Outdoor insulative material?

[hpmer]

I am contemplating building a cooking stove that will be a permanent outdoor fixture.

I know that builders use vermiculite to insulate concrete blocks but I've also heard it traps moisture and so would not be a good insulator in my application.

I've also heard that perlite is great for gardens because it too traps moisture.

Anyone have any experience outside with either or thoughts and/or suggestions?

[canyon]

Have you considered a roof over the works?

[hpmer]

I'm more concerned with dampness infiltrating the insulation and it not drying out. I don't think a roof would solve that issue.

[canyon]

If you have a roof to deal with direct water and you are only dealing with ambient dampness perhaps you just need to do a small drying fire or two prior to renewed use after long idle period. If that doesn't fit your needs try triple wall stainless heat riser?

[Donkey]

Clay is very good at "breathing out" incidental moisture.
If you have a roof and the clay rich parts aren't sitting right on the ground, it's not something I would worry about.

[hpmer]

Donkey,

Do you know what is done on cob structures to keep the clay from washing away in the weather? I see in the book structures that have been around several centuries, so it seems if I could emulate that process I would indeed be ok.

[Donkey]

Don't need to do anything but,
A: keep it from sitting in a puddle, fairly tall foundations are the general rule in really rainy environs. Fairly tall meaning about knee high, which keeps the bottoms of the walls out of the rain splash zone.
B: Wide eves to keep the highest part of the walls in the rain's "shadow"
and C: don't let anything grow up or lean against the walls. Also, don't run gutter downspouts against the walls.

Cob is QUITE good at dealing with horizontally driven rain. If rain hits the walls and SHEETS down, everything is A OK, no problem. If for some reason a channel is formed, by vegetation growing up or leaning on the walls or plugged gutter downspouts, where water is forced into a little stream, THEN you will get erosion at a pretty quick rate. Fortunately it's easy to see and easy to fix.

When I first got into Natural Building and Cob in particular, my first project was an earthen oven. I'm a skeptic at heart, so I decided to not build a protective roof for it and just leave it out in the rain, I wanted to see for myself what would happen. The thing sat in the rain for 3 years before my wife finally had enough and made me fix it, 'cause it was getting to be a bit of an eyesore.
But here's the deal.. It never ruined the stove. All of my sculptural details were washed off, but once the stove got back to a roundish, smoothish shape, that was basically that. I went out after several days of soaking rains and checked it on occasion, what I would find is that the cob was soft and squishy an inch or so in, where it would be hard and just about dry. It seems that the cob, when wet, forms a kind of clay-pan and keeps the inside safe. Seemed that just about no matter how much rain we got, it only soaked in just so far. That said, if I poked a hole in it and just left the hole, THAT would give the water something to "grab", and it would form a rivulet and carve away right at that spot.

[canyon]

One other thing that they did(do) on traditional cob structures to help with weather is a lime rendering(plaster). The lime/sand plaster can take somewhat direct exposure, especially if you apply a lime wash (white wash) yearly.

[Donkey]

Right..
I forgot to mention lime..

DO NOT use stucco, or ANY other render with portland cement. DO NOT paint the walls with anything to "seal" them, like house paint or tar or anything like that. DO NOT use vapor barriers of any kind.
Anything that keeps the walls from breathing naturally will ruin them over time. Portland cement will wick and sweat moisture.

There are lots of really good choices for coverings.
Lime is GREAT, lasts a good long time, stands up to hard driven horizontal wet, takes just about any color, and can be very inexpensive if you do it yourself.
The drawback(s) are that it's pretty easy to screw up, especially if you've never done it before and you MUST use protective clothing to work with it.

Plasters can be made from the same dirt as the cob, pigments and fiber can be added for toughness and color, etc. Earthen plasters stand up better than you would think and their advantage(s) are, no protective clothing needed (you can work naked if you like), really easy to get right, easy to fix, very fault tolerant, ultra inexpensive and more. Earthen plasters will need to be freshened or maintained a little more often than lime, but it can be so easy (and fun even) that it's really not a chore and the neighbors will want to help.

Really, earthen walls are more resilient than we are led to believe. Up in Northern England, where it blows horizontal rain for over half of the year, traditionally they say that raw cob experiences a "loss of face" of about an inch a century.
That is assuming maintenance of the roof, loose the roof and your building can turn back to dirt in a couple/few decades.

[shahborn]

I would try Hardiboard, and then just stucco the outside. I have been trying it and it holds up to heat very well as long as its not exposed to direct flames. Its Also OK for outdoor use. I would just build a container out of it and put the stove in and fill with insulation or just pour in some type of insulative cement mix. You will have to reinforce it in some way if it starts to get heavy but should other wise do the job.

[hpmer]

Feb 12, 2011 18:30:03 GMT -8 shahborn said:

I would try Hardiboard, and then just stucco the outside.

What do you cut that stuff with?

[Donkey]

I've used a jigsaw.. You can also use a Skillsaw, though you need to turn the blade over so that it's teeth rotate the wrong way.. It'll cut cleanly that way.

Don't forget to wear a mask!!