How to insulate a loft - a step by step guide

Insulating your loft is one of the most effective options to help reduce the cost of heating, improve the thermal efficiency of your home, as well as keep your property toasty during the colder months. If your loft is easy to access, doesn’t have damp issues and is not a flat roof, how to insulate a loft is a job worth considering to save on costs.

However, if any existing insulation is wet, or has shown signs of being wet in the past, it’s likely there is a leak somewhere, or that your home suffers from damp and it's not a solid location for DIY loft insulation. In this scenario, you’ll need to instruct a professional installer and potentially a roofer, to inspect the loft before work can be carried out. For DIY novices, insulating with blanket rolls is probably the material to work with when it comes to how to keep your house warm.

How to insulate a loft, step by step

Follow these guidelines for how to insulate a loft...

1. Work out how much insulation you need

Before you can start with how to insulate a loft, you first need to know how much material you'll need. Measure the area of your loft space in square metres by multiplying the length by the width of the space.

‘If there are difficult to reach areas, simply measure the rooms beneath the loft space and work it out that way,’ says Jenny Turner, Property Manager at Insulation Express. Rolls of blanket insulation will come in different widths, so you need to measure the width of your joists and purchase a width of insulation similar to this to minimise cutting.

The blanket insulation should come up to the top of the joists when fitted correctly. Make sure you match the insulation thickness with the joist height. When the surface is even, you’ll lay down more insulation on top. The second layer will need to take you up to the 270mm recommended depth. ‘It’s a good idea to pick up an extra roll or two to be on the safe side,’ says Thomas Goodman, Property and Construction Expert at MyJobQuote.co.uk. Once you have done this, you will be able to work out how much loft insulation costs.

2. Prepare and clear your loft

Firstly, remove all your belongings from the loft, take up any fitted boards attached to the joists, check for loose nails and screws and give the loft a vacuum to get rid of excess dust. Next, check for signs of damp or leaks, if there is, any old insulation will need to be removed to prevent dampness and mould from spreading and a professional installer and roofer should be employed to evaluate the problem.

If existing insulation is dry, then you don’t necessarily have to remove it, you can go ahead with DIY loft insulation over the top of the older materials. Make sure any wiring is dealt with safely. Wires and electrical cables should be kept above the insulation to avoid fire risk, but not stretched. If you don’t think there will be enough slack in the cables to lift on top of the insulation, contact a qualified electrician for advice.

3. Make sure you have the right equipment

When learning how to insulate a loft, it's important to protect yourself adequately. Small particles from blanket insulation can break off when cutting it to size which can irritate the skin (unless using sheep’s wool). Wear long sleeve, full-length clothing, a hat to cover your hair and PPE equipment, available on Amazon, including a face mask, goggles and lightweight work gloves.

You’ll also need crawl boards handy and a knee pad so you can move around safely and comfortably. For the installation process, you’ll need a tape measure, a sharp knife and a pair of sturdy scissors.

4. Lay insulation between the joists

To start with the act of how to insulate a loft, measure the distance between the joists and then cut (if you need to) your insulation to the correct width. Gently roll it out between the joists and lightly press it to fit between the joists. 

‘You need to be careful not to apply too much pressure to avoid crushing it and reducing its insulation value,’ says Thomas from MyJobQuote. Once installed, the insulation should come up to the top of the joists and there shouldn’t be any gaps at the ends.

5. Lay insulation over the joints

Lay an additional layer of insulation at a 90 degree angle to the joists. This second layer should have no gaps between the strips that you lay out. 

If fitted correctly, you shouldn’t be able to see the joists. ‘Don’t overlap the rolls, as this will cause the inefficiency of thermal retention,’ says Jenny from Insulation Express.

6. Insulate pipes and tank

A layer of insulation material should be placed over the water tank and pipes, but not directly underneath them. ‘If you insulate underneath you’ll prevent warmth from the rooms below the loft reaching them,’ says Jenny from Insulation Express. 'This can cause the water in the tank to freeze in cold weather.'

For ease, buy insulation jackets for water tanks and foam pipe insulation, from £1.38 per m, B&Q, which can be cut to size and simply pops onto the pipework.

7. Insulate loft hatch

Cut out a layer of your chosen insulation material. Glue or attach it to the back panel of the hatch. Then attach adhesive draught proofing foam strips around the edge of the hatch door. This will stop draughts coming through and heat escaping.

Remember, that there are loft insulation grants available.