Insulating Render/stucco for thick earth walls

Greetings,
My understanding of earthbag walls is that they have great thermal mass, but a low insulation value.

I live South East UK- a temperate climate where the temperature rarely drops below freezing and rarely gets hotter than 30 degrees celsius.

I'm building a Hyperadobe garden office, and my main concern is keeping it warm through the winter so it's comfortable to work in all year round without increasing my heating costs drastically.

Has anyone tried to insulate earth bag walls with earth or lime plaster mixed with pumice/ perlite to insulate the outside of their earth buildings?
Any views on whether this could work?

It sounds practical, but I have no experience on whether it is. I think a major factor would be strength versus insulating ability of the specific mix, and how thick you can make it both technically and financially.

I might consider, given your mild climate, using an efficient heating system like a rocket mass heater as well as whatever passive solar you can manage so there is makeup for nighttime losses. I think that would be the major issue you will face.

Thanks Glenn....It's pretty tiny...only 10 ' in diameter on the inside, but I have considered trying to adapt a tiny rocket heater to heat a small amount of mass....just enough to take the chill off in the mornings.  I think a rocket heater would be ideal because there's so little smoke ..no complaints from neighbours about ruining their washing.

yep...the strength of the plaster...guess there's no option but to make a test patch. I have put large glass windows in...as big as I dare go.
Thanks again.

I have considered doing something similar for an earth bermed earthbag house.  My reason for insulation on the outside is to control condensation on the inside of the walls.  I haven't figured out what type of insulation to use nor what outside finish to use.  With some really good insulation on the outside and a good passive solar gain design you may be able to capture nearly all the heat you need for your mild climate in the thermal mass of the walls.

Thanks Mike.
On Earthbag building.com they do mention a way to insulate by doing another row of earth bags on the outside filled with pumice/perlite/scoria, but it seems a little time consuming and expensive for a garden office. I would do this if it was the main house. You can even fill the bags entirely with stabilised pumice apparently.
I think I'm going to try filling the tubing with 50% earth-mix and 50 % pumice, plus pumice as an insulating layer in the earth floor, and good thick hemp insulation in the roof. The green roof will also help to keep the heat in/cold out.

With a small rocket stove(with flue) and large double glazed south facing windows it should keep warm enough.

My experience in buildings with thick earthen walls with no other insulation for some 20 winters, in a climate much more extreme than the UK is this: The thick earth walls also act somewhat like insulation, in that the temperature change can travel through them only very slowly. So I think your little earthen-walled garden office might be fine with no insulation. If sitting at a desk in winter seems a little too chilly, you could use a small electric heater near, or heating pad right under your feet, but with your temperate climate, I don't think the building will get really very cold before springtime. At least, its worth trying it for one winter with backup heat available before embarking on the expense, work, and/or area that insulating might consume.

Rebecca....this is great to hear. I'm going to stop worrying so much and take your advice. If it works in the Himalayas it'll work for me.
Thanking you, Glen and Mike for sharing your time and experience. All the best.

As to using pumice or perlite in a cementitious mixture, everything I have read would seem to indicate that as soon as you fill those air spaces in the rock with the cement slurry, you lose the insulating quality of the material. I was really bummed to keep reading this over and over on the net. I do think that your climate makes a difference, and that in some places you can get away with it better than others. We built a 16' ID catenary dome tank house for water filtration equipment from scoria bags, and then had it coated with 3" of shotcrete inside and out. Even in our severe climate, it takes only a light bulb to keep it above freezing. There are no windows for solar gain, and two 6" vents that are open year round to reduce humidity levels. I do not think we could have gotten this kind of performance if we had used pumice-crete (the first choice). Just my 2 cents-

We are in the planning stages for a house. We are almost set on a modified post and beam frame with one inner row of earthbags and one outer row of scoria or perlite. The bags would be contained within the framework. The walls would be plastered with a vapor permeable earthen plaster. This seems to be the best use of both mass and insulation. It will certainly take longer, but we have adjusted our expectations to fit a good outcome, even if it takes more time. I don't know how much time your project is worth.

Hi T Phillips,
I think your double row bags sound ideal, especially since it's a main house. Sounds exciting and I wish you all the best with it.

After searching for more natural insulation ideas I found a lime render/plaster full of bits of (responsibly sourced) cork.
It's from a man called Mike Wye, who's pretty well known for his natural building products and projects in the UK.  The stuff looks promising - here's a link: https://www.mikewye.co.uk/product-category/lime-products/ecocork-insulating-render-plaster/

I think it sounds about right for my small building.

Good luck!

Cath, that render sounds fantastic. I think it would be perfect for our building, also. Please keep us posted!

Lately I've been experimenting with aged horse manure lightly slipped with clay. The manure is very dry when I start and I break up the clumps until it's all loose. I fill a wheelbarrow with water and clay to make a slurry and toss in the manure and mix well. You have to experiment to get the proportions correct. You want to have just enough clay to act as a binder; the less clay you use, the more R value you achieve. So it's the same idea as light clay slip but you get a much finer end product. It reminds me very much of the Celotex insulation board that was sold here in the States in the 70's. It came in 4X8 foot sheets and had a black tar-like waterproofing applied on the surface. I live in a very dry climate so it drys out reasonably quickly, not sure how it would behave in a damper environment. You are lucky to have hydraulic limes available in your market so you could always add NHL to the mix to encourage curing and act as an antimicrobial.  It's a real interesting material to experiment with and it would provide a thermal break thereby keeping the heat in your bags/interior. Not for the squeamish though!!

Just noodling here, but isn't perlite adsorbant?
I know natural building called for a good pair of boots and hat, but still...

Perlite and clay slip is used as an insulater in rocket stoves.  Would the say apply if used as an outside insulation layer for a building?

Alan Loy wrote:Perlite and clay slip is used as an insulater in rocket stoves.  Would the say apply if used as an outside insulation layer for a building?

The test cylinders I made were kind of fragile. I don't think it would wear well on the outside of a building without a layer of plaster protecting it. You could always add more clay and get a stronger result. It works well in rocket stoves because it's kept in place, kind of sandwiched between the riser tube and the 55 gallon drum. And in a damp climate, you would probably have to stabilize the clay with Portland cement or lime. Otherwise it would be very weak when wet.