How to insulate a loft

How to insulate a loft

Save money on your energy bills and prevent precious heat from escaping through your roof by insulating your loft

© MartinPrescott

Anna-Lisa De'Ath

Published: November 3, 2022 at 11:29 am

Did you know that a quarter of heat is lost through the roof in an uninsulated home? Insulating your loft or attic is a relatively easy and effective way to reduce heat loss and cut your heating bills and it doesn’t have to cost a fortune. If it’s fitted correctly, loft insulation should pay for itself many times over in its 40-year lifetime. If your loft is easy to access and doesn’t have any damp or condensation problems, it should be easy to insulate – and in most cases, it’s fairly straightforward to do it yourself.

How much does it cost to insulate your loft?

According the the Energy Saving Trust, here’s how much it costs to insulate a gas-heated home that has a totally uninsulated loft with 270mm loft insulation:

Typical insulation cost:

  • To insulate a detached house will cost £630
  • To insulate a semi-detached house will cost £480
  • To insulate a mid-terrace will cost £455
  • To insulate a detached bungalow will cost £640

How much will loft insulation save on energy bills?

  • For a detached house the savings could be £590,
  • For a semi -detached house the savings could be £355
  • For a mid-terrace house the savings could be £330
  • For a detached bungalow the savings could be £590

Figures are based on fuel prices as of October 2022

How to calculate how much insulation you need

To work out how many packs of loft floor insulation you will require, measure the length and width of the loft space to be insulated. If this is not possible in the loft area due to access issues and the floor below is the same size as the loft, measure that instead.

Calculate the size of the loft area by multiplying the length by the width: length (m) x width (m) = area of loft in square metres (m2). Then subtract 10%m, as the base layer won’t cover the entire loft floor because the rolls are laid between, not over, the ceiling joists.

Divide the area by the coverage per pack, ie. size of area of loft in m2 divided by pack coverage = number of packs required for your base layer (round up to the nearest whole number if needed).

Repeat to calculate how many packs you will need for your top layer, this time without deducting 10% from the area, as the top layer will cover the joists.

How to insulate your loft - step-by-step

Loft insulation safety

Before tackling any loft project there are a few safety pointers you should bear in mind:

  • Lofts often have loose floor insulation and the fibres from that, along with the dust, can be an irritant. To protect against this, wear a long-sleeve top or hoodie, long trousers or overalls, a dust mask, protective gloves, safety goggles and knee pads.

  • To cut down on trips up and down from the loft, make sure you have everything you need to hand before you start. This includes all the tools and materials for project, as well as a fully charged mobile phone in case of accidents.

  • If your loft floor isn’t sufficiently boarded, moving about can be tricky. Don’t be tempted to stand on the plasterboard as it won’t bear your weight, and only stand on the ceiling joists if you have to. For easier movement in the loft, use walk boards (sometimes known as crawl boards) laid across the joists. If a top layer of insulation is already in place on the loft floor, roll it back to reveal the top of the joists and then place the walk boards on top of them. These boards can be lifted and moved as you work around the loft. Have more than one walk board to hand so you can stand on one while you move the other one. Don't rest walk boards directly on top of electricity cables or water pipes.

Insulation rolls are the easiest and most economical way for novice DIYers to insulate the loft. You can also insulate a loft with insulation boards, but these require a higher level of skill to fit and are best left for competent DIYers and professionals to install.

To calculate how much insulation you need

Loft insulation checklist - you will need:

  • Tape measure
  • Screwdriver
  • Pen and paper
  • Loft floor insulation rolls
  • Offcuts of wooden board
  • Panel saw
  • Long knife with serrated edge - a bread knife is ideal
  • Scissors
  • Optional: Insulation guards - also known as heat diffusers, loft caps or loft covers, if you have recessed light fittings

Looking for more handy home tips? Here's how to find the best home insurance in 2023.

Step 1

Before you begin, ensure your loft is a safe space for working.

Check for any electrical or lighting cables, as covering these can pose a fire risk. Without touching the cables, see if there enough slack for them to be lifted on top of the insulation you’re going lay without over-stretching – if so, the cables can be moved, but not yet. Read our section on ‘How to lay loft insulation around cables’ before you start working to find out how and when to do this.  If the cables don’t have much slack, are clipped to the joists or you’re not confident dealing with them – stop and consult a qualified electrician. 

If you have any recessed downlights in the rooms beneath your loft, lift any existing insulation and check to see whether the lights are covered by insulation guards. Lights can be a fire hazard if covered by insulation without a protective guard – if you need to buy guards, we’ll explain how to fit them later on in our section ‘How to lay loft insulation around recessed downlights’. Consult this section before beginning to lay insulation.

Ensure the loft properly lit by hanging an inspection light in a suitable place and wear a head torch if there’s no permanent lighting. Clear as much space as as you can to give yourself a large, safe, work area without any trip hazards. Be aware of, and avoid, any sharp nails that might be sticking out from the roof and beams.

Step 2

Enlist a helper to hand the rolls of insulation up to you into the loft. Don’t remove them from their packaging or cut them before lifting into the loft, otherwise they’ll expand making it really tricky to carry up into your loft. They will also release insulation fibres which can be a messy irritant in the home.

If you’re laying narrow, pre-cut rolls (370/380 or 570mm) or encapsulated insulation, you can move on to the next step. If you’re using ant larger width rolls (1140mm), cut to the required width dependent on the spacing of your joists. While still in their packaging, cut along the partially cut perforations using a panel saw. 

 

Step 3

Start in one corner of the loft and work backwards towards the loft hatch. Place 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, you don’t need to remove its silver wrapping, simply roll it out, silver side up. 

Step 4

Continue unrolling the insulation between the joists across the loft floor area. Once you’ve unrolled your first length, gently press the insulation against the joists so there are no gaps, being careful not to compress it. Remember that the insulation will be compressed from being in its packaging, and so it could take a while to reach its full height. You can 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, begin a new roll and butt the joints closely together. Don’t overlap or compress the insulation. 

Step 5

When you reach a cross beam, cut the insulation flush with the beam, then begin again on the other side, working in from the eaves. Use a long sharp knife with a serrated edge, such as a bread knife, to cut the insulation. To make a straight cut, sandwich the insulation between two boards and, using the top board as a guide. 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 be tempted to compress insulation into awkward corners. 

Continue laying the rolls of insulation across the entire loft area until all the base layer is complete. 

Step 6

Lay the second, or top, layer. There’s no need to cut any wide rolls into narrower ones when laying the top layer across the top of the joists, as 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 neatly butted together. 

How to lay loft insulation around cables

When prepping your loft before beginning your insulation project you should have checked the slack in the cables. Based on that you will have decided whether you were going to handle them yourself or consult a qualified electrician. Follow the below steps if you are confident undertaking the job yourself:

  • Turn off your electrical power supply at the fuse box or 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.
  • Once you’re out of the loft, switch the power supply back on.

 

How to lay loft insulation around recessed downlights

When preparing your loft space before beginning your insulation project you should have checked if you have insulation guards fitted over any recessed downlights. The following steps explain how to fit new guards, if there are none already in place, and lay insulation accordingly:

  • 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 manufacturer’s installation instructions for further details.
  • Fit the insulation around the guard. For the base layer, lay the loft roll over the insulation guard. Continue to lay the base layer and top layer of insulation as explained in the ‘How to lay loft insulation rolls’ section.

Tip: Recessed downlights with LED bulbs emit much less heat than those with older-style halogen bulbs, minimising their fire risk when covered by loft insulation. They’re also more energy-efficient, last longer and are cheaper to run. However, you should still use an insulation guard with LED bulbs as it will help maintain the life expectancy of your lights.

Clean up after insulating a loft

When you’ve finished working in the loft, put your clothing in a refuse sack and shake it outside, in order to remove as many insulation fibres as possible before washing them thoroughly and separately from other clothing. Clean safety goggles with cleaning wipes before storing them away and bin your dust mask (if it’s a disposable one). Use a damp cloth to wipe any areas outside the loft hatch which may be covered by insulation fibres and vacuum the flooring beneath the loft hatch. And don’t forget to wash your hands to stop itchy, irritating fibres spreading to your face or anything else you touch.

Anna-Lisa De'Ath

Anna-Lisa De’Ath is a freelance magazine editor and journalist specialising in homes & interiors, gardening and crafts. She has worked on Prima, Good Housekeeping, Woman & Home, Woman and Woman’s Own. Most recently she was Editor-in-Chief for Your Home and HomeStyle magazines. Anna-Lisa also runs The Crafty Country Fox craft workshops from her Somerset studio. Follow her on instagram @mycraftycountrylife.
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