Recently asked questions on the best way to dry out a crawl space and keep it dry and mold free.

This article series provides a 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.

Q&A on How to Dry Out a Problem Crawl Space & Remove Mold, Rodent Debris

These questions and answers about drying out a damp or wet crawl area were posted originally

at CRAWL SPACE DRYOUT - home - be sure to see that article where we organize the approach to drying out and preventing crawl area water entry.

Crawl space insulation and vapor barrier retrofit details needed

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 LOCATION

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 at CRAWL SPACE DRYOUT.

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".

Watch out: 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.

 

Should I take up plastic cover in my crawl space to let it air out?

I live in a home where there is only 12” crawl space. The ground is a sandy base. There is plastic down but I am not sure if it covers the ground completely.

The sand is wet under it. There are 3 vents on each side of the house. There is no standing water.

Would it help to just remove the plastic and allow the air flow to keep it dry, thus reducing or eliminating the moldy order? On 2018-07-04 by Kevin

by (mod) - NO; If you remove the plastic crawl space cover the moisture from the soil will go up into the building above

Kevin

If you remove the plastic the moisture from the soil will go up into the building above creating moisture and mold risks if not actual problems.

What you want is a complete continuous water and Vapor Barrier over the wet soil and outside you want to take steps to make sure that water isn't running into the crawl space.

Will blowing air in to my wet crawl space fix it?

I have condensation under my house when the guys came to our installation in, so they blew air in over night, the next day it was weter,how do we fix this? On 2018-06-27 0 by Mary Duncan

by (mod) - No, and worse, blowing outdoor air in can increase crawl area moisture

Mary

Indeed blowing outside air (often warm and high in moisture content) into a cool crawl space will cause the crawl space to become more wet than ever. That's because moisture in the warmer outdoor air condenses and accumulates in the cooler crawl space.

Our suggestions for drying out the crawl space are best given in the article above - better than an off-the-cuff reply again here.

Current best practices for many buildings have shifted from venting the crawl space to closing it up, drying it out, and converting it to an insulated, conditioned space.

I would NOT insulate the crawl space before it is dry. Otherwise moisture may cause mold contamination in the new insulation as well as loss of the insulation's R-value.

Trapped crawl space moisture invites mold, rot, insect damage, structural damage.

 

Should I put down plastic overy very wet crawl space ground?

Should I pollyfill on top of my completely wet crawl space

Can I put pollyfill on a existing wet ground ? On 2018-03-04 by Teri

by (mod) - ok to put down poly on wet soil in the crawl space

Teri

You can put down poly on damp or wet soil in a crawl area provided:

1. Any cleanup of sewage, debris, or standing water have been addressed
2. Any grading, gravel placement, sump installation, etc. have been completed
3. You have first taken whatever other steps need to be taken inside the area to stop water from entering or ponding
4. You have addressed outdoor sources of crawl space water entry
5. The poly is installed properly, continuous and properly lapped and secured
   

Should we encapsulate the piers in our crawlspace?

We are doing a full crawl space encapsulation. Should the piers which are part cement and pat wood be completely encapsulated as well?

Reply by (mod) - just the pier base and up a few inches

Lucia

Assuming that the piers are masonry I don't see any reason that they need to be fully encapsulated as long as your plastic Vapor Barrier extends sufficiently high up off of the floor. Sufficiently is probably four to six inches minimum and of course it here to the pier sides.

Why do some websites say crawlspace venting is desirable and others say it creates a problem?

Todd said:
Ok, so I have a question. One website, I see there are hazards of venting:
But in other places, I see venting is a good idea to let out moisture. Where am I missing the point? It ventilation good or bad? or does it depend? On 2017-02-15 by Todd

Answer by (mod) -

There is no single "right answer" that fits all possible building conditions, climates, locations, uses, water entry situation, building materials.

But in general, in most countries where there are climates that include warm moist air during some times or seasons and colder drier air at others, the best practices have given up on conventional crawl space vents as they tend to pump moisture into the crawl area risking mold, rot, insects, etc.

Instead we convert the crawl space to a conditioned space, drying it out and removing sources of water entry - as discussed in this article series.

Problem with crawlspace getting wet after rain or snow-melt

Having a problem with crawl space soil getting damp after heavy rain or slow melting snow seeping under foundation have rain gutters installed and sealed blocks outside foundation at loss where to go to stop dampness to prevent mold and mushroom bloom On 2016-12-29 by Anonymous

by (mod) -

Anon:

First let's be sure that surface grade slopes away from the building so that snow melt drains away not towards the structure.

Next, if still because of soil porosity you're having a water entry problem you might need to take inside (trench, drain, sump) or better, outside steps to de-water the foundation. That includes
- installation of footing drains that slope to carry water around the foundation to daylight
- foundation waterproofing

items discussed in the article above.

What can I put down with stones to absorb crawlspace humidity

What kind of material beside stones can we lay down to absorb humidity On 2016-10-21 by Anonymous

by (mod) - we don't want to absorb humidity we want to keep it from entering the crawlsapce

Anon:

Stones do not absorb humidity to any useful extent in a crawl space.

The best suggestions to dry out the crawl area are in the article above, which I can summarize as

1. fix sources of water entry into or under the crawl area

2. put down 6 mil plastic or an equivalent moisture barrier on the ground if it's a dirt floor crawl area

3. if you convert the crawl area to an enclosed, conditioned space, add a dehumidifier.

Crawlspace gets standing water in wet spring weather - should I add sand?

My basement crawl space was never filled with dirt to the top of the footings, so spring high water tables force water up into the crawl space.

I installed a sump pump that minimizes the inflow to small pools. I have an access door that would allow me to bring in sand to fill up to the top of the footing.

Will this create a weight to keep out water or would it simply absorb the water and leave me with a moist basement anyway. Would covering it over with a vapour barrier help? On 2015-04-07 by Ted anderson

Explanation by (mod) - adding dirt or sand so that the crawl space floor looks less like stone soup wont' stop water

Ted

Regrettably, even adding dirt or sand so that the crawl space floor looks less like stone soup wont' stop water from flowing below the surface nor will it stop high crawl space moisture that may affect the whole building.

Putting down 6 mil poly in the crawl will help as will fixing gutters and surface runoff outside.

But your first step should be making the crawlspace surface smooth enough not to pond standing water and sloped enough to drain to a sump pump area if needed.

Then put down a 6 mil plastic vapor barrier.

Otherwise all of that dirt surface is simply acting as a moisture pump even when the soup is below the surface.

Other crawlspace safety hazards: spider bites and mold

Looking at renting a house that is newer, well built it has a 7 foot crawl space and it looks like it is very well cared for except I noticed the yard was mushy when you walked you foot sank into the mud . I have a mold allergy that can be fatal so we take many precautions.

My husband and our friend who is contractor walked the crawl space which had double polyethylene, black plastic and some standing water in just one corner of the house, there were some white round speck maybe 1/8 to 1/4 inch diameter floating on the waters surface.

Although, 95% of the crawl space was dry.

Sadly, our friend was bit by a spider while inspecting and required medical attentions. So another think that makes me wonder of the safety, What are your thoughts? On 2015-02-08 by Anonymous

by (mod) - our friend was bit by a spider while inspecting and required medical attentions.

Sounds like a flooded soaked yard indeed; You'll want some diagnosis of where the water is coming from and how to direcdt it away from the property.

And I agree that spider bites are a very real crawl space hazard, even in one that seems quite accessible. Don't go back in without wearing appropriate protective clothing, good lighting, and perhaps a consult from your local pest control expert.

Is it safe to buy a wet-crawl space house that has flooded repeatedly?

I want to buy a house that is in a flood zone. The sellers disclosure states that the
garage and crawl space has been flooded 2 x in the last 11 years. There is no basement.

The house does not smell whatsoever. Is it save to buy this house and have the crawl space
"flood proofed"? On 2014-10-02 1 0 by A. Coffman

Reply by (mod) -

A C

Thanks for the question, but I'm sorry I can't give a real answer to whether or not it's safe to buy a house based on a two sentence question.

A better approach would be to have an expert home inspection and to be sure that the inspector helps you assess both the impact of prior floods (such as hidden mold, rot, insect damage, mechanical system damage) and the steps needed to protect the home from future flood damage.

Just what flood proofing is going to involve for your house will depend on its construction and location. For example, some homes are designed with flood vents to let flood water in to avoid foundation damage - but that does NOT prevent water or moisture-cause mold damage in such areas.

...

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

Or see these

Recommended Articles

• CRAWL SPACE DRYOUT - home
  • CRAWL SPACE DEHUMIDIFICATION
  • CRAWL SPACE DRY-OUT PROCEDURE
  • CRAWL SPACE GROUND COVERS
  • CRAWL SPACE INSULATION RETROFIT
  • CRAWL SPACE MOISTURE BARRIERS
  • CRAWLSPACE MOLD ADVICE
  • CRAWL SPACE SAFETY ADVICE
  • CRAWL SPACE SEAL & SANITIZE
  • CRAWL SPACE SEWAGE CLEANUP
  • CRAWL SPACE VAPOR BARRIER LOCATION
  • CRAWL SPACE VENTILATION CODES
  • CRAWL SPACE WATER ENTRY STOP
  • CRAWL SPACE WATERPROOFING

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