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.

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

These questions & answers on crawl space moisture and water entry preventive measures to take inside the crawl space to help keep that area dry were posted originally at z - be sure to review the advice given there.

On 2017-12-29 by (mod) - where do the moisture barriers go and do we need both ceiling & floor plastic?

Claire:

On the ceiling of a crawl space or over a basement the vapor barrier belongs on the warm side - up against the underside of the floor above - to keep from driving moisture into the space below; IMO air leaks are more critical than this feature.

On the floor of a crawl space you definitely want a vapor barrier to keep soil moisture from being driven up into the crawl or basement area. Otherwise soil moisture acts like a pump, exacerbated by the natural chimney effect in buildings, to suck moisture out of the soil and bring it into the building.

See BASEMENT CEILING VAPOR BARRIER

and

See CRAWL SPACE MOISTURE BARRIERS

On 2017-12-09 by Claire - removed the moisture barrier after a sewage flood

So I have a crawl space that has a vapor barrier on the dirt floor and on the ceiling. I had a sewer line break and I’m having the mess cleaned up but the company doing the cleaning said that you do not need both vapor barriers. Is that true? Because of the cleanup I’d like to remove the floor barrier and leave the dirt.

On 2017-11-24 by (mod) - best options for drying out the crawl space

T

It's not the most efficient design to try to dehumidifier crawl space that is partly vented to the outdoors since you are in a sense trying to dehmidify the entire outside

That plus the incomplete moisture barrier on the floor are in my view likely to be the more significant concerns among the problems that you describe

See CRAWL SPACE DRY-OUT PROCEDURE

and

see CRAWL SPACE GROUND COVERS

On 2017-11-22 by TLP

We have recently bought a home. It at one time had a moisture issue in the crawlspace. Supposedly it was remediated.

They put down a vapor barrier (but it stopped shy of a foot from the crawl space wall. They sealed some but not all of the vents and installed a dehumidifier with a continuous drainage line.

The walls of the crawl space have efflorescense on them as well as missing mortar between the bricks.

We are considering encapsulation but I'm wondering if the barrier on the wall will trap moisture behind it causing more issues with efflorescense and mortar deterrioration.

Is covering the floor only, sealing the remainder of the vents and crawlspace door as well as running the dehumidier a better option?

Running the dehumidier now we are able to maintain the humidity level between 45 and 50 rh.

On 2017-07-21 by (mod) - How important is it to also place plastic over the concrete block walls

Usually we see plastic run just a foot or so up walls unless the walls themselves are below ground level and thus are taking in moisture from the soil outside.

On 2017-07-21 by Lee

I laid a moisture barrier on top of gravel in our crawl space. I also have 4 vent fans to keep air circulating which I run about 8 hours a day. How important is it to also place plastic over the concrete block walls or can I get by with just the floor barrier. Thanks.

On 2017-06-25 by (mod) -

Becky,

You will need to both find and fix the water source and dry the area,

Please use the search box just above to find our article on

CRAWL SPACE DRYOUT

for detailed guidance. Take a look at that I go and ask us for more help as needed.

On 2017-06-24 by Becky

I'm in a newly build modular home with dirt floor I have standing water. called several companies and confused as what to do some say to run ditches all around and then put heavy vapor on floor and some say to also cover the walls which do you recommend ?? will not be using the crawl space for anything

On 2017-06-13 by (mod) - lacing heavy items (hot water tank, freezer) on top of the 20mil vapour liner

Great question, Skip. I would want heavy items to be sitting on a concrete or concrete block, not on dirt.

That done, the poly is pretty resistant and can even be run under the base if you prefer; even if the poly is crushed and cut over time you'll see little moisture movement up through the base into the area.

On 2017-06-13 by Skip

I plan on placing heavy items (hot water tank, freezer) on top of the 20mil vapour liner is that going to be ok or will over time destroy it?

On 2016-12-31 by (mod) - should I put down a moisture barrier before I remove the damaged floors

PS

You'll want a moisture barrier on soil in the crawl space - that'll also help you crawl around to inspect, but if you're going to tear up a damaged flooring anyhow, and if the crawl space is tight to access from below, it'll be far easier to inspect framing and install moisture barrier with the floor open from above.

You will want to search InspectApedia.com for and read CRAWL SPACE DRYOUT PROCEDURES

On 2016-12-31 by P. Spencer

I have a bad moisture problem under my new to me house. Which is completely vented on 3 sides of crawlspace. I have some mold and unfortunately rot on my subfloor. I am planning on removing all damaged subfloor and putting new in.

My question is should I put down a moisture barrier before I remove the damaged floors so I can see if I need to replace floor joists. Thanks for the advice, I have been racking my brain trying to fix this problem.

On 2016-05-25 by Nora mccutcheon

Had my crawl space insulated and vapor barrier installed. They sprayed glued the platic together now I have a very bad chemical smell from the plastic how do you get rid of it?

On 2016-02-26 by Rick

Have you ever used or seen shower pan liners used in crawl space?

On 2015-07-17 by (mod) - do I have to dry out the crawl space prior to putting in vapor barrier

Jonathan

If you've removed standing water from a crawl space you can go ahead and put down a vapor barrier; be very careful to seal it or overlap sections by a few feet and to first remove things that may cut the plastic. Use 6 mil poly.

You'll be stopping the water movement up from the soil.

Then go ahead and finish drying out the crawl area as we describe in this article series.

On 2015-07-12 by Jonathan

Our crawl space is damp. I've never noticed any pooling water. Because it is damp, it's not an even floor because my knees sink into the dirt/mud as I crawl through it. I'm looking at the vapor barrier as a solid 'fix'.

My question is

1) do I have to dry out the crawl space prior to putting in vapor barrier and

2) is it enough to put a carpet blower on it to dry it out, or do I need a de-humidifier too?

On 2014-11-15 by (mod) - straw in a crawl space is a bad idea

Patrick

Yes straw in a crawl space is a bad idea as it is gourmet food for mold. I'd remove it.

On 2014-11-14 by Patrick

When my HVAC guy went down in my crawl space he found straw spread under the moisture barrier. Isn't that a bad idea? Should I pull up the barrier and get all the straw out. I have been smelling a moldy smell. inhimpc@aol.com

On 2014-07-11 by (mod) -

Brad

Please see your same question and our posted reply at

CRAWL SPACE DRYOUT - home

On 2014-07-11 by Brad

Two years ago our snowmobile club built a cabin for club members and guests to use as a day lodge/warming hut. The cabin has a full basement with a dirt floor and is 30'x 50' with 10'walls. the upper floor is 30'x30' with a 20'x30'covered deck.

The covered deck extends over the basement and the whole basement is uninsulated. The upper area is insulated and heated by a wood stove that is used most winter days and the building cools to below freezing overnight. access to the basement is via a stairwell off the main floor and is separated by a wall and doorway.

The basement is used for firewood storage and usually has about 10 cords of un-split wood.

The problem we are having,is moisture and frost on the floor joists and on walls of the stairwell. The walls of the stairwell have spots of black mold on them.

Last spring these walls were washed and treated with an anti mold treatment. My question is, what can we do to reduce/eliminate this mold/moisture before it destroys our new building?

The basement is vented to the outside by 6-8"x10" vents. Thanks for any help we can get for this problem.

Question: crawl space insulation and vapor barrier retrofit questions

I live in Syracuse, NY in a 1920 colonial with full basement. I recently put on an 18'x 20' addition for my mom. Bedroom and bath. The crawl space is open to the full cellar via the former cellar window opening. The crawl space has water lines and p-trap for shower as well as heat/cold air runs.

The contractor installed 1 small vent on each side of the addition. Should I permanently close off the vents and turn the crawl space into somewhat heated and conditioned space?

The contractor was going to install batt insulation on the underside of the floor- I told him to hold off.

Should I install 10mil vapor barrier on the dirt floor and use 2-part closed cell spray foam insulation on the interior of the block? Or is it a better method to pour a concrete slab and use the 2-part closed cell foam? I understand I may need to cut a small register in the heat run as well as run a dehumidifier in the warmer months.

I will also be installing a radon mitigation system soon. Thank-you very much for your response. - J.R. - Syracuse

Reply:

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with air and moisture control for a structure that combines crawl and basement areas. That said, here are some things to consider:

First, kudos for packing so many good questions into a small (crawl) space:

The crawl space has water lines and p-trap for shower as well as heat/cold air runs.

Leaving the crawl area open to the basement will make it easy to access, easy to inspect on occasion, and it'll share air (and thus anything else) with the basement. Your comment that there is plumbing and A/C ducting there means there are potential leaks and condensation sources - you don't want them hidden and you want to prevent condensation - at least by insulating the cold water lines.

The contractor installed 1 small vent on each side of the addition. Should I permanently close off the vents and turn the crawl space into somewhat heated and conditioned space?

Your suggestion is conventional wisdom. The problem is that at times blowing air into the crawl adds unwanted moisture, and 2 small vents isn't going to dry anything out anyway, even in the best of conditions. There is just not enough dry air movement into the space - during the part of the year that such an approach might work. So I agree with you.

The contractor was going to install batt insulation on the underside of the floor- I told him to hold off.

If you are certain that the crawl area is dry and you expect it to remain so, you can use fiberglass insulation under the floor, or you can insulate the perimeter - where I prefer solid closed cell foam that doesn't pick up moisture.

Take a look at CRAWL SPACE INSULATION RETROFIT

and at CRAWL SPACE VAPOR BARRIER

Should I install 10mil vapor barrier on the dirt floor and use 2-part closed cell spray foam insulation on the interior of the block? Or is it a better method to pour a concrete slab and use the 2-part closed cell foam?

You may find you're paying quite a bit to have the crawl area professionally foam insulated as it's a tight, hard to work-in area. If you go that route and get bids on the job you may find the bidders don't think it's economical unless you're doing other larger insulation work at the same time. But you could use solid foam as I mentioned above.

And similarly, while you could pour a slab (a "rat slab" they call it around here but don't tell your mother I said that) I prefer to avoid doing retrofits that are in tight (expensive) areas and that can pump a lot of moisture up through the building overhead as the concrete cures. If you go that route be sure to ventilate all that moisture outside during the cure period.

Frankly I think the poly vapor barrier would be easier, cheaper, and for a little-used area, effective.. If you can get 10 mil that's more resistant to holes and tears than the thinner stuff, and I never use less than 6 mil. I find that the material is not that precisely uniform and unrolling the thinner poly I sometimes see fragile thin spots.

Before putting down your poly be sure the crawl space floor is clean of debris, smooth, and pitched to a single drain point so that if necessary you can install a sump in the future. Then run the poly as you describe. Tape any joints or overlap them at least 24". Don't run poly all the way up walls to the sills if you're in a termite-risk area.

Also take a look at CRAWL SPACE GROUND COVERS.

Question: using a sealer and vapor barrier in a crawl space

I live in the Wyoming mountains (9000 Ft) in a very dry climate, 15" of rain a year which includes average 3-4 feet of snow in the winter. The house is 45 years old. There were only two small vents in the crawl space.

When an addition was built on the south end of the house 6 years ago,one vent was blocked by the addition which had a separate crawl space and vapor barrier installed. Last year we had record snowfall & high ground water, within 6" of outside grade.

I had a floor joists sagging in a bedroom and then I found white furry mold on a baseboard on the north end of the house.

I pulled up the carpet and pad and on then inspected the crawl space below the two bedrooms found mold on the joists and subfloor with three joists rotting out under one bedroom and much less mold under the other bedroom which is on the other side of the main support beam..and a very wet space.

The vertical two by fours in the walls and blow-in insulation inspected so far are dry and show no mold.

I had a contractor look at it and have decided to seal off the two bed rooms and remove the floors and joists completely in both rooms, remove a couple inches of dirt, dry out the crawl space, clean all the other visible mold by the methods you describe throughout the entire space, and rebuild the joists with pressure treated wood and new sub floors. And install more piers for support of the floor.

I thought about spraying a sealer or oil based Kilz after the cleaning? I am also planning to put a vapor barrier in.

Due to the dry climate (typically not over 30% RH unless it is snowing or raining and usually less than 20%), put in several vents as well rather then heat it due to the future possibility of high groundwater due to high snow level.

I am also considering a fan to continuously move air, but the wind here blows almost every day due to the altitude in the mountains. Do you have any comments or other suggestions? - J.R. Jay

Reply:

JR even in a dry climate that has just periodic rainfall, leaks and water entry are asking for a mold or rot or insect problem. Your cleanup sounds thorough but I didn't see much about tracking down exactly where the water is leaking in. It's better to keep water out than to let it in and then get rid of it or to try to waterproof the interior against it.

Reader Followup:

I have now found that the water was migrating up from snowmelt to the sill plate on the top of the foundation.

The contractor that did some repair of dry rot rim joist on the north end of the house before it was resided with stucco.

The contractor put Ice and Water Shield on the foundation down about 10 inches into the soil, and then had the Stucco contractor extend the 1/2" styrofoam board 12 inches into the soil as well thinking that all this would seal and insulate the foundation.

When I pulled off the ice and water shield and the blue styrofoam the foundation was soaking wet underneath. I have now removed all that mess and the foundation is drying out.

With the amount of snowmelt that we have in the spring I think that is the main source of problem coupled with the reduced ventilation of only one small vent to the crawl space.

...

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

Or see these

Recommended Articles

• CRAWL SPACE VAPOR BARRIER LOCATION
• CRAWL SPACE DRYOUT - home
• CRAWL SPACE GROUND COVERS
• CRAWL SPACE VENTILATION CODES

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