Underground house with SIPs

I’ve been thinking a lot about underground houses, as I’m sure a lot of permeis do.

Does anyone have thoughts about building a post and beam structure and attaching SIP (structural insulated panels) panels as exterior sheathing.

I know it wouldn’t be an all natural build, and adds a bit more costs to a project.

My thoughts is that it would be a reasonable use of modern materials, since insulation is a pitfall for underground builds it would cover that along with adding a considerable about of structural reinforcement.

I know they make them with PT exterior sheathing for being used for basement foundations, which would make me believe they could withstand lateral pressures.

I think they'd hold the lateral load if there were enough posts, but what's PT? Is it serious waterproofing? Wouldn't take a very big leak to take down a difficult to repair hunk of wall.

PT = pressure treated. Waterproofing is another issue.

Thanks Glenn  :D
I thought that was what it meant.

So I'd say the structure can be made ok, but waterproofing needs to be done REALLY REALLY REALLY well.

My understanding of one of the big benefits of an underground house, is that the first layer of berming acts as thermal mass, and then you put insulation and an umbrella of impermeable material spaced out from the house, and cover that with more dirt.

So it seems like using SIPs in this situation is defeating part of the purpose of building underground. The more I read about efficient, comfortable houses, the more I feel that thermal mass inside an insulated cocoon, is an ideal approach.

That said, there are other reasons for building underground, like protection from windstorms/tornadoes, and a green roof thinks UV is wonderful, whereas, shingles don't.

There is some excellent information about PWF's (permanent wood foundations) that would be applicable to construction of under ground structures.  However, that only covers how to build underground walls out of wood.  It leaves the roof and its supporting structure to ponder as there is A LOT of weight that goes on top of an underground dwelling.

https://www.finehomebuilding.com/project-guides/foundations-and-masonry-work/a-strong-case-for-wood-foundations
https://www.greenbuildingadvisor.com/article/planning-for-a-permanent-wood-foundation

In my opinion, Post and beam, SIP and underground housing just do not go together.
I think its self defeating as mentioned.
I lived for a while in an underground brick garage built under a building.
I used hydronic heating and once the bricks were warm, I kept the heater on in  winter continually.
It was comfortable.
There was no insulation beyond the bricks.

Typical R-Value
Walls  = R-20
Floor = R-30
Roof = R-60

Unlike EPS foam which has a r-value of 4 per inch soil only gives us 0.1 per inch, so soil needs to be 40times thicker. Aka to get that R-20 value for the walls we would need a thickness of 200inch or about 17ft. If you are putting your building directly into the ground with say only the roof exposed then that ok.

The underground house will really shine during the summer months, it will be the perfect temp and at most you will just need to run a dehumidifier and its fan on low, using about 1/10th the amount of electricity that an Air Conditioner would use.

Also in the winder once the floor and wall are warm, it will take a long time for them to cool down and will make it very comfortable, due to it's huge mass.

Just don't forget to vent your kitchen, bathroom and laundry room. And add a ERV to ventilate your house. ERV can save you alot of money. You could even look into a pex-radiant floor heating system.

EDIT: ERV = Energy Recovery Ventilation, The incoming and outgoing air passes by each other and reach  average temperature and humidy of the both air. So less warmth/coolth is lost by the outgoing air and the  incoming ait is tempered. AKA 50% reduction in energy cost.

S Benji, what is ERV please?

I suspect, Energy Recovery Ventilation…but am nor certain.