Friday, December 26, 2014

Design - Footing and Foundation

Footings and Foundation Walls
The footings are not negotiable when pricing construction costs; they are whatever the construction engineer signs off on.  The foundation wall, on top of the footings and under the stick-built walls, is another matter.  In an attempt to cut costs, I costed out several scenarios.  But first, how deep do the footings need to be?

In order to meet code for frost protection in our neck of the woods, the bottom of the footings should be at least 30" from the finished grade.  Under a wood wall, then, the foundation would have to be 30" minus the height of the footing, usually 8".  Then 8" has to be allowed between the grade and the wood members for termite protection.  The minimum height then would have to be 30" - 8" + 8" = 30".
Design similar to ours (minus footings)

Frost Protected Shallow Foundation
However, there is a green alternative called "frost protected shallow foundation" (frost protected shallow foundations).  At least 2" of solid foam insulation is attached to the exterior vertical concrete then more is laid horizontally outward from the wall a few feet.   The horizontal insulation rests on top of the footing (a footing is not shown in the nearby drawing but can be seen in the Amvic pic) and is covered with soil up to a depth allowable for termite protection.  This means that the height of our wall, instead of being 30", can be as short as 20" as follows:  8" for termite protection, 10" for backfill over the insulation and 2" for horizontal insulation.

Amvic Building Systems ICF Technical & Installation Manual
Insulated Concrete Forms
The 10" difference in wall height for the foundation may not seem like much but on a tight budget it is significant.  We will be using insulated concrete forms (ICFs) (our likely choice for ICFs).  They are are stacked in interlocking fashion as a form for pouring the concrete then left in place after the pour to provide 2" of permanent insulation on both sides of the wall.  The cost savings for a 20" wall versus a 30" wall is one third, or $950, for a 163 linear foot wall like ours.  That's 1.31% of the total budget that can be used for something else. And concrete and Styrofoam are not exactly sustainable materials so using less is a green alternative.

Parenthetically, the insulation, watershed umbrella for the AGS system (last of three posts on AGS) lies tight against the ICF on top of the footing and therefore satisfies the requirements for the exterior horizontal component of a frost protected shallow foundation. 

Other Options
I looked at other foundation options in an effort to pinch costs even more but came up empty.  Complete Blocks (Complete Block Company), though tempting, were too expensive despite an offer from the owner for free installation.  Dry-stacked cinder blocks or concrete poured in rented conventional forms were in the ballpark with ICFs but less DIY friendly and, to achieve a thermal performance comparable to the ICF's, an insulation design would have to be improvised, would be labor intensive and would have an outcome very likely to be inferior to that of the proven ICFs.

North Wall
The north (back) wall, which is critical for the AGS system,  will be in contact with the earth to a height of 12' and, because of an interior wall design that accommodates a storage room next to the wall, it will not be well braced against backfilling--essentially mimicking a long straight retaining wall.  Unless something can be worked out with Complete Blocks, there were no sensible alternatives to a 12" thick conventionally-poured concrete wall seriously-laced with rebar and resting on out-sized footings.