How to insulate a loft
In this guide, we’ll walk you through everything you need to know to lay loft insulation. Also known as ‘cold roof’ insulation, insulation is fitted to the floor of the loft trapping the heat in the main part of the house and away from the loft. This is ideal if you don’t use your loft at all, or only for the storage of non-perishable items.
How to lay loft insulation rolls:
Safety first
- Enlist a helper to pass loft rolls to you once you’re in the loft.
- Keep loft insulation a minimum of 75mm away from hot flues (including enclosed chimneys, from a boiler or wood-burning stove).
- Take extra care when working close to power cables.
- Wear a dust mask, gloves and safety goggles when handling insulation rolls.
Before tackling any loft project, check out our guide on loft safety.
Before laying loft insulation
In the loft, remove any boards fixed to your joists with a screwdriver. Clear as much space as possible to give yourself a large, safe work area. Use any available wooden planks or loft boards to create a flat storage area for a few rolls close to where you’ll be working.
Cold water tanks
Check for any existing insulation under the cold water tank (if you have one). If the tank is elevated by at least 300mm - leave it where it is. If the tank isn't elevated by at least 300mm - remove it. Don't block the heat coming up through the ceiling as it can actually help keep the tank warmer and prevent freezing during cold winter weather.
Cold water tanks should be insulated separately, read our how to guide for more information.
Electrical cables
Check for any electrical or lighting cables. Covering these with insulation can pose a fire risk, so it’s important to assess them before laying insulation. Without touching the cables, look to see how much slack is in the cables. Is there enough slack for it to be lifted on top of the insulation you’re about to lay without overstretching it? If so – the cables can be moved. If the cables don’t have much slack, are clipped to the joists or you’re not confident handling them, consult a qualified electrician.
Recessed downlights
If you have any recessed downlights in the rooms below your loft, lift any existing insulation and check whether the lights are covered by insulation guards. Lights can be a fire hazard if covered by insulation without a protective guard acting as a barrier between the two. Buy as many guards as are needed. We’ll show you how to fit them later in the section ‘How to lay loft insulation around recessed downlights’.
How to lay loft insulation rolls
The following steps advise on how to insulate the loft floor with rolled or encapsulated insulation. This includes between, and over, the joists and creating base and top layers.
1. Cut the insulation to size
Cut any larger width rolls (1140mm) to the desired width dependent on your joist’s spacing. Still in their packaging, cut along the partially cut perforations using a panel saw.
2. Lay the insulation
Start in one corner of the loft and work back towards the loft hatch. Position your walk board at right angles to the adjacent joists. Lay the first loft roll at the eaves, leaving a 25mm gap away from the eaves for ventilation to help prevent condensation.
If you’re laying encapsulated insulation, there’s no need to take it out of its silver wrapping. Roll it out, silver side up.
Continue unrolling between the joists across the loft floor area. Once you’ve unrolled your first length, lightly push the insulation against the joists so there are no gaps, taking care not to compress it. Remember that the insulation will be compressed from being in its packaging, and so could take a while to reach its full height, as advertised. Gently ‘fluff’ it up with your hands to help it reach its maximum depth.
If one roll isn’t long enough to reach to the other side of the loft, start a new roll and butt the joints closely together. Do not overlap or compress the insulation.
3. Cut around cross beams
When you reach a cross beam, cut the insulation flush with the beam then start again on the other side, working in from the eaves.
To cut the insulation, use a long sharp knife with a serrated edge, such as a bread knife. Sandwich the insulation between two boards and, using the top board as a guide, run the knife along the edge for a straight cut. Use a pair of large scissors for small or difficult cuts, for example around pipes.
For narrower areas, cut the insulation to the required width and lay it between the joists in the same way. Don’t pack or compress insulation into awkward corners.
5. Lay the top layer
Lay the second, or top, layer. Don’t cut any wide rolls into narrower ones when laying the top layer across the top of the joists. This will help speed up the laying process and reduce any joins and gaps.
Lay the first roll of the top layer at right angles to the base layer. Continue until the top layer is complete. Ensure all joints are snuggly butted together.
How to lay loft insulation around cables
Turn off your electrical power supply at the fuse box/ consumer unit before lifting any electricity cables. Carefully lift the cable to sit on top of the insulation. Do not stretch it. By laying the cables on top of the insulation, it will also make it easier to find and access them again.
If there isn't enough slack in the cables or you're not confident moving the cable, contact an electrician.
First, fit an insulation guard around the recessed downlight. Check that the downlight’s transformer is placed clear of the insulation. And that the downlight doesn’t touch the inside of the guard. Follow the product installation instructions for further details.
For the base layer - lay the loft roll over the insulation guard. Continue as per ‘How to lay insulation rolls’.