Foundation discussions can get heated. For some reason, builders often dig in their heels when the topic of slabs versus crawl spaces versus basements comes up. It’s time to declare a truce.
It’s perfectly possible to build a great house on any one of these three foundation types, as long as everything is properly detailed. Each type of foundation has advantages as well as disadvantages. If you have a foundation type that you prefer, that’s great. I’m not going to try to change your mind.
Basements are handy
Before the development of central heating systems and the electrical grid, most cold-climate homes in North America included a basement or cellar. Why? Because a cellar was the only room in the house where temperatures wouldn’t drop below freezing during the winter. Homeowners could store potatoes, carrots, and turnips there without the risk that these foods would be spoiled by frost.
Of course, most of us no longer have to worry that the food stored in our kitchen cabinets will freeze. However, basements still have virtues:
- Plumbing repairs and plumbing changes that accompany remodeling are much easier in a home with a basement than in a home with a slab.
- A basement is an excellent location for mechanical equipment like a furnace, water heater, or HRV. If the basement is properly detailed, equipment located here will be inside the home’s thermal envelope, where it belongs.
- A basement is an excellent location for ducts. Unlike a vented crawl space or a vented attic, a basement is inside a home’s thermal envelope. When HVAC contractors install ducts in a basement, they can stand upright rather than crouch; for most contractors, easy access usually leads to higher quality work.
- If a house has a basement, it’s easier to put the water heater and the furnace near the center of the house than if the house is built on a slab. This keeps distribution lines (ducts or hot water tubing) short, improving the energy efficiency of these systems.
- Basements are useful in areas where the tornado risk is high. If you plan to install a safe room to help your family ride out the next tornado, the best place for the room is in a basement.
- If you need to build a frost wall that extends below the frost line, an 8-foot-high frost wall — one high enough to create a basement — provides more usable space for homeowners than a 4-foot-high or 5-foot high frost wall that merely creates a crawl space.
- Most homeowners find that basements provide a useful location for frost-proof storage of rarely used possessions.
- If you are building on a small lot in a neighborhood with height restrictions, a finished basement can provide needed square footage that can’t be designed into the house any other way.
Basements are often wet
Even though basements have advantages, plenty of people still hate basements. Here are some of the reasons why:
- Basements tend to be damp. The reason is simple: when you dig a big hole in the ground, nature likes to fill it up with water.
- Although there are construction details that you can use to keep water out of your basement, the required details are somewhat fussy and expensive.
- The easiest way to assure that soil moisture doesn’t enter your house is to make sure that every room is above grade. If your house design requires a mechanical room or a big walk-in closet to store rarely used possessions, there’s no reason that you can’t build these rooms above grade. Since most homeowners think that above-grade space is more desirable and versatile than below-grade space, why waste money on a basement?
- Basements are much more likely to have high radon levels than above-grade rooms.
- Crawl space foundations and slab-on-grade foundations cost less than basement foundations.
If you’re planning to build a basement
Let’s assume that you’re familiar with the pros and cons of basement foundations and you’ve decided to include a basement. Here are some issues to consider.
How should I build my walls? In most areas of the country, concrete block (CMU) basement walls have gone the way of the dodo, and that’s a good thing. Poured concrete walls are preferable in all respects to CMU walls.
If you want a basement wall system that includes insulation, you may want to specify the use of insulated concrete forms (ICFs) or a precast system like Superior Walls. Either of these options is likely to cost more than a site-insulated poured concrete wall, however, so these choices only make sense if you have a compelling reason to prefer them.
How tall should the walls be? Basement walls are usually 8 feet high, but they often feel shorter. Most basements have pipes and ducts that are installed below the floor joists. If a homeowner ever wants to finish the basement, these ducts and pipes dictate a low ceiling. That’s why it often makes sense to specify 9-foot basement walls for a new house.
How can I keep water out of my basement? Fixing a wet basement is expensive — especially if the repair work requires excavation of the soil on the outside of your foundation. In general, it’s better to spend a few hundred dollars on waterproofing details during new construction than thousands (or tens of thousands) of dollars on future repairs.
If you are building a new home on a basement foundation, you should specify:
- Wide roof overhangs to keep rain away from the foundation;
- Gutters at the roof eaves; these gutters should be connected to solid (non-perforated) conductor pipes that convey the roof water far from the house (either to daylight or a dry well);
- A 4-inch-thick layer of crushed stone under the basement slab as a capillary break; the crushed stone layer needs to be vented through the roof to help control radon;
- A layer of horizontal rigid foam on top of the crushed stone to insulate the slab from the cold soil below;
- A layer of polyethylene above the rigid foam (directly under the concrete slab) to act as a vapor barrier;
- At least one 4-inch-diameter drain pipe running horizontally through the footing, to connect the crushed stone layer under your basement slab with the exterior footing drain;
- A capillary break (for example, an asphaltic dampproofing compound, UGL DryLok, or elastomeric paint) between the top of the concrete footing and the foundation wall;
- A ring of perforated drain pipe on the outside of the footing, surrounded by crushed stone and wrapped with filter fabric to make a “burrito,” drained to daylight, to a distant drywell, or to an interior sump;
- An application of dampproofing compound or waterproofing compound on the exterior side of the concrete foundation walls;
- A layer of dimple-mat drainage board installed on the exterior side of the foundation walls; failing that, the foundation should be backfilled with coarse, free-draining material like crushed stone, topped with an 8-inch layer of dirt (ideally, dirt with a high clay content);
- Closed-cell foam sill seal between the top of the foundation walls and the mudsill, to reduce air leakage and to act as a capillary break.
Do basements need to be air sealed? Absolutely. Common air leakage sites include the crack between the slab and the walls (yes, air can migrate through soil and crushed stone); unsealed sumps (all sumps need an airtight lid); and the rim joist area. For more information, see Air Sealing a Basement.
Where should I put the wall insulation? Either interior or exterior wall insulation can work, but it’s important to get the specs and details right. Here’s a link to an article that will help you decide what type of insulation to install and where to put it: How to Insulate a Basement Wall.
Can I skip the horizontal layer of rigid foam under the basement slab? Some builders omit the layer of rigid foam under basement slab, arguing that homes don’t lose enough heat through below-grade slabs to justify the expense of the foam. They forget that the main reason to include the rigid foam under the slab — at least in cold climates — has nothing to do with energy savings; the foam is there to keep the slab warm enough during the summer to avoid condensation or moisture accumulation in the slab. If the slab stays dry during the summer, the basement is less likely to smell damp and moldy. That’s why it makes sense for builders in Climate Zones 4 through 8 to install horizontal rigid foam under a basement slab.
Do basements need to be actively conditioned? If your basement is unfinished, there is usually no need to include any supply air registers connected to your furnace (or baseboard radiation connected to your boiler). If a furnace, boiler, or water heater is located in the basement, the incidental heat given off by these appliances is usually more than enough to take the chill off the basement in winter.
If your home is heated with ductless minisplits, you may have no heating appliances in your basement. In that case, your basement may get cool during the winter — especially if there is a heat-pump water heater in the basement. (Heat pump water heaters lower the air temperature of the room where they are located.) Under these circumstances, your basement may need a little bit of space heat, depending on the types of activities that occur in the basement.
When calculating air changes per hour at 50 Pascals (ach50) after a blower-door test, is the volume of the basement included in the house volume? Yes. The conventional way to calculate air changes per hour at 50 Pa (ach50) for houses with basements is to include the basement in the volume calculation.
Should I install a dehumidifier? If you are building a new house with a well-detailed basement, your basement should be dry, and you shouldn’t need a dehumidifier. That’s good, because dehumidifiers are energy hogs.
However, if you live in an older home with a damp basement, you may find yourself between a rock and a hard place, with two unpleasant choices: hiring a contractor to make very expensive basement repairs, or operating a dehumidifier. (You have my sympathy.) In such a case, you may find it necessary to install a dehumidifier as a stop-gap measure. For more information, see All About Dehumidifiers and Fixing a Wet Basement.
Is it a good idea to turn my basement into finished space?
Below-grade rooms always carry a risk of possible water entry. If you are building a new house, and you want the house to include a home theater or a bar, it’s usually best to design your house so that these rooms can be above grade.
If you live in an older home, and you know that your basement stays dry, you may be tempted to transform your basement into finished space. As long as you understand the expense and the risks of this approach, you can certainly turn your basement into almost any type of room you want. For more information on the steps you may need to take to finish your basement, see Build a Risk-Free Finished Basement.
When it comes to the advisability of transforming a basement into finished space, opinions differ. Here’s my opinion: basements are different from above-grade space, and that’s OK. In many homes, the basement is an informal space where people can retreat when they want to relax or make a mess. A basement is a good place for a ping-pong table, for a sturdy shelf to hold home-made jam and pickles, or for a work bench to tinker on small projects.
If the basement is finished into a carpeted home theater, and the work bench is banished to the garage, the house hasn’t necessarily been improved. In my opinion, the trade-off probably isn’t worth it.
An honest, unfinished basement with a concrete floor is preferable in all respects to a finished room with damp carpeting.
Martin Holladay’s previous blog: “Borrowing a Cellulose Blower From a Big Box Store.”