Can I use Earthwool ceiling insulation as subfloor insulation?

Question

Hi Bunnings team,

I’ve started insulating my subfloor, and wasn’t sure if I should use earthwool subfloor insulation, or if I could get away with using earthwool ceiling insulation? (mainly as the subfloor earthwool insulation at Bunnings only has an R value of 2.5, and don’t want to use any polyester insulation as that would be silly in the case of a fire).

Due to the house structure, there is a depth of approx 450mm where I could stuff in plenty of insulation. Therefore, I was wondering if I could use earthwool ceiling insulation of R5 or 6?, which is  much thicker. As this is earthwool, I assume there would be no moisture issues? Or am I mistaken, as the thickness could increase the risk of moisture being trapped?

What is the difference between earthwool ceiling and subfloor anyway? Can’t be much difference, or is there?

Should I also cover the mesh with the aluminium insulation foil, or does that increase the risk of trapping moisture even more (even if the foil is breathable)

Many questions I know! :) Thank you for your help

JacobZ · Answer

Hi @johan ring,

I've just spoken with the Technical team at Knauf, who produce Earthwool.

They've said that you can use the ceiling insulation in a subfloor application, but the density that I mentioned above would come into play. The subfloor insulation is denser and has a wind resistant coating, which means it will not be affected by wind; however the ceiling insulation is a lighter product without a wind coating so if there is any wind within the floor cavity, you can lose some of that r rating that you are chasing.

Looking at the floor cavity, assuming that image is yours, I can't imagine you would be getting roaring winds, so this wouldn't be too big a risk as long as you fix the insulation in place firmly, but it is still worth noting.

So, essentially, yes, you can use ceiling insulation in this area, but if wind were to rip through, it can reduce the r rating.

They also mentioned that aluminium faced insulation products are reflective in nature and with the subfloor being closed, it wouldn't really be exposed to reflective heat, so there isn't really much point using it here.

I hope this helps to answer your questions. Please let me know if there is anything else I can assist with.

Jacob

JacobZ · Answer

Hi @johan ring,

While it is physically possible to use ceiling insulation under a floor, the products are designed a bit differently for their intended environments. Ceiling insulation is thicker, softer and is made to sit flat in a relatively still, dry space. Underfloor insulation is more rigid because it designed to be friction fit between joists, so it needs to hold its shape, as well as being able to cope with airflow and exposure from below.

Theoretically, more, or thicker insulation would increase thermal performance, however in a subfloor application where air flow and moisture are major concerns, I am not certain whether it would cause issues with trapped moisture or not.

Unfortunately, I can't say I am 100% certain whether this approach would be viable or not and I'd like to err on the side of caution. So, if you could please bear with me, I will reach out to our suppliers' technical teams for their advice when they reopen tomorrow and get back to you when I have an answer.

Jacob