Offgridburbia

The Earth Berm Cottage (EBC) is an earth sheltered tiny house. The walls are light wood framed (2×6 studs on 16″ centers). We’re infilling the stud bays of the side and back walls with cob. The front (non-bermed) wall will be infilled with Light Clay Straw (LCS). So, why put mud in the walls?

Thermal reasons for putting cob in the wall

Cob is NOT a good insulator. Is has an R-value of less than 1 per an inch where light clay straw is somewhere in the 1.5- 2.0 range, fiberglass insulation batts are at 3.7 and foam panels are in the neighborhood of 5.

Cob has a lot of mass and thus great thermal capacity. It’s like a thermal battery that absorbs warmth or coolth and releases it later. So, how do we charge this battery that is the wall? We’ll look at a couple of situations.

Charging during Warm Weather

Six feet below the surface the ground temperature remains fairly stable. That temperature should be close to the area’s average annual temperature. In central Texas our average annual temperature is around 70° F.

To get the cob in the walls to absorb that coolth from the ground we isolate it from the surface temperature by installing continuous exterior insulation on the walls. We’re also putting a lot of insulation in the roof along with 6 inches of soil. Our site also has a really good tree canopy providing a lot of shade to keep the ground around the cottage cool. The foundation and floor are not insulated allowing the cob to absorb the coolth from the ground.

In a colder climate where the ground temperature is say 55° F you would want to insulate the foundation.

Charging during Cold Weather

The uninsulated foundation/floor that to provides coolth will also act as a heat sink to any heating effort but if the ground temperature is around 70° F that’s not too bad for an inside winter temperature. To take the edge off of really cold days I’m planning to install a wood stove. The cob walls will absorb the radiant heat from a wood stove and the exterior insulation will keep the heat from escaping into the berm. The heat stored in the cob will radiate back into the cottage.

Structural reasons for putting cob in the wall

Walls vs Living Roof

We’re planning to install a lightweight living roof which will weigh 60-70 lbs per square foot. A typical roof is designed to accommodate a permanent load of around 20 lbs per square foot.

The 2x6s studs we have supporting the roof are resistant to the compression force of the roof bearing down on them but they susceptible to bending. That’s why with light wood framing the members are installed fairly close together unlike in a post and beam or timber frame construction where larger dimensional lumber is used.

The plywood sheathing give the stud walls shear strength and ties them together making them more of a monolithic unit rather than individual members. The cob in the stud bays furthers this monolithic effect even further reinforcing the studs against bending. While the cob doesn’t hold up the roof it is a structural component in a supporting role sense.

Walls vs The Berm

Between the weight of the earth of the berm and hydrostatic pressure there will be a some lateral force exerted on the walls. Cob in the walls play less of a role here. The monolithic aspect of the cob infilled wall will resist force parallel to the walls.

The bigger concern with earth an earth sheltered structure is force perpendicular to walls. Cob has a lot of mass that will help a little but here are the main factors keeping the walls from collapsing inward.

1) Good drainage: Hydrostatic pressure of water pooling against the wall is more of an issue than the weight of the berm. I’ll put more details in a future post but for now I’ll say that I put in drainage in to move water around the structure and downhill from the cottage.

2) The roof structure and weight: The rafter and beams are secured to the walls with structural fasteners. In the picture below it looks kind of like a grid. The rafters and beam combined with plywood sheathing and the weight of the living roof will make the walls resistant to being moved around.

3) Intersecting interior walls: The exterior back wall is 18’ long but there will be an interior wall intersecting close to mid way. This interior wall will be infilled with cob and act as a buttress. When back filling we’ll also temporarily brace all of the walls. Once back filled as long as drainage is taken care of there shouldn’t be much lateral force on the walls.