Study on Vented vs. Sealed Crawl Spaces



you this first crawlspace we see a large puddle sitting on top of the ground Polly in the second view we're looking at water wicking through the below grade walls all of this extra moisture in the crawl spaces leads to a new building problem crawlspace mold closed bass mold ranges from light spotting to heavy staining and more and more homeowners and builders are becoming concerned about crawlspace mold because of the potential health effects that may be having on people that live in these homes here's some keys to think about to control crawlspace mold mold will grow on wood in any other organic material when excess moisture is present so what you need to do is to eliminate water leaks and standing water and particularly when it comes to crawl spaces is to keep the crawlspace moisture levels below 70% here's our field study test site it's a 12 home sub development located in Eastern North Carolina and the town of Princeville these homes are located on a cul-de-sac Street the house size is ten thousand and 40 square feet and look at the back of the houses we can see that the site is well graded for drainage the home site was elevated approximately three to four feet above grade to raise it above the 500-year floodplain during the experiment set up all crawl space related moisture issues were resolved these efforts included correcting construction grading and drainage problems sealing any foundation holes that could allow water entry and fixing plumbing leaks the crawl spaces in these test homes averaged about two feet high and virtually all of the wall area in the crawl space is above grade 10 standard size 8 by 16 inch foundation vents ventilate the crawl spaces looking at this site plan you can readily see how the experiment was set up the four houses or the control group the control homes are wall vented crawl spaces with the foundation vents left open all the time and be sure to notice how a hundred percent of the earth floor and these crawl spaces is fully covered with a ground vapor retarder of six mil polyethylene coming back to the site plan the remaining eight houses how to experiment group these crawl spaces were converted to closed crawl spaces for the field study we did this by carefully air-sealing the exterior walls of the crawlspace in this shot you can see workers using rigid pink foam board to seal off the foundation events as well as the wall hole for the duct connection to the outdoor packaged unit in addition to the wall sealing work we install crawlspace liners to minimize water vapor entry from the earth floor and through the masonry walls the liners were created by extending the six mil ground poly up the walls at the top edge the wall poly stops three inches short of the wood sill this provides us with a clear termite inspection strip we use duct mastic to seal all edges and seams of the poly liner to monitor the moisture performance of the field study we rely on small battery-operated data loggers these sensors which are installed in the crawl spaces the homes as well as outdoors record the temperature and relative humidity every 15 minutes in the first two years of operation we have now accumulated over 5 million reading let's take a look at a summary of this data we can use this graph to compare relative humidity conditions for the summer of 2002 the first line to draw in on the graph is the outdoor conditions since this test site is in the hot and humid climate area of Eastern North Carolina it's not at all surprising to see that outdoor humidity levels stayed high most of the summer now let's display what happened in the control group of woven in crawl spaces as this line draws in notice how closely the crawlspace moisture levels follow the outdoor conditions these crawl spaces stayed damp with relative humidity levels above 70% most of the humid summer finally we can show the experiment group performance and sharp contrast to the wall vented crawl spaces the moisture level in the closed crawl spaces did not follow outdoor humidity conditions instead the closed crawl spaces stayed dry with relative humidity levels below 60% all of the time this next graph shows what absolute humidity conditions were for an entire year and the field-test absolute community is a measurement of how much water vapor is in the air and this graph reveals that during the winter both the wall vented enclosed crawl spaces reach their driest conditions and they ended up performing about equally well since winter ends up resulting in drying conditions for both groups crawlspace moisture control efforts really need to focus on the wedding effect that hot humid weather has on wall vented crawl spaces now let's summarize this graph information and table form from June to August 2002 the wall vented crawl spaces state above 70 percent relative humidity 79 percent of the time and they stayed above 80 percent relative humidity 39 percent of the time while the closed crawl spaces never got above 60 percent relative humidity to effectively prevent mold blooms crawlspace moisture levels should be kept below 70 percent relative humidity these results also bring into question the effectiveness of ground vapor retarders in controlling crawlspace mold many building professionals have thought that a full coverage layer of 6 mil poly on the ground was the main strategy needed to control crawlspace moisture the field test results do not support this belief when it comes to mold prevention despite the carefully installed hundred percent coverage ground poly excess moisture continues to enter and remain in the wall vented crawl spaces this excess moisture creates a damp mold friendly climate and humid weather conditions in addition to space moisture measurements we also record wood moisture content a total of ten pin moisture readings are periodically taken in the crawl spaces during data collection trips the pin moisture readings parallel the space moisture readings the closed crawl spaces have lower wood moisture content than the wall vented crawl spaces particularly in the summertime dry wood framing discourages surface mold wood rot and insect infestation the field test is operated for several years now the experiment is operated long enough that the moisture performance characteristics have become stable and predictable here are the key moisture results the chief finding is that the closed crawl spaces stay significantly drier than the wall of into crawl spaces second the foundation vents had excess moisture to the crawl spaces during wet and humid weather particularly in the summertime this excess moisture promotes mold growing conditions the last finding is that brown vapor barriers by themselves do not keep crawlspaces dry enough to prevent pole growing conditions you

41 Comments

  • Matt Vaughan

    April 15, 2019

    This is EXACTLY WHY no one should buy a SLAB HOUSE! Slab houses should be BANNED. Heck, if nothing else, it is an obvious RED FLAG of a cheap, lazy ass builder. I do hvac for a living and 80% of homes with mold in the hvac unit are slab homes even though the percentage of actual slab homes to crawl space homes is flip flopped, meaning slab houses are only about 20% of homes. Mathematically, if 1/5th of houses have 4/5th of mold, then that means a slab home is 25 times more likely to have mold.

    Reply
  • Kelly White

    April 15, 2019

    I've lived up here in Wisconsin in mobile homes that don't have any vapor barrier they have skirting around them and I've lived in houses with crawl spaces not sure what's under there or how they were built but I ain't never had none of these troubles that y'all are having, I never even gave a thought to any of them kinds of troubles, I guess somebody must have built them right.

    Reply
  • Lisa Walker

    April 15, 2019

    Everytime I watch one of these videos I see the underpart of the houses with no protection, the insulation just hanging down.
    Ours isn't like that. We have a 3-4 ft crawlspace and the underside of our house has a pretty thick plastic vapor blanket. O ly openings were to get to pipes or electrical for alterations.
    On the bottom we have gravel and another pretty thick plastic blanket on top of the gravel.
    We also have a drain tile all.along the edge of the crawlspace and a sump to expel the water.
    We do have moisture and close our vents in the winter and open in the summer.
    We had a company come and look at an encapsulated system after we'd had our house for five years. He thought it would be full of mold and crappy. He stated he was surprised to find out it wasn't.

    Reply
  • Eric Lundgren

    April 15, 2019

    Slab on grade or get a basement

    Reply
  • S G

    April 15, 2019

    Great work!! Well done and very informative. Thank you!

    Reply
  • jdstud6

    April 15, 2019

    So the crawlspaces were just sealed up, with no dehumidifier? I've been getting quotes and most want to remove old moisture barrier and insulation, then put down much thicker moisture barrier, seal up vents, and add a dehumidifier. Of course doing all of that is pretty expensive.

    Reply
  • Maurice Landry

    April 15, 2019

    I am going through this now in Eastern NC as well… 75% rh in my crawl space… I just finished sealing my foundation vents and just ordered a Aprilaire 70 pint dehumidifier that will automatically drain to the outside…I have 80% vapor barrier covering per code and will leave it like that for now and will monitor my levels for awhile… I was quoted ridiculous amounts to fully encapsulate like $15000. Ain’t gonna happen.

    Reply
  • Coden11

    April 15, 2019

    What about dry rot?

    Reply
  • jegogdotorg

    April 15, 2019

    Here in San Francisco the city required a vented crawlspace. But it never gets hot and humid, and it stays about 50% RH all year…

    Reply
  • Steve Woodward

    April 15, 2019

    FYI, the goal is actually below 60%RH as that is the threshold when condensation begins to occur. If the crawlspace is sealed, you must install an engineering control to maintain efficient RH (dehumidifier, exhaust fan, etc.) Emphasis on "must".

    Reply
  • bab008

    April 15, 2019

    It's amazing they are still putting vents in new construction as of late 2018 in NC. Close and seal those vents year round!

    Reply
  • JASON JACKSON

    April 15, 2019

    Man I’m having a huge problem here in FL with high humidity in crawl space. And have tried some many things. Idk what to do.

    Reply
  • Tom Jones

    April 15, 2019

    Complete encaprulation is also not necessary. Many companies over sale the importance of total encapsulation on houses that do not require it

    Reply
  • Tom Jones

    April 15, 2019

    Powered ventilation will take advantage of the lower humidity during evenings and stops pulling when humidity is high. For 20% of what a closed crawl space , you can successfully keep humidity levels below 70. You actually can keep them down below 60 if a proper system is used. $8000 or $1600 your choice

    Reply
  • Middi 321

    April 15, 2019

    Would mold grow on the other side of the non condition side of the sealed bearers?

    Reply
  • Boneyfreak

    April 15, 2019

    Vapor barriers help some but foundations are permeable. There are more than a few factors contributing to excess moisture. Biggest culprit being exterior grade and concrete sidewalks and such creating swimming pools along the foundation. Seen this a million times. A lot of the US the soil clay content make perforated pipe &/or fabric filtering useless. If you must build a concrete sidewalk swimming pool next to your foundation install plastic or metal grates wide enough for a spade shovel to clean out easily across it every 8 ft or so filled with round drain rock which is far easier to remove once clogged than crushed rock which compact rather well making cleanout rather difficult.

    Reply
  • Samuel Hishmeh

    April 15, 2019

    Were dehumidifiers installed in the closed crawl spaces?

    Reply
  • Olds 68

    April 15, 2019

    One thousand and forty square feet not ten thousand

    Reply
  • Brian Lewis

    April 15, 2019

    thank you!

    Reply
  • Joe Scheller

    April 15, 2019

    so i have a doublewide mobile home, and they recommend venting the crawl space open to the the dirt or gravel under the home. it has to breath. not?????

    Reply
  • John Register

    April 15, 2019

    My A/C duct work (insulated) now sweats so I cut away my vapor barrier under it to allow moisture to pass on through the soil instead of collecting on top of the barrier. If I encapsulate and seal off outside vents will the duct work still sweat and collect on top of my new moisture barrier? I do plan on installing a dehumidifier and seal up the outside vents.

    Reply
  • Mike McCarthy

    April 15, 2019

    To vent or not to vent – I was confused till I watched your video. I watched so many videos and read so many conflicting reports re: venting and ground sealing but your in depth study has been the only one that made sense for me – THANKS. I will be folowing your advice and closing my vents. One question I would like to ask and would app[reciate a reply after closing all vents but one, do you think it makes sense to put a small extraction fan in one vent and run periodically when out side conditions are not so humid? Thanks.

    Reply
  • Dove's DIY Construction & Reviews

    April 15, 2019

    Instead of sealing the ground and the walls, couldnt we just seal the wood by covering it with a vapor barrier which would serve 2 purposes. One would be to seal off moisture to the wood and the other would be to help hold up fiberglass insulation since it likes to sag over time. Your or anyones thoughts on that?

    Reply
  • Coastal Dry Home Mold and Crawl Space Solutions

    April 15, 2019

    To all who have asked questions here pertaining to this video or to crawl spaces in general, we had responded to each and every one of them but for some reason many of these replies do not appear to be posting properly. Please feel free to contact us directly at [email protected] if you'd like. Thanks!

    Reply
  • homayoun Shirazi

    April 15, 2019

    Based on this study, it appears that placement of 6 mil polyethylene moisture barrier did not reduce moisture in the crawlspace but vent closure did reduce relative humidity in the crawlspace.
    May be the home owner should forego this added expense and simply seal the vents only.

    Reply
  • Sharon Lee

    April 15, 2019

    I'm dealing with this issue right now and I hope to get advice on this forum.  I just built a 1000sf home on pole frame with concrete footers.  It has a crawl space with dirt surface.  My builder tried to take short cuts towards the end and he hated getting in the crawl space.  After he left, I began having moisture issues and began inspecting the crawl space.  I have repaired all plumbing leaks.  I did notice that the OSB on the interior walls of the crawl space were wet.  The builder did NOT put down the 6 mil moisture barrier, instead it was left in a ball in the corner of the crawl space.  Also the rigid board insulation was just cut and fitted to the wall, not sealed or anything like that.  There are 4 vents in the crawl space and they are not sealed at all.   On the outside of the crawl, the exterior walls are covered but I found on closer inspection that they are not covered with a moisture barrier but instead, the builder used my Geo-Textile Driveway fabric to cover the exterior crawl space walls.  I live in very southern Indiana.  So far I have already spread the moisture barrier on the ground but I need to know if it would benefit me to cover the exterior walls with a moisture barrier since I plan to seal the rigid foam board and to seal the vents.  Also if you totally encapsulate the crawl space, do you have to worry that beneath the foam board, there could be moisture on the walls?  Thank you in advance.

    Reply
  • HezUhlive22

    April 15, 2019

    "Ten thousand forty square feet"…hehe

    Reply
  • Rich Wold

    April 15, 2019

    Appreciate the straightforward study that clearly establishes how to keep crawl spaces dry. I've been preaching the value of closing vents year round for a long time and wish I would have found this video years ago.

    Reply
  • dollarbillyboy

    April 15, 2019

    so it seems best to close your vents in the summer open in the winter …

    Reply
  • Rol Saave

    April 15, 2019

    how were you allowed to sale homes that were being used for this experiment…

    Reply
  • Anne Tinnell

    April 15, 2019

    I want to encapsulate my crawlspace. My father in law is against this and said it is against building codes. I have watched many videos about encapsulation and encapsulation makes sense to me. What would you say to him?

    Reply
  • Curman

    April 15, 2019

    Great video. Interesting study. We are in process of purchasing a home in the triangle area. The block foundation has efflorescence staining. I believe the crawler is vented. There is minor mold on joists consistent with the age of the home (ten years). Will closing the vents and covering the block wall with poly be enough to take care of this? Also, what is your opinion on crawler dehumidifiers and fan ventilation? Thank you.

    Reply
  • lawrence porter

    April 15, 2019

    The home were buying is near Philadelphia. It has a half basement half crawl. The crawl is open to the basement thru 2 large pass through areas. Currently there is a heavy mildew smell. Should I close off air vents to outside still ? currently there is no vapor barrier over the dirt. Thanks so much

    Reply
  • mshelton26

    April 15, 2019

    Ã

    Reply
  • P. L.

    April 15, 2019

    would be interesting to see humidity levels for houses with closed vents and 6mm vapor barriers

    Reply
  • r r

    April 15, 2019

    What about Radon, isn't there a chance you trap more of it ?

    Reply
  • Jonathan Trivette

    April 15, 2019

    I live in the mountains of NC, Ashe County to be exact. We don't have nearly as many high humidity days here as you do just a few hours away in the Piedmont, Triad or Triangle areas. I am building my new home now w/ a crawl space and I am constantly going back and forth as to whether I should install a closed or vented crawl space. Do you have any recommendations for my area?

    Reply
  • Paul Trigger

    April 15, 2019

    It's possible for mold to form in as low as 50% RH if the air is stagnant enough (ie a closed crawl), although it takes much longer. Also this video only applies to certain climate zones like the east coast. If you live on the west coast the main source water in crawlspaces is direct penetration, not water vapor.

    Reply
  • Matt Garrett

    April 15, 2019

    Has there been any study on exsisting homes with vapor barrier…then air sealing vents and putting a air supply line in from the supply side? Where can I get the data trackers for my crawl> Thanks guys … informative video.

    Reply
  • Judy Hefren

    April 15, 2019

    I've heard that there is a possible risk with sealed crawl spaces with deterioration of the cement/brick foundation as the moisture is trapped between the liner and the brick/concrete. Has this been studied?

    Reply
  • Toshlito

    April 15, 2019

    thanks for the info

    Reply

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