Before you start, check for things like damp and rot in the loft. If you spot anything that concerns you, call in an expert, unless you’re confident you can deal with it yourself.
Mineral wool insulation is often used in lofts, but it can irritate the skin, throat and eyes (unless encased in foil), so wear gloves, goggles, a dust mask and overalls when fitting it. You may prefer eco alternatives, such as insulation made from sheep’s wool or recycled newspapers or plastic bottles - try B&Q for these.
Before buying insulation, measure the floor joists in the loft and try to get insulation the same width or bigger - cut it with a saw when it’s rolled up to make life easier. The first layer of insulation should be laid between the joists, with a second layer on top, laid at 90 degrees, up to the required depth (see below). When joining pieces of insulation, butt them together.
The recommended depth for mineral wool insulation is usually 27cm, but this may vary for other types of insulation, so check the packaging or direct with the manufacturer. You don’t have to start from scratch if you already have insulation (in good condition) in place - just put the new stuff on top.
If you want to use the loft for storage, you’ll need to board it, but don’t put boards directly on top of the insulation because they will compress it and make it less effective. Instead, use loft legs, which raise the boards above the insulation so it’s not compressed, or rigid insulation, which can’t be compressed.