Mineral wool loft insulation

How it works

Mineral wool traps still air between millions of fine fibres. Still air is a poor conductor of heat, so a deep, fluffy layer slows down heat escaping from the rooms below. That is also why it must not be squashed: compress it and you push out the air that does the insulating.

Glass wool or rock wool?

Glass wool is lighter and springier and is the usual choice for loft rolls. Rock wool (stone wool) is denser, handles heat and fire especially well, and also deadens sound better, which is why it is often used between floors and in party walls. In a loft, either does the job.

How it’s laid

The first layer goes between the ceiling joists, filling the gaps to the top of the timber. A second layer is rolled out across the joists at right angles, which covers the timber so it doesn’t act as a cold bridge, and takes the total to the recommended depth. The eaves are left clear for ventilation, and pipes and tanks are lagged because the loft will be colder.

Pros

• Usually the lowest-cost material for a loft floor
• Non-combustible
• Quick to lay
• Doesn’t rot and isn’t eaten by pests (though they will nest in it)

Cons

• Itchy to handle: gloves, long sleeves and a mask are a must
• Takes up a lot of depth, so you can’t board straight over it; see raised loft boarding
• Loses performance if it gets wet or squashed

Thermal conductivity (the “lambda” value, in W/mK) tells you how well a material resists heat: the lower the number, the less thickness you need. Figures here are typical ranges; the product’s own declaration of performance gives the exact value.