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A guide to DIY loft insulation

A pile of around a dozen rolls of loft insulation waiting to be laid.
29 February 2024

A straightforward job for a competent DIY-er …

Insulating your loft is a simple and effective way to reduce heat loss, keeping you warmer in the winter and cutting energy bills. Fitting the insulation yourself can help keep the cost down.

And what will it cost?

The table below shows typical figures based on a loft of a typical 3-bedroom semi, and using mineral fibre. Other materials may cost more. A typical payback time is about 1 to 5 years, depending on the fuel used.

Installing the full 270mm£250
Topping up from 120mm to 270mm£25

What type of insulation material should I use?

The most common form of loft insulation is mineral fibre which comes in rolls that are laid out at the floor level of the loft between the joists. Loft insulation can also be made from sheep wool, hemp fibre, flax, cellulose or recycled materials like plastic, glass or newspaper.

There are alternatives such as rigid boards, blankets (where insulation is sealed inside a foil bag) and foam panels that squeeze between joists. Some materials are better insulators than others. So you could use a thinner layer of a solid insulation board to achieve the same results as a thick layer of mineral fibre, for example.

For this guide, we’ll assume that you’re using standard mineral fibre insulation. If you want to compare materials, you can use the figures on the product packaging or get the figures from manufacturers. Some retailers provide leaflets comparing the relative properties of different materials. Look out for:

Before you start

You’ll need to know how many rolls to buy – most retailers and manufacturers can calculate this for you if you provide them with the floor area of your loft and the depth you need.

In most properties you can measure the floor area using the length and width of the rooms beneath.

For mineral fibre, the recommended depth is 270mm. So if you’ve already got 100mm of insulation in the loft (about level with the height of the joists), you’ll need rolls that are 170mm thick to bring your insulation up to standard. Also buy some draught-proofing strips for the edges of the loft hatch.

Check that your loft hatch is large enough for you to comfortably get through with a roll of insulation. If it’s very small, you’ll need to get it widened by a competent joiner. Be sure you have a sturdy and secure ladder to get yourself safely in and out of the loft. You’ll also need a couple of loose planks or loft boards to stand or kneel on once you’re up in the loft. Be very careful to only put weight on the boards so you don’t damage the ceiling or yourself. Check there’s enough lighting and ventilation for you to work.

Get the gear

We recommend that you use goggles, a mask that covers your mouth and nose, and gloves. Most DIY stores sell all of these. You’ll also need a tape measure and a knife or long scissors to trim the insulation. Wear overalls if you have them.

Plastic googles, a face mask and a pair of work gloves.

Before you get going …

Make a plan

Aim to start at the corners and edges and insulate towards the hatch so you have more room for the cutting and joining. If no insulation is in place already, you’ll need to lay the rolls between the joists first and begin the second layer once the whole of the first layer is done.

Always read the manufacturer’s instructions and take care when cutting and unpacking the insulation.

Most insulation is compression packed and will expand once the wrapping is removed, so only unpack the rolls once they’re up in the loft. It’s also much easier to cut the insulation while it’s still rolled up. You may find that the packaging has a line showing where to the cut the insulation so it fits neatly between the joists.

Don’t squash the insulation to fit into corners. Trim it to fit properly, and where one roll ends and another begins, push the ends together leaving no gap.

For ventilation, leave a gap of at least 25mm around the edge of the loft. Do not lay the insulation all the way up to the eaves. If your boiler flue goes through the loft, leave a gap around this as well.

Install the first layer to roughly the depth of the joists. To minimise heat loss and give an even coverage, fit the second layer across the joists at right angles

Showing a double layer of insulation, with the top layer laid at right angles to the layer below.

Don’t forget the loft hatch itself. Insulate the back of the hatch by using a square of insulation, wrapped in polythene. Fit it to the hatch using staples or tacks. Then draught-proof around the frame so it fits snugly.

Water and electrics

If you have a cold water header tank or cylinder in your loft, you need to reduce the risk of it freezing by lagging it, but do not put insulation under the tank. Also lag all the water pipes so they don’t freeze.

It’s a good idea to retain a walkway to any water tanks and boilers in case they need servicing. Make sure the walkway is easily identifiable – once the insulation is in place it can be very difficult to see where the joists are, so fit walk-boards if necessary.

Make sure any electrical cables are lifted above the insulation and that recessed light fittings are not covered.

Ventilation and condensation

Once the loft is fully insulated, the air temperature in the loft will be considerably cooler. Because cold air cannot hold as much moisture as warm air, condensation is more likely to form. Condensation can create a damp problem, so make sure your loft is adequately ventilated and that air can circulate. Be careful not to cover air vents, and make sure that the insulation is not pressed up around the outside walls or eaves.

Check the loft from time to time to spot any condensation problems. You may find additional ventilation is required. If you are concerned about condensation making items stored in the loft damp, consider using plastic boxes with fitted lids.

What if I want to use my loft for storage?

If you need to use your whole loft for storage then topping the insulation up to 270mm may not be practical. This is because if you squash the insulation down underneath loft boards it becomes less efficient at keeping the heat from escaping. One option is to raise the height of the joists to allow for 270mm of uncompressed insulation and placing boards on top. But this will increase the cost of the job considerably, will reduce height of the loft space and may need a professional joiner.

A collection of random objects that might be found in an attic: a space hopper, a hideous vase, a cardboard box and an oil painting of the Madonna and Child.

Think about what you would like to store and whether frequent access is required. If it’s just a few boxes, you could fully insulate most of the loft and just leave a small boarded area near the hatch. You could also consider using rigid insulation boards rather than mineral fibre.

Bats in the loft?

Bats are protected by law. You may be committing a criminal offence if you capture, injure or kill a bat. It is even an offence to disturb a roost or obstruct the bats’ access to their roost.

If you have bats in your loft, we recommend you contact the Bat Conservation Trust who can provide free advice. In Wales, contact Natural Resources Wales.

Close-up of three lesser horseshoe bats hanging in a loft. Photo by John Black and Bat Conservation Trust.
Lesser horseshoe bats (photo: John Black / Bat Conservation Trust)

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