Attic Insulation Materials Compared

Mineral wool, cellulose or boards: the difference matters less than getting the depth and the air sealing right.

For a cold loft, where insulation sits on the ceiling joists rather than under the roof slope, the practical options are mineral wool batts, blown cellulose, and rigid boards. Mineral wool is cheap, non-combustible and easy to handle, but only performs if it is laid without gaps and never compressed. Blown cellulose fills awkward shapes and around cables better than any batt, which is why it often outperforms a nominally better material in a real loft. Rigid boards give the highest performance per millimetre and are the right choice only where depth is genuinely constrained.

Depth Beats Material Choice

The difference in thermal conductivity between mainstream mineral wool and cellulose is small, roughly 0.035 against 0.038 watts per metre kelvin. The difference between 150 mm and 300 mm of either is enormous. If your budget forces a choice between a premium material at modest depth and a standard material at full depth, take the depth every time. Current practice for a cold loft is around 300 mm, laid in two layers with the second running perpendicular to the first so the joists are covered rather than left as thermal bridges.

Air Sealing and Ventilation

Insulation slows conduction; it does nothing about air leaking through gaps around loft hatches, downlights and service penetrations. Seal those first, because warm moist air escaping into a cold loft is what causes condensation on the underside of the roof. Equally, do not block the ventilation path at the eaves. Insulation pushed tight into the eaves stops the airflow that keeps the roof structure dry, and the resulting damp does more damage than the heat you saved is worth.

Expert Tip

Never insulate underneath a cold water tank in the loft, and do insulate its lid and sides. The whole point is to let a little warmth from below keep the tank above freezing while stopping evaporation and heat loss elsewhere.

Summary

Pick the material that suits the access and the shape of your loft, then commit to full depth in two crossed layers. Seal the air paths before you lay anything and keep the eaves clear. Our Attic Insulation Calculator converts your loft area and target depth into the number of rolls or bags to order.