I first met Sukita Crimmel about  6 years ago when I was building my home-the first permitted and bank financed clay-straw insulated home in Oregon.  Her passion is natural building and we’ve worked since on other projects.  Her specialty is earthen floors and her work was featured in the NY Times. This spring she asked, if she could hold a ‘mud floor’ workshop this summer in a small cabin I’m building behind our house.  I leaped at the idea.

When we built our home, I wanted to have an earthen floor in one of our rooms.  Unfortunately, my engineer nixed the idea for structural reasons.  Now I could have an earthen floor in my cabin.  Though normally the floor is one of the last items installed, I wanted to go ahead now.

My cabin is small—12’ x 16’—but has long history.  We lived in a very small house before we built our home.  I wanted a “man cave” but knew we were moving in a couple of years. So, I constructed 2 x 4 wall panels with T1-11 sheathing.  I screwed the panels together to form the walls and attached the rafters with screws and

Cabin Without Doors And Windows Installed

brackets.  Everything was fastened with screws including the flooring and sheathing.  The roofing was a heavy tarp.  The interior walls and vaulted ceiling were painters drop clothes stapled to the studs.  I even hung pictures on the walls.  When we moved I unscrewed everything, hauled it to our home site and stacked it under more tarps.  It sat there, deteriorating slowly, until my crew and I rebuilt it.

Before she could hold her workshop I needed to do the ground work.  There was additional fill needed to level the area.  The stem wall required insulation.  I planned to have radiant floor heat and needed some type of insulation under the floor.  I also had to install the PEX (crossed linked polyethylene) tubing for the floor heat.

The fill was no problem.  My teenage son and friends loaded a pile of mixed gravel and dirt left over from the septic system installation into the space.  I had scraps of 3” XPS (extruded polystyrene) foam (R=15) insulation left over from building the house that worked well to insulate the stem wall.

Preparing the base for an earthen floor.

The floor insulation was a different story.  Though I had several sheets of 2” XPS (R=10) lying around, I was reluctant to use it.  The blowing agents used in the production of XPS are toxic and contribute to climate change.

I consulted with Sukita.  She tried loose perlite but found it difficult to work with because it was too ‘squishy’.  My next idea was white pumice which has an R value of approximately 1.5.   An air-filled volcanic rock commonly found in the Northwest, it is available at garden centers but the size sold is 3/8″ and smaller.  Too small—’squishy’ again.  My local garden center did have red volcanic rock similar to pumice.

Volcanic Pumice-like Rock

At 1″ to 3/4″ in size and with sharp edges it packs well and provides a good substructure for the earthen floor.  Assuming a R-value slightly less than pumice my 2″+ to 3″ of red rock base should have a R-value of 3 to 4.  That’s equivalent in value to about 1″ of EPS (expanded polystyrene) foam insulation.  I’m fine with the low values for this project because it will see limited use in the winter.

The red rock is one of many layers of my floor.  The deepest layer is “drain rock”– 1” to 2” gravel left over from my foundation drainage.  It provides a capillary break to prevent ground moisture from wicking up from the earth.  Next is a laver of gravel and dirt—the bottom of the gravel pile.  I tamped all of that with a motorized plate tamper.  The next layer in this ‘lasagna’ floor is the 6 mil polyethylene vapor barrier.  I used two layers of ‘poly’ because the red rock is sharp.  A bed of sand over the ‘poly’ is a good idea but I did not have enough depth .

Red Volcanic Insulation

Next I installed and tamped the insulating red rock.  Final layer before the tubing was some cast away filter fabric I’d saved.  It prevented the earthen floor mix from settling into the insulation rock.

PEX tubing will not lay flat in a floor.  It needs to be firmly fastened to a substrate.  One simple way is to use 6” welded wire mesh.  It is used for reinforcing concrete slabs.  Though available in rolls, it is much easier to use flat sheets which are 42” x 84”.  The tubing is attached to the wire mesh with concrete rebar ties at 12” to 18” spacing.  There’s a special little tool for twisting these ties and the process is quite quick once you get the hang of it.  I installed the tubing in a serpentine

Radiant Floor Tubing Is Attached To Concrete Wire Mesh

pattern with the tubes spaced about 1’ apart.

You do not want any joints in the floor.  The final step is to pressurize the PEX to about 100 psi and keep it pressurized until you’re ready to connect it.  This assures that there are no initial leaks and that none occur during the pour.

Tubing Is Pressurized

I’m ready for the first phase of the workshop, pouring the base coat of earthen floor.