Have you used mineral wool as a floating floor resilient layer?

9 Jul 2018
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United Kingdom

I'm a new member here, but have been floating in the background for quite a while now.

I'm putting acoustic insulation and UFH into the floor of my first floor flat. I'm planning an approach inspired by an (acoustic) detail mentioned in building regulations document E, 3.102, which suggests a floating platform floor sitting on mineral wool batts.

This detail suggests densities of 60-100 kg/m3 for the wool @25mm thick.

My main question: Does anyone have experiences of doing this? Does it result in an unstable or spongy floor finish?

More details: My plan is to sandwich the following: existing chipboard base -> 25mm RW5 rockwool with gaps cut for 16mm UFH pipes in aluminium spreader plates (0.5mm thick for good conduction) -> screedboard20 (for mass and thermal conductivity) -> epoxy finish/ (DIY) 4mm timber laminates. My thermal modelling suggests this should work nicely.

Talking to Rockwool they strongly recommend against their RW5 wool (100kg/m3) as being too spongy, and urge me to use the much more (3x) expensive rockfloor product (140kg/m3, different binder?). However, this product is spec'ed to go under much thicker screeds (so may be overkill). Also an old data sheet (https://www.just-insulation.com/downloads/rockwool/Datasheets/Rockwool-Universal-Slab-datasheet.pdf ) suggests that RW5 has an elastic limit of 11.3 kN/m2 which compares well with my loading (0.2kN/m2 screedboard plus some live load up to 1.5kN/m2). The literature I read suggests that even at stresses approaching 0.3*elastic limit rockwools creep to only a few percent over 10 yrs. So I'm inclined to go with the cheaper product (which should also have better acoustic absorption coefficients), but am of course wary of ignoring expert advice (though unfortunately they are unwilling to cite examples or data to show why it won't work - it may just be a quality assurance issue without full data).

Another question that is less important I think: the document-E detail is keen on having overlapping boards in the upper mass layer. Why is this so important? I can't imagine a single thickness board system with interlocked, glued joints being acoustically that much different (at least in thickness modes).

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the document-E detail is keen on having overlapping boards in the upper mass layer. Why is this so important?

If you don’t do this, with regular boards when you stand at the edge of one board it will sink relative to the one next to it.
Tongue & groove joints would help, but may not be sufficient with a layer as soft as rockwool underneath. (Your screedboards seem to have some sort of “click” system???)
Two layers, staggered and glued to,each other, will be much more solid.
if done properly you're be surprised how firm it is
it was a common detail in timber frame flats ,15mm plank ,rock wool 22mm chipboard
Thank you both. Yes, I see what you mean about the possibility of flexure at joints (the screedboard20 does indeed have a clicking tongue and groove, not sure how much stiffness this adds).

Bobsuerita - It's good to hear that it has been used extensively - your message gives me some confidence that the tongue and groove might be sufficient. Are you able to give any other advice on how to "do it properly"?
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the best advice i can give is go and buy a roll of breathable membrane and lay over the rockwool it helps to stop in trapping in joints also glue the groove not the joint
A final update.

I ended up using Rockfloor thermal rather than RW5. The rockfloor data sheet I was sent showed it to have a much higher compressive strength : "...A gradual breakdown of the structure does however begin at approximately 40kN/m2"

On talking to the screedboard manufacturer, they strongly recommended the stiffer product. The floor feels good now - firm and not bouncy. The UFH arrangement works well.

I now understand why overlapping boards is important - it adds stiffness to the overall floor to avoid excessive deflection when you stand near a board edge. With the screedboard this is provided by the tongue and groove edge details.

Thanks to everyone!

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