What is the best insulation to use?

I’m planning to insulate my suspended floor. It’s about 70 sq m. Without really thinking about it, my go-to insulation product would be some sort of insulation board like Celotex or Kingspan. These are readily available, hydrophobic and have a great R value.

However, I’m wondering what the environmental impact of these products is and whether there are more sustainable alternatives out there.

Any advice?

Well, the embodied carbon and VOC content will be pretty horrible, but I must admit that I have used them (seconds actually) recently on a temporary project due to cost, convenience and simplicity. For a more permanent solution we all should be using natural fibre insulation - there’s even a trade association for the manufacturers of these products - using materials such as wool, wood fibre, hemp, cork, recyled clothing etc. Take a look: https://asbp.org.uk/natural-fibre-insulation
The key thing I’ve learnt is to have a systemised approach as one measure impacts on another, so it’s really difficult to achive meaningful efficiencies unless you consider detailing, thermal bridging, air circulation and other inter-related aspects. Unintended consequences coudl lead to condensation, damp, mould, poor air quality etc.
Hope that helps!

Thanks @Locky. The issue that I have with most natural fibre insulation is that it is not hydrophobic. This is really important to me because I live on a steel boat and any water absorbed by the insulation will rust the hull from the inside out! This is something that the insulation board does very well. You can literally soak Celotex in water for years and it doesn’t absorb one drop. I have actually fished old Celotex boards out of the river and used them for insulation and they are completely fine :joy:

Thanks for the link though, really useful. It seems like there are plenty of alternatives out there that would work in lots of cases.

Given your challenging requirements (i.e. hydrophobic) you could consider closed-cell polyurethane spray foam insulation. This is effectively the same material as the rigid boards such as celotex but because it is applied in situ it will conform to the shape of the hull and provide a continuous layer so there are no cold bridges or gaps.

Obviously it still has all of the environmental drawbacks of PU foam but it will at least provide better insulation than rigid boards. If you decide to go with it makes sure you find an accredited installer. Quality control procedures during installation are critical because the two part mixture (polyol and isocyanate) needs to react together fully to become safe. If there is an incomplete reaction the foam is of poor quality and poses a significant health risk.

Have you considered coating the steel hull in some way to keep it dry so that you have more options for insulation?

Thanks @EcoEngineer. My initial post was actually about floor insulation but what you say about the effectiveness of spray foam for insulating a steel hull is completely correct.

The PU spray foam option is probably the best bet if it can be done. Unfortunately it usually requires vehicle access as the 2 part mixture also needs to pass through a heater and the specialist equipment for this is usually within the van. Sadly we have no vehicle access :frowning: . Thanks for raising this option though.

With regards to coating the hull, the only coating that would work would be spray foam, as this would also act as insulation, the issue is that steel is cold and completely water-impermeable so any air that contacts the hull will form condensation. If condensation forms it is important that the insulation does not trap it against the hull as this will cause it to rust from the inside out. Needless to say, this would be very very bad. This is why, if you cannot stop air reaching the hull completely, the insulation needs to be completely hydrophobic ie. insulation board.