Safe to put cistern under straw bale house?

I’m living at an eco-village in the Northeast with a bunch of green building techniques in practice (straw bale, cob, timber frame, passive solar, freestanding and attached greenhouses, etc).

One of the houses, they are looking for folks to invest in a septic system so it can become a legal residence. It’s a two-story, 2-bedroom straw bale and cob house, currently not framed out with a kitchen but could be.

The interesting design feature of this house is it has a rainwater catchment system built into a poured concrete foundation. This cistern gathers and holds rainwater from two adjacent houses, and stores it for release on demand to water nearby gardens. This catchment is accessible through a wooden trapdoor in the floor of the main room.

This strawbale house currently has a mold problem.

In theory, it is possible to sufficiently seal a cistern so it’s not a problem for strawbale house? If so, how would one do this?

How would one assess if the water and mold problem were from the ground and not the cistern?

If the problem is the cistern, and the cistern is emptied and water catchment moved elsewhere, would this solve the mold problem?

Thanks in advance!

As a builder, I would not imagine the cistern would be creating any moisture problems if the following steps were followed;
- A waterproof membrane was wrapped around the cistern.
- a waterproof membrane was installed under the floor slab of the house.
- edge of floor slab is above ground level.
Lack of the latter will allow any ground moisture to come through, from that edge.
But I would check the roof first, that is a common point of entry for mouldy homes.

Lots of basements in the northeast have earthen floor basements that flood due to the water table but they don't have a mold problem

Two story high, makes me think that the roof overhangs are not big enough.
I wonder if your walls are growing mushroom fruit bodies in addition to mold?
Which areas of the house is growing the most mold?

Hi Amma,

Welcome to Permies.

John and Bengi, thank you for the building replies.  I will take a look at the roof overhangs, and inquire more about how the cistern is sealed.  For a 2-story building, what length of overhang do you recommend?

Something else that concerns me is that the cob exterior to the strawbale is very, very close to the ground in some areas.  (The house is on a slight incline, so the amount varies from a few centimeters to almost a foot).  I need to inquire in this interior strawbale is this close to the ground in places as well, or if it's just the cob facade.  Curious... what do you recommend as a good distance from the earth for the cob and the strawbale?  We can't change the strawbale distance (unless maybe we excavate down, but that causes other problems with rainwater flow).  The other cob and strawbale houses I've seen and helped build, we had a good foot and a half of mortared stone or cinderblock at the ground level, so the cob was nowhere near the earth.  I need to ask them for the building plans.

John Dean... thank you for the welcome!