Vapour movement through different floorboard finishes

[info_45]

I know that there are many, many floorboard finish questions on the forum which I have read with interest. But this question doesn't seem to have come up - perhaps because it is not relevant!?

A contractor is offering Blanchon Primer and Blanchon Initial ( I believe this is the Water-Based Polyurethane option) or Osmo Hardwax Oil. The contractor said that most people go with the Polyurethane option but I am torn and am currently awake, flip-flopping between the two. I understand there are pros and cons with both.

As some of my other posts on here will show, I've had a huge amount of builders waste removed from the floor and added some air bricks, but also insulated (mineral wool plus breathable membrane). The insulation especially makes me a little uneasy, because of the perceived potential for altering moisture movement and causing the timber to rot. I asked a question here a few weeks ago about whether filling between boards would essentially 'seal' the floor, but that was my misunderstanding of the concept of how water moves through materials.

So my questions (and this could also be based on a misunderstanding) if I went for the Water-Based Polyurethane option, am I restricting water vapour movement through the building fabric, and thus undoing some of the good work in improving ventilation etc.? Or is such movement so minimal relative to other things that I shouldn't be concerned? The fact that nobody is really asking this question makes me think that it is another example of me overthinking!

I was leaning towards the Osmo oil before reading a very strong opinion on it elsewhere on the forum!

 

[fernicarry]

I'd imagine floorboards are pretty impenetrable to begin with so I don't think its something to get overly worried about. A suspended floor won't be subject to capillary action, rising or penetrating damp, unlike a solid floor or solid walls so shouldn't be having to expel a large amount of moisture. There is the potential for interstitial condensation where warm air from inside leaks into the floor void, cools and condenses but you've mitigated that by making the insulation breathable and having good air flow underneath.

There's lots of opinions on here on which finishing approach to use, more from an appearance and longevity pov. PU seems to be less loved than others. I'm very happy with shellac and wax. Water beads up on it nicely, which I guess means I've made it waterproof to some degree!

 

[info_45]

Thanks!

I've certainly done my best to improve the ventilation under there, but the set up hasn't really bedded in yet. I suppose if you're relying on water moving through the floorboards to keep the floor dry then perhaps something is amiss. I'm also conscious that, when the floor was removed and there was some heavy rain a few months ago, the soil did become damp in the area around the bay window. This must have been happening forever and it was just that I had never seen it before. I also understand its not uncommon, but it's made me very aware of potential issues with moisture and wanting to do my utmost to make things better.

 

[fernicarry]

I suppose if you're relying on water moving through the floorboards to keep the floor dry then perhaps something is amiss

Indeed. Given how shallow the footings are on our buildings its not surprising that water can get around them and make the solum wet from time to time. Its a bit scary to think that there is water "inside" the building but so long as its not constant. If it was then maybe you need to think about directing water away from the building. You've taken steps to ensure the floor is isolated from the ground by clearing away the debris and I'd wager that's more than the majority of home owners ever do.

Why do you think your ventilation hasn't bedded in?

 

[info_45]

Why do you think your ventilation hasn't bedded in?

I suppose just because its so newly installed, and whilst in theory I think i've made some quite major improvements it must be a real readjustment for how the house functions. For all that, in theory, everything was worse before, there seemed to have been a bit of an equilibrium reached whereby the entire floor hadn't fallen through and was actually pretty solid in most places (if a bit wonky!). And with the insulation in place, its actually quite a faff to check what is happening under there.