I have encountered several piles of cedar sawdust that have persisted outside for many years, simply because it was in a well drained spot with some rain protection. It's a very resilient material. It is also free. Vermin don't seem to like it. I live near a mill where it is given away by the truck load.
I'll start off using it around a simple greenhouse/spa building. This building will sit on a well drained rubble trench. The rock in the trench will protrude a couple feet beyond the walls, sloping downward. Sawdust will be dumped on top of it. The sawdust will be protected from rain by rubber pond liner.
That's about it. A drawing will soon appear. And there it is. The flag stones protect the rubber from sun damage and they look better. The rubble should continue 6 inches beyond the grade beam. Landscape fabric could separate the rocks from the sawdust.
In general, if sawdust is used as an insulation material, it must be "within" the umbrella of the roof and well protected from any moisture contact, or accumulation. Where you have it (note it should also be segregated from the rubble by filter cloth or "geotex" at a minimum) it will just turn to compost and not insulate at all (marginal at best like a loamy soil. I is also going to to get way to compacted in the configuration.
When you think saw dust, think well protected (yet inspectable) wall cavities, and some open attic applications. I the interstitial void of "stake wood" or "kubbhus" architecture has some merrit as well.
A few years back, I demolished a railway station that sat on untreated timbers placed directly on gravel. A four foot overhang kept the gravel dry. 110 years after construction, I got $5 per foot for the wood. It was very dense Douglas fir.
As a Historic Conservation and Restoration Artisan, I agree with you on many points and can also attest to the "durability" and "rot resistance" of wood when:
1. Allowed to dry after wetting. (Think wooden boats which when just marginily cared for will last 50 to 100 years-more if well cared for and of proper design and wood species.)
2. If the wood is protected from moisture in a well drained manner.
I see structure (barns, mills, log cabins, and farm houses most commonly) that are 150, 200 and in some of the Japanese Minka 400 years old. So I agree wood can be underestimated by many who don't know its true potential.
My worry in this case is the novice that reads things here on Permies and gets an idea that something is "good practice." or "reliable advice" without really reading, researching and vetting a concept that is not well understood or proven.
You opened by say you are going to "INSULATE THE FOUNDATION," which I will stress again, from your cross section elevation drawing of the structure will not be achieved any better than if you placed "pine needles or "oak leaves," in such a configuration. You are going to get candescent moisture on the pond liner which will wet the saw dust thoroughly enough to begin decomp (even if it takes 10 or 20 years.) You are also really going to need the geotex to keep the saw dust out of the gravel, and the compaction of the sawdust is going to greatly inhibit the R factor (insulation modality) of the saw dust.
I am not speculating on this, I am taking this from first hand observation of historic dismantles of Ice Houses, and other "Sawdust" related applications near grade level. By all means progress with your plans and report back your findings over time. I would note that it is better than nothing being there, yet perhaps not the most prudent and effective approach for the materials you are employing in this context.
Only thing I would add to this is that the insulation value will drop considerable with time, so even if it isn't yet rotting it will be losing its ability to insulate quickly. Your basically starting on the back side of a bell curve as far as performance goes. Soo, in short order you will have lost much of the value and wasted a lot of time and energy even if it doesn't rot.
There are plenty of lumber cutoffs available as well. To avoid the compaction problem, I'll use lots of thick chunks and fill the voids with sawdust. As stated in the original post, this is a durability test. If I can keep it dry enough to retain some usefulness as insulation, I'll lay it 4 ft thick against an earth sheltered structure. There's no end to the supply of scrap wood and sawdust. Call it carbon sequestration, or call it Penelope.
If anyone has ideas on how to make this work better, that might be interesting to look at.
If it doesn't rot, it will remain pretty much the same over time, just like the 800 year old stuff that I used on a roof. Those trees fell a century before my roofer split the shakes. If it rots, the R-value will drop.
I am sorry Dale...you are simply wrong on this one, and taking these materials completely out of context. I am sorry you can't see that.
You cannot expect even a rot resistant species like cedar at grade level (or below it) to behave in the same fashion as a "roof shake." That is just silly, and I can not believe you even made that comparison? I have seen 2000 year old shrines in Japan...I know wood intimately and can tell you that within the same temple compound there are cedar posts (and saw dust) that are decomposing at grade and under it in only 20 years (as the temple master and Shinto Priests expected them to.)
To avoid the compaction problem, I'll use lots of thick chunks and fill the voids with sawdust. As stated in the original post, this is a durability test. If I can keep it dry enough to retain some usefulness as insulation, I'll lay it 4 ft thick against an earth sheltered structure. There's no end to the supply of scrap wood and sawdust. Call it carbon sequestration, or call it Penelope.
You are creating a giant "wick" up to an earthen wall by this description...
If anyone has ideas on how to make this work better, that might be interesting to look at.
Well you won't like it (it would seem) nor agree to the observation thus far shared...but simply put...don't do it... as you are wasting the materials and using comparative modalities that are not applicable to your suggested concept.
It is a long and drawn out concept of creating a natural "thermal break or barrier" on the outside of "mass earth walls" (or within them) and a challenge to do correctly even with advanced skill sets. If you are up for it, we can discuss that, but not until you can see the very large flaws in the current plane... If you can not see that, then I say good luck, and let us know how long it last...hoping we have missed something in your plan, and that it works as well as you think it will. I would be pleased (for your sake and efforts) to be wrong...
And, I think I may have cracked it. Rather than covering the sawdust with the liner, I think the liner should have an open top that allows the wood and sawdust to poke above ground. Plate glass is available for free. A glass lid could protect the insulation from rain while allowing the material to dry out completely each summer. I'm only going a couple feet deep and the bottom will rest on fabric which rests on well drained cobbles. There should be plenty of days even during the winter, when the wood could shed moisture. I suppose a 6 mil poly loop could encase all but the materials that are exposed to the air. This effectively puts the material into a narrow little greenhouse with a ground sheet. There would be small gaps under the glass lid to allow water to evaporate.
I'm quite stubborn. When I have an idea, my goal is to find a way to make it work. I'm still going to do a small experiment in the manner of the original drawing, but I expect the open top to be better.
This drawing shows it all done in glass. A glass wall separates the sawdust from the soil and a glass lid caps it. The glass wall could be vertical. The goal is to prevent the soil in the greenhouse from being cooled by the frozen ground. It can freeze up to a foot deep in bad years.
There is no earthen wall. This is for a greenhouse with an earth floor.
O.k. "green house," sorry I must have missed that...So no insulated walls, got it
The current drawing is much better and closer to achieving your goals. The sawdust can be "dense packed" but not made supper solid either. Larg chucks of wood depri is not going to help in any way so, do not bother. All sawdust would be much better if you can get it.
It's something I expect to complete within a couple hours.
Hmmm....either you are crazy efficient or....I would think at least an entire day (more like two) if you want a thorough job of it.
Now you are getting much warmer! (or better insulated )
And, I think I may have cracked it. Rather than covering the sawdust with the liner, I think the liner should have an open top that allows the wood and sawdust to poke above ground.
Don't be so stubborn that you do not see the obvious though my friend...Most times it is better to take a few steps back, try and remember what was done before, and not reinvent a wheel (or make something work) when there is a better solution to the challenge.
I am currently designing a small timber frame home for a client. They want a green house-solarium on the side of the structure. We are actively having the exact conversation for real "right now" in their design here in Vermont (very cold!) I most cases in vernacular folk architecture of the most enduring types you have larger over hangs and often "engawa" or porches that go all around a structure further carrying the liquid and snow precipitant further away from the structure and its foundation. What I am proposing to the client is similar to what I would suggest to you:
1. Build your green house.
2. The grade beam in mine would either be a rot resistant species of heavy timber (to support the timber frame above) or a stone with lime-cobb mortar to the 600 mm level then an timber plate, or large plinth stones to take the timber frame armature that will support the green house.
3. If I was to insulate with saw dust, I would have to trap the gravel (20mm) stone and rock (50 mm to 150 ) within a gabion baskets system beneath the sill plate or plinth stones to keep the saw dust in a more vertical format. The other would be to dig to mineral soils, dig a grid for sill and plinth stone locations (at least 300 mm deep into the mineral soil) with drain channels "to light or exit of grade" and fill these with packed 20 mm gravel. Set the plinth stones, fill the areas between with cobb walls, the my cobb floor (I don't like cobb for greenhouse floors and try to get clients to go with gravel...your (their choice). Now you can create a "shroud around this with your saw dust, and hop it stays dry.
4. Here is the big difference between your drawing (which will still wick ground moisture up through the saw dust.) I would build 3 season hoop frames (or plastic - glass enclosed veranda) around the entire greenhouse further extending the "precipitant shedding" roof line well past my sawdust insulation. Making very sure that whatever plantings and watering I do is kept away from my sawdust insulation. If it is compacted and/or wetted even a little bit the insulative qualities go to about zero.