Finish a Basement

How can you finish a basement with less risk of mold growth? Too bad more builders and contractors don’t know how to do this!

Basements were originally intended as buffer zones between the soil and the living areas of the house – good for wine and root cellars but otherwise left unfinished. Modern life has changed that, often for the worse because the risk of mold is increased. Mold can grow on building materials, especially in wall cavities.

Homeowners may assume their basements are dry and finish them, but the owners do not realize that concrete is not waterproof. Moisture migrates through the foundation wall and may foster the growth of mold on the paper backing of sheetrock. Unless you live in a desert, finishing a basement raises the risk of hidden mold growth.

Tip: When I give guidance to someone looking for a new home, one of my first suggestions is: “No below-grade finished spaces.”

That said, many homeowners still want to finish their basements. Here are some suggestions for finishing a basement with reduced risk for mold growth.

CAUTION: Please consult with your contractor or architect prior to taking action. I am neither!

Check for foundation cracks and efflorescence

  • Check for foundation/slab cracks. If any are more than ¼” wide, consult a foundation specialist. Seal cracks less than ¼” wide. Wire-brush them off, both from inside the basement and on the exterior foundation. Apply a waterproof expandable foam sealant.
  • Gauge the amount of efflorescence present. “Efflorescence” is whitish lime and other mineral deposits around the inside perimeter walls of the basement. If significant deposits are found on the lower portions of foundation walls, there could be deterioration of the concrete from the exterior, particularly with block foundations. Use of a dehumidifier increases this process because it pulls moisture through foundations walls, and lime is water soluble. The worst basement I saw for efflorescence had ½” to ¾” mineral deposits across lower basement walls. The homeowner thought it was mold, but it wasn’t.

Ideally, only waterproofed basements should be finished.

  • Waterproofing would be costly and would involve digging down to footings around the perimeter of the house, section by section, wire brushing off the foundation, applying a thick coat of wet-dry tar, and then wrapping the tar in six mill polyethylene sheeting. A professional contractor would know how to avoid buried pipes and wires. Check with a waterproofing contractor prior to taking action.
  • Even if walls were waterproofed, the slab is still porous. Although there are various methods of waterproofing a slab (easiest in new construction), none are inexpensive. The slab may need to be removed in sections, with a moisture barrier laid down before pouring a new section of slab. Or, a moisture barrier might be laid down, with risers and a new floor platform built on that.
  • Another thought is to seal the slab with porcelainized ceramic tile or porcelain tile. Although not providing the same seal as porcelain, Marmoleum is another green option.
  • A lower-cost finishing treatment would be attractive painted concrete, or polished concrete, with washable scatter rugs. You might check for least toxic products similar to epoxy at healthy hardware stores.
  • For green construction supply stores, refer to the Resources tab.

Is a drainage system advised?

  • Prior to considering a floor covering, it may be necessary to have an interior subterranean perimeter drain installed. This should be done only if there is a high water table.
  • When the water table is not high, a subterranean exterior drainage system, known as a “French drain,” is a better option. This system reduces water intrusion through the foundation, rather than recycling water after it enters the basement.

For standard, non-waterproofed basements, careful selection of finishing materials is essential when you want to finish a basement. 

  • The objective is to use materials that mold doesn’t like to grow on. This will lower the risk of future mold growth.
  • Mold mostly likes to grow on wood-based products, including the paper backing of drywall. Special attention needs to be given to wood-based products.
  • All biodegradable surfaces (every square inch of wood in a basement or crawlspace) should be encapsulated, thus making these surfaces unavailable to mold growth. If all cellulosic surfaces are made unavailable to mold, there would be less concern with relative humidity (unless furniture is present and then dehumidification may be essential).

Take steps to reduce wicking of moisture through the foundation into wood framing materials.

  • When constructing walls, take precautions to reduce the amount of wicking into building materials.
  • All accessible surfaces of sill plates and studs should be thoroughly encapsulated with two coats of Caliwel ( or other encapsulant. If metal sill plates and studs are present, they should be grounded electrically to a dedicated ground rod.
  • If there is room, set up framing 18-24″ out from the foundation, so that you can walk behind the walls. A fan could keep air moving behind the wall. Encapsulate vulnerable materials (the backing of drywall and studs and sill plates, plus ceiling joists and subflooring). Here, air may be the insulator.

Install insulation as needed.

  • Fiberglass insulation is not recommended below grade, yet there is no ideal, reasonably-priced alternative. One alternative would be board insulation. Look for the best R-value – maybe polystyrene board, without fire retardants (which are showing up in human tissue). Check requirements with the local building code. Confer with your contractor about installation and sealing requirements. Green building supply stores offer least-toxic spray foam insulation, such as Touch’nSeal.
  • Some clients have been happy with another least-toxic insulation, Air Krete, a mineral-based insulation with magnesium oxide, This insulation dissolves in water which in a way is good. If there is a leak, the water will pass through the insulation instead of getting trapped behind it and spreading out– but then you have to either have an installer stop by to patch the missing area of Air Krete or order some Air Krete pieces to patch them.

Consider the range of wall products when you plan to finish a basement.

  • Check into DragonBoard, a magnesium-oxide based drywall-type of product. (You just heard magnesium-oxide mention with Air Krete insulation.) This could also be used in place of drywall in a new house. DragonBoard is recommended for very chemically sensitive individuals.
  • Or use WonderBoard or other cement backer board for walls. (Skim coats may be required prior to painting.) A newer product is Georgia Pacific’s Dens Armor, a mold-resistant, fiberglass-backed wallboard.
  • If regular drywall is used instead of cement backer board, encapsulate the unpainted back side with two coats of Caliwel prior to installation.
  • Green and purple board should be avoided. Though widely thought to be mold-resistant, mold will grow on it. Maybe it will take 8 hours to start growing instead of 2 hours.
  • Sometimes not putting up walls at all is a good alternative. Have the kids paint murals on the walls (seen at a private school) or paint the walls pleasing colors. At one basement I inspected, you would have thought you were walking into a western saloon…and there were no installed walls. The foundation walls were artistically painted.

Take steps to reduce wicking of moisture into wall materials.

  • Leave a half inch of space between the bottom of the backer board and the slab.
  • Cover the half inch gap with resin (plastic) base molding. Vinyl-based molding may have an objectionable smell. Or, encapsulate the unfinished surfaces of wood base molding.

What paint should be used to finish a basement?

  • Paint is sort of a ticklish subject now. Please confer with manufacturers for the latest information. My understanding is that the no-VOC paints use silver or other ions as mildewcides (antimicrobials) but that the European Union is outlawing this practice out of concern that ions could become airborne.
  • The paint recommended to chemically sensitive clients by The Mold Doctor, Marilee Nelson, is EnviroSafe Paint (contact: Jim Lee, not the other EnviroSafe paint company). Check that this paint is ok to use below grade.
  • When painting, use a no- or low-VOC paint. Most manufacturers offer no- or low-VOC choices. Benjamin Moore’s Aura line reportedly has no-VOC in pigments.
  • Check first for personal sensitivities to paint. Unless you live in the desert, any below-grade paint should have a mildewcide, or mold may grow on it. I saw a least-toxic paint without mildewcide used in a basement on Long Island and within two months, it was covered in Penicillium.

Control relative humidity when you finish a basement.

  • To protect furniture from mold, control relative humidity with an EnergyStar dehumidifier. Relative humidity downstairs should be kept under 50%. Use a hygrometer (relative humidity meter) to monitor relative humidity.
  • I’ve had several individuals provide positive feedback about their experiences with the Wave ventilation system, which costs a lot less to run than a dehumidifier. One woman said it took the musty air out of the basement within a day or two. There is controversy about this product, so do your on-line review before contracting for one. Please give me feedback if you go ahead with The Wave.
  • Unless a dehumidifier is necessary to protect furniture in a finished basement, a dehumidifier may not be recommended. It draws moisture through foundation walls, helping to leach out lime. Over time, the foundation may experience degradation, particularly if it is a block foundation.

Be careful of storage in a basement.

  • Don’t feed lunch to mold. Vulnerable items are better stored in an attic.
  • In a below-grade space, store items in plastic containers to protect them from mold. Books belong upstairs, unless they are in a dehumidified space.

Improve your indoor air and water quality and reduce exposure to mold and electromagnetic fields