To berm or not to berm?

As I've thought about the house I plan to build about a decade and a half from now, although my design still borrows a lot of ideas from Earthships, lately I am less and less seeing advantages to berming along the pole-facing side of it.

It seems like a lot of energy spent pushing dirt around (or digging partway into it) to achieve thermal mass, which I imagine I could get by easier means*. That pole-facing wall would still have to be insulated and water- and vapor-barriered**, and placing soil in contact with all or most of the height of an exterior wall strikes me as a way to introduce complications that wouldn't be there otherwise, for the same reason slab-on-grade is simpler and invites fewer problems than basement foundations.

Is something missing from my thinking on this? What is so great about berming that I should reconsider?

*Examples of easier means of introducing thermal mass: thick/heavy wall material such as cob/adobe/rammed-earth or even just brick; earthen or tile floors; etc.

**Insulating the wall means I lose some of the advantage of the berm-as-thermal mass anyway!

As I understand, you would not insulate between the back wall of an earthship and the berm, so that the berm becomes part of the mass. I would put insulation and a water-shedding membrane beneath the surface of the berm to isolate most of that mass from the air temperature. That is definitely the case with the wofati design.

For a given amount of mass, piling dirt seems easier than mixing cob or filling earthbags or tamping tires. You just need the structure to be capable of holding back the berm and being durable.

Yes, berming introduces one set of complications at the same time it eliminates another set. You need a level of waterproofing for a bermed wall that you don't need for an exposed wall. But with the right materials, you completely eliminate maintenance of the exterior while adding a significant amount of thermal barrier or tempering. Berming may make it easier to achieve drainage away from the structure on the uphill side.

Thanks for the well thought-out replies, Glenn!

Glenn Herbert wrote:As I understand, you would not insulate between the back wall of an earthship and the berm

I thought the updated Earthship designs do call for insulating there. Maybe I'm misremembering?

I would put insulation and a water-shedding membrane beneath the surface of the berm to isolate most of that mass from the air temperature. That is definitely the case with the wofati design.

Wouldn't insulating the berm's surface reduce its effectiveness as a solar battery?

For a given amount of mass, piling dirt seems easier than mixing cob or filling earthbags or tamping tires.

Good point, though I think excavating/piling/tamping a berm still seems like a pretty huge energy input compared to at least some of the more conventional massive wall construction methods (e.g. the various brick-based ones). Plus with a berm you need to build a very strong wall anyway, so you're already looking at a not-insignificant additional energy input from that.

You just need the structure to be capable of holding back the berm and being durable. [...] Yes, berming introduces one set of complications at the same time it eliminates another set. You need a level of waterproofing for a bermed wall that you don't need for an exposed wall. But with the right materials, you completely eliminate maintenance of the exterior while adding a significant amount of thermal barrier or tempering.

With berming you might eliminate maintenance if there are no problems down the road. Seems to me there's a decent risk of running into problems, in which case you're looking at excavation or other costly forms of remediation. An exposed wall can be potentially very low-maintenance, e.g. if you use corrugated metal siding which, facing the nearest geographic pole, would only need to be scrubbed of algae once every few years, and if there are problems with an exposed wall they are much easier to address. Meanwhile I would not expect a berm to provide much of a thermal barrier given the r-value of tamped dirt, especially when wet.

Berming may make it easier to achieve drainage away from the structure on the uphill side.

To an extent, yes, I agree. I think there are many ways to achieve good drainage, without berming, that aren't too difficult though. But I can't claim to be experienced in these things.

I have not studied earthship design, just read and watched videos, so new developments are out of my scope.

An uninsulated berm as thermal battery would not get meaningful direct solar gain - what comes in in summer would be at least balanced by what goes out in winter. To be a thermal battery, it needs to be connected with the house interior and isolated from the weather. Heat should go from the interior to the berm/storage mass in summer. The idea of the umbrella is to keep the soil under it dry, so if wet earth is a factor, the design has failed. Likewise with wall waterproofing, there should not be a need for it separate from the umbrella. There would be no need to tamp a berm, as settling after construction would not hurt anything. You just allow a few inches extra height.

Glenn Herbert wrote:I have not studied earthship design, just read and watched videos, so new developments are out of my scope.

Same here, just thought I remembered seeing/reading that the newest iterations included insulation between pole-facing wall and berm.

An uninsulated berm as thermal battery would not get meaningful direct solar gain - what comes in in summer would be at least balanced by what goes out in winter. To be a thermal battery, it needs to be connected with the house interior and isolated from the weather. Heat should go from the interior to the berm/storage mass in summer. The idea of the umbrella is to keep the soil under it dry, so if wet earth is a factor, the design has failed. Likewise with wall waterproofing, there should not be a need for it separate from the umbrella. There would be no need to tamp a berm, as settling after construction would not hurt anything. You just allow a few inches extra height.

Ah, that makes sense.

So then it really comes down to which set of trade-offs you prefer. These days I think I am inclined more toward those of a mass-rich wall...

I am not an expert brick maker by any means, but I have made a few. It seems quite labor and time intensive any way I can think of doing it on a home scale. Moving many tons of earth for a berm wall with a rented excavator and less than 5gal of diesel per day is very easy comparatively. Moving material to make bricks or moving premade bricks would likely take a similar amount of energy and expense.

Ben Zumeta wrote:I am not an expert brick maker by any means, but I have made a few. It seems quite labor and time intensive any way I can think of doing it on a home scale. Moving many tons of earth for a berm wall with a rented excavator and less than 5gal of diesel per day is very easy comparatively. Moving material to make bricks or moving premade bricks would likely take a similar amount of energy and expense.

I agree, and this makes sense, but keep in mind with a berm you don't get out of having to make the wall anyway. It's not like the berm takes the place of the wall. The berm implies an additional labor input, not a replacement one. (And that's just while it's being built; if there are problems later then remediation into a berm seems likely to dwarf the labor input needed to remediate problems with an exposed wall.)