Plastic moisture barrier installed in a crawl space following mold remediation © Daniel Friedman at Crawl Space Moisture Barriers
Where & how to put a water or moisture barrier in crawl spaces

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Stop crawl space water entry & control crawl space moisture using moisture barriers:

Plastic or other materials placed on crawl space floor and sometimes foundation walls can make a big difference in crawl space moisture levels. But for a crawl area moisture barrier to work successfully, additional steps both inside and outside the building are also recommended.

This article series describes the steps needed to get into, inspect, clean, and then dry out a building crawl space.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to Keep Out Crawl Space Water & Moisture - Interior Measures for Crawl Space Moisture Control

Puddles on plastic in a crawl space © Daniel Friedman at InspectApedia.comHere we focus on measures to take inside the crawl space to help keep that area dry.

Crawl Space Surface Slope & Drainage Specifications

Be sure that the dirt floor moisture barrier slopes to a drain.

Before putting down plastic to keep moisture down in a crawl space, be sure that the crawl space floor surface is smooth - we don't want puddles of water below nor above a poly vapor barrier that we may be planning to put down.

[Click to enlarge any image]

In addition it's important that the smooth crawl space floor surface also slope gently towards one or more drain points where, if necessary, you'll be able to install sump pumps to remove any water that might enter in the future. Slope on the crawl space floor can be as little as 1/8" per foot.

A great many installations we see, even in company product literature, show multiple low spots in a crawl space, each of which is subject to ponding and water retention from leaks or other unanticipated water entry into a crawl space or basement.


Crawl Space Floor & Wall Moisture Barriers: When, Where, How, & Why to Install Moisture Barriers, Heat & Dehumidification Equipment in the Crawl Space

Plastic moisture barrier installed in a crawl space following mold remediation © Daniel Friedman at

Provide a moisture barrier over any dirt or gravel crawl space floors and extending up foundation walls but not up to nor in contact with wood framing or sills.

Moisture barriers may be 6-mil poly or other special basement sealer products such as special moisture-sealing concrete coatings.

The crawl space moisture barrier system shown here uses mechanical fasteners and adhesive to secure the plastic barrier to the foundation walls near the tops (but short of wood framing to reduce insect attack risk).

We install moisture barriers to stop the "pumping action" that draws moisture into a building through the foundation walls or through the crawl space dirt or concrete floor.

Crawl space moisture pumping occurs as moisture evaporates off of the indoor surface into building air. This process invites movement of additional water molecules through the outside surfaces and into the building.

Should the crawl space plastic go under or on top of the gravel in a crawl space?

Crawl space gravel © D Friedman at

Some installers like to put down the plastic then cover it with gravel, arguing that the gravel will "protect" the plastic from damage. That approach is shown at left.

In our opinion, placing gravel on top of the crawl space plastic is a bad idea.

  1. First, gravel on top of the plastic won't work well. The gravel, especially sharp gravel, is more likely to puncture the plastic ground cover.
  2. Second, if gravel is on top of the plastic we can't see that the plastic was placed over an adequately smooth sloped crawl space floor - the risk is future puddles of water that collect on top of the plastic but hidden from view under the gravel - an ongoing crawl space water and moisture trap.
  3. Third, we can't even see the plastic to know that it was properly placed and is continuous.

As you may have noticed in our first photo at the start of this section (Keep Out Crawl Space Water & Moisture), if you can see the plastic you will see any puddles that are forming on top of it.

Use heavy 6-mil poly or stronger plastic on top of whatever else is on the crawl space floor, and walk or crawl with care. Also see CRAWL SPACE GROUND COVER.

Mold remediation completed in a problem crawl space © Daniel Friedman at

Choose your crawl space plastic vapor barrier material carefully. Some plastics can give off an obnoxious odor themselves - an effect which may become suddenly noticeable and more significant than you anticipated once you've spread a smelly plastic out over a large crawl space area below a building.

It may take some time for such odors to dissipate. (We don't have data on possible health effects of the volatiles and gases comprising the odor from smelly plastic vapor barriers. Contact us if you do.)

The white reinforced plastic material shown in the photos here was not producing any noticeable smells at the time of our inspection.

Our photo shows a successful cleanup of a very moldy crawl area that had a gravel and dirt floor. The framing was cleaned, dried, then sealed with a Fosters™ clear fungicidal encapsulant, and a heavy poly vapor barrier was placed on the crawl area floor and sealed neatly throughout.

Even high quality workmanship such as this job may not last however, if the building owners do not take the outside steps to keep water out of the crawl area.

See CRAWL SPACE GROUND COVERS for additional details about choosing ground cover material for a crawl space and about installing it effectively.

Advice About Pouring (Placing) Concrete on Crawl Space Floors

Controlling moisture due to crawl space in hot climates (C) Carson Dunlop Associates

Moisture warning when installing concrete in a crawl space: Poured concrete or similar dense coating on crawl space floors and lower walls is a crawl space sealing and cleaning process offered by some mold and flood damage remediators as an alternative to the plastic vapor barrier.

Before pouring concrete or similar coatings into a confined space under a building you should determine if the surge of moisture that will be generated under the building is going to be a problem for other building areas.

If so, you'll need to take steps to direct that moisture outside rather than up through the building.

The sketch of the role of a dirt floor crawl space in building moisture and mold problems (above) is courtesy of Carson Dunlop Associates. In hot humid climates, venting a crawl space is actually asking for trouble: moisture-laden warm air entering a crawl space leads to accumulated condensation and water and to a building moisture and often building mold problem.

Watch out: It is almost always preferable to keep water from entering a building rather than allowing it to enter and then working to get rid of it. But if you are going to do something permanent like pour a crawl space slab, and if your building has had chronic crawlspace water entry, consider including the measures described at PERIMETER DRAIN SYSTEMS.

It is much less costly to install a perimeter drain system before a concrete slab has been poured than afterwards.

This article series describes the steps needed to get into, inspect, clean, and then dry out a building crawl space.

We give a step by step crawl space entry, inspection, cleanout, dryout and keep dry guide explains how to get into or inspect a crawl space even if there is no ready access, how to assess crawl space conditions, how to stop water that is entering the crawl area, how to dry out the space, how to clean up and if necessary disinfect or sanitize the crawl space, and how to keep out crawl space water and moisture in the future.

Reader Question: Building code & standard requirements for flame spread properties of polyethylene vapor barriers

Recently our town building inspector required a flame spread rating of 75 or better and smoke of 200 or less for crawl space moisture barrier. Do you know if this is really in the IBC and do you know of 6 mil or heavier moisture barriers that meet the requirement? - K.W. 30 October 2015

Reply: ASTM & NFPA Standards pertinent to flame spread of polyethylene vapor barriers

The building inspector on site is the final legal authority in requiring specific building features for compliance with local building codes, though one might mount a challenge and though some building code officials will happily accept alternative designs if they are specified by and signed-off on by a design professional such as a licensed engineer or architect.

A review of some polyethylene products that offer flame-resistant ratings includes a flame-resistant polyetylene product that is essentially a three-layer laminate: two layers of a high-strength polyester film that is fire-retardant/resistant laminated with a layer of polyethylene (or in some product descriptions a "molten polyethylene".

There are pertinent standards and some building codes that discuss flame spread in thin polyethylene sheeting such as the 6-mil poly that may be used in a crawl space: ASTM and NFPA. I include descriptions of these blow.

ASTM-E84 "Standard Test Method for Surface Burning Characteristics of Building Materials",


4.1 This test method is intended to provide only comparative measurements of surface flame spread and smoke density measurements with that of select grade red oak and fiber-cement board surfaces under the specific fire exposure conditions described herein.

4.2 This test method exposes a nominal 24-ft (7.32-m) long by 20-in. (508-mm) wide specimen to a controlled air flow and flaming fire exposure adjusted to spread the flame along the entire length of the select grade red oak specimen in 51/2 min.

4.3 This test method does not provide for the following:

4.3.1 Measurement of heat transmission through the tested surface.

4.3.2 The effect of aggravated flame spread behavior of an assembly resulting from the proximity of combustible walls and ceilings.

4.3.3 Classifying or defining a material as noncombustible, by means of a flame spread index by itself.

Source: - - excerpting from ASTM:

(Here is some additional information on applying this standard: )

NFPA 285: Standard Fire Test Method For Evaluation Of Fire Propagation Characteristics Of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components


This standard provides a standardized fire test procedure for evaluating the suitability of exterior, non-load bearing wall assemblies and panels used as components of curtain wall assemblies that are constructed using combustible materials or that incorporate combustible components for installation on buildings where the exterior walls are required to be non-combustible.

Fire propagation characteristics are determined for post-flashover fires of interior origin. NFPA 285 requires both visual observations made by laboratory personnel conducting the test and temperature data recorded during the test. It is important to note that NFPA 285 is an assembly test.


NFPA 701: Standard Methods Of Fire Tests For Flame Propagation Of Textiles And Films


This standard establishes test methods to assess the propagation of flame of various textiles and films under specified fire test conditions. Two distinct test methods differentiate fabrics of different densities. The test methods apply to textile materials used in interior furnishing for public occupancy buildings including curtains, window shades, draperies, table linens, textile wall hangings, as well as to fabrics used in the assembly of awnings, tents, tarps and other similar architectural fabric structures and banners.


Building Codes & Other Standards Citing Flame Spread & Combustible Materials Involving Vapor Barriers, Retarders, Water Barriers

Some flame-spread resistant 6-mil plastic poly vapor barrier products including sold at Home Depot - listed later, below.

Product Sources: flame-retardant poly coverings:

HUSKY 20 ft. x 100 ft. 6 mil Flame Retardant Plastic Sheeting

and here is an MSDS for a flame-retardant sheeting from Poly-America.

Global Plastics also lists a range of such products.

Thank you for asking - I'd welcome any further comment, questions, photos etc.


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