Is it better to have a crawlspace or slab-on-grade foundation system? This is a common question among those planning to build or add onto their house. A crawlspace continues to be the more common type of foundation, but slab foundations are becoming increasingly popular. While there are advantages and disadvantages to each type of foundation, the choice ultimately comes down to a combination of personal preference, costs/benefits analysis, short-term vs. long-term advantages, and site-specific conditions accruing to where the new addition or custom home is being built. Continue reading this crawlspace vs. slab comparison to learn which one will best suit your needs.
What Is a Slab-On-Grade Foundation?
A slab-on-grade foundation is a construction method in which the concrete slab that will serve as the foundation for the building is formed from formwork that is set into the ground. The concrete is then poured directly into the formwork, leaving no space between the ground and the concrete slab itself. This method of foundation construction is more commonly used in more temperate climates where: 1). seasonal freezing and thawing of the ground not of concern, 2). the site is not susceptible to a high water table, and, 3). there is no need for mechanical (heating &/or cooling ductwork) to be installed beneath the floor of the building.
What is a Crawlspace Foundation?
Basically a shallow basement, a crawlspace foundation is an elevated structure – roughly 1.5 to 2 feet high – and as the name suggests, with just enough room for someone to crawl through rather than stand in it. The crawl space is the area between the ground and the home’s main level. Generally speaking, a crawl space usually provides only sufficient room for underfloor access.
In addition to elevating the home off the ground – unlike the slab foundation – a crawlspace allows relative ease of access to MEP utilities (i.e. mechanical/electrical/plumbing) such as insulation, air conditioning and heating ductwork, plumbing runs, and electrical wiring. Crawlspaces can be especially convenient in areas with heavy rains or a high water table where excessive water can build up. The off-the-ground support keeps the floor framing away from damp conditions that can compromise or cause damage to the building’s structure and/or building systems.
Crawl Space Foundations are Better for Sloped Lots
Slab foundations with shallow footings (steel-reinforced concrete pads that support a foundation) are limited to flat or nearly flat lots where only minimum excavation is required to pour the concrete slab. When a lot is sloped, however, a slab would require extensive excavation because the high side of the lot would have to be dug out and made level with the low side. Crawlspace foundations are a better choice for slopes because less excavation is necessary (excavating, forming, and pouring concrete for only the perimeter footings, not for an entire slab).
Some benefits of the slab-on-grade foundation technique are that it is quite sturdy while also being relatively inexpensive. Moreover, slab-on-grade foundations are considered less vulnerable to infestation from termites and other pests because there is no hollow space between the ground and the bottom of the structure. A major benefit of a concrete slab foundation ease of construction, thus speeding up this phase of the construction process by days or even weeks. Slab foundations contribute significant thermal mass to the energy performance of a home, thus significantly moderating diurnal temperature swings. Finally, slab homes are typically able to be built closer to the ground than homes with crawl space foundation systems, thus reducing the number of stair risers needed to enter the home. Easy access is advantageous for those who are less physically able.
Slab foundations have some disadvantages that should be considered. The disadvantages of slab foundation include: • Slabs admit of no access beneath the slab for utility lines and wiring. Gas, sewer, and water lines laid down when the slab is poured may eventually break down over time and need replacement. It is impossible to check the pipes regularly so you must watch for signs of a leak. These include wetness on the floor, water pressure dropping, and discoloration of either interior or exterior walls. • Unless specifically mitigated, in cooler climates and/or during the winter the slab can act as a heat sink, transferring the building’s interior heat into the ground beneath the building. • Water intrusion can be a concern. If the slab foundation is located at a too-low elevation, if the area around it is not designed to properly drain, or if the underslab waterproof elements and not detailed and installed properly, water can either intrude or percolate into the building’s structure, its systems, or the interior.
While slab-on-grade foundations are relatively inexpensive and also enable their construction to go more quickly, they require both careful detailing and on-site implementation to work effectively in preventing the recognized downsides.
Crawlspace Foundation Advantages
• Crawlspaces area the most suitable for sloping lots since the footers can be installed in “steps”, thus accommodating the prevailing slope. Also there is no need for the overexcavation a slab would require.
• As has been discussed, crawlspaces offer access to below-floor utilities.
• They are easier to inspect for termites or other pests.
• Depending on the site conditions (flat vs. sloped) and/or preference, the height of the space may admit of additional storage.
• Crawlspaces allow decks or porches to be built well above grade.
• A crawlspace is far less likely to suffer damage from tree roots.
• Subfloors are usually wood-framed, making them more comfortable underfoot than the hard concrete of a slab-on-grade.
Crawl Space Disadvantages
• More expensive to construct than a slab foundation.
• Construction takes longer than a slab foundation.
• To reduce the risk of accumulating excessive dampness, crawl space foundations systems must be properly detailed and ventilated.
• The wooden subfloor system must be adequately insulated, and properly detailed and executed to prevent heat transfer from the living areas into the crawlspace.
• Because it entails an open crawlspace, this type of foundation system requires a more intensive inspections and maintenance regimen in order to avoid potential insect or rodent intrusions into the crawlspace.
Both slab and crawl space foundations offer a range of advantages and disadvantages, but ultimately it comes down to a few key points. Where are you building the house? Will the site be on a slope, prone to shifting soil, or earthquakes? Is the region dry, humid, or prone to flooding? Are first-costs a significant consideration? Are you considering ductwork, electrical runs, or plumbing lines under the slab and, if they break, how will you repair them? All these factors need careful consideration before choosing between slab-on-grade vs crawlspace.
For Further Reading:
• “The Advantages & Disadvantages of a House Built on a Slab vs. a Crawl Space”, an article at SfGate.com: https://homeguides.sfgate.com/advantages-disadvantages-house-built-slab-vs-crawl-space-36388.html
• Angie’s List has published an article, “Which Foundation Is Better for Building a Home: Slab or Crawl Space?” at: https://www.investopedia.com/articles/investing/013017/pros-and-cons-buying-house-slab.asp
• “Slab or Crawlspace”, and article at Houzz.com: https://www.houzz.com/discussions/2304109/slab-or-crawl-space