Berming the Earth Berm Cottage (Part 1)

The cottage has finally transitioned to being the earth berm cottage. Quite a bit when into the berm so I’ll post about it in a series of three articles.

Part 1: Waterproofing, exterior insulation, siding, drain and vent system

Part 2: Building a lime stabilized cob retaining wall, installing a french drain and an earth tube cooling system

Part 3: Building the berm along with many more retaining walls

Below is the conceptual drawing of the cottage I made before starting construction over a year ago. If you’ve been following the build you can see I’ve made several changes. Keep this image in mind as we go along.

Waterproofing the exterior wall

The first step was waterproofing the exterior walls. As a recap, the walls are stud framed with 2x6s, sheathed with ¾ inch ground contact plywood and infilled mostly with cob.

Sample section of wall
Taping plywood seams

To waterproof the walls I used an elastomeric liquid membrane that is typically used for waterproofing basement walls or shower enclosures. I put mesh tape over the plywood seams used a roller to put on the first coat. After that I used a paint brush to make sure all the screw hole and seams were well covered. I put a second and third coat on the lower portion of the wall since the berm will only come up 5-6’.

Drain pipe stubs fitted with flashing collars

For the drain/vent pipe penetrations in the lower sections I fastened vent pipe flashing collars to the wall with screws with neoprene washers. I don’t have a picture but I also put butyl tape around the perimeter of the pipe flashing before fastening it down. Afterwards, I applied a healthy dose of liquid membrane along the edges.

The membrane is the last line of defense to keep water out of the cottage. I’ll be taking several other precautions so hopefully it won’t have to shed much moisture.

Exterior Insulation

I decided to use continuous exterior insulation to isolate the walls from ambient surface temperatures. The walls are infilled with cob which has great thermal capacity. To maintain a comfortable temperature inside the cottage I want the walls to absorb heat energy from the ground which in our area at a depth of 6+ feet maintains a temperature of around 68° F year around. So, the walls and ceiling of the cottage are insulated but the floor is not.

For insulation I used 4’ x 8’ x 1.5 inch thick sheets of Rockwool Comfort Board 80 which is a rigid mineral wool product. Mineral wool is made from molten minerals spun into fibers. It has many good properties the most important in this case being vapor permeable which means it won’t trap moisture against the walls. The second aspect that I like is that it has been suggested from sources I think are reliable that Rockwool comfort board is termite and/or carpenter ant resistant. Note, the manufacturer does not make this claim. However, I have some first hand experience with foam insulation panels and know that the carpenter ants in my area absolutely love it. Mineral wool has a very different consistency and feel than foam so hopefully, it does the trick. I will still have to be vigilant for termite and carpenter ant infestation.

On the downside, Comfort board 80 is very expensive (about the twice the price of foam) and difficult to find. I had to go through a building supplier about 30 miles from me who had to truck it from the manufacturer 200 miles from them.

The material is pretty easy to work with. I cut it with an insulation knife which is essentially a steak knife and tacked each panel to the wall with screws and roofing discs.


Since the side and back walls will only be partially bermed, the upper 2-3 feet of the cottage walls will be visible. I couldn’t bring myself to install cement board siding so I used pine lap boards. Since the roof eave is fairly deep and there are gutters the wood should be well protected from moisture.

First, I attached 1×4 furring strips vertically. This provided attachment points for the siding, held the insulation in place and provided an air gap to allow the siding to dry out if it does get wet.

The siding is almost finished here. I’ll attached a length of 1×6 pressure treated board below the bottom row

Drain/Vent System

Another important detail to take care was installing the drain and vent system for the kitchen sink and half bath. Since the back and sides are bermed I was able to run the drain pipe straight out the wall. The vent pipe runs straight up and over the roof line. I strapped the drain pipes to the furring strip and ran them underground where the retaining wall will begin.

The drain pipe coming from the kitchen sink is currently capped. Eventually there will be a gravel basin here

The kitchen drain pipe is currently capped but will eventually run to a gravel basin like the one I installed for the half bath sink. I dug a roughly four foot by four foot by two feet deep hole and then half filled it with river rock. Then I ran the drain pipe into an irrigation valve box and filled around the box with more river rock. I put down the old tarp to keep sediment from filling in the space between the river rock before covering it with dirt. I estimate this drain basin should be able to handle at least 8 gallons of water a day when it shouldn’t see more than three.

That’s it for part 1. I’ll plan to post part 2 in a week.