Correct way to install XPS insulation boards on top of plywood?

[_Alex_]

Hi,

 

I will be installing Herringbone engineered wood floors (10mm thick) on top of timber subfloor on a first floor on a house. As the Herringbone wood floors are small, the manufacturer insisted on gluing them down. I would like to put as well some insulation between the engineered wood floors and the timber subfloor (unfortunately, the correct way to insulate, by putting a thick thermal insulation material below the timber subfloor between the joists is ruled out).

 

I have an idea on how to install each layer, but the XPS.

 

Option 1: The manufacturer mentions to just lay them: Underwood installations using a Fastwarm XPS insulation board are laid directly on to a clean, dry level subfloor in a brickbond pattern (boards are joined together using our heat resistant duct tape). (source https://www.fastwarm.com/fastwarm-10mm-xps-insulation-boards-11107-p.asp ). The different layers would be as below:
    ---------------------------------------------
    Herringbone floor (10mm)
    ---------------------------------------------
    Plywood (4mm)
    ---------------------------------------------
    XPS boards (Fastwarm) (10mm)
    ---------------------------------------------
    Timber subfloor (18mm)
    ---------------------------------------------

 

Option 2: I read in several forums the best way to install such panels for wood floors is by gluing them down with a polystyrene adhesive (solvent-free). In that case I would avoid gluing them directly to the timber subfloor, adding another thin plywood layer, with the layout being:
    ---------------------------------------------
    Herringbone floor (10mm)
    ---------------------------------------------
    Plywood (4mm)
    ---------------------------------------------
    XPS boards (Fastwarm) (10mm)
    ---------------------------------------------
    Plywood (4mm)
    ---------------------------------------------
    Timber subfloor (18mm)
    ---------------------------------------------

 

My questions are:

1. Should option 1 or option 2 be avoided for some reason?
2. Would option 1 lead to squeaky / raised floors as nothing is holding the XPS boards down?
3. Would option 2 end up cracking the XPS panels as they adhere to wood which can warp with time?

 

Any feedback / opinions will be welcomed.

 

Thanks in advance,
Alex

 

Edited February 10 by _Alex_
Grammar

[Beau]

Could you use a tile backer board XBS insulation like Jackoboard and just bond the herringbone straight to it? Ive only tiled onto it but would have thought with the right adhesive you could bond it to wood. Might be worth having a read of theirs and similar products specs

[_Alex_]

On 10/02/2025 at 06:59, Beau said:

Could you use a tile backer board XBS insulation like Jackoboard and just bond the herringbone straight to it? Ive only tiled onto it but would have thought with the right adhesive you could bond it to wood. Might be worth having a read of theirs and similar products specs

Thank you @Beau! this could be a great option.

 

After reading some sections of the Jackboard installation guide, it seems the boards have to be either glued down or fastened. This makes me think the XPS panels should be installed in a similar way, instead of just laying them down.

 

Best regards,

Alex

 

[JohnMo]

Question why are you adding a thin bit of insulation. Is this just causing issues and not resolving anything.

 

If your sub floor squeaks fix the issue. The insulation isn't going to.

[marshian]

1 hour ago, JohnMo said:

If your sub floor squeaks fix the issue. The insulation isn't going to.

 

He didn't say his floors squeaked

 

He asked the question "would option 1 lead to squeaky floors?"

[marshian]

On 09/02/2025 at 23:45, _Alex_ said:

I would like to put as well some insulation between the engineered wood floors and the timber subfloor (unfortunately, the correct way to insulate, by putting a thick thermal insulation material below the timber subfloor between the joists is ruled out).

 

No offence intended here - why is the best way to insulate a suspended floor by insulation under the floor ruled out

 

I'd much rather do that when there is a std floor than have to do it later and ruin a herringbone floor

 

Having insulated almost all the entire of my ground floor from underneath in a 2 ft crawl space due to a range of finished flooring above (Kitchen and Utility room tiled, Front Hall, hallway, downstairs loo and dining room 25mm Oak floor.) I'd choose ripping up a std wood floor and insulating from above in a heartbeat!!!

[_Alex_]

Thank you so much @marshian and no woreries,

 

The reasons behind ruling the correct way of doing it are varied unfortunately: lack of experience on this specific topic, not very happy to cut open the downstairs floors during winter, and lack of time as this at least in my DIY opinion, would take much longer than laying down some XPS and plywood layers.

 

Just in case, I finally opted to follow Option 1 but I used nails (ring-shank) on the plywood layer as the glue was not strong enough. Time will tell if it was an acceptable choice.

 

Alex