Underlayment, Moisture Control and Adhesives
Underlayment, moisture control and adhesives are where floors fail invisibly. This domain tests moisture test methods and limits, membrane and underlayment selection, adhesive families and their open and flash times.
Moisture testing
- Concrete: ASTM F1869 calcium chloride test, reported in lb/1000 ft²/24 h, and ASTM F2170 in-situ relative humidity probes. Run at least three tests in the first 1000 ft² and one more per additional 1000 ft².
- Typical adhesive limits are about 3 lb/1000 ft²/24 h or 75-80% RH, but the adhesive manufacturer's number governs.
- pH: high alkalinity attacks some adhesives; measure, then prime or install a barrier.
- Wood: flooring and subfloor moisture content should be close, typically within 2-4%.
Moisture control systems
- Slab vapour retarder beneath the concrete, plus surface-applied epoxy or urethane moisture barriers where readings exceed limits.
- Cement backer board embedded in thinset, and waterproof membranes in wet areas.
Underlayments and cushion
- Plywood, fibre-reinforced sheet, self-levelling compound, foam, cork, rubber.
- Carpet cushion: rebond or foam of correct density and thickness for stretch-in; glue-down carpet uses cushion approved for that adhesive or none.
Adhesives
- Pressure-sensitive adhesive: apply, let solvent flash until clear and tacky, then lay. Trapped solvent causes bubbles and debonding.
- Families: hard-set, modified thinset (ANSI A118.4), unmodified (A118.1), epoxy (A118.3).
- Respect open time and working time at 18-29°C, then roll resilient with a 100 lb roller within the working time.
- Rubber and vinyl need their specified adhesive; never assume one adhesive covers all resilient.
Worked example
A calcium chloride test returns 4.5 lb/1000 ft²/24 h against an adhesive limit of 3 lb. The floor cannot simply be installed with a stronger adhesive. The correct response is to apply a manufacturer-approved epoxy moisture barrier at the specified thickness, allow it to cure, and either re-test or use the barrier system's written moisture allowance before installing.
Traps
- Testing a slab before 28 days of cure and treating the falsely low reading as final.
- One test in a corner for a whole floor.
- Laying into wet adhesive with no flash time.
- Using unmodified thinset over a membrane that requires modified.