What types of foundations are most commonly found? What is each foundation’s characteristics? What type is under your home? How can you best insulate your foundation? What are the most important thing to remember about foundations?
The Conundrum of Foundations and the Building Shell:
As an energy auditor, the foundation poses something of a puzzle. They are part of the building shell but no one pays attention to it (at least not homeowners until the flood seasons). The building shell is the enclosure around the house that controls the flow of air, heat and moisture. You know, the walls and roof. But the average homeowner rarely thinks about the basement in terms of heat and moisture controls.
Concrete does a spectacular job at one role of a building envelope (providing structure) while brutally failing two others (controlling heat and moisture). So how does a giant stone sponge with almost no insulating value work as part of the building shell? Generally, very poorly…making them a good energy savings opportunity (with many caveats).
Types of Foundations
We’ll explore these issues in depth in other posts. Right now let’s review the common ways that concrete is poured under your home. The three most commonly found foundations in residential construction are slab, crawlspaces and basements. A fourth type, pier foundations, are a less common variant that can make sense in certain climates and soil conditions.
Slab Foundation
A slab foundation (in construction called ‘slab on grade’) is pretty much exactly what it sounds like: a concrete slab formed directly on the ground. Slab foundations are a less costly option, usually poured on site with XPS foam board insulation and a vapor barrier underneath. These are most common in the South where ground moisture makes a full basement more problematic.
Slab foundations are an obvious choice for radiant floor heating and require minimal excavation work. They work better than other foundations in high water tables (assuming properly installed vapor barriers and drainage). The main drawbacks are that they make closet space a premium and the plumbing and water lines are often buried. Older slabs can be uninsulated, needing expensive retrofit work.
Crawl Space Foundation
Another foundation type that combines the minimal storage space of slabs with the poor insulation of full basements is a crawl space. The main advantage of a crawlspace is that the plumbing and water pipes (and possibly heating system) can be easily accessed.
Because crawl space generally have dirt floors, they can be a massive source of interior water vapor. An uninterrupted vapor barrier covering the floor and sealed to the the foundation walls is essential. The walls should be insulated with a vapor barrier product such as XPS foam board or closed cell spray foam.
Lastly, crawl spaces are subject to the same moisture, rot, foundation cracking and flood issues that a full basement would be. Homeowners rarely enter their crawl space so problems can persist well after they have become an issue.
Basement Foundation
Full basements are essentially very deep crawl spaces. They are most common in the North, providing plenty of additional storage space and if proper moisture and insulation controls are installed, extra living space. They follow the same structure as crawl spaces (footing, slab, a formed concrete walls and drainage systems). They are generally waterproofed against water infiltration and to retard water vapor infiltration.
Basements can be a versatile space if properly designed. Like I noted earlier, many many homeowners pay no attention to their basement beyond, ‘no, it is not flooded’. By adding proper thermal, air and vapor barrier systems in the basement, it can be a warm and dry space. One last advantage of basements is that since people use their basements, problems are often discovered and addressed quickly.
More Information
Understanding Basements from Building Science
This is a Building Science article detailing some of the moisture and thermal transfer issues occurring in basements.
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