Insulation foil between timber sub-floor and plywood sheets?

[_Alex_]

Hi,

 

I want to install Herringbone engineered wood floors (10mm thick) gluing them onto plywood sheets. I have timber sub-floors.

 

I wanted to know: Is is possible to install insulation foil (like Sonic Gold) between the timber sub-floor and the plywood sheets? If that is possible, what would be a recommended maximum thickness for the insulation foil?

 

Thanks to anyone giving advice,
Alex

[JohnMo]

9 hours ago, _Alex_ said:

between the timber sub-floor and the plywood sheets?

Do you mean compressed between two layers of wood?

[_Alex_]

11 hours ago, JohnMo said:

Do you mean compressed between two layers of wood?

Hi,

 

Yes, the layers would be:

• Timber sub-floor (18mm).
• Insulation foil on top of the timber sub-floor.
• Plywood sheet on top of the insulation foil, nailed down to the timber sub-floor (with the nails going through the insulation foil).
• Herringbone engineered wood floors on top of the plywood sheet, glued down to the plywood sheet.

Thanks,

Alex

[JohnMo]

Would give zero insulation value unless installed with air gap either side and allowed to fully expand.

 

Do PIR insulation of same thickness

 

[_Alex_]

Thanks!

 

For me to understand: I would use the same layout, but replace the insulation foil with PIR, is that correct? (timber sub-floor | PIR board | plywood sheet | engineered wood floors).

 

Also (and pardon my ignorance), some PIR boards are made of foam which would make the floor unstable. Are there any boards offering thermal and acoustic insulation (and ideally not flammable) which are solid and rigid enough to put below the plywood sheets? Perhaps https://www.sruinsulation.co.uk/acoustic-floors/rockwool-rockfloor-acoustic-slab-25mm ?

 

Thanks again,
Alex

Edited January 7 by _Alex_
(not clear explanation in original message)

[saveasteading]

Topps Tiles, backing board.

There are others like this. Jackoboard. Wedi board. It is much denser, made for under tiles, and it has thermal qualities which the foil would not.

And/ or mineral wool in the space below.

[torre]

Have you ruled out insulating below your existing timber floor? You'll be able to fit far more insulation.

 

It sounds like you're already raising your existing floor level a fair bit with both ply and 10mm flooring and something in between, is that going to cause problems elsewhere? Including the 25mm insulation you linked would mean losing overall best part of 50mm headroom throughout, including all the doorways and make your bottom/top stair a tripping hazard due to uniform height.

On a positive note those dense foam underlays like sonic gold are very stable and do a great job of deadening sound (have used under a floating floor before now)

[Redbeard]

12 hours ago, _Alex_ said:

Also (and pardon my ignorance), some PIR boards are made of foam which would make the floor unstable.

 

Unless something has passed me by *all* PIR boards are made of foam. I have done a number of floating floors on PIR,on XPS and on rigid wood-fibre, and in none of those cases has the floor felt anything other than solid. No discernible bounce whatever. I have never done one with rigid rockwool, though, so unfortunately I cannot comment, except to say that if I were trying it as an experiment I would start with the external wall insulation grade of rockwool.

 

BTW the web says this about thermal conductivity of the RW boards you mention: "Thermal performance. ROCKWOOL Thermal RockFloor has a thermal conductivity of 0.038 W/mK. ROCKWOOL Acoustic RockFloor has a thermal conductivity of 0.040 W/mK".

 

So just over half as good as PIR for a given thickness, but it has the acoustic properties which PIR neither has nor claims.

[_Alex_]

Thanks a lot @JohnMo, @saveasteading, @torre and @Redbeard!

 

I understand the correct way of achieving proper insulation is through the subfloor void but unfortunately that will take time, effort and skill beyond what I have. I will reach as well two people I know who work in construction (although they are not specialized on flooring) for more feedback.

 

I had one last question: In the case of the floor layout below

---------------

Engineered wood (10mm)

---------------

Plywood (around 10mm max)

---------------

Professional Sonic Gold underlay (7mm)

---------------

Timber sub-floor (18mm)

---------------

 

Does anyone know of any material which could replace the plywood (meaning it is about 10mm thickness maximum, rigid, can be nailed down to the sub-floor and can stand large weight loads on top) which has also good thermal properties? 

 

Thanks everyone for the huge help!

Alex

[Redbeard]

4 hours ago, _Alex_ said:

Does anyone know of any material which could replace the plywood (meaning it is about 10mm thickness maximum, rigid, can be nailed down to the sub-floor and can stand large weight loads on top) which has also good thermal properties? 

Sadly not, if you really need it to do exactly what ply would do. Even if you could find a rigid board claiming insulation qualities at that thickness it would be likely to have all the insulation value of a thin thing. I am aware that anything is better than nothing but I feel that no 10mm board is going to give you an exciting effect in the lay-up you describe.

[Iceverge]

Thermally you're not going to achieve much here in terms of reducing conductive heat losses. The build up is just too thin. 

 

This is what you have currently. 

 

 

 

This is what you are proposing. Sonic gold isn't magic. It'll perform much the same as any thin layer of insulation but it's K value isn't great at 0.048W/m2k. 

 

 

Ideally you would lift the existing floorboards and insulate between the joists, lay something like Egger p5 chip board on top and then your floor. 

 

 

 

 

However if you've ruled this out then I would really concentrate on air sealing all the drafts from the floor. This really is the biggest win thermally and in terms of comfort. I wouldn't bother with the Sonic gold. Its mostly marketing guff. 

 

Like mentioned above you could glue the wood floor directly to a tile backing board like Jackoboard which has a K value of 0.034 W/m2K. Lay the boards and seal all joints fastidiously with sealant or tape. Seal them to all perimeter walls similarly to completely eliminate all drafts. This will be your biggest win. The elimination for the need for a layer of ply or OSB will buy you some more insulation depth too. 

 

Here's 12mm. 

 

 

 

And here's 20mm if you can fit it. 

 

 

 

 

Both are better than the sonic gold in terms of heat loss but like I said before. The really really big gain is to be had in airtightness and the elimination of drafts. 

 

 

 

 

 

 

 

 

 

 

 

Edited January 9 by Iceverge

[_Alex_]

On 09/01/2025 at 14:31, Iceverge said:

Thermally you're not going to achieve much here in terms of reducing conductive heat losses. The build up is just too thin. 

 

This is what you have currently. 

 

 

 

This is what you are proposing. Sonic gold isn't magic. It'll perform much the same as any thin layer of insulation but it's K value isn't great at 0.048W/m2k. 

 

 

Ideally you would lift the existing floorboards and insulate between the joists, lay something like Egger p5 chip board on top and then your floor. 

 

 

 

 

However if you've ruled this out then I would really concentrate on air sealing all the drafts from the floor. This really is the biggest win thermally and in terms of comfort. I wouldn't bother with the Sonic gold. Its mostly marketing guff. 

 

Like mentioned above you could glue the wood floor directly to a tile backing board like Jackoboard which has a K value of 0.034 W/m2K. Lay the boards and seal all joints fastidiously with sealant or tape. Seal them to all perimeter walls similarly to completely eliminate all drafts. This will be your biggest win. The elimination for the need for a layer of ply or OSB will buy you some more insulation depth too. 

 

Here's 12mm. 

 

 

 

And here's 20mm if you can fit it. 

 

 

 

 

Both are better than the sonic gold in terms of heat loss but like I said before. The really really big gain is to be had in airtightness and the elimination of drafts. 

 

 

 

 

 

 

 

 

 

 

 

 

Thank you so much @Iceverge for your huge hep, this incredible message rounds all the other messages in this thread and makes this much easier to understand for me.

 

[_Alex_]

I think this thread was extremely helpful and I want to thank everyone for all the help. I am sure other non-expert DIY people like me will come across it in the future and all the information will be of great help.

 

Best regards,

Alex