Laying blanket insulation is usually a pretty straightforward affair, once you’ve done the preparatory work of clearing the decks and sealing any wire/pipe/vent entry points with mastic.
Having calculated the amount of insulation you need and bought the necessary number of rolls, you can now take it up into the loft and open your first roll. (Don’t be tempted to open it up to have a look at it before you get it up there. It expands when the wrapping is breached!)
Place the free end between a pair of joists, pointing into the eaves, but don’t block the ventilation gap – remember what we said about allowing proper circulation of air through the loft? You can either trim the end of the roll down to a bevel edge (cheaper, but more fiddly) or else fit eaves vents.
You can now unroll the blanket along the run of the joists, remembering to keep electrical wires and fittings clear of the blanket to avoid overheating. Make sure it fits snugly, but don’t press it down too hard – the air within the blanket adds to the insulating effect, and expelling the air by compressing it lessens the insulation. If the roll’s a bit wider than the joists, let it curl against both of them to left and right.
When you come to the end of your first roll, butt the start of the next one up against it and roll it out towards the other side of the loft. Don’t forget to make sure that air is allowed to circulate through the other set of eaves!
If there’s an obstacle that gets in the way of the blanket (like the cold-water tank or the top casings of light fittings, for instance), or an odd space that needs filling, you can trim the blanket to fit using a large knife or suitably sized shears/scissors.
Likewise, you may want to cut a suitably shaped piece of blanket to insulate your loft hatch cover; you can use gaffer tape, drawing pins or adhesive to hold it in place.