs Hiring a Tradesperson
£240.00 – £450.00
£8.00 – £15.00 per m²
£96.55 – £306.55
Shopping List (Mix & Match)
Insulation — Between Joists (100 mm)
- 4 × 1 roll (8.30 m²)Loft Roll 100 mmHomebase£47.96
Insulation — Across Joists (170 mm)
- 7 × 1 roll (4.80 m²)Loft Roll 170 mmHomebase£87.50
Sundries
- 1 × 3 masksDust Mask (FFP2)Screwfix£7.99
Insulating your loft is one of the most cost-effective home improvements you can make. The Energy Saving Trust estimates that a well-insulated loft can save you up to £590 a year on energy bills. The recommended depth is 270 mm of mineral wool — typically 100 mm between joists and 170 mm laid across them.
It's a straightforward DIY job that most people can do in a day. The main challenge is working in a cramped, dark space, and handling itchy mineral wool — but with the right protective gear and a headtorch, it's very doable.
Assumptions
- Loft is empty and accessible with standard joist spacing (400 mm or 600 mm centres).
- Two layers: 100 mm between joists + 170 mm across joists to reach 270 mm total.
- Standard rolls are 1140 mm wide × 5.4 m long (approx. 6.15 m² per roll at 1140 mm width).
- 10% waste allowance for cutting and fitting around obstacles.
- No existing insulation present (full installation from scratch).
What Affects the Cost
Loft insulation is one of the cheapest home improvements pound-for-pound. The main cost driver is simply the area of your loft — more square metres means more rolls of insulation.
The difference between regular and premium mineral wool is relatively small in percentage terms. Premium rolls may have better handling properties (less itch, easier to cut) but the thermal performance is virtually identical at the same depth.
If you need to clear your loft of stored items first, or install loft legs and boarding afterwards to keep storage space, that's an additional cost not included in this calculator.
Frequently Asked Questions
How thick should loft insulation be?
The UK government recommends 270 mm of mineral wool insulation. This is typically achieved with 100 mm between the joists and 170 mm laid across the top of the joists at right angles.
Can I lay new insulation on top of old?
Yes, if the existing insulation is in good condition and dry. Simply lay the new rolls on top. If the existing insulation is damp, mouldy, or compressed, it's better to remove it first.
Do I need to insulate under the water tank?
No — leave the area directly under a cold water tank uninsulated. This allows a small amount of heat to rise from the room below, reducing the risk of the tank freezing in winter. However, insulate the top and sides of the tank itself with a tank jacket.
How much can I save with loft insulation?
According to the Energy Saving Trust, insulating from 0 mm to 270 mm can save approximately £590 per year for a detached house, £355 for a semi-detached, or £150 for a mid-terrace (2024 figures).
Is loft insulation itchy?
Mineral wool (glass or rock wool) can irritate skin and lungs. Wear long sleeves, gloves, an FFP2 dust mask, and goggles. Premium rolls with a tissue facing are less irritating to handle.