Passive Solar Composting Toilet Part 1- The Design Process

We will finally (after almost two years waiting for paper work to come through) be starting work on our barn conversion next week!

We decided that we needed a toilet on the land so that the builders who are coming to demolish the inside don’t leave little presents all over the place for us.  I thought that this would be a good opportunity to try out “Light Straw Clay” also known as “Slip Straw” as we want to make the interior walls this way.  We dont have any previous experience with this type of building (or any type of building for that matter) so it will be fun and we can get a feel for technique.

Light Straw Clay is a system that was originally used in Germany.  I really like the sound of it because it looks really easy.  All you have to do is make some “clay slip”(clay mixed with water), toss it over some straw like if you were dressing a salad and then pack it into a form.  Repeat the process until your  wall is finished, leave to dry, remove the forms and presto!  You have a wall that is ready for plastering and is well insulated.

Passive Solar Design

I have been studying passive solar design for a couple of years now and our composting toilepassive solar imaget project is a great excuse to put all of the theory into practise.  It will be an exterior loo, so it could get chilly in the winter and probably not the nicest place to be in if its just a dark and sombre outhouse.  I have designed it so that the sun will shine in and heat it up in the winter, making it a comfortable, warm and bright place to do your duties.

For those of you that are still wondering what Passive Solar Design is; it is basically the use of the heat of the sun shining onto something that will store that heat (mass, normally stone, brick, adobe, etc) to warm  a space.

The Design

I first designed the porta potty in a program called Google Sketchup as it lets me make a scale model and then place it in the exact virtual geographical location to see how the light and shadows react at different times of day and throughout the year.  This is great because it allows you to make changes, try out different configurations until you have something that works.

Our passive solar composting toilet design.

Here it is!

This is the design that I came up with for the shed that will house our composting toilet.  Its form is dictated purely by function.
The roof is slanted at 21.6 degrees which is the angle of the sun at solar noon on the winter solstice (21st of December) in our region.  I have set the roof at that angle so that the sun shines directly inside through the large window at the top giving light and heat which will be captured by the light straw clay and earthen plaster (mass) and then released slowly as night falls.  The light straw clay also acts as insulation which will help to maintain the temperatures fairly constant on the inside.

The overhang is designed so that as the sun gets higher in the sky (closer to summer) it will stop shining straight in and  wont heat the space any more.   If I have done everything correctly the space will be warm and bright in the winter and cool and shady in the summer.

In my next post I will show you my progress with making the frame.

I am hoping that we will be ready for the winter solstice so that I can see if my maths (which is rubbish) is correct and feel the warm sun on my face while depositing a little packet of future fertility for our fruit trees.

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