Wooden flooring compatibility with UFH pipes

[Adsibob]

I have purchased two types of engineered wooden board flooring:

1. planks which are a sandwich of 4mm oak - 6mm birch ply - 4mm oak (total thickness 14.5mm, I guess the adhesive creates that extra 0.5mm). planks are 230mm wide and about 1.8m long
2. herringbone which is not a sandwich construction and has only two layers, a top 3mm layer of oak on a 11mm thick layer of birch ply, this has a much smaller footprint at 85mm x 425mm

 

These are both to be installed over foil faced underfloor heating boards made by Cellecta from extruded polysterene with pre-routed grooves for the pipes. Cellecta say it's fine to do this as that is what their boards are designed for, but they of course caveat that advice with "check with your flooring manufacturer".

 

The flooring manufacturer says that is is "essential" that the subfloor on which the wooden floor is laid does not exceed 27 degrees C and that we should test this with temperature probes in the actual subfloor rather than rely on room stats. His concern about the subfloor suggested above is that where the pipes are exposed, there will be slightly more heat than in the majority part of the boards that don't have exposed pipes, and that this will expose the wooden flooring to uneven heat which could cause problems. His recommendation is to lay an additional "6mm or ideally at least 9mm" plywood deck to "protect the flooring from the UFH".

 

This is all rather annoying as the flooring is marketed as being compatible with UFH. My architect thinks the flooring manufacturer is being unduly cautious because wood shouldn't warp at that temperature, the differential between the part of the UFH boards where the pipe is and the parts where there is no pipe should not be that unhomogenous when there is foil to carry the heat, and the whole point of engineered boards is that they are more stable.

 

9mm is out of the question as I agree this would be overkill for this situation.

 

Should I lay 6mm ply in between the UFH boards and the wooden flooring both where I have herringbone and where I have planks or is this also overkill?

 

Or should I only lay the 6mm ply under the herringbone on the basis that it is only a 2-layer construction, but skip the ply for the planks which are 3 layer?

 

 

[PeterW]

I’d use 6mm everywhere as it will stop flexing on joints (ie where a joint is over a pipe) but also to stop damage to the cellecta  panels when the floor is being laid. 

[saveasteading]

18 minutes ago, Adsibob said:

not a sandwich construction and has only two layers, a top 3mm layer of oak on a 11mm thick layer of birch ply

I was about to say that any engineered floor needs at least a third layer as a 'balancing' layer to prevent warping.

But ply should do that. How many ply, and how thick and which direction of grain is the outer one.?

 

Quite a risk to lay all this and then it warps or splits or shrinks leaving gaps, and you have no guarantee. I suggest you ask the supplier to clarify whether it is, or is not, suitable for UFH. It cant be both. If not confirmed in writing then they must take it back.

Or perhaps your Architect will guarantee it.. haha.

 

I once laid herringbone using tiny wooden tiles on a mesh backing. They expanded more than the expansion strip specified, and buckled into mounds. so I re-laid them with even more expansion edge, and then they shrank and there have since been characterful gaps and lips, probably moving with the seasons.

[Adsibob]

28 minutes ago, PeterW said:

I’d use 6mm everywhere as it will stop flexing on joints (ie where a joint is over a pipe) but also to stop damage to the cellecta  panels when the floor is being laid. 

Okay, that sounds sensible. What temperature should I run the UFH water at to ensure the 6mm ply doesn't heat up to more than 27C ? We are using 16mm pipes at 150mm pipe centres. (And it's a gas boiler based system on the second and third floors of a well insulated, but not passive standard, semi detached house.)

Edited November 16, 2021 by Adsibob

[ProDave]

Is that flooring "structural"?  I very much doubt it at that thickness, so it will expect a firm flat stable base to be laid on.  The UFH system does not fit that description on it's own so well need something like ply to provide that base.

 

Our own Oak flooring was bought specifically as a structural floor board and is a lot thicker about 22mm iirc and being suitable to lay directly on the UFH and be a structural board to span the 400mm centre joists was a requirement given to the supplier.

[Adsibob]

1 minute ago, ProDave said:

Is that flooring "structural"?  I very much doubt it at that thickness, so it will expect a firm flat stable base to be laid on.  The UFH system does not fit that description on it's own so well need something like ply to provide that base.

 

Our own Oak flooring was bought specifically as a structural floor board and is a lot thicker about 22mm iirc and being suitable to lay directly on the UFH and be a structural board to span the 400mm centre joists was a requirement given to the supplier.

No. We have Egger 22mm chipboard flooring down as our structural floor on top of posi joists. After a lot of rangling with my builder to use the correct glue, he eventually did a great job and it's solid.

[Adsibob]

The other consideration is how to stop the additional plywood layer squeaking. A lot of effort has gone into making a squeak proof subfloor, but will this now be at risk of squeaking from the addition of the plywood? Or does 6mm thick plywood come in T and G that won't squeak?

Maybe I should do a biscuit mix instead, but I thought I would get extra performance benefits from the Cellecta? I only mention the biscuit mix because the floor manufacturer sounded more relaxed about installing over UFH that is inbedded in screed. How long does 25mm of biscuit mix take to cure though (if that's the depth one lays for 16mm pipes?

[Makeitstop]

41 minutes ago, PeterW said:

I’d use 6mm everywhere as it will stop flexing on joints (ie where a joint is over a pipe) but also to stop damage to the cellecta  panels when the floor is being laid. 

 

I would deffo go this route too.

 

With timber products, it's not simply a question of heat per se, but more about what moisture the product holds prior to installation, and, what the relative humidity is going to be once its in service. In the main, a kitchen is likely to be subject to more than adequate moisture to never get below that 6 - 8% moisture content within the boards. However, when you introduce direct heat to it, and especially over a prolonged period of time, "almost any" excess moisutre in those boards WILL be driven off, which is likely to result in at least some dimensional change, however slight. The problem you have there is that with flooring, it's such a close fitting product, that you want to keep it that way to look and perform at its best. So, maybe a barrier to stop it becoming "quite" so warm is a good idea.

 

Of course, engineered flooring is a very different beast to solid timber, and is generally far more stable. Having said that, some of that stability is still dependant on levels of moisture within it prior to putting it into service, and especially the RH it's exposed to once it's in service doing its job.

[Makeitstop]

10 minutes ago, Adsibob said:

The other consideration is how to stop the additional plywood layer squeaking. A lot of effort has gone into making a squeak proof subfloor, but will this now be at risk of squeaking from the addition of the plywood? Or does 6mm thick plywood come in T and G that won't squeak?

Maybe I should do a biscuit mix instead, but I thought I would get extra performance benefits from the Cellecta? I only mention the biscuit mix because the floor manufacturer sounded more relaxed about installing over UFH that is inbedded in screed. How long does 25mm of biscuit mix take to cure though (if that's the depth one lays for 16mm pipes?

 

I would say that t&g ply is more likely to squeak than square edged, because there's a larger area in contact with the neighbouring piece. I would personally lay the ply but with a fractional gap of maybe a mil or so.

 

 

Edited November 16, 2021 by Makeitstop

[Adsibob]

8 minutes ago, Makeitstop said:

 

I would say that t&g ply is more likely to squeak than square edged, because there's a larger area in contact with the neighbouring piece. I would personally lay the ply but with a fractional gap of maybe a mil or so.

 

 

Thanks @Makeitstop (good name btw) but that would require fixing the plywood to the extruded polystyrene cellecta - how would I do that? With their foil face, I can't glue it.

[PeterW]

4 minutes ago, Adsibob said:

Thanks @Makeitstop (good name btw) but that would require fixing the plywood to the extruded polystyrene cellecta - how would I do that? With their foil face, I can't glue it.

I’d use carpet adhesive …! Spray both sides and it will not move once down 

[Adsibob]

19 minutes ago, PeterW said:

I’d use carpet adhesive …! Spray both sides and it will not move once down 

Wow, never knew it was so sticky. The other option is to switch out the Cellecta boards for their Tile Backer version, which are identical but instead of foil face they have a membrane that can accept tiles. But then I lose the benefit of the foil.

[saveasteading]

17 hours ago, Adsibob said:

I lose the benefit of the foil.

what is the purpose of the foil? I don't know the product or context.

[Adsibob]

3 hours ago, saveasteading said:

what is the purpose of the foil? I don't know the product or context.

It's an aluminium foil which is supposed to "spread" the heat from the pipes across the surface of the insulation boards. The theory is that the boards are highly resistant to heat transfer because they are such good insulators. Foil is the opposite. So buy boards that have a sheet of foil adhered on the top of them and it gives somewhere for the heat to transmit to. This then makes the heat rise in a more uniform way, which should be good for the wood given the manufacturer's main concern was the unevenness of the heat produced by the pipes.

Alternatively it may be marketing bull.