Floor Slab Design

Post date: Dec 6, 2014 4:25:57 PM

Unlike previous posts about things such as lighting fixtures that can be easily changed if they are not right the first time, this one involves the very foundations of the house. If we get this wrong, the rest of the project is doomed. (And I am acutely aware of this because my 90-year-old house in Seattle has weak foundations and is still settling.) Now I think I have found the difficulty with the Solar Slab design — the one that uses concrete blocks under the cast concrete floor slab to store heat.

In cold areas designers and builders have migrated to the Frost Protected Shallow Foundations (FPSF).

This system insulates the foundations, reducing the depth required to prevent frost heaves. At the same time this design was developed, building contractors discovered that they could avoid building and removing forms on the inside of the foundation walls if they thickened the slab around the perimeter. Then they could pour the footings, foundations, and slab all at one time - saving a lot of time and effort. I have not found evidence that the thickened edges of the slab are required structurally, but they will improve shear strength at the perimeter.

However, with the Solar Slab design (see illustration at left - from James Kachadorian's book), the slab rests on top of concrete blocks laid sideways, so the gravel base within the foundation walls must be flat all the way up to the edges of the floor. This requires that the foundations be poured within forms, the forms stripped from the foundation walls, and the gravel pad leveled and compacted within the foundations after they have set.

There are two sides to this arrangement. While a monolithic footing-foundation-slab is easy to build, there is no way to thermally isolate the floor slab from the foundation walls. The foundations and footings will be heated by the building from within. That requires extra insulation on the outside (and possibly beneath) the foundation, but insulation is cheaper than concrete.

On unstable soils, such as those in river bottoms like my lot, a monolithic slab-and-footing can reduce uneven settling because the footings and slab are structurally connected (by the same path that enables heat loss). Making the footings wider (24” is recommended, rather than 18”) will help, but that, too, means a concrete pour for the footings separate from the pour for the foundation walls.

But wait! The architect suggested a while ago (which, typically, I forgot) using ICF (Insulated Concrete Forms). That still means three separate (or maybe two - more later) concrete pours, but it eliminates building and stripping forms. Not only that, but the insulation of the forms can provide the thermal break between foundation wall and floor slab. Typically the forms are made of extruded polystyrene (XPS) with insulating value of R-10 on each side of the concrete. That would mean an R-10 thermal break around the perimeter of the slab, plus the foundation itself would be insulated from the outside by another R-10.With ICFs, the footings are poured and and that concrete sets first. The forms are then assembled atop the footings. It might then be possible to prepare all the under-slab utilities, the gravel base inside the foundations, the Solar Slab concrete block ducts, and the slab and foundation reinforcing steel, and finally pour the slab and foundation walls together. I would still probably tie the slab and foundation together with 1/2" rebar dowels that pierced the inner wall of the ICF, like the illustration on the right, but with the insulation continuing between foundation wall and floor slab.

Another method (this article describes a variety of methods), one that requires a crew of four or five that work well together, pours the footings, then sets the ICF and pours the foundation stem walls before the footings have set. Some vendors provide components that enable placing the ICF on top of the footing forms first, then pouring footings and stem walls at the same time. These methods eliminate the cold joint between footing and stem wall but require an experienced crew.

So this is a big compromise on my part. Is it worth — to me — to use this heating design at the extra expense of building separate foundation walls and providing an insulation break between foundation and slab? This will probably require some heavy discussions with the building contractor.

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footing, foundation, and slab tied together