Frost Protected Shallow Foundation

I've been spending many man-hours working on the plan. We spoke with a concrete contractor regarding the footers, and he had some interesting facts to share with us. The first fact was the two foot [24 inch] frost depth required for footers here. I had been thinking about the amount of concrete we'd need to adequately support those big posts, and had settled on 12 inch depth, 36 inch square footers, but to avoid using excess concrete, we were planning on laying each footer on a bed of ruble 12 inches thick. The local code, however, requires a depth of 24 inches for a footer. No matter, we can accommodate that. But I recalled something I had come across before regarding shallow foundations, and frost protection I'd like to share.

A frost-protected shallow foundation system was started in Scandinavia. They use rigid foam insulation set around the outside of the foundation to protect against frost heave. Such shallow foundations reduce costs several ways. First, you have less excavation and backfill. You can use significantly less concrete by not needing to pour footings below the frost line. Since we plan to use in-floor radiant hydronic heating,
it just makes sense to combine the insulation of the surrounding foundation with the slab insulation.

Post Footings

I have been working on finalizing the footing plans for the twenty posts we need to put up for our barn. I have a grave concern for rising damp. The antique wood we have, and the new posts we are making out of our felled poplar trees deserve to remain dry. I have seen the damage done to those components of the barn that were in contact with ground, and the amazing preservation afforded those posts held up off the ground by a simple rock footing. The rock, having been much denser than concrete, didn't exhibit the rising capillary action of water near as much.

This capture from SketchUp shows a compound footer made up of 12 inches of compacted stone and a 36"X36"X12" concrete filled FastBag from FabForm. We will dig out 20 holes, 12 inches deep, by 36 inches square, to hold the rock, [Thank goodness for the back hoe!], compact it with a hand tamper, and set the bags over the rock. That will give us effective 2 foot deep footers with half the concrete.The bag is waterproof, and will prevent ground moisture from entering the concrete. We will place a moisture barrier under each post, likely a square of EPDM, to serve as a backup.