Questions and answers about putting down plastic or other ground covers in a crawl area in an effort to control water and moisture under a building.

We also present research on the toxicity of exposure to off-gassing from some crawl space plastic barriers. We also debate the benefits and shortcomings of covering only part of a dirt crawl space floor with a plastic barrier.

This article series explains how and why to use ground covers to control crawl space moisture.

FAQs: Crawl Space Ground Covers for Crawl Space Moisture Control

These questions and answers about poly or other ground covers in crawl spaces were posted originally at CRAWL SPACE GROUND COVERS - you'll want to read through Steve's advice given there.

[Click to enlarge any image]

Readers dealing with damp or wet crawl spaces should start reading

at CRAWL SPACE DRYOUT PROCEDURES.

On 2021-03-31 - by (mod) -

@Shannon,
yes I can certainly best practice to cover all of the crawlspace floor and even to run slightly up the walls of the crawlspace. Partial coverage will reduce moisture movement out of the soil into the crawlspace but of course it won't stop it if there are openings.

Is it possible that part of your crawlspace was not accessible?

On 2021-03-31 by Shannon

Is it normal to only install 80% vapor barrier in a crawl space? Shouldn't the barrier cover the entire ground under the house.

On 2020-11-21 - by (mod) -

If under the plastic you can't remove it without picking up the ground cover - plastic; scrape up and shovel up the sawdust after removing larger debris.

If on top of the plastic, collect and bag the debris then sweep and vacuum up the sawdust.

On 2020-11-20 by Whitney

What is the best way to remove construction debris, sawdust, etc. from a crawl space that has ground cover down already?

On 2020-07-18 - by (mod) -

Selena

I need a bit more info. to sort this out and offer sound advice.

Are we looking at the top of the foundation wall in your photo - bottom of photo is top of concrete block wall?

If so, yes you can staple your 6 il poly to that horizontal joist edge,

But I'm a little concerned about trapping moisture behind the poly in the fiberglass if there is any possibility of outdoor water or moisture entering that cavity.

If that's the case then we'd not put poly over the fiberglass - as any moisture getting into it from outside needs to escape to the crawlspace air (or back into outdoor air)

Give me those construction details and we'll discuss this further.

On 2020-07-16 by Selena W.

My crawlspace walls have pink fiberglass insulation atrached to them. I want to add a new 6 mil plastic vapor barrier. Can I put the vapor barrier all the way to the top of the wall over the fiberglass? Do I have to pull away the fiberglass and attach the plastic sheet to the wall underneath somehow?

On 2019-07-02 - by (mod) -

Install a sump pump and pit in the low point of the floor

On 2019-06-30 by davidneal9996

Put gravel over barrier in winter when building house a lot of water got in. Pumped it out now there is water under the gravel on plastic. How to get that out?

On 2019-04-28 by (mod) - off-gassing from poly crawl space barriers may be toxic

No, it's sensible IF you can access the area safely.

I did the very same thing in a horribly tight crawl space in a 1920's Poughkeepsie home, crawling into the space, unfolding and smoothing out 6-mil clear polyethylene sheeting as I went ahead - making the crawl easier.

I pushed the poly everywhere I could reach, made it flat, ran it a foot or so up walls, and made sure there would be no areas that would puddle if water got into the space.

I figured I was reducing the rate of moisture movement into the space and thus reducing moisture movement up into the home from the crawl - possible mold issues - even though there were some spots I couldn't even reach using a long stick to push poly ahead.

But

Watch out: I was sickened by fumes off-gassing from the polyethylene sheeting itself.

After that nauseating job I made sure to smell the poly before buying it and also to air it out outside in sunlight.

I don't know if I was breathing respiratory irritants or carcinogens or other toxins. In 2013 Mülhaupt (2013) pointed out that there was actually rather little research on the respiratory hazards of offgassing from such material. But the very next year just such hazards were cited by another researcher (Meng 2014).

Research on Toxic or Hazardous Offgassing from Polythylene Sheeting - PEVA Plastic Toxicity vs PVC

• Lithner, Delilah, Åke Larsson, and Göran Dave. "Environmental and health hazard ranking and assessment of plastic polymers based on chemical composition." Science of the Total Environment 409, no. 18 (2011): 3309-3324.
• Mülhaupt, Rolf. "Green polymer chemistry and bio‐based plastics: dreams and reality." Macromolecular Chemistry and Physics 214, no. 2 (2013): 159-174.
• Meng, Tingzhu Teresa. "Volatile organic compounds of polyethylene vinyl acetate plastic are toxic to living organisms." The Journal of toxicological sciences 39, no. 5 (2014): 795-802.
  Abstract:
  
  Volatile organic compounds (VOCs) in polyvinyl chloride (PVC) plastic products readily evaporate; as a result, hazardous gases enter the ecosystem, and cause cancer in humans and other animals.
  
  Polyethylene vinyl acetate (PEVA) plastic has recently become a popular alternative to PVC since it is chlorine-free.
  
  In order to determine whether PEVA is harmful to humans, this research employed the freshwater oligochaete Lumbriculus variegatus as a model to compare their oxygen intakes while they were exposed to the original stock solutions of PEVA, PVC or distilled water at a different length of time for one day, four days or eight days.
  
  During the exposure periods, the oxygen intakes in both PEVA and PVC groups were much higher than in the distilled water group, indicating that VOCs in both PEVA and PVC were toxins that stressed L. variegatus.
  
  Furthermore, none of the worms fully recovered during the24-hr recovery period. Additionally, the L. variegatus did not clump together tightly after four or eight days’ exposure to either of the two types of plastic solutions, which meant that both PEVA and PVC negatively affected the social behaviors of these blackworms.
  
  The LD50 tests also supported the observations above.
  
  For the first time, our results have shown that PEVA plastic has adverse effects on living organisms, and therefore it is not a safe alternative to PVC.
  
  Further studies should identify specific compounds causing the adverse effects, and determine whether toxic effect occurs in more complex organisms, especially humans.
  

On 2019-04-28 by Chris

Thanks Dan. A full encapsulation seems likely to be expensive (especially if it’ll need repair after we eventually fix the skirting around the house), but I can reach most of the crawl space easily.

Would it be foolish to spend a few hundred dollars on materials to DIY “most” of the dirt covering rather than thousands on a complete fix?

We haven’t seen evidence of bulk water or mold problems; the house just gets humid and the clay soil stays damp for a long time after it rains.

On 2019-04-26 by (mod) - benefit of partical coverage of dirt crawl space when not all is accessible

Sure Chris,

By covering part of a dirt floor with poly sheeting you reduce the moisture uptake into the building and reduce mold and moisture problems.

But in my OPINION it would be odd to do just part of the proper job.

And yes, as your question implies, we cannot assume that 80% coverage would reduce the problem by 80%.

Think of it this way:

Suppose you cover 80% of the dirt with poly and then next year find that water is entering the crawl space, perhaps even puddling atop the poly. If the area is wet or dangerously humid, thje fact that the opening through which water entered is no more a guarantee of significant problem reduction than would be a crawl space that floods from a 1/2" burst water pipe instead of a 2" water pipe.

Is there benefit in covering most, but not all, or the crawl space dirt floor with poly sheeting? I guess my question is: does 80% coverage give a 80% reduction in moisture, or will it be significantly less effective due to the uncovered areas?

On 2018-06-30 by (mod) - OK to see water under the crawl space plastic?

Jean

In my OPINION (I'm not a soils nor water engineer), as long as the water under your crawl space moisture barrier is being kept out of the crawl space the barrier is doing its' job. I'm not shocked to hear that you see water under the (presumably plastic) barrier.

I would take steps to dry out the crawl area including soils under it, starting with fixing any roof runoff, gutter, downspout spill points (at the foundation), surface grade and surface runoff problems first.

And I would be sure the crawl space barrier is intact, working, not leaking.

On 2018-06-30 by Jean

I live in the Pacific Northwest where it rains a lot. Our crawlspace has a ground cover on the dirt floor. There is no water on top of the ground cover. I notice though that it is squishy in some places when I walk on the ground cover. Is it okay to have damp soil underneath the ground cover? I am wondering if I have a drainage problem or not. Thanks.

On 2018-02-18 by (mod) -

Yes as the soil is surely moist. That's why we want the moisture barrier to be complete and not leaky.

On 2018-02-15 by Erny

We have had our entire crawl space worked on. First cleaned out, trench dug by the block walls with 2 sub pumps and a dehumidifier installed along with rock against the block walls. Black plastic was laid up to the block walls. We now have moisture droplets under the black plastic. Is this normal?

On 2017-09-08 by george Penick

Western NC mts- log home on block foundation, I'm sealing the crawl space, should I run the plastic UP the blocks and then glue the foam board to the plastic, or the other way around? Foam is not faced.

On 2016-09-09 by (mod) - Can I put carpet over vapor barrier in the crawl place?

Physically, probably yes.

But it's a terrible idea: Why would you want to put carpet into a place that's at risk of collecting dampness, insects, mice, or other pests?

I'd not recommend it.

On 2016-09-09 1 by Anonymous

Can I put carpet over vapor barrier in the crawl place?

On 2015-09-01 by (mod) - ok to encapsulate a wet moldy crawl space with no cleaning first?

Ben

You are starting at the right place by stoping outside sources of water entry or moisture entry into the crawl area, particularly roof drainage and surface drainage defects.

"Encapsulation" without cleaning and without fixing the source of mold growth in a building would be an improper approach. And with proper cleaning encapsulation is often not necessary. It's appeal is it's fast, profitable, and easy compared with the real work of cleaning and of stopping water entry.

Sure you can put down plastic and install a dehumidifier that is drained outside or to a drain or sump.

See CRAWL SPACE DRYOUT

On 2015-09-01 by Ben

We live in the Southeast and our crawlspace has consistent high humidity. We have no standing water but the block walls do show signs of moisture in spots. We also have a few small places of surface mold growth. We would like to reduce crawl space humidity but the cost of "encapsulation" is very high and we are searching of other less costly reasonable options if they exist.

We are having our gutters inspected as well as our foundation irrigation system to start.

And then in terms of the crawlspace itself, we are considering putting down a new thicker vapor barrier on the dirt floor, closing all crawlspace vents and installing a dehumidifier. Would this be a reasonable approach? This would cost less than half of the "full encapsulation" procedures. Thanks.

Question: how to dry out a dirt crawl arear in a summer-use home

(June 27, 2012) Andy R said:
I have a dirt crawlspace in a summer home in Northern New York.

We can not run heat in the crawlspace year round since we only use the house in Summer. We mostly covered the ground with plastic for the winter, and sealed it up, but when we opened the camp this summer the basement was very wet and there was mold. In this case should we increase ventilation?

Also should we remove the plastic too? Any help would be great.

Reply:

Andy I'm mixed up - is it a crawlspace or a basement? Are we discussing two areas or one?

If you leave electricity on year round you can run a dehumidifier in the enclosed crawl or basement, provided you can make sure the condensate it produces will safely drain away without that drain freezing up. That would put a little heat into the basement or crawl area too.

I'd also look with care at where that moisture is coming from. If you covered the ground with plastic such that the soil is not pumping moisture into the basement/crawl area, then the water is coming from elsewhere - perhaps outside roof spillage or surface runoff - those need to be addressed.

(June 28, 2012) Andy R said:

... , the area is a crawlspace with a dirt floor. The plastic we have down is only covering about 90 percent of the ground. We have a french drain around the house to deal with water run-off. Also we can't run the dehumidifier year round because it gets really cold there and it would freeze. Heating the crawlspace is not really an option in the winter. Any thoughts?

Reply:

Andy,

- a french drain around the house does not deal with water runoff effectively; it relieves pressure of water around the foundation AFTER water has run down the foundation wall, leaking through it at every opportunity. When clients give me an answer like this I worry that they've predefined their understanding of a problem in a way that gets in their own way of solving it.

- Passive ventilation, can help some dry a roof cavity, but it won't help and will sometimes hurt the moisture level in a crawl space.

- If it is not possible to get rid of unwanted moisture in a building because no equipment of any kind can be operated, then your best option is to stop water and moisture from entering the structure.

That may mean a combination of a more informed examination of where water is coming from and how to direct it away from the building (start with roof and surface runoff), combined if necessary with far more costly excavation and installation of waterproofing under and around the foundation - converting it into a boat, and capped by great attention to detail to make sure that once your boat has been built, there are no leaks into it from above (roof, plumbing, windows, doors, siding, etc.)

(June 28, 2012) Anonymous said:
Thanks ....

We have a tin roof that just runs off onto the ground around the house. Would it be worthwhile to add gutters to make sure that water is taken away from the house? Also our house is on a slope would it be helpful to build a trench around the house for any runoff down the hill?
Thanks again for you help with this.

Reply:

Andy, yes and no. Adding gutters plus downspouts that get water 10-12 ft away and to a location where it keeps going would be very important - but if the home is unattended the gutters will clog and spill over at the foundation all over again.

Gutter screens can reduce but not eliminate the frequency of gutter cleanout needed. It depends on how many leaves or whatever are blowing onto the roof.

An intercept drain on the upslope side of the building can intercept groundwater and direct it away;

A swale on the upside of the home can intercept and reroute surface runoff, though in heavy frozen snowfall it may stop working;

If water stains are high on the foundation wall I'd start with roof runoff control and a swale.

If water stains are just low on the foundation the problem may be subsurface ground water.

For unattended buildings and barns whose metal roofs and ice/snow conditions knock off gutters every year, we had perfect success with a combination of buried geotexiles and careful subsurfaace (below the waterproof layer) contouring to a drainage area and surface contouring (plus gravel under the eaves) as an expensive alternative to roof gutters

Question:

(Mar 28, 2013) Charles said:
We have experienced a very wet winter, with greater then 24" of rain fall in our area. I have been battling a very wet crawlspace for several months. My down spouts remove the rainfall to an area about 15ft down hill from my house (The drain pipes do work. I checked)

The backyard is sloped in such a way that carries surface runoff around my house. The sump pump that I put in removes more then 100 gallons per day. I have a dehumidifier and poly down to control the moisture. I believe that i have a spring that introduces the water to the crawlspace. What more can i do to control the moisture levels and control the water intrusion.

How do i find someone in my area that could help me waterproof the crawlspace (Atlanta area). If i have a spring can an engineer control the water. I am worried that mold and termites are in my near future. Thanks in advance for any additional information.

Reply:

Charles,

See CRAWL SPACE DRY-OUT PROCEDURE for detailed help in drying out the crawl area.

For the case you describe you may need to install a sump system that is deep enough to drop the water level below the crawlspace floor. But don't forget to check outdoors for other water sources. Too often people blame a spring or rising ground water but fail to see roof spillage by the foundation.

Question: roots in the crawl space, fungus growing under the poly vapor retarder

(Mar 31, 2013) Patti said:
When I moved in the partially covered dirt crawl space was dry with no water issues. I sealed the vents, put own 10 mil poly right up the sides and spray foamed the walls. There is passive heat down from the house through floor vents. Its a happy place to find the cable wires and water pipes.
My concern now 3 yrs later is that I can see roots, moisture and now fungus growing below the poly. Crawl space is still dry...... but looking scary beneath it. What should I do?

Reply:

Patti,

Roots mean there is still moisture entering the soil under the poly - though it sounds as if you did mostly the right thing.

Keep the poly intact, tape or cover and then tape any splits or tears;

Seal openings between the crawl space and the living space above to minimize potential movement of air, moisture, mold spores upwards into the occupied space.

See CRAWL SPACE DRY-OUT PROCEDURE

Question: how to insulate an inaccessible crawl area

(July 30, 2014) Darlene said:
I have a home that was built around 1901.

The back half of the house is goes from a foot to 5" or so off the ground. To make matters worse the house sets on rocks and is open under the house. What can I do to insulate this space. The hardwood floors set on the beams so air just flows up in the house. The hardwood floors are to beautiful to cover.

Reply:

Darlene

In your question we are literally stuck between a rock and a hard place - rocky ground under the home, only 5" of clearance at the low end - and a wood floor above.

If I were certain (which I am not at all) that the space below the floor were completely dry and would reliably remain so (which I am not)

I might try pumping closed cell insulation into the space. I can't recommend this approach but it's on the menu. I would first want to know more accurately where the foam was going.

A properly designed solution might take any of several expensive approaches:

1. Raising the whole structure to provide room below

2. more likely: carefully removing the flooring from above to give working space, preserving the floor boards, installing plastic over the ground, inspecting for insect and rot damage and repairing as needed, then solid foam insulation between the floor joists, then floor replacement. That's an approach I've followed successfully in the past.

Question: ground cover below a mobile home

(Aug 21, 2014) Anonymous said:
We bought a mobile home 66' x 14'. The crawl space underneath is not heated and there is nothing covering the ground. We live in Canada where it is hot and humid in the summer months and very cold in the winter months. Could this be a problem. Thank you

Reply:

Seal openings in the mobile home floor that could permit moisture or mold passing up into the occupied space;

Inspect and complete any missing insulation under the floor.

Inspect and repair the "belly wrap" or barrier protecting the floor insulation on the crawl space side.

Question:

(Mar 18, 2015) Travis Simons said:
I didn't know that putting a ground cover would reduce moisture from capillary rise by up to 90 percent. That is quite the increase. It seems like that would be the one of the best things you could do to control the moisture in your crawl space.

Reply:

Travis

Any break in the moisture path is helpful in controlling moisture in a building, but it is even more critical to understand that unless soil is covered in a typical (damp or wet) crawl space, the combination of temperature differences, humidity differences, and air movement upwards out of the crawl area, make the crawl space a water pump pushing moisture up into the building.

Question: put the plastic vapor retarder over saturated dirt in a crawl space?

(Apr 14, 2015) lexi63 said:
should a water barrier ( large black plastic ) , be put over saturated ground ? or should it be completely dry before covering it with the plastic and sealing the crawl space with the skirting ? common sense tells me the ground shoild be as dry as possible to do this procedure , not do half the job, install water barrier, then allow flooding to take place then close the crawl space up with saturated ground . please advise .

Reply:

You want to remove standing water, fix the sources of crawl space water entry, install a sump if necessary, seal openings between crawl area and the occupied building above, and defer insulating the floor over the crawl space until that area has been dried out and if necessary cleaned of mold growth. Then you can insulate.

Best current practices stop outside air movement in and out of the crawl space - as that's another source of moisture during humid weather.

Question: use heat pump to condition the crawl space area?

(July 24, 2015) JohnHomeowner said:
Can I use my heat pump to supply conditioned air beneath the house? I am purchasing a home built in 1980, the current heat pump is junk so I thought I would remove all the old ducting and encapsulate the crawl space before I replace the heat pump. My plan is to remove all floor insulation, install polystyrene foam boards along the foundation wall and then encapsulate.

My question is will I need a dehumidifier, of can I open the supply and return ducting and supply sufficient exchange air?

My HVAC guy thinks the dank crawl space air will be pumped throughout the house; my contention is the air will cease to be dank when you remove the moisture and dry out the space. What are your thoughts?

Reply:

You will be unnecesssarilyi increasing heating or cooling costs; why not just seal the area, put down a poly vapor barrier, and run a dehumidifer?

Question: humid crawl space in the Southeastern U.S.

1 September 2015 Ben said:

We live in the Southeast and our crawlspace has consistent high humidity. We have no standing water but the block walls do show signs of moisture in spots. We also have a few small places of surface mold growth. We would like to reduce crawl space humidity but the cost of "encapsulation" is very high and we are searching of other less costly reasonable options if they exist.

We are having our gutters inspected as well as our foundation irrigation system to start.

And then in terms of the crawlspace itself, we are considering putting down a new thicker vapor barrier on the dirt floor, closing all crawlspace vents and installing a dehumidifier. Would this be a reasonable approach? This would cost less than half of the "full encapsulation" procedures. Thanks.

Reply:

Ben

You are starting at the right place by stoping outside sources of water entry or moisture entry into the crawl area, particularly roof drainage and surface drainage defects.

"Encapsulation" without cleaning and without fixing the source of mold growth in a building would be an improper approach. And with proper cleaning encapsulation is often not necessary. It's appeal is it's fast, profitable, and easy compared with the real work of cleaning and of stopping water entry.

Sure you can put down plastic and install a dehumidifier that is drained outside or to a drain or sump.

See CRAWL SPACE DRYOUT at inspectapedia.com/structure/Crawl_Space_Dryout.php

...

Continue reading at CRAWL SPACE GROUND COVERS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Crawl Space Moisture Barrier Articles

• CRAWL SPACE GROUND COVERS
• CRAWL SPACE GROUND COVER FAQs
• CRAWL SPACE MOISTURE BARRIERS
• CRAWL SPACE VAPOR BARRIER LOCATION
• CRAWL SPACE VENTILATION CODES

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