Rob Rosen is diving into a basement remodel, a job that involves digging out and removing a concrete slab to provide more headroom so the basement can be turned into usable living space.
He’ll reinforce the footing and foundation as needed, but when it comes time to build a new floor for the basement, Rosen wonders whether he can go with something other than a concrete slab.
“I would love input from someone who has put a vapor/water barrier, some foam insulation between 2×4 sleepers and then used some form of wood flooring that would work for higher humidity situations,” Rosen writes in a Q&A post at Green Building Advisor.
There’s nothing that Rosen can find in the Washington State building code that would prevent him from trying it.
“Concrete is soooo not environmentally correct if not necessary,” he writes.
Can Rosen swap wood for concrete and still get a durable, structurally sound basement floor? That’s the issue in this Q&A Spotlight.
Stopping the migration of moisture from the ground is key
Keeping ground moisture out of the basement is important, but it is the rigid foam insulation and the vapor barrier that actually do the work, not the concrete, says GBA senior editor Martin Holladay. In general, Holladay writes, the rigid foam goes down first, followed by the vapor barrier. “Above the polyethylene, the finish materials are up to you,” he says, “and and your local code inspector, of course.” [Editor’s note: As pointed out by Mark F. in Comment #5 below, a basement slab often has a structural function: It prevents the foundation walls from being forced inward by soil pressure. GBA readers should consult an engineer before omitting a basement slab.]
In terms of durability and cost, Holladay favors concrete, but he adds: “If you are willing to replace any components that rot, or if you don’t mind introducing pressure-treated lumber (with associated chemicals) into the interior of your house, go ahead and experiment. If these materials rot in the future, you’ll need to replace them. You’ll have to consider the wooden components as sacrificial layers.”
Agreed, writes Howard Gentler. Although a concrete slab would do a good job of supporting any lumber above it, it won’t block any moisture by itself and Rosen might well save both money and effort by using wood instead. “I think you can do what you are considering,” he says.
Rosen replies that he was considering adding “ribbons” of concrete on 8-ft. centers to support 2x floor framing. “The 2xs would be supported by the earth and the concrete in case things shifted,” he says. “I don’t believe that I can use pressure-treated wood on interior living space[s], and don’t really know that code guys would be happy with this setup… only because it’s atypical.”
Use the right lumber
If Rosen uses pressure-treated material as part of the assembly, Flitch Plate says, it should be stamped for ground contact, and it should be allowed to dry before it’s installed.
Plate refers Rosen to specifications for treated lumber published by Wolmanized. Although lumber treated with chromated copper arsenate (CCA) has been banned for most residential applications, it’s still permitted for permanent wood foundations. The specs indicate that it should have a chemical retention level of 0.60 pounds per cubic foot (pcf). In contrast, above-grade pressure-treated lumber would need a retention level of only 0.25 pcf.
“That is a lot of copper, by weight, making it heavy and dense — a real bear on saw blades,” Plate writes. “Use only stainless-steel fasteners; preferably hex heads and star drives. Fasteners are expensive. Galvanized is not reliable, and who wants to use a hammer anyway, these days.”
For the vapor barrier, Plate suggests a cross-linked polyethylene material called Tu-Tuff.
Pressure-treated material is expensive, Gentler adds, and in this case it may not be necessary at all.
“I don’t think you are planning for the wood to be in actual ground contact, just near it, but above layers that will be excluding most moisture,” Gentler says. “It sounds like you will be using concrete piers for support of the wood. Make sure something non-permeable is between the wood and concrete, since the concrete wicks moisture to the wood.”
Trouble ahead with resale?
One thing to consider, advises Robert Hronek, is whether a concrete-less basement would affect the resale value of the house. “You will be making a big investment, but I would be concerned about the resale value and the marketability of the home,” Hronke says. “You might call an appraiser, a real estate agent and, if you have a good relationship, a lender.”
If a lender balked at offering a loan, Rosen wouldn’t be able to sell the house. “Personally, I would walk out of the house and not make an offer,” he says. “Ask the [real estate] agent how hard it would be to sell. Ask the appraiser how hard it would be to appraise.”
Plate agrees that resale and mortgage issues are a “different and valid concern.” But while pressure-treated material has earned a “bad rap,” basements are fully code compliant if they are built to engineering and materials standards.
Exactly, adds Donald Endsley. “The use of concrete is mainly because it can handle bulk water, and water vapor issues, IMHO,” Endsley says. “As long as those are 100% taken care of I see no reason wood won’t work.”
Endsley’s grandfather, in fact, built a house in Florida with a wood foundation. “Parts of that foundation were still there when he sold the place,” he says. “It lasted 90 some years, but only because that soil was extremely well drained. The parts that had to be replaced were due to adding plumbing to the house and having it subsequently leak.”
If galvanized steel is OK, why not pressure-treated lumber?
Jack Woolfe offers a link to Polycore Canada, which makes a basement flooring system that can be used in place of concrete. Factory-made sections of galvanized steel and expanded polystyrene insulation have an R-value of 18, according to the company’s web site. With no large equipment and no specialized tools, a crew of three could install a 1,500-square-foot floor in a single day.
“Polycore Canada makes a floor system similar to what you’ve described, except they use galvanized sheet-steel sleepers instead of pressure-treated wood,” says Woolfe. “I suspect PT wood would work okay too.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
I think the default basement floor is concrete because it’s a self-leveling material with really high compressive strength and it does not care about being wet. I am not saying it has great moisture-managing properties, but as an inorganic material, it can wet and dry repeatedly without compromise to its other properties (contraction/expansion, compressive strength, etc.).
Any floor assembly can be set up to handle soil moisture and soil gases (such as radon). Basement floors can get wet from soil moisture or an internal leak, so I would not install any basement floor system that is inherently more moisture-sensitive without having a lot of confidence in historic high water table information, foundation perimeter drainage system, and leak protection for hard-piped appliances: clothes washer, dishwasher, and ice-makers.
Having done more than one basement retrofit (including my own home) involving casting a concrete slab, neither the expense nor the difficulty were issues I considered, mainly because other framed systems seemed just as much if not more cost and work.
On the other hand, assuming equivalent strategies for managing moisture and radon, the Polycore Canada system looks pretty slick. There is no reason that any framed system won’t work, so long as it sees the same or similar conditions as other below-grade framed systems: protection from liquid water (bulk and capillary) and vapor permeability to allow drying to the interior.