June 24, 2026 · 7 min read · TQF team

Subfloor prep and moisture barriers: The details that ruin Chicago floors

The concrete moisture trap

In Chicago, concrete slabs—especially in basements or new builds in Bridgeview and Schiller Park—are essentially sponges. They pull moisture from the soil via capillary action. If you lay hardwood directly over a slab without testing, you are gambling with the warranty.

We require two types of tests for concrete: Calcium Chloride (CaCl) or Relative Humidity (RH) probes. A standard RH probe should not exceed 85% for most engineered products. If the slab is "green" or excessively damp, a moisture mitigation system is mandatory. Skipping this leads to cupping (edges curling up) within the first two winters.

Flatness tolerances: The 3/16" rule

A common failure point is "bounce" or hollow spots. Most manufacturers, including Kährs and Mirage, specify a flatness tolerance of 3/16" over a 10-foot radius.

If the subfloor exceeds this, you have two choices: grind the high spots or use a self-leveling underlayment (SLU). Installing a 7" wide plank over a dip in the concrete creates a pivot point. Over time, the adhesive bond breaks, and the floor develops a rhythmic "click" or a visible dip. We don't recommend "filling" small gaps with glue; the subfloor must be mechanically flat.

Vapor barriers and mil specs

For float installations or nail-down over sleepers, the vapor barrier is the only thing stopping hydrostatic pressure from hitting the wood.

Polyethylene film: Use a minimum of 6 mil thickness. Anything thinner (like 2 or 4 mil) is prone to punctures during the install, creating "leak points" that cause localized crowning.

Liquid barriers: For glue-down installs on concrete, a liquid moisture barrier is applied to the slab to seal the pores before the adhesive goes down.

Without a verified barrier, the wood will absorb moisture from the slab, expand, and eventually buckle against the walls.

Acclimation and Wood MC

Chicago's humidity swings are violent. Wood is hygroscopic; it wants to match the equilibrium moisture content (EMC) of the room.

Before installation, we recommend using a pin-type moisture meter to check the Wood Moisture Content (MC). For most domestic hardwoods, you want to see a delta of no more than 2-4% between the flooring and the subfloor. If you bring planks from a cold warehouse into a heated home and install them immediately, they will shrink as they dry, leaving 1/16" to 1/8" gaps between boards by February.

Matching the method to the subfloor

The choice between glue, nail, or float depends entirely on the substrate:

Concrete Slab: Glue-down with a high-quality adhesive (like Bona or Loba) is the gold standard for stability, provided the RH is under 85%.

Wood Joists: Nail-down is standard. Ensure the subfloor is 3/4" tongue-and-groove plywood or OSB.

Basements: Engineered hardwood with a reinforced core (like Bjelin or DuChateau) is preferred over solid wood due to the higher moisture risk.

Get the right samples

Specing the right product for your specific subfloor requires seeing the wear layer and core construction in person.

We provide professional samples for all our brands, including RIVA, Lauzon, and Coswick. Samples are pickup only at our Schiller Park or Bridgeview showrooms. There is a $50 refundable deposit per sample; samples are not free and are not shipped. This ensures the samples return to our inventory for the next contractor.

If you have photos of your subfloor or recent moisture test results, bring them into the showroom. We can help you spec the correct barrier and adhesive to ensure the install lasts 30 years, not three.