Enclosing structure on piers, want to do earthen floor

Hi, this is my first post, and I can’t seem to find anything relating to this in your forums or on the internet. I am looking for some thoughts on our plan to enclose a round wood framed structure on piers and make it a sunroom, with earthen floor, in Wyoming.

structure is/will be tied into existing 288 sf shed/cabin. addition is 140 sf. The addition is currently being built on concrete piers poured from below frost depth (48 inches).

We were unsure if we would get to framing in walls before our snow really started falling but it looks like we could. So the question is more of a what are my options/is the following a bad idea.

The piers stick out of the ground 4-6 inches.  We have an abundance of small to medium sized rectangular granite blocks. We were thinking of building a stone and lime mortar
“Walls” connecting the piers and putting our base plate on top of that, also tied in directly to the supporting Timbers, then framing as normal with straw bale insulation and cob and lime plasters. We were then thinking of utilizing mesh to screen critters out and putting in the natural floor over that with some kind of insulation as well, building it up 6 inches or so to match the height of the concrete and keep everything level. Do we need to dig a trench for the stone walls and do gravel first as a drainage solution if we have good berm and slope away from the exterior on all sides? Is using the existing piers and essentially stone “infill” beneath the base plate an acceptable support structure if there will be windows and strawbale insulation ( these will be cut out o 9 inches not 18) ? It is primarily a sunroom, so more windows than straw or insulation…

Just looking for some guidance. I have been reading lots of posts by others and the answers seem all over the place but not to specifically hit on what I am worried about (shifting of the walls or windows/stability). Again the main frame is to the appropriate depth for a foundation but I am not finding info on building walls on this type of timber framed “porch” that don’t involve building a deck. A deck would also tie the walls to each other. But we are hoping to do a natural floor and avoid a wooden deck/floor structure. Hopefully that makes sense. If floor joists of some kind are necessary to tie the walls to one another, would burying them in the earthen floor be acceptable?

Thanks I’m advance for any and all help. We are trying to be creative here with materials we had on the property. I realize it’s unconventional, and that is what we are going for. But, we do want to make sure it lasts and is sturdy. Again, thanks!

Hi Elyse,
I am not a building expert, so please take this with a grain of salt.

You mention cement piers at least 4 feet in the ground. I'm assuming these were done with the right spacing and sizing to hold up the structure you are building. If this is the case, and your frost level is less than 4 feet. Then I don't think you will have much problem with shifting or need to be too concerned with how deep the fill-in wall goes. If you are not going to go down to below the frost level with it, then I would go just deep enough below ground level to avoid erosion. If the piers are 6 inches above ground level, I wouldn't bother going much more than a foot for the total height of the fill-in wall. 6 above and 6 below. I don't think the frost would shift those enough to effect the doors and windows if the rest is sitting on piers.

Insulation under the earth floor is definitely needed.

Please forgive me if you have already asked yourself this question, but I feel the need to at least ask. Due to the depth of the piers and your talk of insulation and cold weather, I'm assuming you are in a cold climate. Do you think the earthen floor is the best option? If you have looked at various options and this fits the best for your house and climate and needs, then I think you are all set. However, I know that sometimes I fall in love with a particular technique or particular building type, and I try to use it because I like it... rather than because it is a good fit. When I hear of earthen or cob based building and floors in cold climates I am always leery. I wonder if there are some other options that might be a better fit. Maybe not. I just have to be the guy on the shoulder asking you to think about it from a different perspective.

Hi Elyse,

Welcome to Permies.

Hello!
I'm not entirely sure what kind of construction style you're actually aiming for with the floor.
Is there any way you could show some drawing diagrams of what you're envisioning, or describe it again in a different way?
What do you mean by 'Natural Floor'?

Because right now it sounds like you're building pier-and-beam, which generally comes with a crawlspace.... but later you talk about filling up that space with insulation and asking if stone infill is a support structure... and then talk about burying floor joists?

This is what I picture when you say you have piers:

^ There's the deep-set piers that go at the 4 corners, and sometimes around the outer perimeter of the house. You can add a stone/masonry wall around the outer perimeter to stabilize, and then there are smaller piers just sunk a bit into the earth directly under the house where frost (usually) can't reach, which supports the other floor beams.

We were thinking of building a stone and lime mortar “Walls” connecting the piers and putting our base plate on top of that
The base plate should be braced primarily on the piers themselves.
If you want to add walls, or solid footings to help support the base plate beams to prevent future sagging, that's good! (see pic above) But, the primary weight bearing should be on the Piers. That's the point of them.

Do we need to dig a trench for the stone walls and do gravel first as a drainage solution if we have good berm and slope away from the exterior on all sides?

ALWAYS have extra drainage, regardless of other berm and slope. Under-house drainage is often for water pressure pushing water up through the earth, not just direct rainfall from above. Make sure the grading of the under-house earth also slopes toward whatever drainage channels you have, so water won't pool if it does get under there.
Adding well-draining stone, proper under-house grading and some drain pipes/channels to divert water to outside is essential, yes.


Is using the existing piers and essentially stone “infill” beneath the base plate an acceptable support structure if there will be windows and strawbale insulation
What stone infill? I thought you were building stone walls.

If floor joists of some kind are necessary to tie the walls to one another, would burying them in the earthen floor be acceptable?
You don't want to bury wood as a general rule, and you want to minimize dirt-to-wood contact. Timber is usually up on steel or stone/concrete to keep it off the ground, with a barrier between wood & cement to reduce water wicking & rotting, if you want the build to last a very long time.
As I said earlier, the area under your house can become wet because of hydrostatic pressure - nearby rainfall pushes water through the earth sideways, not just down - and so it can seep under your house and into any dirt it touches. This is why cracking or poorly sealed basements can leak water during rainfall - the entire earth is getting filled with water, and the walls need to resist the immense pressure of thousands of gallons pushing sideways against it in the earth. Like a reverse-pool.

Moisture wicking into straw bales MUST be avoided at all costs with straw bale construction - they rot easily, and the heat generated by the decomposition process can get intense enough that it reaches the ignition point for straw - lighting the walls themselves on fire. Barn fires are often started because someone took wet hay or straw and piled it with the dry stuff.

I am not finding info on building walls on this type of timber framed “porch” that don’t involve building a deck. A deck would also tie the walls to each other. But we are hoping to do a natural floor and avoid a wooden deck/floor structure
This doesn't make sense to me. Deck and porch are the same thing. Please explain, or use different terminology.

strawbale insulation ( these will be cut out of 9 inches not 18)
^ That's not going to work. Cutting the bales in half lengthwise will make them fall apart instantly. That's not suitable for building a wall.
For areas with lots of windows, you can use Leichtlehm, or “light clay.”  It's basically straw mixed with clay mud, which is applied into a form and tamped down like a less-intensive rammed earth. (Rammed straw?)
It can be used to create thin exterior or interior walls, insulation and ceiling panels between rafters, or insulation below adobe floors. The straw/clay provides less insulation than bales, but offers greater flexibility in constructing walls of different widths

Matt, John, Toko,

Thank you for all the input. It is helpful on a lot of levels. Matt, I appreciate asking the hard question about preference vs. practicality. I have thought about this and still don’t know. We have thought of pouring a slab after the fact (like next year) but also don’t have another way in and out of the cabin and would need to find somewhere else yo sleep or commit to camping for a bit. The main reason for coming up with what we were going to do, was it’s free and we can do it now or later. There is a portion that would be stone and leave us a way in and out while the rest dries. I’ll keep thinking on it though, as I have wondered if it is actually a good idea at all.

Toko, thank you for the detailed interest and explanation of post and beam building. I know the language I use is not always what a builder would say so I think I gleaned some info on how to be less confusing in the future from your response. The main questions aren’t how to build post and beam, but how far and what kind of deviations are acceptable—Assuming that the piers are built correctly and that the main support structure is the piers. The largest span between piers is 9.5 feet. The stone wall was referenced as “infill” because that was one of the main questions. It won’t really be a support structure as the plate will be on the piers but it will also be sort of support because it is filling the space between them and under the piers. Does that make more sense? I can share a picture if I can get it to upload.

Thank you for all the thoughts on straw bake and the useful information regarding combustion. We have considered light straw clay in addition to some other kind of insulation behind it as well. I’ll have to write down the other works you used for that because that was new to me too.

Thanks for all the input so far and the welcome to the community.

E