I love the look and price of cob. I love it's thermal mass properties too. Awesome stuff....but as far as I have read, it is not weatherproof. For those of us living in the wet areas of the world, it would be a bad choice. Yet, I am determined to make an out-door rocket mass heater/stove/water heater thing to impress myself and the neighbors. I have been contemplating, and it was also suggested, that I use a soil/cement slurry. I'm not sure what ratio I should use, I was thinking it would have to be 1/2 and 1/2. Anyone have experience in this? Will our butts get muddy after a rain (which is typically at least once a week)?
Can something like this be painted with a surface-adhering cement? Will putting a water-proofing layer (such as a surface adhering cement) on top of this cause bad things to happen? Moisture comes from both the ground and the sky here.
To begin, it seems that soil-cement proportions can vary between 1:5 and 1:10, thus very little cement is needed.
Ratios of Portland Cement to Aggregate This site has more info on working with soil-cement as well.
No matter what material you use for the thermal mass of an outdoor bench, you would want to have a waterproof top surface to avoid damp seating surfaces. Imagine a steam iron pressing your pants as you sit on it! If you have access to flat stones, those would be ideal and attractive, otherwise large floor tile or dense concrete pavers might work well. I would tend to go with masonry (brick, block, or stone) for the sides (all on a well-drained base) with loose gravel fill up to the duct, and either more gravel or a cob-and-stone lasagna above it. I might put waterproof insulation on the sides and possibly underneath to focus the heat up to the seat. Just keep the insulation away from the hotter areas to avoid risk of combustion. If you have full-width stones or other good waterproofing for the top, I think cob as strictly a thermal fill inside would work fine. The greater the proportion of stone or gravel or sand to clay, the more stable it will be.
Cement doesn't work too well with cob / soil. It's best to use lime, though of course you go back to the 'not water proof'. I'd use cob and something like linseed oil, not sure how that works with heat. That would need researching.
As with all soil / cob builds. It really depends on your local soil so what works for me, or doesn't work for me, may not be the same for you.
I agree, try and find a roofing solution to your project while keeping the original 'open air' aspect.
Sounds like fun. Don't forget, cob has been used in England for centuries. A very wet and cod country. Can't be that bad.
No- roof is not an option. That would ruin the whole point. I could put water-proof pillows or blankets on top to give it a sort-of roof, but when the down pours happen, the bottom will get wet and when the snow melt happens, the whole thing will still get wet. It sounds like the cob can be water "resistant." I'm surprised that cob and cement don't mix (pun intended). You see: soil cement is used like all over the place for a hard yet not quite cement hard thing. I know you should never cover adobe (what I think of as a close relative to cob) with cement or something else more waterproof because the moisture won't wick away and the whole thing will just melt, but mixed together I would have thought that problem would go away. Though, it is relatively easy to mix in lime versus cement. Yes - temperature changes will be dramatic! So something that expands and contracts much without flexibility will probably be a huge problem too. I can't imagine an outside cover of linseed oil having issues. I may have to look into that or some other oil topping. I'm not sure about the consistency of dirt yet. I have to import fill so it might be anything.