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
What Does Moisture Barrier & Underlayment Include?
Moisture barrier and underlayment install is the final prep scope before the finish floor goes on — vapor retarder, foam, cork, or decoupling membrane matched to the specific finish-floor manufacturer install spec, plus seam taping, edge wrapping at the perimeter, and the perimeter expansion gap that floating floors require. Handis covers install from $500 on a small bath or laundry to $1,800 on a full main-floor cork and decoupling membrane scope. The underlayment is the warranty-critical interface between the substrate and the finish floor — wrong underlayment voids the floor warranty on day one, so we confirm the spec in writing against the manufacturer's published install guide before any product is ordered.
6 Mil Polyethylene Vapor Retarder over Slabs
6 mil polyethylene rolled across the full concrete slab, seams overlapped 6 inches and taped with vapor-barrier seam tape (Tyvek tape or matched manufacturer tape), perimeter wrapped 2 inches up the wall and trimmed flush after the finish floor installs. Used under engineered hardwood and laminate floating floors on slab-on-grade or basement slabs. Slab moisture-tested first with a calcium chloride test or relative humidity probe (per ASTM F2170) so the vapor barrier matches the actual moisture vapor emission rate of the slab.
Asphalt-Saturated Rosin Paper or 15-Pound Felt under Nail-Down Hardwood
Asphalt-saturated kraft (rosin) paper or 15-pound asphalt felt rolled under nail-down hardwood on plywood subfloor — the moisture-buffering layer required by every premium nail-down hardwood manufacturer (Mirage, Lauzon, Mercier, DuChateau, Bona). Seams overlapped 4 inches, stapled to the plywood through the paper layer, perimeter trimmed flush to the wall. Rosin paper is the lighter-weight standard for most modern nail-down installs; 15-pound felt adds heavier moisture buffering where the home has known crawlspace humidity.
Foam Underlayment with Integrated Vapor Barrier under Floating Laminate and Engineered
3 mm or 5 mm foam underlayment with integrated vapor barrier (Bestlaminate Pro Vapor 3 in 1, Roberts Super 60, QEP Sound Reducing) rolled under click-lock laminate and floating engineered hardwood. The foam provides cushion and sound dampening; the integrated vapor barrier sheet handles slab moisture without a second polyethylene layer. Seams aligned and butted (never overlapped, which creates a hump the finish floor reads through), perimeter wrapped 2 inches up the wall for expansion. We never use plain foam (no integrated vapor barrier) over a slab; the floor will fail.
Cork Underlayment under Hardwood on Upper Floors for Sound
3 mm or 6 mm cork underlayment rolled under nail-down or floating hardwood on upper floors where sound transmission to the floor below matters (second floors in condos, townhomes, and multi-story homes with rooms below). Cork provides the highest sound dampening of the residential underlayment options (typical IIC rating of 50 to 60 with the right matched assembly) and is dimensionally stable across temperature and humidity cycles. Adhesive-set with cork-compatible adhesive (Roberts 1407, Wakol PU 280) where the manufacturer spec requires bonded install; loose-laid where the manufacturer permits.
Decoupling Membrane under Tile (Schluter DITRA, Wedi Subliner)
Schluter DITRA or Wedi Subliner decoupling membrane troweled to the substrate (cement board or sound plywood) before the tile install, with the polyethylene fleece face up to receive the thinset that bonds the tile. The decoupling layer breaks shear stress between the substrate and the tile so movement at the substrate does not crack the tile or open the grout lines. Used under porcelain and ceramic tile on any substrate where movement is expected (most residential floors, almost universally above-grade). Also functions as a waterproofing layer in bathroom floor installs where the manufacturer spec is followed in detail.
Slab Moisture Testing on Every Concrete Install
Concrete slab moisture is the most common cause of engineered hardwood cupping and laminate joint failure in Seattle basements and slab-on-grade additions. We run a calcium chloride test (CaCl) for a 60 to 72 hour read on the slab moisture vapor emission rate, or a relative humidity probe per ASTM F2170 for a fast read where the slab is freshly poured. The reading goes on the quote alongside the matched vapor barrier or a moisture mitigation primer (Mapei Planiseal VS, Ardex MC RAPID) if the slab reads above the finish-floor manufacturer spec.
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.
Matched to the finish-floor manufacturer install spec, confirmed in writing
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, Schluter DITRA or Wedi Subliner under tile. We confirm the match in writing on the quote against the manufacturer's published install guide before any product is ordered. Wrong underlayment voids the warranty on day one and we never let that happen.
Slab moisture tested before any vapor barrier goes down
Concrete slabs in Seattle basements and slab-on-grade additions get tested before any underlayment rolls out. Calcium chloride test for a 60 to 72 hour read on the moisture vapor emission rate, or ASTM F2170 RH probe for a fast read where the slab is freshly poured. The reading goes on the quote with the matched vapor barrier or moisture mitigation primer if the slab reads above the spec. Skipping the test is the most common cause of engineered hardwood cupping in Seattle.
Seams taped, perimeter wrapped 2 inches up the wall
Every seam on the moisture barrier and foam underlayment gets taped with vapor-barrier seam tape (Tyvek tape on polyethylene, matched manufacturer tape on foam). Perimeter wrapped 2 inches up the wall so the vapor barrier is continuous to the wall, trimmed flush only after the finish floor and the baseboard install. An untaped seam or an unwrapped perimeter is a moisture path that defeats the entire underlayment.
Foam never overlapped, never gapped — butted to the next strip
Foam underlayment is butted to the next strip at the seam, never overlapped (overlap creates a hump the finish floor reads through) and never gapped (a gap creates a soft spot the click-lock joint walks across). The roll alignment is confirmed at every seam before the next strip rolls out. The detail nobody sees is the detail that decides whether the floor lasts.
Decoupling membrane troweled to spec and cured before tile
For tile installs, Schluter DITRA or Wedi Subliner is troweled to the substrate with the matched thinset, fleece face up, rolled tight, seams butted. The membrane cures per the manufacturer spec (typically 24 hours) before the tile install rolls in. Skipping the cure window or skipping the matched thinset voids the decoupling and the tile warranty on day one.
Insured, background-checked, one-year project warranty
Handis carries general liability and workers' compensation; every technician has cleared a background screening. One-year project warranty on the underlayment install workmanship — vapor barrier seam, foam alignment, cork install, decoupling membrane bond, perimeter wrap. A finish-floor failure traced to our underlayment install (a missed seam tape, a foam overlap that humped through the boards, a decoupling membrane that lifted because the thinset coverage was short) gets the underlayment redone at no cost. The finish-floor itself carries the manufacturer warranty; we name both warranty paths on the quote.
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.