Wetroom floor - dry rot – insulate or not?

I’m making a wetroom & trying to work out the structure.

My main worry is about the floor. I had a serious case of dry rot once before in a previous house.

The wetroom floor is between the 1st floor and the loft conversion, suspended timber (now heavily battened up for strength), then it will be ply on top, then Wedi board, then tiles. I’ve added a lot of glass fibre insulation between the joists, and am just about to start fixing the ply down. Now I’m wondering about that insulation. Dry rot takes off when the temperature & dampness are right & there’s no air circulation.

Currently there’s a bit of a gap between the insulation & the ceiling below (and I’ve got the choice of letting internal air circulate through there, or since there’s an airbrick, I could make it external air). But the top of the insulation is snug up against the ply – no airgap – no circulation. Will it get humid in normal use? What if I get any sort of water leak in the future from the wetroom – might take weeks or months for it to dry out?

I’m thinking that maybe I’d do better to take out the insulation & have some decent ventilation, rather than have all that timberwork all closely wrapped up all the time?

Any thoughts, anyone?

I have a feeling that you're a little paranoid about dry rot. Why do you think it'is more prone to dry rot that any other floor in any other house?

The first goal is to prevent water getting into the sub-floor.

The second is to ensure that you know about it when it happens, so that it can be remedied before a lot of damage happens. Arguably, a lot of insulation may well soak up water and delay or prevent water appearing at the ceiling below.

What is the purpose of the insulation? Is it for sound?

Thanks for your thoughts.

Yup – paranoid is about right – the previous bout was very expensive (moneywise & physical disruption to 5 rooms in the house – plaster hacked off, new floor & bathroom, skirtings & door surrounds taken out & replaced, heavy doses of fungicide in kid’s bedrooms, … ) – not something I’d willingly repeat.

I looked it up afterward & the general opinion was that it likes some warmth & damp, but that is the case in a lot of situations. If additionally there’s no air circulation, that’s when it can take off. In my case, there was a tiny water leak under the bath, and it was all panelled in – no ventilation – and no finding out about it until appeared several metres away. Gone through the walls & into other rooms by then.

I guess if I was just getting some insulation between a ceiling & the flooring above, there shouldn’t be any problem? That’s what you do when you insulate your loft floor, although any dampness there can usually escape upwards if there’s no floorboards. Here I will have a floor. But not too worried about this as many people insulate between floors.

I agree about ensuring that no water gets there from the wetroom. Doing my best, but a few years down the line …?

I’m just basically wondering whether I’m compounding the problem if I ever do get a leak. Insulation means no ventilation & holding the water in place for much longer than if it was just an empty void. As you say, I’d find out about a leak a lot quicker too.

I’m not really sure about the reasons for the insulation. The architect put it in the plans & BC approved them. I’ve never asked BC about dry rot as it’s only just occurred to me, but whenever I talked to them before, I got the idea that it was a necessary part of 30min fire protection.

I like to treat the timber under a bathroom floor with Cuprinol Green, which gives good protection against rot and insects. Apply several flowing coats, each before the previous is fully dry. The new formula doesn't smell much, but leave the timber exposed for a week or so if you can.