Tuesday, August 9, 2011

What's the Greenest Foundation System for a New House in Denver?

The conventional wisdom in cold climates for basements (since we stopped hand-digging them) has been "if you need to go down 4' for a crawlspace foundation, why not just dig another 4' and throw in a basement? The marginal costs are very low per square foot, definitely less than building the second floor. Historically, however, unfinished basement square footage is not allowed in the multiple listing service database.

Habitat for Humanity in the Denver area even takes some criticism because they usually choose volunteer safety over basements. No one can get hurt by accidentally falling into a crawlspace foundation. For the same reason, they have eschewed the second floor until recently as vacant land has become scarcer.

My cost analyses, however, are showing me that a frost-protected monolithic slab is more cost-effective than a basement as long as the land is cheap. Once the price of land reaches about $20/sq. ft., then a basement may be required by the homebuying market.  In other words, the neighborhood is so expensive that the buyers expect the extra square footage of a basement.

The tipping point in favor of slab-on-grade over crawlspace is that the slab can be the finished floor. Stained concrete is still trendy, bulletproof, and saves at least $3/sq.ft. on your floor system.

My concrete floor is six years old, had zero maintenance*, and looks just like the day we moved in.

The thermal mass and the way it buffers the temperature is just a bonus.
Unfortunately, slabs are all but incompatible with modular construction.   Houses from modular builders always have a wooden joist floor system for shipping rigidity, so there's no opportunity for a slab floor.

A Slab is green because:

A slab is much easier and less expensive to insulate properly than a crawlspace or a  basement.
No ventilation required, and no water problems if it's a few inches above grade.   It uses less money and resources to construct.   It has less chance of failure or air quality problems.

The biggest drawback is the lack of easy remodeling.

*Cleaning is not maintenance.


  1. Great article and I'd like to point out my recent experience with modular home builders - they will go well out of their way to sell a structure. They re-arranged their standard floor templates, finishes, fixtures and anything we asked them for. We needed to reduce weight so that the crane could reach out further to set our unit and the modular factory agreed to ship the unit with the siding & shingles stacked in the living room. That way we could manually remove them prior to picking & setting. I'd be willing to bet good money that they would install a temporary stabilization frame inside their units if a customer wanted to build on a frost protected slab.