How to Insulate a Loft: Rolls vs Boards, Step by Step
Insulating a loft is one of the most straightforward upgrades you can make, and it is a job most DIYers can do in a weekend. This guide shows how to insulate a loft step by step, helps you decide between rolls and rigid boards, and shows what you need to buy. It covers a standard cold loft, a loft you board for storage and a loft you are converting.
Quick answer
For a normal cold loft, lay mineral wool rolls in two layers: the first between the joists, the second across them. Aim for about 270mm in total, which is the depth recommended by the Energy Saving Trust (see our 270mm loft insulation guide). If you are converting the loft into a room or boarding it for storage, rigid PIR boards give the same performance in less depth.
Need a quick quantity or thickness check? Use our free loft roll calculator. It uses the products we sell, prices the list live, and you can add it to your basket. Results are estimates — call 03330 908479 if you want us to check your sizes.
Step 1: Choose rolls or boards
| Mineral wool loft roll | PIR insulation board | |
|---|---|---|
| Best for | Open, unboarded cold lofts | Loft conversions, boarded floors, limited depth |
| How it is fitted | Unrolled between and across joists | Cut to fit between joists or rafters, or laid over them |
| Depth needed | More, because the material is less efficient per mm | Less: PIR has a declared thermal conductivity of 0.022 W/mK, against about 0.042–0.044 for our loft rolls (the number in each roll's product name) |
| Cost per m² | Lowest | Higher, but you may need fewer millimetres |
| Can you walk or board over it? | No, it compresses | Rigid boards take boarding on top if laid correctly |
| Our range | Loft insulation rolls | PIR boards |
For more on the choice, see our loft roll vs PIR guide and loft insulation boards guide.
Step 2: Check the loft before you start
- Look for damp, leaks and condensation. Fix any roof leaks first. Wet insulation does not work properly.
- Check for ventilation at the eaves. Do not push insulation into the eaves so that it blocks the air flow. Leave a gap to keep the roof ventilated.
- Look at wiring and downlighters. Keep insulation clear of any light fitting that is not rated to be covered, and lift cables on top of the insulation rather than burying them. If you are unsure, ask an electrician.
- Plan around the cold water tank. Insulate the tank and pipes above it, but leave the space under the tank uninsulated so that heat from the room below keeps it above freezing.
- Plan your access. Walk only on the joists or on a temporary board. The plasterboard ceiling will not take your weight.
Step 3: Gather materials and kit
You need loft rolls (or boards), a sharp knife or insulation saw, a tape measure, a straight edge and a work light. Wear gloves, long sleeves, goggles and a dust mask when handling mineral wool. A kneeling board saves your knees.
Step 4: Lay the first layer between the joists
Unroll the first layer between the joists, snug to the sides without squashing it. Tight fits matter, as gaps let heat through. Where the roll is wider than the joist spacing, trim it to size. Cut it around pipes and cables. If you are using a roll with a 100mm thickness, such as ODE Lunaflex 100mm at £22.47 per roll (10.83m²), this layer will be the first.
Step 5: Lay the second layer across the joists
Lay the second layer at right angles over the top of the joists, so it covers the tops of the timbers. This reduces heat loss through the joists, a form of thermal bridging (see our thermal bridging guide). A 200mm roll such as ODE Lunaflex 200mm (£26.73 per 6.84m²) or Knauf Loft Roll 44 200mm (£31.29 per 6.84m²) works well here. Do not compress the layers. Insulation works by trapping air.
Step 6: Insulate and seal the loft hatch
The hatch is a gap in your insulation. Fix a piece of insulation to the loft side of the hatch door, fit a draught seal around the frame, and make sure the hatch closes firmly. We sell access panels but not loft hatches, so for a new hatch you will need to source it elsewhere.
Step 7: Boarding for storage
If you want to use the loft for storage, boarding directly on the joists compresses the first layer and leaves no room for the second. Raise the boarding on a platform above the insulation, or fit PIR boards between the joists and board over them. Our PIR in the loft guide explains how, and our OSB boards suit the floor deck. You can cut PIR cleanly with a fine saw, see how to cut PIR.
Worked example: a 20m² loft to 270mm and above
For a 20m² loft with 100mm between joists and 200mm across (300mm in total), using ODE Lunaflex rolls at the live prices on 28 October 2026 (ex VAT, and prices vary):
| Layer | Rolls needed | Cost |
|---|---|---|
| 100mm (10.83m² per roll) | 2 rolls | £44.94 |
| 200mm (6.84m² per roll) | 3 rolls | £80.19 |
| Total | £125.13 |
Use our insulation calculator for your own loft size. See the full loft insulation installation guide for more detail.
Insulating a loft conversion
If the loft is becoming a room, the insulation follows the roof slope instead. It normally sits between the rafters and is topped by insulated plasterboard on the inside, with a clear ventilation gap in a cold roof. For example, 100mm Eurothane GP (£36.83 per sheet) can be cut to go between rafters. See our roof insulation collection, and check the U-value target in the PIR U-value calculator before you buy.
Frequently asked questions
How thick should loft insulation be?
The Energy Saving Trust recommends 270mm of mineral wool for a standard cold loft. Boards can achieve the same performance with less depth.
Can I insulate a loft myself?
Yes. Take precautions with protective gear, and avoid the cold water tank, wiring and eaves ventilation points.
Should I insulate between the joists or over them?
Both. Lay the first layer between the joists and the second across, which covers the joists as well.
Do I need a vapour barrier in a loft?
For a standard cold loft with a normal ceiling, you generally do not need to add one, as the priority is keeping the loft ventilated. Refer to the product instructions or a professional for your situation.
Prices are shown ex VAT and vary, so check the product page before you order. We have a £400 ex VAT minimum, with free delivery in England. British Gypsum (including Gypframe), Celotex and Knauf can also go to mainland Wales and mainland Scotland (£400 ex VAT minimum plus £40 + VAT delivery). Access panels can also go to mainland Wales and mainland Scotland, with their own charges (£15 + VAT under £300 ex VAT, free from £300). Other ranges, including suspended ceiling tiles and grid, are delivered in England only. We do not deliver to Northern Ireland or the Scottish islands. Insulation and plasterboard orders have a £400 ex VAT minimum and free delivery (see delivery information). Outside England? Call us or contact us for a quote. Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.