How to insulate a loft

Based on a clear guide from a trusted UK source. This walk-through draws on the popular "How to Install Loft Insulation" video from B&Q, which covers the process from prep through to finishing neatly around the hatch. Worth watching before you haul those rolls up the ladder — it makes the two-layer approach much easier to picture.

1. Check what you already have up there

Before buying anything, pop up into the loft with a torch and a ruler. Measure the depth of any existing insulation. Current UK guidance recommends a total depth of around 270 mm. If you have 100 mm of old mineral wool, you are adding a top-up layer, not starting from scratch.

Old insulation that is damp, compressed flat, or clearly disturbed should be replaced rather than built on. Take a moment to look for any signs of a leak too — no point trapping moisture under a fresh layer.

2. Sort the loft floor before you start

Move stored items off to one side, or bring them down. You need a clear run across the joists to work safely and lay the insulation flat. While you are up there, check that any pipes and cold water tanks are accessible — these will need lagging separately.

If the loft has boards down, lift the section you need to work on. Do not assume the old boards can stay in place beneath the new insulation — they often prevent the lower layer from sitting properly between joists.

3. Wear the right protective gear

Mineral wool insulation is itchy stuff. It irritates skin, eyes, and the back of your throat. Wear long sleeves, disposable gloves, a dust mask rated to at least FFP2, and safety glasses. Wash your hands and arms thoroughly once the job is done.

It is also worth putting a temporary board or two across the joists to kneel on. Putting a foot through a ceiling is an expensive afternoon. Take your time moving across the loft.

4. Lay the first layer between the joists

Cut the insulation roll to length using a craft knife or scissors — leave it in the packaging until you actually need it so it does not expand prematurely across the floor. Lay it flat between each pair of joists, pushing it gently down so it sits snug at the bottom without being compressed.

The first layer typically sits at 100 mm deep, matching the joist depth. Do not overfill the gaps. Compressed insulation is less effective than insulation allowed to sit at its natural thickness.

5. Lay the second layer across the joists

This is the step that catches people out the first time. The second layer runs at right angles to the first, directly across the top of the joists. This covers the joists themselves — which would otherwise act as cold bridges — and brings you up to the recommended total depth.

Keep each run continuous where you can. Butt the ends of rolls tightly together rather than leaving gaps, and use leftover offcuts to fill awkward corners around the eaves.

6. Keep the eaves clear

Do not push insulation right into the eaves. The gap between the roof rafters and the top of the wall needs to stay open to allow air to circulate and prevent condensation building up in the roof structure. Most rolls come with an eaves ventilation guide; follow it.

Some lofts already have rigid eaves baffles fitted. If yours does not, it is worth adding them before laying insulation — a small job that protects the roof long term.

7. Insulate the loft hatch

The hatch is a classic heat leak that most people forget. Stick a layer of insulation to the top of the hatch board with adhesive, making it at least 100 mm thick. Then fit a draught-proof seal around the frame so warm air cannot sneak around the edges.

If the hatch has a pull-down ladder, there are purpose-made insulated covers available that box the whole thing in. Worth considering if you are doing a thorough job.

8. Lag any pipes or tanks in the loft space

Once you have insulated the floor, the loft itself will be colder than before — because the heat from the rooms below is now staying in the rooms. Any water pipes or cold water tanks sitting above the insulation will be exposed to lower temperatures. Lag them properly before you close up.

Foam pipe lagging is cheap, quick to fit, and cuts easily with a bread knife. Cold water tanks need a proper insulation jacket. Do not skip this step in properties that might sit unheated for periods.

When to call a handyman

Call Richard if access to the loft is awkward, if the existing insulation looks damaged or damp, or if there are signs of a roof leak that need investigating first. Loft insulation itself is straightforward — it is the getting up there safely and the associated checks that sometimes need an extra pair of hands.