Eye On Housing
NAHB analysis of the Survey of Construction (SOC) shows that 56.8% of all new single-family started in 2016 were built on slab foundations, 27.4% had a full/partial basement, and 14.5% selected a crawl space. The gap between slab and full/partial basement foundation adoption rates is growing: the share built on slabs steadily increased from 46.0% in 2000 to 56.8% in 2016, while basement foundations dropped from 36.8% to 27.4%.
The foundation types for residential construction are closely related to climate conditions, especially the frost line. Homes in colder areas, where building codes normally require foundations built below the frost depth, are predominately constructed with full or partial basements. The division with the highest share of full/partial basements in new homes is the West North Central (80.2%), followed by the New England (80.1%), the East North Central (65%) and the Middle Atlantic (64.1%). In these divisions, full/partial basement foundations offer additional finished floor areas at a marginal increase of construction cost.
In the West South Central (97.0%), the South Atlantic (70.0%) and the Pacific (65.2%) divisions, new homes with slab foundations are the most common. This trend is particularly noticeable in the West South Central division where new single-family homes have been primarily constructed with slab foundations over the past ten years. The high water line in these divisions makes the slab foundations more popular. Moreover, warmer climate and therefore a shallow frost line requirement can also make building on slab more cost-effective, compared to full/partial basements.
Foundations in the East South Central division included 46% crawl space with an increasing trend and 35% slabs with a decreasing trend. Since 2003, the share of new homes built on crawl space foundations increased from the lowest level of 17.6% in 2004 to the highest of 46.4% in 2016. On the contrary, the share of homes built on slab foundation declined from the highest of 61.0% in 2004 to the lowest of 34.8% in 2016.
I think the Pacific region needs to be broken down to more districts. To say that the majority of foundations are slabs does not recognize that Washington state does very little of these, at least on the western side of the state. Having been in the home building industry for over 40 years in this area, I can only recall one or two single family developments that had slab floor foundations. The mix is usually crawlspace or walkout basements.
Regarding footings below frost line . We have been using frost protected shallow foundations (IRC 403.3) for 8 years in Montana with good results. Suggest insulating above the IRC code levels which are pretty low , The IECC is about double the IRC levels. After backfill it’s hard to add any more insulation. .