Aaron Yarbrough

Welcome to Permies!

Axel Oehrli wrote:My question is, can I just stack the rubble and compact it really well?

Yes, although it's nice to have smaller stones at the top of the trench to make leveling the foundation easier.

Axel Oehrli wrote: Also, do I need to dig out a separate drainage line too? How do I go about this?

It depends on how the water flows around your site. With large roof overhangs and if you divert water away from the foundation you will probably be fine without it.

Here is a video of a rubble trench foundation I did a few years ago. I use large stones and pieces of urbanite we had on the property in addition to to some smaller stone I brought it.

Hi Jen,

Since cob is a poor insulator you need pretty thick walls (like 1-2 feet) to get enough thermal mass for a cob wall to really be effective. So you would probably need to furr out your 2x4 or 2x6 and extend the perimeter of your foundation.

Protecting the existing drywall is kind of a Catch-22. On one hand it would need to protected from the moisture of the cob during construction to prevent it getting moldy. But then you want the wall to be vapor permeable when its complete so moisture doesn't become trapped in the wall. House wrap is supposed to be water resistant yet vapor permeable so that would probably be your best bet.

If you like the aesthetic of cob I think I would insulate the stud bays with something like wool or rock wool batts and then do a lath and lime plaster finish on the exterior. It would look like cob but without having to make any changes to the wall system or mess with the the electrical.      

14 months and it looks like my replacement chest freezer is failing.

Since the last one I got a bigger generator that doesn't surge whenever the freezer kicks on. The one in our guest cottage is still working fine and it is over four years old now. They both have the same type of external thermostat. Could it be some sort of electrical issue like a faulty neutral?

Anyway, we have guests coming in this evening and I'm away at a board game convention next week so I probably won't mess around too much with troubleshooting and just run over and buy a new one.

TLDR: Clay is fine in light clay straw in almost any environment provided you have generous roof overhangs. Clay based plaster is probably fine provided the aforementioned generous roof overhangs but I think lime plaster is better.

The long response:

With significant roof overhangs an exterior clay based plaster should be fine. We did our whole exterior light clay straw house with earthen plaster and it's held up fine for the past five years. There is one fairly exposed corner with only two feet of roof overhang that I re-plastered a few months ago. Our climate is probably drier than yours but we still get ~32 inches of rain a year and when it usually comes in the form of blowing wind storms.

That being said since building our cabin I've switched to mostly using lime plasters for a few reasons. Note, these are anecdotal observations and maybe over time I've just become a better plasterer.

1) Lime based plaster dusts off less than clay plaster
2) I get less shrinkage with lime plaster than clay plaster.
3) Hairline cracks in lime plaster will heal themselves to an extent.
4) Once lime plaster cures it is pretty hard wearing
5) If you're adding a tint to your plaster it's easier to maintain a consistent color.

Regarding cost and causticness

A lime plaster is mostly sand (~80%) so a 50 pound bag of type S hydrated lime  (~$20 in my area) will go a long way. You could probably plaster the interior and exterior of a 200 sqft house with less than six bags. I've found that contact with hydrated lime mostly just dries out my skin. I'll typically wear light gloves and safety glasses when I'm plastering.



 

James Bradford wrote:Permablitzes ... educate me ... sounds fun.

A permaculture oriented work party. Often they involve earthworks and/or tree/shrub planting but I've also been to ones where we installed rain water catchment and earthen floors. Usually, activities are labor intensive and low skill. Typically the host provides lunch and sometimes there's a speaker or too. It's a good way to meet other people in the permaculture community.

Pre-pandemic there were about half dozen permablitzes a year in the central Texas area and it's slowly picking back up.

Welcome to Permies! With Hill County Natives closed I'm glad to see you're keeping the central Texas native plants trade alive. I live on the south side of Austin but I'll give you a shout when I plan to be up in your area. When you settle on your 30 acre property are you planning on hosting any permablitzes?  

Welcome to Permies!

Definitely trending in the right direction. Good on you for tracking progress.

Mark ruhl wrote:
However the point of this post is me trying to find and studies on the efficiency of the 3 materials compared to eachother. Aircrete is water permiable so naturally I wonder about what materials I’d need to keep moisture out of my house. Anyone have insight on which of the 3 is the MOST energy efficient

Hey Mark,

Welcome to Permies!

I don't have any experience with aircrete but straw bale, light clay straw and cob with earthen/lime finishes are all very vapor permeable so any moisture that ends up in the wall can escape. To keep moisture out of the house I think large roof overhangs are more important than wall systems, especially overhangs over windows and doors.

Regarding thermal properties straw bale and light clay straw have an R-value 1.5-2 per inch. Cob has a very low R-value (~.5 per inch). We live in a warm humid climate where we use mechanical cooling for about half the year so I like to use light clay straw or straw bale on the exterior walls and cob in the interior (e.g. earthen floor, trombe walls).  The cob is in perpetual shade and in the case of the earthen floor absorbs heat/cool from the ground keeping it cooler inside. In a colder climate you can orientate your house so the majority of your windows face sunward so the low winter sun can shine through and heat up your interior cob elements.

Welcome to Permies! Glad you found us. The Greening the Desert project is one of the things that first got me interested in permaculture.