Insulating Earth Plaster

​Earth and clay, in and of themselves, are not insulating materials. They have thermal mass, which means they store heat (or cold), but don’t reduce the transmission of heat energy from inside to outside (or vice-versa). Nevertheless, you can make earth plaster more insulating if you need to. More on how to do that, and how well it insulates, later in the post.

straw-heavy plaster

Insulating a House Basic Overview

Before I get to the plaster game though, a quick overview of insulating houses in general: The first thing you need to worry about is not the walls, it’s the roof. That’s where 60% of your heat goes. High ceilings are notorious for energy wastage, as you have a fat layer of completely unused heat that skulks under the ceiling. Your floor is the next worst culprit for heat loss. This doesn’t mean you don’t want to insulate the walls. You do. Though sometimes insulating the north facing wall (or south facing in southern hemisphere) is enough. But insulating the walls without sorting out the roof is a bit pointless.
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R Values​

What is all this jabber about R and U values? If you’re a newbie builder you may be wondering. Insulation levels are measured in R values. Here’s a neat little infographic for US climate zones showing the kind of R values you would need for your roof, floor and walls for a conventional house in various climates. All building materials have an R value per inch, though be careful because calculating R values is a bit more complicated than just multiplying by inches. Some insulating materials work in different ways (for example, if you compress certain materials, or mix them into something like clay plaster, then you reduce their capacity to insulate). Nonetheless, the graphic gives a basic idea of what’s going on with R values.
 
Then there are U Values, which rate how energy efficient a given system will be. Me? I want to get back to earth plaster, but if you feel like geeking out on U Values as well here’s more info.
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Insulating Plaster

Ok, back to the plaster. First, let's be clear; Mixing insulating materials into your earth plaster alone isn’t going to cut the mustard in seriously cold climates. You’ll want to use the Other Ways to Insulate an Earthern Wall listed in the next section. But if you live in a temperate climate, or a place where you need a little extra oomph in the winter months, insulating plaster can work a treat.

• Sawdust/straw: If you mix a decent quantity of straw or sawdust into your plaster (instead of the sand), things are soon going get warmer in your mud world. Please note: That plaster needs to be pretty straw/sawdust heavy to have a decent effect.
• Cork granules: Ah, cork is a goodie. It has a very high insulating value, is also superb for sound-proofing, and, counter-intuitively, it’s relatively fire-resistant as well. But best of all, cork is sustainable. The cork tree’s bark can be harvested every 9 years. You can buy cork granules and mix them into your plaster instead of the sand.
• Paper adobe (R2.5-3 per inch): Paper adobe, as suggested by the name, is where you mix re-pulped paper with clay and earth to create a paper-rich adobe. Paper adobe has a high R value, so it’s worth experimenting with. 3-4 inches of this is going to help keep you toasty. To make it, shred lots and lots of newspaper finely. Let it soak in water overnight, then mash it with some kind of blender/mixer. Finally, when it’s a sticky pulp, mix it into your adobe.
• Perlite? I was a bit dubious about perlite, because I wasn’t sure you could compact it enough in the plaster for it to work sufficiently as an insulator. Perlite is a volcanic glass with an R value of 2.7 when used pure and compacted. It hasn’t been tested much in earth plaster, and the R value would certainly reduce depending on the ratio of perlite to plaster. Craig Ray's cob house in New Zealand (below) boasts a mix of perlite and paper pulp in the plaster with splendid results. He specified that he added two layers of plaster: the first is perlite-heavy, the second is a weather protecting layer with sand. His climate is very wet, with cool but not freezing winters. I’ve no idea to what extent the paper pulp was doing the insulating, but the house is snug.

Note: When using insulating plaster, you need to apply a thick layer (at least 3 inches). It's important to add the plaster to the exterior walls of the house for maximum insulation (though if you’re restoring a listed building, this may not be possible, in which case you’ll have to insulate the interior wall instead).

Craig Ray's lovely mud home in New Zealand

Insulating earth plaster mixed with perlite and paper pulp (photos courtesy of Craig Ray)

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Other Ways to Insulate an Earthen Wall

If you live in a climate which remains below zero for months at a time, earth plaster alone is not going to be enough. I'm not discussing whether or not you should have built a mud home in a cold climate here, nor going into massive detail. There are a few ways of insulating earthen houses. Most of these require attaching a light lath or frame over your wall, and stuffing it with some sort of insulation such as:

• Wool (R3.5 per inch)
• Cotton batts (R3.7 per inch)
• Cellulose (R 3-4 per inch)
• Slip Straw (see Sigi Koko’s nice FB post on this).

 
Alternatively, you can cover the exterior of your mud house in straw bales (R 1.45 per inch).

straw bales

I'm sure there other natural, insulating earth plaster methods out there. If you've tried something else, feel free to add it in the comments.

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