# Vapor barrier | Floor Prep Guide

> What a vapor barrier under a floor does, how NWFA classes it, what a complete record shows, the red flags, and what manufacturers' documents ask for.

Source: https://floorprepguide.com/vapor-barrier

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# Vapor barrier

Other names for this test: vapor retarder, moisture barrier, poly film, vapor barrier underlayment.

[What the test measures](#measures) · [What a valid report looks like](#report) · [Red flags](#red-flags) · [Questions to ask the installer](#questions)

## What the test measures

A vapor barrier, or vapor retarder, is a layer that slows moisture moving up from a slab or a crawl space into the floor. It is not a reading but a material, chosen by how much vapor it lets through. NWFA’s installation guidelines class it by permeance:

- Class I
  “Class I vapor retarder ≤0.1 perm.”[1](#ref-1)
- Class II
  “Class II vapor retarder >0.1 perm and ≤1.0 perm.”[1](#ref-1)
- Class III
  “Class III vapor retarder >1.0 perm.”[1](#ref-1)

Over concrete, whether one is needed often turns on a moisture test; see [Concrete moisture test: the RH probe (ASTM F2170)](https://floorprepguide.com/astm-f2170-rh-test) and [Calcium chloride test (ASTM F1869)](https://floorprepguide.com/calcium-chloride-test). What the floor’s own document asks for is in the table under [Red flags](#red-flags).

## What a valid report looks like

A complete record names, at minimum:

- the moisture test result that the choice rests on, where the floor goes over concrete;

- the material used, by product name, with its class, permeance or thickness as its maker states it;

- where it went: over the slab, under a wood subfloor, or over the crawl space ground;

- how the seams were overlapped and sealed;

- the product document it was chosen against.

NWFA’s installation guidelines also say when each class is called for. Where a record says what was used, these are the points to hold it against, quoted from NWFA:

- Concrete, by test result
  “Install a Class I impermeable vapor retarder when calcium chloride readings are greater than 3 pounds, relative humidity readings are greater than 80%, or calcium carbide readings are greater than 2.5%.”[1](#ref-1)
- Concrete on and below grade
  “In on- and below-grade applications, due to the ever-changing moisture variability with a concrete slab, and the likelihood of sub-slab moisture barrier degradation over time, a Class I impermeable vapor retarder is always recommended.”[1](#ref-1)
- Sheet materials
  “Class I underlayment sheet-good vapor retarders may include 6-mil polyethylene film or other premium polymer sheet goods, closed-cell foam pads, or peel-and-stick membranes.”[1](#ref-1)
- Liquid-applied materials
  “Class I liquid-applied vapor retarders may include rolled- troweled- or sprayed-on sealers, epoxies, or urethanes.”[1](#ref-1)
- Wood subfloor over an unconditioned space
  “A Class II vapor retarder (sheet-good or liquid-applied) may be used on wood subfloors over unconditioned spaces to slow the rate at which potential moisture-laden air moves through the assembly and into the wood flooring, unless otherwise directed by the flooring manufacturer.”[1](#ref-1)

This is NWFA’s guidance, not the floor document’s; where your product’s document names a barrier, that document applies.

## Red flags

Vapor barrier: published limits by floor type, from manufacturers’ documents.

| Floor type | Published limits | Documents | Conditions |
| --- | --- | --- | --- |
| Rigid core vinyl | vapor retarder required directly beneath the slab crawl space ground covered with a vapor barrier moisture mitigation (6-mil poly film or MSI underlayment) above 90% RH moisture mitigation (6-mil poly film or MSI underlayment) above 80% RH 6 mil poly film moisture barrier over concrete required 6 mil vapor barrier over all exposed crawl-space soil slab built so ground water vapor cannot penetrate a moisture barrier/mitigator like 6 mil poly considered 6 mil black polyethylene over 100% of the crawl space earth 6 mil Poly Film Underlayment required | 6 documents [2](#ref-2) [3](#ref-3) [4](#ref-4) [5](#ref-5) [6](#ref-6) [7](#ref-7) | on- and below-grade concrete [4](#ref-4) wood subfloor over a crawl space [4](#ref-4) [5](#ref-5) slab above 90% RH [6](#ref-6) slab above 80% RH [7](#ref-7) concrete subfloors [2](#ref-2) on- and below-grade slabs [4](#ref-4) subfloor not cured [3](#ref-3) crawl spaces [3](#ref-3) floating method over concrete substrates [3](#ref-3) |
| Glue-down LVT | moisture-vapor barrier coating where moisture or alkalinity exceed the limits working moisture barrier/retarder under the slab required slab built so ground water vapor cannot penetrate crawl space ground covered with a vapor barrier 6 mil black polyethylene over all crawl-space earth vapor barrier covering the crawl space ground effective vapor retarder directly under the slab | 4 documents [8](#ref-8) [9](#ref-9) [10](#ref-10) [11](#ref-11) | with K-99HM adhesive [8](#ref-8) on- and below-grade concrete [9](#ref-9) wood subfloor over a crawl space [9](#ref-9) [10](#ref-10) crawl spaces [11](#ref-11) on or below-grade slabs [11](#ref-11) |
| Laminate | 6-mil virgin polyethylene, edges overlapped 8 in. "6mm" virgin polyester, as printed vapor barrier over crawl-space concrete not required on a wood subfloor vapor barrier over concrete, seams overlapped 8 in. or more crawl space ground covered with a vapor barrier vapor retarder below the wood subfloor system vapor retarder directly under the slab vapor barrier always used over a concrete slab poly vapor barrier required over concrete (printed as 6mm) 6 mil vapor barrier over sheet vinyl on concrete 6 mil virgin polyethylene over the crawl-space soil, seams overlapped at least 8 in. required over any subfloor containing concrete, even when covered poly sheet under the slab plastic over the dirt crawl space minimum 6-mil polyethylene (plastic) sheeting | 6 documents [12](#ref-12) [13](#ref-13) [14](#ref-14) [15](#ref-15) [16](#ref-16) [17](#ref-17) | concrete subfloor [14](#ref-14) [17](#ref-17) printed as "6mm ... virgin polyester"; not corrected [17](#ref-17) crawl space with a concrete surface [14](#ref-14) wood subfloor [14](#ref-14) concrete subfloors [13](#ref-13) [15](#ref-15) wood subfloor over a crawl space [12](#ref-12) wood subfloors [12](#ref-12) on- and below-grade concrete [12](#ref-12) concrete slab [17](#ref-17) over sheet vinyl on a concrete subfloor [17](#ref-17) crawl space with a soil surface [14](#ref-14) concrete covered by vinyl, VCT, terrazzo or ceramic tile [14](#ref-14) below-grade basement [17](#ref-17) mobile home with a dirt crawl space [17](#ref-17) concrete subfloor; products with preattached acoustical underlayment [16](#ref-16) |
| Engineered hardwood | moisture retardant system if excessive moisture is present or anticipated crawl space ground covered with a vapor barrier slab built so ground water vapor cannot penetrate 100% black polyethylene, at least 6 mil, or Class C membrane (ASTM D1745) underlayment pad with moisture barrier glue-down with a vapor abatement system adhesive system with a vapor control component choosing an adhesive system with moisture abatement properties is prudent 6 mil black polyethylene over the crawlspace floor vapor barrier over the wood subfloor recommended membrane with vapor permeability from 0.7 to 10 (ASTM E-96 A) sealer, or glue containing a sealer sealer/retarder when calcium chloride is above 3 lbs membrane required | 4 documents [18](#ref-18) [19](#ref-19) [20](#ref-20) [21](#ref-21) | glue-down installations [18](#ref-18) crawl space [21](#ref-21) [19](#ref-19) on- and below-grade concrete [21](#ref-21) floating installation [19](#ref-19) below-grade basement [19](#ref-19) glue-down over a concrete thermal mass with radiant heat [19](#ref-19) glue-down on concrete [19](#ref-19) crawlspace or pillar foundation [20](#ref-20) nailed or stapled installation [20](#ref-20) glued installation (engineered floors only) [20](#ref-20) concrete subfloors [20](#ref-20) floating installation (engineered floors only) [20](#ref-20) |
| Solid hardwood | at least 10 mil poly film between the ground and new slabs 6-20 mil black polyethylene ground cover, joints lapped 6 in. and taped moisture retardant under a plywood subfloor fastened to concrete moisture retardant under a floating plywood subfloor moisture retardant over the screeds at least 6 mil black polyurethane film over earthen crawl spaces, seams overlapped and taped vapor retarder (or further curing) above 80% RH subfloor covered wall-to-wall with a moisture inhibiter: 15 lb. asphalt saturated felt, Silicone Vapor Shield paper, Aquabar B or a liquid applied Class II vapor retarder moisture inhibiter required over wood subfloors felt paper or equivalent over the entire floor surface 6 mil black polyethylene over the crawlspace floor vapor barrier over the wood subfloor recommended membrane with vapor permeability from 0.7 to 10 (ASTM E-96 A) sealer/retarder when calcium chloride is above 3 lbs | 4 documents [22](#ref-22) [20](#ref-20) [23](#ref-23) [24](#ref-24) | below the slab, new construction [22](#ref-22) crawl space [22](#ref-22) plywood fastened to concrete [22](#ref-22) floating plywood subfloor over concrete [22](#ref-22) screeds over concrete [22](#ref-22) earthen crawl spaces [24](#ref-24) concrete under an added wood subfloor; the floor may not be installed directly over concrete [24](#ref-24) crawlspace or pillar foundation [20](#ref-20) nailed or stapled installation [20](#ref-20) concrete subfloors [20](#ref-20) |

The limit that binds you is in your product's installation document.

1. **Concrete: a moisture result above NWFA’s figures, and no Class I retarder named.** NWFA’s sentence on when Class I is called for is the first point under [What a valid report looks like](#report).

2. **Any floor type: the record names a barrier but not the product document it was chosen against.** The documents read ask for different materials, placed differently; the table lists them. The limit that binds you is in your product's installation document.

3. **Laminate: the seams were not overlapped as the product’s document asks.** One laminate guide, Pergo’s, gives the overlap with the material:

   > On concrete subfloors, use a non-recycled 6-mil. (.15mm) non-recycled (100% virgin) resin polyethylene film as a vapor barrier, overlapping edges 8", or follow the directions for installation of an equivalent vapor barrier.

   Pergo Installation Essentials Guide (Mohawk), PDF p. 2[14](#ref-14)

4. **Laminate: the barrier on the record does not match what the product’s document prints.** A record should follow the document’s own words. Swiss Krono’s installation FAQ prints its requirement this way, quoted as printed:

   > Swiss Krono specifies that the vapor barrier for its laminate floors must be 6mm thick and made of virgin polyester.

   Swiss Krono USA installation FAQ, web page[17](#ref-17)

   Where the record and the document’s words differ, ask the installer what was used and which page of the document it rests on.

## Questions to ask the installer

1. Which product’s installation document did you follow for the vapor barrier, and what does it ask for?

2. What material did you use, and what is its class, permeance or thickness?

3. Over concrete: which moisture test result did you base the choice on?

4. Where exactly did it go, and how were the seams overlapped and sealed?

5. Over a crawl space: is the ground covered, and with what?

References (24)

References

1. 1 NWFA Installation Guidelines. NWFA. Revised © 2025. [https://nwfa.org/wp-content/uploads/2026/02/NWFA-Installation-Guidelines.pdf](https://nwfa.org/wp-content/uploads/2026/02/NWFA-Installation-Guidelines.pdf). Read 2026-11-13. [↩a](#m-class-0)[↩b](#m-class-1)[↩c](#m-class-2)[↩d](#p-0)[↩e](#p-1)[↩f](#p-2)[↩g](#p-3)[↩h](#p-4)

2. 2 Armstrong/AHF Rigid Core SPC Locking Installation. Armstrong Flooring (AHF). [https://www.armstrongflooring.com/cdn/installation/afi/AF-Rigid-Core-SPC-Locking-Installation-System.pdf](https://www.armstrongflooring.com/cdn/installation/afi/AF-Rigid-Core-SPC-Locking-Installation-System.pdf). Read 2026-11-13. [↩a](#c-rigid_core-0)[↩b](#k-rigid_core-4-0)

3. 3 COREtec Pro SPC Residential Installation Guidelines (Shaw/USFloors). Shaw (USFloors COREtec). Revision: 11052024. [https://pdmsview.shawinc.com/USFloors/COREtec-Pro/Installation/Residential-Installation-Guidelines-for-SPC-Produc](https://pdmsview.shawinc.com/USFloors/COREtec-Pro/Installation/Residential-Installation-Guidelines-for-SPC-Produc). Read 2026-11-13. [↩a](#c-rigid_core-1)[↩b](#k-rigid_core-6-0)[↩c](#k-rigid_core-7-0)[↩d](#k-rigid_core-8-0)

4. 4 Mannington ADURA Max Install Guide. Mannington. rev. 05.16.2024. [https://assets.ctfassets.net/r69d9lchftro/qCw4Uqxa3cq2fI1bKVby9/76c789d4ccc805eb4069ffb98068d7b4/ADURA_Max_InstallGuide2024_01F.pdf](https://assets.ctfassets.net/r69d9lchftro/qCw4Uqxa3cq2fI1bKVby9/76c789d4ccc805eb4069ffb98068d7b4/ADURA_Max_InstallGuide2024_01F.pdf). Read 2026-11-13. [↩a](#c-rigid_core-2)[↩b](#k-rigid_core-0-0)[↩c](#k-rigid_core-1-0)[↩d](#k-rigid_core-5-0)

5. 5 Mohawk SolidTech Installation. Mohawk. Rev. October 2018. [https://mohawk.blob.core.windows.net/mohawkflooring/pdf/install/SolidTech_Installation.pdf](https://mohawk.blob.core.windows.net/mohawkflooring/pdf/install/SolidTech_Installation.pdf). Read 2026-11-13. [↩a](#c-rigid_core-3)[↩b](#k-rigid_core-1-1)

6. 6 MSI Everlife Rigid Core flyer. MSI. [https://cdn.msisurfaces.com/files/flyers/rigid-core-everlife-lvt-flyer.pdf](https://cdn.msisurfaces.com/files/flyers/rigid-core-everlife-lvt-flyer.pdf). Read 2026-11-13. [↩a](#c-rigid_core-4)[↩b](#k-rigid_core-2-0)

7. 7 MSI Hybrid Rigid Core Installation Guide. MSI. [https://cdn.msisurfaces.com/files/flyers/hybrid-rigid-core-installation-guide.pdf](https://cdn.msisurfaces.com/files/flyers/hybrid-rigid-core-installation-guide.pdf). Read 2026-11-13. [↩a](#c-rigid_core-5)[↩b](#k-rigid_core-3-0)

8. 8 Karndean K-99HM Adhesive Installation Guide. Karndean. 240902. [https://www.karndeancommercial.com/globalassets/karndean-commercial/blocks/b2b-brochure-listing-block/usa/install-guides/us-karndean-k99-hm-adhesive-installation-guide-240902.pdf](https://www.karndeancommercial.com/globalassets/karndean-commercial/blocks/b2b-brochure-listing-block/usa/install-guides/us-karndean-k99-hm-adhesive-installation-guide-240902.pdf). Read 2026-11-13. [↩a](#c-glue_down_lvt-0)[↩b](#k-glue_down_lvt-0-0)

9. 9 Mannington ADURA Flex Installation Instructions. Mannington. Copyright © 2026. [https://assets.ctfassets.net/r69d9lchftro/72wp40QH7Sjze2rfjb2mBs/eda3a968f13c3aebe68000069be092bd/ADURAFlex_Installation_Instructions_2026.pdf](https://assets.ctfassets.net/r69d9lchftro/72wp40QH7Sjze2rfjb2mBs/eda3a968f13c3aebe68000069be092bd/ADURAFlex_Installation_Instructions_2026.pdf). Read 2026-11-13. [↩a](#c-glue_down_lvt-1)[↩b](#k-glue_down_lvt-1-0)[↩c](#k-glue_down_lvt-2-0)

10. 10 MSI Everlife Acclima Glue-Down Installation. MSI. [https://cdn.msisurfaces.com/files/flyers/acclima-gluedown-installation.pdf](https://cdn.msisurfaces.com/files/flyers/acclima-gluedown-installation.pdf). Read 2026-11-13. [↩a](#c-glue_down_lvt-2)[↩b](#k-glue_down_lvt-2-1)

11. 11 Shaw Resilient Residential Installation Guidelines, glue-down. Shaw. Revision: 04012022. [https://pdmsview.shawinc.com/Shaw-Residential/Shaw-Residential-Installation/Resilient-Installation/Glue-Down/Resilient-Residential-Installation-Guidelines](https://pdmsview.shawinc.com/Shaw-Residential/Shaw-Residential-Installation/Resilient-Installation/Glue-Down/Resilient-Residential-Installation-Guidelines). Read 2026-11-13. [↩a](#c-glue_down_lvt-3)[↩b](#k-glue_down_lvt-3-0)[↩c](#k-glue_down_lvt-4-0)

12. 12 Mannington Restoration Collection Laminate Installation. Mannington. Copyright © 2026. [https://assets.ctfassets.net/r69d9lchftro/4DiXQqXdjMnyAJ4HL02eit/d9d8899188d5e7b4d8d8f0236622add0/Restoration_Collection_Laminate_Installation_Instructions_2026.pdf](https://assets.ctfassets.net/r69d9lchftro/4DiXQqXdjMnyAJ4HL02eit/d9d8899188d5e7b4d8d8f0236622add0/Restoration_Collection_Laminate_Installation_Instructions_2026.pdf). Read 2026-11-13. [↩a](#c-laminate-0)[↩b](#k-laminate-5-0)[↩c](#k-laminate-6-0)[↩d](#k-laminate-7-0)

13. 13 Mohawk RevWood Plus Installation Instructions. Mohawk. [https://mohawk.blob.core.windows.net/mohawkflooring/pdf/install/Revwood_Plus_Installation_Instructions.pdf](https://mohawk.blob.core.windows.net/mohawkflooring/pdf/install/Revwood_Plus_Installation_Instructions.pdf). Read 2026-11-13. [↩a](#c-laminate-1)[↩b](#k-laminate-4-0)

14. 14 Pergo Installation Essentials Guide (Mohawk). Pergo (Mohawk). [https://mohawk.blob.core.windows.net/pergo/pdfs/Installation_Essentials_Guide_Combined.pdf](https://mohawk.blob.core.windows.net/pergo/pdfs/Installation_Essentials_Guide_Combined.pdf). Read 2026-11-13. [↩a](#c-laminate-2)[↩b](#k-laminate-0-0)[↩c](#k-laminate-2-0)[↩d](#k-laminate-3-0)[↩e](#k-laminate-10-0)[↩f](#k-laminate-11-0)[↩g](#f-pergo)

15. 15 Quick-Step Studio installation (Unilin). Quick-Step (Unilin). Edition Date: 04/01/2020. [https://pdf.lowes.com/installationguides/604743166056_install.pdf](https://pdf.lowes.com/installationguides/604743166056_install.pdf). Read 2026-11-13. [↩a](#c-laminate-3)[↩b](#k-laminate-4-1)

16. 16 Shaw Repel 2G Laminate Installation. Shaw. Revision: 01312022. [https://pdmsview.shawinc.com/Shaw-Residential/Shaw-Residential-Installation/Laminate-(1)/Lock-N-Place/Repel/2G-2G-Laminate-Installation](<https://pdmsview.shawinc.com/Shaw-Residential/Shaw-Residential-Installation/Laminate-(1)/Lock-N-Place/Repel/2G-2G-Laminate-Installation>). Read 2026-11-13. [↩a](#c-laminate-4)[↩b](#k-laminate-14-0)

17. 17 Swiss Krono USA installation FAQ. Swiss Krono. [https://www.swisskrono.com/us-en/faq/installation/](https://www.swisskrono.com/us-en/faq/installation/). Read 2026-11-13. [↩a](#c-laminate-5)[↩b](#k-laminate-0-1)[↩c](#k-laminate-1-0)[↩d](#k-laminate-8-0)[↩e](#k-laminate-9-0)[↩f](#k-laminate-12-0)[↩g](#k-laminate-13-0)[↩h](#f-krono)

18. 18 Bruce Engineered 3/8-9/16. Bruce (AHF). LA-295727-422. [https://www.bruce.com/cdn/content/sites/2/210049.pdf](https://www.bruce.com/cdn/content/sites/2/210049.pdf). Read 2026-11-13. [↩a](#c-engineered_wood-0)[↩b](#k-engineered_wood-0-0)

19. 19 Hallmark Engineered Wood Installation. Hallmark. 05-2026. [https://hallmarkfloors.com/pdfs/Installation-Instructions-Eng-Wood-WEB.pdf](https://hallmarkfloors.com/pdfs/Installation-Instructions-Eng-Wood-WEB.pdf). Read 2026-11-13. [↩a](#c-engineered_wood-1)[↩b](#k-engineered_wood-1-1)[↩c](#k-engineered_wood-3-0)[↩d](#k-engineered_wood-4-0)[↩e](#k-engineered_wood-5-0)[↩f](#k-engineered_wood-6-0)

20. 20 Expert by Lauzon Limited Warranty and Installation Guide. Expert by Lauzon. October 2025, LII-MK-ST-874-C. [https://lauzonexpert.com/wp-content/uploads/Expert-by-Lauzon_Installation-and-warranty-guide-7.pdf](https://lauzonexpert.com/wp-content/uploads/Expert-by-Lauzon_Installation-and-warranty-guide-7.pdf). Read 2026-11-13. [↩a](#c-engineered_wood-2)[↩b](#k-engineered_wood-7-0)[↩c](#k-engineered_wood-8-0)[↩d](#k-engineered_wood-9-0)[↩e](#k-engineered_wood-10-0)[↩f](#k-engineered_wood-11-0)[↩g](#c-solid_wood-1)[↩h](#k-solid_wood-7-0)[↩i](#k-solid_wood-8-0)[↩j](#k-solid_wood-9-0)

21. 21 Mannington Hardwood Install Guide. Mannington. Copyright © 2025. [https://assets.ctfassets.net/r69d9lchftro/1z2pM4YyPCtbRibzxxpYEu/1898b6f3500c462b163ada0fa0d68865/Hardwood_InstallGuide2025_02.pdf](https://assets.ctfassets.net/r69d9lchftro/1z2pM4YyPCtbRibzxxpYEu/1898b6f3500c462b163ada0fa0d68865/Hardwood_InstallGuide2025_02.pdf). Read 2026-11-13. [↩a](#c-engineered_wood-3)[↩b](#k-engineered_wood-1-0)[↩c](#k-engineered_wood-2-0)

22. 22 Bruce Solid 3/4" Strip and Plank. Bruce (AHF). LA-295726-120, © 2020. [https://www.bruce.com/cdn/installation/212080.pdf](https://www.bruce.com/cdn/installation/212080.pdf). Read 2026-11-13. [↩a](#c-solid_wood-0)[↩b](#k-solid_wood-0-0)[↩c](#k-solid_wood-1-0)[↩d](#k-solid_wood-2-0)[↩e](#k-solid_wood-3-0)[↩f](#k-solid_wood-4-0)

23. 23 Mirage Installation Guide, Solid Prefinished Hardwood Floors (Mirage Classic). Mirage. [https://media.planchersmirage.com/webfolder_download/ea857cc59594ba5743ea8f81d0bb81d1/file/2d7a6951401a1af90be5f366c1f36ca448debdab/file.pdf](https://media.planchersmirage.com/webfolder_download/ea857cc59594ba5743ea8f81d0bb81d1/file/2d7a6951401a1af90be5f366c1f36ca448debdab/file.pdf). Read 2026-11-13. [↩](#c-solid_wood-2)

24. 24 Mullican Solid Wood Flooring Installation. Mullican Flooring. [https://www.mullicanflooring.com/jm-resources/tech-docs/installation/SolidInstallationInstructions.pdf](https://www.mullicanflooring.com/jm-resources/tech-docs/installation/SolidInstallationInstructions.pdf). Read 2026-11-13. [↩a](#c-solid_wood-3)[↩b](#k-solid_wood-5-0)[↩c](#k-solid_wood-6-0)

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Documents last read 2026-11-13; the sources page lists each one with its read date.

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