Moisture Barrier & Underlayment
Handis moisture barrier and underlayment install is the final prep layer between the substrate and the finish floor — vapor retarder, foam, cork, and decoupling membrane matched to the specific finish-floor manufacturer install spec so the warranty stays valid — from $500 for a small bath or laundry underlayment install to $1,800 for a full main-floor vapor barrier and cork underlayment scope. The 1971 Ravenna basement where the engineered hardwood went down over a slab in 2015 without any vapor barrier and the boards cup at the perimeter every winter. The 1924 Queen Anne second-floor renovation where the downstairs neighbor reports every footfall because the original install put the new hardwood on rosin paper without any sound underlayment. The 2005 Renton kitchen where the floating laminate failed at every seam because the foam underlayment was missing the integrated vapor barrier on top of the slab. The 1956 Ballard ranch where the new bathroom tile cracked along every grout line three years in because no decoupling membrane went between the cement-board substrate and the porcelain. The right underlayment is the cheapest insurance on any flooring project and the wrong underlayment voids the manufacturer warranty on day one. We name the underlayment on every quote, confirm it against the finish-floor install spec in writing, and tape every seam at the edge so the moisture barrier actually works.
Service
How a Moisture Barrier & Underlayment Install Works
Six sequential steps from finish-floor spec read to final perimeter seal — the actual sequence on every Handis underlayment install.
Read the Finish-Floor Manufacturer Install Spec
The finish floor's published install spec dictates the underlayment — 6 mil polyethylene over slabs for engineered floating, rosin paper or felt under nail-down hardwood, integrated-barrier foam under floating laminate, cork for sound on upper floors, decoupling membrane under tile. The matched underlayment goes on the quote against the manufacturer's published install guide. Wrong underlayment voids the warranty on day one, so the spec match is confirmed in writing before product is ordered.
Test Slab Moisture if the Install Is on Concrete
Concrete slabs get tested before any underlayment rolls out. Calcium chloride test for a 60 to 72 hour read on slab moisture vapor emission rate, or ASTM F2170 relative humidity probe for a fast read on a freshly poured slab. The reading goes on the quote with the matched vapor barrier or moisture mitigation primer (Mapei Planiseal VS, Ardex MC RAPID) if the slab reads above the finish-floor spec.
Substrate Prep — HEPA Vacuum, Damp Mop, Confirm Dry
Substrate (concrete slab or plywood) HEPA-vacuumed to remove every grain of dust and debris that would create a void under the underlayment. Concrete slab damp-mopped if needed and confirmed bone-dry before product rolls out. Plywood checked for fastener heads sitting proud (set flush) and any joint that needs adhesive-sealing before the rosin paper or foam goes on.
Roll Out, Align, Cut to the Room
Underlayment rolled out across the room, cut to the wall with a utility knife, aligned so the seams stagger across the joist run (for plywood substrates) or run perpendicular to the finish floor direction (for most floating floors). Each roll length cut to butt the next roll, never overlap (overlap on foam underlayment creates a hump the finish floor reads through).
Tape Every Seam, Wrap the Perimeter Up the Wall
Every seam taped with vapor-barrier seam tape (Tyvek tape on polyethylene, matched manufacturer tape on foam, contractor sheathing tape on rosin paper). Perimeter wrapped 2 inches up the wall on the moisture-barrier and foam layers so the vapor barrier is continuous to the wall. The wrap is trimmed flush after the finish floor and the baseboard install, never before.
Decoupling Membrane Troweled and Cured for Tile
For tile installs, the decoupling membrane (Schluter DITRA, Wedi Subliner) is troweled to the substrate with the matched thinset, fleece face up. Membrane rolled tight to the substrate with a roller, seams butted (never overlapped, never gapped), perimeter cut to the wall. Membrane cured per the manufacturer spec (typically 24 hours) before the tile install rolls in.
Moisture Barrier & Underlayment Pricing
Final pricing is labor plus underlayment product (typical 6 mil polyethylene runs $0.05 to $0.10 per square foot, foam underlayment with integrated vapor barrier runs $0.40 to $0.80 per square foot, cork runs $1.50 to $3.00 per square foot, Schluter DITRA runs $1.60 to $2.20 per square foot; product passes through transparently on the quote). Slab moisture testing (calcium chloride test or RH probe) is $150 to $300 per test as a separate line item where the install is on concrete. Mitigation primers (Mapei Planiseal VS, Ardex MC RAPID) are a separate scope quoted when the slab moisture reading is above the finish-floor spec. Request a free estimate for an accurate quote.
Send a phone photo of the substrate and tell us the finish floor (brand, model, install method) — we will match the underlayment to the manufacturer spec and quote in writing.
Why Handis for Moisture Barrier & Underlayment
Most finish-floor warranty claims in Seattle homes trace to the underlayment layer the homeowner never thought twice about. The engineered hardwood that cupped in the basement because the installer skipped the slab moisture test. The laminate that walked apart at every seam because the foam underlayment was missing the integrated vapor barrier. The hardwood on the upper floor where the downstairs neighbor reports every footfall because no cork went under the install. The tile that cracked along every grout line by year three because no decoupling membrane was put between the substrate and the porcelain. The right underlayment is the cheapest insurance on a flooring project and is required by every finish-floor manufacturer warranty. We match the underlayment to the published install spec, document the match in writing, and tape every seam at the edge so the moisture barrier actually works.
Estimate
Tell us the room, the rough square footage, the substrate (concrete slab, plywood subfloor, existing sound floor), and the finish floor going down next (brand, model, install method — nail-down, glue-down, floating click-lock, bonded tile). A phone photo of the substrate and a screenshot or photo of the finish-floor install spec page from the manufacturer's website helps us match the underlayment without a second round of questions. We send a written estimate with the underlayment named, the slab moisture test included if applicable, and the manufacturer install-spec match confirmed in writing.
Customer Reviews
Moisture barrier and underlayment reviews from real Handis customers.
1971 Ravenna basement engineered hardwood install. The previous owner had laid hardwood without any vapor barrier and the boards cupped every winter. Handis demoed the old floor, ran a calcium chloride test on the slab, laid 6 mil polyethylene with taped seams, then installed foam underlayment with integrated vapor barrier on top. Two winters in and the new floor is flat across the whole basement.
1924 Queen Anne second-floor renovation. We were laying nail-down white oak and the downstairs neighbor was concerned about footfall noise. Handis recommended 6 mm cork underlayment over rosin paper and the assembly tested at IIC 58 from the install. Downstairs neighbor cannot hear us walking. Floor looks and sounds exactly like we wanted.
2005 Renton kitchen replacement after the original laminate failed at every seam. Handis showed me that the original install was over plain foam with no integrated vapor barrier on top of the slab — exactly the failure mode the manufacturer spec warned about. We did the new install with proper integrated-barrier foam and the slab moisture-tested first. Three years and the new laminate is dead tight at every seam.
1956 Ballard ranch master bath tile install. Handis put Schluter DITRA between the cement-board substrate and the porcelain tile because the floor had visible movement under load. The decoupling membrane handled all the shear stress and the grout lines have stayed tight three years in. No cracks at the perimeter, no opening at the bath-to-shower transition.
1948 Wallingford craftsman dining-room nail-down hardwood install. Handis recommended 15-pound felt over the original plywood subfloor because of the known crawlspace humidity (we did not have a sealed vapor barrier under the house). The felt buffered the moisture for the first two years of acclimation and the floor has been dead flat through every season since.
Frequently Asked Questions
Common questions about moisture barrier and underlayment install — pricing, the matched-to-warranty spec, slab moisture testing, sound underlayment, and decoupling membrane under tile.