Does it matter if my loft insulation is squashed?
Mineral wool works because of the air trapped in it. Compress it and you remove the air, so squashed insulation performs closer to its compressed thickness than its original one.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 2 min
Short answer
Yes, it matters. Insulation squashed under boards, boxes or a water tank is doing a fraction of its job in that area. It does not recover fully once compressed for a long time, but it is easy to fix by lifting the load off it and replacing the worst of it.
People are often surprised that a material can be ruined by being pressed. The reason is that the fibres are not the insulator — the still air held between them is. Squash it and the air goes, so a hundred millimetres of quilt compressed to twenty-five behaves much more like twenty-five millimetres of insulation than like a hundred.
Step by step
- Look for where things sit on the insulation.Under boards, under stored boxes, under a cold water tank, under anything walked on. These are the areas doing least, and they are usually the areas nobody looks at.
- Check whether it will recover.Lift the load and see. Quilt compressed briefly springs back. Quilt that has been under boards for years often does not, and stays permanently thinner.
- Replace insulation that has stayed flat.It is inexpensive, and there is no way to restore the loft in it. Cutting new quilt in and disposing of the old is straightforward.
- Remove the cause, not just the symptom.New insulation under the same boards is compressed within a week. Raise the boards, or move the storage, first.
- Do not compress insulation to fit it between joists.Quilt is meant to be laid loose and lofted. Squeezing a wider roll into a narrower gap reduces its performance rather than concentrating it.
- Watch the awkward places.Around the hatch, at the eaves, under pipework and around water tanks are where insulation gets pushed, trodden and squashed without anyone noticing.
- Treat rigid board insulation differently.Rigid foam boards do not rely on lofted fibres in the same way and are made to be loaded in some applications. This guide is about mineral wool quilt, which is what most lofts have.
Questions people ask
How much performance is lost?
Roughly in proportion to how much thickness is lost, because the trapped air is doing the work. Halving the thickness costs you most of the benefit of that half.
Can I fluff it back up?
Recently compressed quilt can be teased back to something near its original loft. Long-term compression is usually permanent, and replacement is cheap.