Floor unheated space

[Hunneberg]

Hi, I am rebuilding the porch for the guest cottage and have now cast a small footing around the entire area for the walls to rest on. I've placed Platon mat on the outside, so moisture migration from the ground outside should be minimal. I'm now about to build the floor and have read that one should have an inner or outer ventilated space underneath, but I think since the space is so small (between 2-7cm approx.), could one fill it with LECA balls and skip vents and other things? Do I still need to lay plastic? The floor is intended to be built with wooden beams and stone wool, OSB board, and then a vinyl mat on top.

Thanks

 

[klaskarlsson]

Hello, Rebuilding the porch for the guest cottage and have now cast a small ledge around the entire perimeter that the walls will rest on. Platon mat on the outside so moisture migration from the ground outside should be fine. Now I'm going to build the floor and have read that you should have an interior or exterior ventilated space underneath, but I'm thinking since the space is so small (between 2-7cm or so) Could you fill it with LECA balls and skip the vents and other things? Do you still need to lay plastic? The floor is intended to be built with wood studs and stone wool, OSB board, and then a vinyl mat on top.

Thanks

If you look at how, for example, hybrid foundations are built, you fill the space with rigid insulation and then building plastic on top of this, and then the floor.
https://www.attefallshus.se/hybridgrund/

The problem is that the concrete can draw up ground moisture, which then spreads to the space that needs to be ventilated, or "pushed out" by being warm (insulated below and on the outside).

If you fill it with insulation that does not absorb significant water (e.g., xps) and then a building plastic on top, you should both maintain reasonable insulation over time and not pull up moisture into the inner construction.

 

[Hunneberg]

IF you look at how, for example, hybrid foundations are built, you fill the space with foam insulation and then a construction plastic on top of this, and then the floor.
[link]

The problem is that the concrete can absorb ground moisture, which then spreads to the space that you then need to ventilate, or "push out" by it being warm (insulated underneath and outside).

If you fill it with insulation that does not significantly absorb water (like xps) and then a construction plastic on top, you should be able to both maintain reasonable insulation over time and not draw moisture into the inner construction.

It is not a concrete slab but the space inside the casting is only a ground fabric on top of existing well-drained ground. There should thus not be anything that draws moisture there, but it might still result in condensation when it is temporarily heated and then cooled down again?

 

[klaskarlsson]

It's not a concrete slab, but the space inside the casting is just a ground cloth on top of existing well-drained ground. Thus, it should not draw moisture there, but maybe it can still become condensation when temporarily heated and then cooled again?

No, not a slab, I understand that, but it's through the edges that the concrete can draw up water over the decades. But that's why you have building plastic on top, instead of directly on the ground like regular crawl spaces, if you have a hybrid foundation, as I understand it.

 

[mexitegel]

I would have suggested isodrän underneath and now see that this is exactly how the hybrid foundation is built (capillary-breaking cellular plastic, I interpret as just that).

How high is the curb you cast? One concern might be condensation forming against the uninsulated part of the wall.

 

[Hunneberg]

I would have suggested Isodrän underneath and now see that this is exactly how the hybrid foundation is constructed (capillary-breaking cellplast is how I interpret it).

How high is the foundation ledge you poured? One concern might be condensation forming against the part of the wall that is uninsulated.

The foundation ledge is only about 10cm, and then there are Leca blocks on top, and the floor will attach to the part that is Leca. The ground underneath varies in height between about 2-7 cm, and I'd prefer not to disturb it more since there's slate and a risk of it collapsing, filling in the gaps between the stones that may open later. Hence, the idea with Leca balls, which can be spread evenly over uneven ground.. It seems you agree on having plastic before the studs and insulation, and it shouldn't be a problem since they will be situated between the construction site underneath and the plastic sheet above?

 

[mexitegel]

Yes, then I agree with leca and then Isodrän as high as possible. You fill with Leca around the Isodrän also where it is difficult to cut evenly. In this construction, you place age-resistant plastic on the leveled layer of Leca.

 

[Hunneberg]

Yes, I agree with leca and then Isodrän as high as possible. You fill with Leca around the isodrän where it is difficult to cut evenly. In this construction, you place age-resistant plastic on the leveled layer of Leca.

Thinking and googling a bit more about the floor. The problem is that I can't fit any Isodrän boards as it's very low in some places (about 2-3cm), and I don't want to disturb the crushed stone/soil, creating gaps that water can seep into and undermine. Regular leca, according to searches, is both moisture-retaining and absorbent, which would mean that I would draw up the moisture that might be on the ground and bring it closer to the structure. I can lay construction plastic on the leca balls before the floor beams, but then I wonder if there's any advantage compared to just having air? And how tightly must the plastic be secured to avoid issues?

 

[Hunneberg]

@klaskarlsson & @mexitegel do you have any thoughts on whether it's possible to solve this without fitting isodrän or a capillary-breaking layer? As I wrote before, placing leca under plastic would likely just lift the moisture since leca absorbs moisture. Placing the plastic at the bottom and out to the walls and then filling with leca there and up towards the underside of the joists and insulation? Is there any point, or is it better to have a small space with air? I find it hard to imagine how different temperatures in the space, and inside or outside, could affect it.. thanks in advance!