All About Basements

Rare in warmer regions, basements are nearly universal in the north. Why? Because the ground freezes in the north. When water in the ground freezes, it expands by about 9%.

If that water is under the house footings, that expansion can cracks and heave concrete. The colder the climate you’re in, the deeper down the ground freezes. In climate zone 5, which extends from New York across the middle of the country, the frost depth is 3 feet of more. The solution is to build with the footings deeper than the frost depth, and if you’re already excavating 3 feet, going a couple of feet deeper to get a basement doesn’t cost that much more.

Older basement walls were built with dry-laid stone. Later houses had mortared stone or brick basement walls, and concrete, both block and poured, replaced stone and brick completely in the 20^th century. Insulating concrete forms (ICFs), permanent rigid foam forms that are filled with concrete showed up in the 1980s and offer insulation as well as strength. Most recently, precast concrete panels that are craned into place and connected with stainless steel bolts have been used for basement walls.

Keeping Basements Dry

There are several reasons basements have a reputation for being damp. Dig a hole in the ground and depending on how well-drained the soil is and the level of ground water, the hole is likely to fill with water. Houses built on sandy, well-draining soil and houses that are more elevated have fewer water issues. Another cause of basement moisture occurs in humid climates, where warm, moist, summer air condenses on contact with cool basement surfaces. It may seem counterintuitive but opening basement windows in humid climates can make basements wetter. Your best bet is to close the windows and run a dehumidifier.

To handle bulk groundwater, pipes and gravel are installed around the footings during construction. If the house is on higher ground, the drains are usually outside the footings and pipes that lead out of the ground lower down dispose of the water. On flatter sites, the drains are often inside the footings under the floor slab and lead to a sump pump that ejects the water outside.

Sump pumps must drain at least 10 feet from the foundation, or the water will just come back in (Likewise with roof downspouts). The ground around the house should slope away at least 1 foot for every 10 to keep surface water away. It’s rare for the soil used to backfill next to the house during construction to be properly compacted, and it often settles over time to the point that it slopes toward the house. If this happens, you may need to regrade.

Even with good bulk water management, soil tends to be relatively damp. Vapor pressure can force this moisture through basement walls. Not even a poured concrete wall is waterproof. In the past, builders attempted to combat dampness with marginal success by applying liquid asphalt to the outside of basement walls. Beginning in the 1990s, self-adhering membranes and spray-applied coatings were developed that do a much better job than liquid asphalt.

Using a Basement for Living Space

Unless you’ve dealt with water issues, basements can be moldy, unpleasant places. Before spending money finishing a basement, be sure it isn’t going to flood and that you have a plan to deal with summer humidity or you may be setting yourself up for a moldy disaster.

Another consideration is ceiling height. Basements often have less than 8 feet from the slab to the floor joists above, and beams and ducts may be lower. The final height below such protrusions must be at least 76 inches, and the overall finished ceiling height must be at least 7 feet. If you’re building new, having a 9-foot basement wall instead of the more typical 8 feet can be a good investment.

If you plan a bedroom in the basement, you’ll need to add a code-compliant egress window and escape path or an exterior door for the space, as well as carbon monoxide and smoke detectors. If you want a basement bathroom, it will likely need an ejector pump, a job for a pro.

In some areas, carcinogenic radon gas collects in basements. Always test for radon before finishing and have a professional install a ventilation system if called for. In newer homes in areas with known radon, these systems are installed during construction. Also, have basement water heaters, boilers, or furnaces tested for proper combustion to be sure they aren’t leaking carbon monoxide.

Choose Materials Well

The work you do to a basement depends on its use. If all you want is storage space or a workshop, then handling bulk water, coating the walls with a moisture-blocking paint, and adding a dehumidifier can make the space pleasant enough.

If you want living space though, finishing becomes more involved, and largely revolves around keeping mold at bay. Mold needs both moisture and organic material (for food) in order to grow. So, insulate basement walls with products such as rigid or spray foam instead of batt insulation, which uses organic binders. Use paperless drywall for the wall surfaces. There are also proprietary snap-together wall finish systems for basements. Plumbing and wiring often run below the first-floor joists, and these mechanicals need to be moved into the joists if you want to drywall the ceiling. That’s expensive, and suspended ceilings can be a good alternative.

Many types of finish flooring require a wood subfloor, which can support mold. Be sure there’s a plastic vapor barrier or rigid foam insulation with taped seams below. Alternatively, you can paint the concrete slab, or tile it, or go over it with snap-together vinyl flooring. Finish the space off with PVC moldings.

It’s a good idea to dehumidify finished basements. A portable dehumidifier works but extending your home’s existing air conditioning into the space is better, as is having a ductless mini-split added.

Handled properly, a finished basement can be a valuable addition to a house.