The Role of Precast Foundations in Green Residential Building

Steve Farrell

 Premise; as essential elements of sound building practice, foundations are expensive in time, money, and are carbon intensive. Further, foundations require a considerable sum of human capital or brain-space, to execute.

For such an important element of construction practice, foundations get scant attention. For me the foundation is the hard part – muddy, time consuming, expensive, and the part that stands between where I am and where I want to be.

Where I want to be is framing! Framing is fun!

So what can be done to manage this required element more efficiently and with less negative environmental impact? One option is precast concrete foundations.

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 Pros of Precast Foundations;

1.     Speed; a precast foundation for a 50’ x 30’ footprint can be set in one day!

2.     Ease of installation; Footings, foundation, insulation, interior studs, exterior finish (potentially), and waterproofing all in one step.

3.     Continuity of insulation; in some systems the insulation is continuous, uninterrupted by studs. R-24 is possible with precast (one could add additional insulation as well). Precast was recently used on a Net Zero Energy (NZE) house in the Triangle area of North Carolina.

4.     Reduced carbon or embodied energy; precast foundations are still energy intensive as primarily concrete and insulation elements – just less so than conventional foundations practices like poured block or poured concrete or even Insulated Concrete Forms (ICF).

5.     Speed of installation allows for quicker site repair; this can reduce runoff, erosion, etc. and can open up projects to re-vegetating in a planting season that may be missed with a conventional system.

6.     Hedges against weather and labor disruptions. Rainy, snowy weather can have a cascading effect on building starts as everyone’s work gets pushed back.  Utilizing precast may allow for an otherwise ill-advised winter start.

7.     Structural integrity; a precast system is inherently more sound than a block system. This comparison would obviously not hold for a cast-in-place system.

Cons of Precast Foundations:

1.     Non-local: As industrial systems, precast foundations take money out of the local economy (although there are two factories in North Carolina).  You may find your mason disparaging your character if you spec precast exclusively.

2.     Exterior finish may not be acceptable in all situations; covering a precast foundation with an exterior finish material negates one of the pros listed above.

3.     The header of most precast foundations presents as a potential thermal short circuit; this could be addressed with a top-chord bearing floor truss to bury this detail in an insulated cavity.

4.     In some systems there are additional electric challenges; armored cable or stud grommets with Romex are required to pass wire through metal studs. Installing boxes and wall-mounted fixtures is somewhat non-standard.

5.     Insulation is on the wrong side: ideally, building insulation is on the outside of the building envelope. This is not possible with precast panels unless one adds additional exterior insulation and subsequent finishes.

6.     Not suited to every site; very steep or multiple stepped foundations can be tricky with precast; typically, a grade delta of about 0’ (level) or 8’ to 10’ across the house seat are best suited to precast.

7.     Complicated footprints (multiple bays, more than 10 corners, etc.) are not well suited to precast.  As with any building system, more corners cost more than fewer corners.

8.     Not a good fit with a crawl space; (but then what is?)

A careful reader will observe that costs were not listed in either the pros or the cons; I’m of the opinion that a precast foundation ends up costing about the same as a conventional system, all in (for a straightforward house). If an additional exterior finish system were to be installed on a precast foundation a conventional system may be the more cost effective way to go.

If, however, one prices the cost of borrowing, and the risk of being exposed to the weather and the vagaries of the masonry trades, a precast foundation may be worth considering.