Cut a piece of glass-fibre or mineral-fibre blanket or thick expanded polystyrene sheet to fit above the loft hatch.
To fix the blanket, hammer about two nails along each edge of the door, depending on its size. Tie string over the top of the material and loop it around the nails to hold it in place. Do not pull the string so tight that it squashes the blanket.
Alternatively, cut a piece of tough brown paper large enough to cover the blanket. Fix the paper over the blanket, holding the edges in place with drawing pins.
If you are using expanded polystyrene sheet, stick it to the door with adhesive designed for the purpose, such as Evo-Stik ceiling tile adhesive or Unibond polystyrene adhesive.
Make sure that the hatch door is a tight fit. Fix a foam, rubber or flexible draught excluder to the rebate so that damp air cannot pass through into the cold loft and cause condensation.