Building into a Hillside (or Below Ground)

Lots of peeps are asking questions about building into the side of a hill, or semi-underground. I’m in a good position to answer this, because all my stone huts have been built into the rock face. In fact the barn uses the rock of the land as one of the walls. This is quite common in both Spain and Portugal, but brings with it some issues, the main one being damp.

Semi submerged

Why build into the landscape at all? Despite the drawbacks people have been building underground for eons (there are 10 000 year old underground cities in Turkey). Why? Well there are quite a few perks to being a little lower down.

Advantages

• Shelter: You are much less prone to wind damage which is important in places with hurricanes and the like.
  
• Aesthetic: You blend into the landscape better and your house becomes a part of the land.
  
• Invisibility: You are less conspicuous, so for all those on the run it’s an attractive option.
  
• On a small plot, it can be a good use of land space too.
  

Disadvantages
The most obvious drawback is that underground houses are notoriously prone to damp, or worse flooding.

How to build underground
The key to a semi-submerged house is two-fold. You need a vapour barrier and decent drainage. Whether you are considering an earthbag bunker or a stone root cellar, the same applies.

1.Vapour barrier
I’m usually quite ambivalent about vapour barriers in natural homes, but this is the clear exception. You definitely want a vapour barrier (plastic tarp) on the exterior of your outside walls if you are underground. The barrier should separate your structure from the earth/rock outside. Some people even add a double barrier.

Separate your walls from the ground. The black is the vapour barrier.

2. Drainage
Dig a decent trench all the way around your structure, line it with geotextile membrane and fill it with rubble (rubble trench). You could also add a french drain here, but do it properly (see this article). The main point about drainage that seems to get forgotten is gravity. You need a decent downward slope, and no amount of perforated plastic tube is going to help you if said tube is laid flat, or nearly flat. I often think a decent rubble trench lined with geotextile membrane sloping well downwards, works better anyway.

The rubble trench. Line it with geotextile membrane to help keep the dirt from seeping in and clogging it up.

3. Guttering
This is super important. Make sure the water running off your roof, isn’t sinking into the ground around your building. Add guttering all the way round your roof eaves and channel that water away from the house and the ground surrounding it.

Extra note: There’s no getting away from it really. Submerged buildings often feel a bit damp. A good solution to damp is a wood burner. It will dry a building out very fast. But of course that’s only a valid option in a cooler climate. If you’re in the warm then think long and hard about ventilation and possibly a dehumidifier.

Which types of sustainable building suit going underground?
Now, there will no doubt be those who disagree, but having seen a couple of examples of strawbale rot in the damp, personally I wouldn’t be itching to stick strawbale underground. Nor would I prefer to use wood. The safest options are stone and earthbag because they can handle the wet. Hempcrete might also be OK but I haven’t seen any examples of that yet.

Marvao in Portugal built into the landscape.

Related links:
Earthbag root cellar: https://www.wildernesscollege.com/earthbags.html
Underground earthbag room: https://youtu.be/8dM2It_T3nE
Rubble trench: https://cordwoodconstruction.org/rubble-trench-the-basics
Do you want a french drain or not? https://www.estormwater.com/drainage/french-drains-and-their-downfall

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