Interesting question! I also live in ant country so this seems like a concern worth consideration.
It would seem to me that if there's nothing to eat down there and limited points of entry, the ants may not bother.
I wonder how an application of borax and/or DE would fare, if pored between the rocks?
As long as the top layer is sealed completely, would you be disturbed by them? Be it Tadelakt or even just a beeswax/linseed oil finish that should effectively seal out ants or other critters. Your floor would have to be ant proof to be water proof, I would think.
These probably aren't the best solutions... Maybe we'll get lucky and Jay C. Whitecloud or some of the other wise and generous natural building experts hereabouts will stop in to offer what's worked for them, and what traditionally limits that kind of problem.
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We ran into a similar situation when building our house. There was a large fire ant hill in the middle of the house location. I dug in a 4 pound box of borax into the ant hill its self with a backhoe (the colony went about 5 feet deep). Then before placing the gravel for the floor base, I dusted the ground with another two boxes of borax (over about 700 square feet). So far there haven't been any intruders through the floor. I don't think the two boxes dusted over the floor area was excessive. The ant hill area was well dusted but most of the rest of the floor barely had any. If I had to do it over again, I would use another box or two. That said - there haven't been any issues.
Do you have a reason to think they will be a problem? Do you have fire ants or unusually many ants in your area? I've lived in a house with earthen floors for some 20 years, and though I certainly see an ant in the kitchen once in a while, I don't see them very often. There is occasionally an ant's nest in the garden right outside my kitchen door, and the first time I saw it I was alarmed and smoked it with incense, but then the next year I decided to wait and see. They never actually became a problem, so now I let them be. I've never had the mouth of an ants' nest appear inside the house, and if I did, I guess I'd try the incense-down-the-hole trick again, or sprinkle boric acid powder around it. I don't worry about it.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
In this area i think there's just one ant type : ordinary type.
Yesterday i've dug a trench for a foundation, 80cm (32") deep in heavy clay and found many ants hanging out at the bottom ...
I don't know, the last thing i need is for the wife to come and bang a big cooking pan against my head when she gets overrun by ants (she dislikes crawling insects with an ancestral visceral passion) just because i wasn't cautious from the beginning.
Though I have not seen an ant inside my earthen floor, we experienced a subterranean termite "bloom" last spring. I believe I brought in the colony myself in some firewood, and the next was built near my firewood storage. All I ever saw of the colony was the relatively few adult winged members who emerged through very small holes (like 1 mm) that they chewed in the oiled floor to emerge to fine a new home. When a new emergence began (they tended to happen late morning for several weeks in a row) I usually let a chicken into the house to make quick work of them. But, if I wasn't around, then I would return home to see a small pile of clear wings (which the adults quickly shed upon emergence). As our house contained no suitably moist wood for food, I suppose they died trying to find a new home. I have no idea. We shall see if I have another "bloom" this year. The holes in the floor are so small as to be basically invisible unless I am searching for them, so it would be difficult to introduce borax. Not sure what long term control options would work, but it sure can be entertaining to watch the chickens clean up.
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