Suspended floor insulation material choice

[montyjohn]

Hi, I've been doing some research on what insulation material to use for my suspended floors.
I'm looking to make sure I'm not making a error leaning towards CosyWool, as I've never used it before so don't know the pitfalls, so looking for opinions and experience.


I've discounted PIR, glass etc as they either don't absorb water, or don't breathe.
I've kept rockwool in as it's slightly better than glass.

After factoring wastage I get material costs as follows:

• Rockwool: £13.75/m²
• Hemp: £14.71/m²
• CosyWool: £15.8/m²
• Woodfibre: £32.95/m²

In terms of thermal characteristics they are almost identical

• Rockwool Flexi: 0.038–0.039
• Hemp batts: 0.038–0.040
• Woodfibre batts: 0.038–0.040
• CosyWool: 0.039

Cosywool (which is 75% sheepswool and some plastic binder) jumps out as a winner. Almost as cheap as others in the list, but far better properties in terms of water absorption, and also very breathable.

I may be able to get better prices on all of these if I shop around more (woodfibre seems a bit pricey), but just want to confirm if my conclusion is sensible.

More research figures below:

Rockwool Flexi (100mm)
• Type: Stone wool batt
• Material cost: £11/m²
• Waste factor: 20%
• Installed cost: £13.75/m²
• Moisture buffering: Low
• Breathable: Yes
• Compressible: Low–medium
• Notes: Needs cutting; robust; affordable
• Link: https://www.wickes.co.uk/Rockwool-Thermal-Insulation-Roll---100-200mm/p/148843

Hemp Batts (100mm)
• Type: Natural fibre batt
• Material cost: £12.5/m²
• Waste factor: 15%
• Installed cost: £14.71/m²
• Moisture buffering: Medium–high
• Breathable: Yes
• Compressible: Medium
• Notes: Great balance of cost, breathability, ease
• Link: https://www.celticsustainables.co.uk/indinature-indiloft-hemp-insulation-batts

Woodfibre Batts (100mm)
• Type: Rigid board
• Material cost: £19.77/m²
• Waste factor: 40%
• Installed cost: £32.95/m²
• Moisture buffering: Medium–high
• Breathable: Yes
• Compressible: Low
• Notes: High waste; awkward from below; not ideal
• Link: https://www.insulationsuperstore.co.uk/product/steico-flex-wood-fibre-insulation-2-737m2-pack.html

Thermafleece CosyWool (100mm)
• Type: 75% sheep wool + binder
• Material cost: £15/m²
• Waste factor: 0–5%
• Installed cost: £15.8/m²
• Moisture buffering: Very high
• Breathable: Yes
• Compressible: High
• Notes: Zero waste; easiest to fit; excellent moisture behaviour
• Link: https://www.insulationuk.co.uk/products/thermafleece-cosywool-roll-100mm-x-370mm

 

[fernicarry]

I was under the impression that Indinature is a good bit cheaper than Thermafleece but I haven't priced the latter recently. I suppose its partly a factor of the wastage when comparing a batt against a roll but I always find somewhere to use the offcuts so have never had any wastage. Haven't used rockwool but isn't it nasty to handle?

I'd think about the properties of the material in terms of how easily you can do the job. Sounds like you are doing it from below which is mostly how I've done it too often with only inches of clearance. Hemp has a good balance of stiffness so it stays put, versus squashability so it can be flexed into place and sits nicely. I feel like it will keep its shape better than wool in the long term so I only put in supports every foot or so. Wool would probably need continuous support so not only additional materials but quite the bun fight trying to hold it in place whilst you get the support under it.

Also think about how easily you can cut the material and how good an edge you will get. Hemp batts are really tough and can't be torn but if you compress them hard and cut with a long carving knife type blade then you get a perfect factory looking edge. Wool you'll be tempted just to rip it and gloss over the raggedy edge but I imagine the same cutting technique works for it. Wood fibre I've found near impossible to cut accurately and the edges are very easily damaged so didn't take well to being bashed into position but that was a very rigid batt which to be fair was intended for overboarding. Steicoflex might cooperate better and I might give it a try next time I'm doing a floor from above. From below hemp is the winner for me.

Do you think you'll need as much as 100mm? I've been using 80mm.

 

[montyjohn]

Indinature is coming out more expansive at the moment, but that may change once I really start searching for the best deals.

Rockwool is ok, not like that horrible fibre glass stuff. Still have bad memories. it's the lack of moisture absorbing that puts me off.

I am doing it from below, but I have a basement everywhere where I have suspended floors, so I'm hoping it will be pretty easy. Whatever material I will use I'll need woodwool, lime and a breathable geotextile so the cost of support doesn't really come into it.

Only thinking of 100mm as I have 4" x 4" secondary joists throughout and why not maximise the performance.

The reason I was leaning for sheepswool is I figured I can be a lot less precise with my cuts, and provided I cut the roll 20mm too wide, it will be fine.

I assume hemp need to be cut more precisely.

That was my main reason, but may be misguided.

 

[skierhughes]

I've used sheep wool rolls for vertical insulation and I found it very easy to use, it stayed put easily, was easy to cut accurately, but I did what you say and gave it an extra inch to ensure it was a good fit, though my cuts were only length wise (as in my cuts were across the width not length), as it was two rolls wide for width ways. I used a large pair of upholstery scissors to cut it.

 

[montyjohn]

I think it's probably a tie between Hemp or Sheepwool.
I tried totting up what an order of Hemp would cost and the delivery was £180.

So it may come down to what's available locally.

 

[claraclattermole]

Is there a potential clothes/carpet moth issue with wool insulation or is it treated ?

 

[montyjohn]

You may have a point

https://www.telegraph.co.uk/news/2017/09/01/grand-designs-dream-home-ruined-biblical-plague-moths/