Thoughts on insulated backer boards?

[Landshark]

Hi All,

First time post here after reading about lots of different options for my kitchen floor refurb.

I want to add electric underfloor heating in the kitchen, but as it's a 70s uninsulated concrete slab I don't want to spending lots of cash to heat up the worms.

I'm tending towards covering the slab in 50mm of Marmox multiboard or similar, then the UFH mat + leveling compound, then limestone titles.

Does anyone have any experience or thoughts with this setup? My concern is the weight of the Kitchen island causing the insulation to deform an develop cracks. Is that a valid concern, or do the board tend to be solid enough for all domestic loads?

Adam

 

[Ajax123]

Depends how heavy the island is. If you know that you can work it all out

 

[Landshark]

Hi @Ajax123 . My guestimate is about 450kg over 2.8m2 which puts it at 1.5kN/m2.

The BBA certificate for Marmox gives a compressive strength of 400kN/m2, but the deflection limit is 1.5kN/m2.

Does that mean I am at risk of movement and therefore tiles cracking?

 

[Ajax123]

Hi @Ajax123 . My guestimate is about 450kg over 2.8m2 which puts it at 1.5kN/m2.

The BBA certificate for Marmox gives a compressive strength of 400kN/m2, but the deflection limit is 1.5kN/m2.

Does that mean I am at risk of movement and therefore tiles cracking?

The compressive strength on insulation usually refers to the pressure it takes to compress it by 10% of its depth. It's usually measured in kPa and 1kPa/m2 will be created by 1kN over 1m2. So 400kN/m2 is pretty strong. Kingspan type insulation is usually around 150kPa for context. In terms of the level of deflection created by your unit itll be next to nothing as a dead load. It would be a problem if you were to drop it in place from a height but I'm guessing thatll not happen. And dont party heavily with a lot of fat blokes on top of the island once installed

The deflection limit normally refers to the amount of deflection allowed in the subfloor system. Its calculated by the span divided by 300 or 330 depending on the structure. For domestic structures the design load is based on 1.5kN/m2 ( for commercial its normally 5). That load should not cause the floor to deflect by more than the span/300 otherwise the floor will fracture or sheer. Uts important when designing suspended floors. If it's a ground borne concrete floor underneath I'd say you wont have an issue.

Marmox board is pretty tough stuff but if you still have concerns give them a call and ask them.