Cob house flooring and moisture barrier

I'm planning the construction of a cob tiny house. It will be built on a concrete slab that is already in place. I was thinking of having a wooden floor, but from what I understand, there will need to a moisture barrier between that and the concrete.
What are the options for this?
Ideally, I'd like to stay away from plastic if at all possible.
If it is not possible, I need to understand how that would work. as if it is a sheet of plastic material, I'm guessing that the fixings that will attach the floorboards to the concrete will pierce the plastic.
Is that the case? and if so, would that be a problem?

thanks

For a non-plastic moisture barrier, birch bark is the traditional one. It has been used for more or less exactly what you describe, as a barrier between the foundations and bottom logs in log houses etc, as well as for a lot of other things.

As for your other question, do you actually need to attach the wooden floor to the concrete slab at all? Your floor will be between the cob walls, right? Wouldn't the cob hold the floor in place well enough on its own? You could just put your choice of moisture barrier on the slab, lay your floor beams on that, and build whatever type of floor you want on top, and then anchor it with the walls.

Oh, and welcome to Permies!

Plastic is a superb product for the application you speak of.
Tell us more about the situation;
- is the soil damp?
- how will you seal the cob so water does not enter?
- Is the slab edge above the ground?

I think the danger is from trapped condensation causing rot issue in your floors. It depends a lot on your climate too. If you have plenty of ventilation under a suspended wooden floor, it will be drafty but no rot.
If you need insulation in a damp cool climate at some point in the heat transfer between warm inside and cool outside you are likely to get condensation which needs to be considered somehow. My thought would be to do as Eino suggests use the concrete to support the cob, suspend the floor joists above that (you can suspend insulation underneath if needed) and have vermin proof vent holes in the cob to help it all breathe. A good roof overhang will help keep the foundation dry.
Welcome to permies!

I’m working with a natural builder renovating an old stone house at the moment. The first floor was mostly compacted earth and had moisture issues.
We dug it out about 40cm, will put down sharp gravel, then a slab of 5cm lime and sand and hemp mix. The gravel should break the capillary effect for rising moisture.
On top of the slab we will tile or put wooden flooring.

I too have been looking into vapor barriers between walls and foundation that did not use plastic. I think you have a couple good ideas already, but could I suggest metal? Metal roofing or even aluminum flashing would be plenty to break the capillary action between cement and other materials.

John C Daley wrote:Plastic is a superb product for the application you speak of.
Tell us more about the situation;
- is the soil damp?
- how will you seal the cob so water does not enter?
- Is the slab edge above the ground?

-The climate is seasonally quite wet and hot and dry in the summer
- The cob will be lime rendered
-the slab is about 4 inches above the ground

I am really happy with my concrete floor though if I wanted a wood floor I would go with a floating floor system.  Have you looked into a floating floor?

Mowgli, your plans are good,
I would add a good drain around the slab edge to get rid of moisture.
A wide eave on the roof to protect the walls
A big tank to collect rainwater
You could constructure a floor frame with insulation and build a wooden floor on that.
Where are you building?

John C Daley wrote:Mowgli, your plans are good,
I would add a good drain around the slab edge to get rid of moisture.
A wide eave on the roof to protect the walls
A big tank to collect rainwater
You could constructure a floor frame with insulation and build a wooden floor on that.
Where are you building?

Wide eave, drain and rainwater collection are already on the plan.

It's in Portugal

Post 16/04/2025 08:25:39     Subject: Cob house flooring and moisture barrier
I’m working with a natural builder renovating an old stone house at the moment. The first floor was mostly compacted earth and had moisture issues.
We dug it out about 40cm, will put down sharp gravel, then a slab of 5cm lime and sand and hemp mix. The gravel should break the capillary effect for rising moisture.
On top of the slab we will tile or put wooden flooring.

Hola Benjamin

We are in very similar circumstances as you. We are renovating an old stone 2 floored Alpujarran house and need some fresh ideas for materials to use on the second floor. Unfortunately the ground floor was built with concrete and a plastic moisture barrier but we want to change the next stage of building to bio construction. We also nee to build the roof and pillars to support this.

So my question is what materials can you recommend to use on the second floor floor. In order to support pillars a perimeter of concrete might have to be used as a strong foundation and then bio construction materials for the rest of the floor but will they shrink and crack at different rates? 2 possibilities for floor material might include
arcilla expandida, lime, sand, and cement. Or the other option is sand, cal and ground hemp.

It would be great to hear some of your own experiences and the materials you used.

Many thanks
Daniel

Hi Daniel.
It’s nice to have a fellow permie in the area.
When it comes to your construction, I actually don’t know how similar they are.
We were renovating a first floor (touching the earth).
I don’t really understand the situation with the roof you’re trying to support. Is an architect involved in that? Sounds like you need one.

Daniel,

How the second floor is constructed? Is it a reinforced concrete slab or you will have to install some wood or metal beams?
Traditionally two level masonry homes had wooden ceiling on which wooden floor would work the best. They could also have vaulted ceilings built from bricks or stones and tiles would be used. In more modern times they started casting reinforced concrete slabs.

Being in hot summer Mediterranean climate I would avoid wood as much as possible due to fire risk.