How do I choose insulation material?

Insulation is usually compared on thermal performance alone, and in an older building that is the wrong first question. What the material does with moisture determines whether the job improves the house or quietly rots it.

Safety

Step by step

1. Start with where it is going.A ventilated loft, a solid wall, under a floor, or a flat roof. Each has different constraints on thickness, moisture and fire, and the material follows from that rather than from a performance table.
2. Understand what the numbers mean.Lambda is how well a material insulates per unit thickness — lower is better. U-value is the performance of the whole build-up. Comparing lambda alone ignores how much thickness you can actually fit.
3. Use mineral wool where there is space.Glass or rock wool. Cheap, non-combustible, easy to fit, and the default for loft insulation between and over joists. It performs poorly if compressed, so it needs room.
4. Use rigid boards where thickness is limited.PIR and phenolic boards insulate far better per millimetre, which matters in a floor, a flat roof or an internal wall where every centimetre costs floor area. More expensive and combustible, so check the fire requirements.
5. Consider natural fibre for older buildings.Wood fibre, sheep's wool, hemp. They handle moisture by absorbing and releasing it, which suits solid-walled buildings designed to breathe. More expensive and considerably safer in that context.
6. Think about the moisture path before the performance.In an older solid-walled house, a vapour-closed insulation traps moisture in the wall. This is how insulation causes rot, and it is why solid wall work needs proper advice.
7. Check the fire requirements.Combustible insulation has restrictions depending on the building and where it goes. Mineral wool is non-combustible; most foam boards are not. This matters more in flats and near escape routes.
8. Do the cheap measures first.Loft insulation and draught-proofing save more per pound than anything else. Work down the list by cost-effectiveness rather than starting with the most technical job.

Tips

• Insulation only works if it is fitted without gaps. A small unfilled gap disproportionately reduces performance — fitting quality matters as much as material choice.
• Check for grants before paying. Loft and cavity wall insulation are frequently subsidised and sometimes free depending on circumstances.
• Do not compress mineral wool to fit. Its value comes from the trapped air, and squashed into a gap it can lose half its performance.

Questions people ask