Best way to insulate internal floor void

[SanMan]

Hi

 

I want to insulate an internal floor void. I have to do this from underneath as the floor above is finished plus the room below needs a new ceiling. The joist centres vary from 290 to 350 mm, so I gather Celotex would be a nightmare, also because the joists are frequently tapered. The joists are 200 mm tall. 

 

So I was going to use 100 mm Rocksilk slabs. 

 

My questions:

 

1. Should I opt for foil faced? What is the benefit? Which way should the foil point - up towards floor or down towards ceiling? The main aim of the exercise is to reduce downward heat loss from the UFH floor above. 

 

2. Do I need an air gap between the insulation and the floor above? Or is the 100 mm void left between slab and ceiling below sufficient, especially because the ceiling will have downlights. 

 

3. What is the best way to secure Rocksilk slabs from underneath, to stop displacement, sagging and ensure good contact with the floor above? 

 

Thank you. 

[Olf]

On 12/12/2022 at 09:19, SanMan said:

I want to insulate an internal floor void

With extra detail in the original thread I now don't think it will be beneficial, but for reference:

 

 

On 12/12/2022 at 09:19, SanMan said:

Should I opt for foil faced? What is the benefit? Which way should the foil point - up towards floor or down towards ceiling?

Foil face is radiant heat reflector, airtight barrier and (marginally) they equalise surface temperature. Radiant heat transfer between the floors is not going to be big (low surface temperatures and small temperature difference between them), so the extra cost of foil facing may not pay back. Airtigtness internally does not matter. So all in all not worth it internally, unless difference in price negligible (or other factors in place, eg supply of non-faced, having leftovers etc)

For the reflection to work you need an air gap (but that will be the case anyway, explained below) and point it to the underneath of the floor.

 

 

On 12/12/2022 at 09:19, SanMan said:

What is the best way to secure Rocksilk slabs from underneath, to stop displacement, sagging and ensure good contact with the floor above?

 

All the wools are intended to be supported from the bottom: even if you pack it between the joists, the panels can sag anyway as you already noticed. Addidng noggings wil reduce sagging, but not eliminate it - and that at a lot of expense in time and materials. Glueing is pointless, as the boards themselves have poor structure and can leave just the very top layer attached, with the rest going loose. I think it is safe to asume that ultimately the insulation may shift all the way down until it sits on the ceiling board, if not - that's a bonus.

[SanMan]

20 hours ago, Olf said:

With extra detail in the original thread I now don't think it will be beneficial, but for reference:

 

 

Foil face is radiant heat reflector, airtight barrier and (marginally) they equalise surface temperature. Radiant heat transfer between the floors is not going to be big (low surface temperatures and small temperature difference between them), so the extra cost of foil facing may not pay back. Airtigtness internally does not matter. So all in all not worth it internally, unless difference in price negligible (or other factors in place, eg supply of non-faced, having leftovers etc)

For the reflection to work you need an air gap (but that will be the case anyway, explained below) and point it to the underneath of the floor.

 

 

 

All the wools are intended to be supported from the bottom: even if you pack it between the joists, the panels can sag anyway as you already noticed. Addidng noggings wil reduce sagging, but not eliminate it - and that at a lot of expense in time and materials. Glueing is pointless, as the boards themselves have poor structure and can leave just the very top layer attached, with the rest going loose. I think it is safe to asume that ultimately the insulation may shift all the way down until it sits on the ceiling board, if not - that's a bonus.

Thank you for your replies. 😊