Encapsulate and dehumidify.

Lots of hand labour.

But you know that already.

That....looks like a job for a smaller framed person than myself.

and someone not claustrophobic.

To preclude mold/mildew growth, you should limit the relative humidity in the crawl space to 50% +/- . To that end, you need some combination of reducing the amount of moisture entering the crawl, and dehumidification of the space to deal with what moisture you can't exclude. I'd speak with local contractors about how to install a soil moisture vapor retarder; that will reduce the size and cost of dehumidifying the space. For dehumidification, I suggest you install a continuously-operated exhaust fan and an opening in the 1st floor to allow conditioned air from the living area to be drawn through the crawl and exhausted to the exterior.

You don't have to encapsulate the crawlspace - just install a high capacity commercial quality dehumidifier (expensive - ~$1500, but a lot cheaper than encapsulating). Yes, it will run almost all the time but that's what it will take to keep the humidity down.

The problem with installing a dehumidifier in a space like the OP's that puts the "crawl" in crawl space is access for maintenance and poor air circulation. The latter can result in areas of high relative humidity despite installation of a dehumidifier. I think a better alternative is installing an exhaust fan (which can be installed on the exterior of the home) and drawing air from the living space at one or more locations on the far end from the exhaust.

Just to translate the fancy words, a ground vapor retarder is just a 6mil polyethylene sheet of plastic. It can be bought at Home Depot for an $80/1000sf roll and installed with scissors.

And you cannot just put an exhaust fan in. It would require insulating the exterior crawl space walls to mitigate risk of condensation (you are bringing in cool air from the transfer grille to above).

The best bet for a quick fix or simple DIY is as suggested for a good dehumidifier. Also note you do not have to insulate the walls then.

There are a variety of materials which can serve as soil moisture vapor retarders including polyethylene sheeting. 6 mil poly is a great value-for-the-money option.

In my experience, there's not much heat transferred through uninsulated masonry crawl space walls--at least in my coastal VA (mixed-humid) climate zone. Summer design temperatures for gulf locales aren't much different from those in my area. Summer design temperatures for Norfolk, VA are 91 F d.b./78 F w.b. Summer design temperatures for Houston, TX are 94 F d.b./ 79 F w.b. Like I said, not much different. I find the temperatures of vented (with floor above insulated) crawl spaces in summer in my area to be within 15F +/- of conditioned indoor air temperatures. Based on physics, I would expect the temperature difference for an unvented, uninsulated crawl space to be less. Assuming an indoor temperature of 72 F and an indoor relative humidity of 50%, the dewpoint of indoor air would be 50 F. Assuming a 15 F temperature difference, the crawl temperature would be 57 F. The OP stated the relative humidity in the crawl is 95% to 98% The dewpoint for those conditions would be between 55.6 F and 56.4 F Introducing air into the crawl space from the living space which has a lower (50F) dewpoint is not a recipe for condensation.

Or you could just follow code. IRC R408.3 Unvented Crawl Spaces.

- Requires Class I vapor retarder: That is 6mil, going to 10mil or 20mil offers no more moisture control, just more puncture proofness in a crawl space that is not accessed.

- Either an exhaust fan or tying into HVAC both require insulating the exterior walls, a dehumidifier does not. It is no different than code requiring insulating basement walls, venting attic spaces, GFCI receptacles, or . . . . etc. As with any requirement, there is a normal use that doesn't meet the "stringent end" of the requirement, but it is there as it does get met at times and should be accounted for. So someone's experience at a specific location and specific time does not translate to all times and all locations for all people. Thus, to lessen risk you follow the "stringent end" of code.

The construction of the OP's crawl space doesn't comply with modern code requirements with respect to clearances, soil moisture vapor retarder, insulation or ventilation. I doubt local code officials would require an all-or-nothing approach if the OP wants to implement changes to improve the status quo. If they do, then the OP would need to start by jacking up the house...

Conditioning the crawl by supplying air from the supply side of the HVAC system is a very different case than supplying air from the living space. I don't recommend it. Air supplied from the HVAC system air can be at or near dewpoint temperature and condensation can result. That's not the case nor is insulation required when air is supplied from the living space, provided the crawl space is at or below the temperature of the living space. With no insulation in the floor, no openings to the exterior and no appreciable cooling load for the crawl, I would expect that to be the existing condition.

The 6mil vapor barrier we used was produced to our dimensions as one piece by a commercial construction supplier. It came as a roll when unfolded. Looks like you can position it and partially unroll it. Then use poles or whatever to push it to unroll the rest of the way. You can add pea gravel to some of it.

Tim, that is a very surprising response. A few things:

- You do not get to pick and choose which code items you follow. It is not a matter of what the code official inspects or what you can get away with, it is a matter of code following design principles for health, safety and welfare. The requirements of R408.3 are based on design guidelines to address moisture mitigation in any climate. You can parse dew points, relative humidity, temperature, barometric pressure, and the current orbital location of Saturn, but those all change and to lessen risk of failure, design guidelines are put in place.

- Code does not specify a height of crawlspace, only the size of access points into it. Only if your wood joists are closer than 18", or beam closer than 12" to the exposed ground, do you need to use preservative treated wood.

- When you have have a naturally ventilated crawl space you do not need a vapor retarder on the ground, though it would help. When it is unvented, you do and it is required.

- The OP has a moisture problem. Since it is unvented, a vapor retarder will see results. Since it is within the building enclosure, mechanical moisture mitigation must occur. If air movement (exhaust or HVAC) is chosen for that, condensation is a risk, and you personally can't control the temperature of the air coming from upstairs in a house of an unknown location, owners, and lifestyle, nor the dewpoint from Mother Nature. So you would have to insulate walls. Or instead, as suggested, to use a dehumidifier. All of which code addresses.

In Virginia, we have a process for requesting code modifications to the residential code and we also have a separate existing building code. The existing building code is intended to permit improvements to existing structures recognizing that is often impossible to meet the full requirements applicable to new construction.

In the OP's case, the crawl space is uninsulated, unvented, has no soil vapor retarder and limited height under the joists to do anything. It's impractical and unreasonable to impose requirements applicable to new construction and unless I missed something they are not required to do anything. That shouldn't preclude them doing something to improve the status quo. The OP indicated some indoor air quality issues. Implementing an exhaust ventilation strategy which continuously pulls air into the crawl from the living space and exhausts to the outdoors will help to reduce the relative humidity in the space and will hopefully make a noticeable improvement in indoor air quality-- even if no other modifications are viable.

I would get an estimate on encapsulation and dehumidifier, then save up for it. I spent $4200 a few years ago - it seemed way overpriced but was totally worth it. The inside of the house is completely comfortable now. Before encapsulation, the carpet reminded me of a wet sponge - or recently shampooed carpet that never dried. (I have since removed the carpet.) Encapsulation was an absolute necessity IMO. (My crawl space is much taller than yours.)

Pros in your area would know how much they can do for you in that short crawl space. Dehumidification will keep your joists in good shape as well as improve indoor comfort. The company that did my encapsulation was recommended by my HVAC company and they actually specialize in pest extermination. The encapsulation is an additional service they offer. Ask HVAC and exterminators for recommended encapsulators, as well as foundation companies. All of these people would be familiar with crawl spaces.

In the meantime, the vapor barrier over the dirt seems like it would help the dehumidifier operate better, as long as the vents are sealed. If you can dehumidify that space, you should feel more comfortable in your indoor space. But an expert would be able to assess the entire space to know what works and what will backfire or be unsafe. My house had one big masonry skirt with vents and a porch - they sealed the vents (with foam, I think), blocked off under the porch with a vapor barrier wall. They Installed the dehumidifier and pump and a new electrical outlet, routed the water line out one of the vents into the ground outside. And they gave me a remote to monitor crawl space humidity from the inside of the house.

There's nothing wrong with adding a dehumidifier to a crawl space. Given the low headroom in the OP's crawl, it may not be possible to install one and have the necessary height for a gravity condensate drain. Ditto for access needed for routine maintenance. If you install it between floor joists, then air flow--and removal of moisture-- is likely to be affected.

A better strategy, in my opinion, is to install a crawl space exhaust fan either on the exterior of the home where it's accessible or on the interior near the access door. I recommend drawing air from the living space and not the a/c based on my experience in a mixed-humid climate with summer outdoor conditions similar to the OP's.

"It's impractical and unreasonable to impose requirements applicable to new construction

There's nothing wrong with adding a dehumidifier to a crawl space.

install a crawl space exhaust fan / drawing air from the living space"

I'm unclear of your position. Literally sitting a dehumidifier down there is as easy as it gets. Laying plastic is pretty easy and cheap too, and both are common sense that coincidentally matches requirements for new construction. If clearance is a problem, the Sylvane compacts get good reviews.

The OP has an issue with elevated relative humidity in what appears to be an unvented, uninsulated crawl with poor access to some unquantified portion of the crawl. That condition can be ameliorated a number of ways. It's likely that the OP's home is grandfathered, in which case there's no obligation to upgrade to full compliance with the applicable code for conditioned crawls in new construction. Unless their home is condemned pending some particular upgrade, they can choose from a menu of options including multiple options.

Adding a soil vapor retarder to whatever portions of the crawl they can will produce some benefit. I recommend it. A dehumidifier would provide a benefit as well, but the dimensions of the crawl may preclude that option and/or require a condensate pump. My experience is that dehumidifiers are expensive to install and expensive to operate. Condensate pumps are best avoided if possible. Except for foam insulation of rim joists, insulation of crawl walls in the south provides little benefit in my experience, albeit a code requirement for new construction.

My recommendation, based on 25 years of designing, building, and retrofitting crawl spaces as conditioned crawls in my coastal VA (mixed-humid) climate zone (and having lived on the Texas gulf coast) is to install a crawl space exhaust fan with air supplied from the living area in lieu of a dehumidifier. That should help minimize both the relative humidity in the crawl and potential migration of mold and mildew spores from the crawl to the living area. I believe it will be a lower cost alternative than installing a commercial-grade dehumidifier which responds to the OP's statement " I would love to encapsulate, but I don't think I can afford to hire that out on this old house." And it's easy to know whether or not the exhaust fan is operating. I've been in numerous crawl spaces where the installed dehumidifiers weren't operating for some time and the homeowners were unaware.

The target should be to maintain 50% r.h. in the space if possible so any mold spores in the space remain dormant.

3onthetree said " When you have have a naturally ventilated crawl space you do not need a vapor retarder on the ground, though it would help."

We have a 1958 built house in NE Oklahoma with a crawl space with less clearance than the OP. The crawl space has natural venting and no vapor barrier. I hung a smart, recording hygrometer near the middle of the space. I "forced" all the vents open and the humidity went up 4-5%. After 3 days, I closed all the vents and the humidity returned to the previous level of 90-92%. A local foundation company wants $2,500+ to install a 6-mil barrier.

We are in a similar situation but they are saying there is NO room for a dehumidfier and do not suggest mechanical fans. Not sure how we are supposed to get air flow. What did you do?

@Mz Goldstein

The Sylvane SanteFe is only 12" tall.

You may also have the joist space to position a dehumidifier between joists, so maybe a $125 big box store unit that is around 24" tall will fit.

How does this remedy not cause negative pressure in the living space?

During 24 years military service living all over the country, the only place I lived where crawl space was an issue was Charleston, SC.

When we bought we found the HVAC condensate was draining into the crawl space. Fixed that.

We found that there was no vapor barrier and the vents to the outside had been sealed shut.

We paid to have a vapor barrier installed and the vents were replaced with vents that were open in warmer weather, and automatically shut in cold weather.

Dehumidifiers are notoriously expensive to operate and we did not even consider taking conditioned air from the upstairs and pumping it into the crawl space.

We rejected any approach that included electricity to move air - up or down.

YMMV, we were 100% happy with the results we had.

"How does this remedy not cause negative pressure in the living space?" Unless the home is very tightly constructed (e.g., a SIPs home,) the ventilation flow rate should be comparable or less than the natural air infiltration rate. The 2022 International Residential Code requires a conditioned crawl space ventilation rate of 1 cf/50 sq. ft. of footprint. For a 2,500 sq. ft. single-story home, that would be 50 cfm which is about the capacity of a powder room vent fan.

.

ETA: By way of clarification, the IRC has a number of options for ventilation of sealed, conditioned crawl spaces. One option is to install an open return grill in the floor so any air introduced into the crawl space is returned to the HVAC system. That makes it pressure neutral. I don't know about you, but I don't think returning air from a crawl space--which could contain soil gases, insects, mold spores, etc. into the living area. I don't install return grills.

That would be a hefty additional electric cost and also an increase in HVAC load would it not?

There are plenty of case studies which show that sealed, conditioned crawl spaces reduce HVAC load, the latent load in particular. Sealed, conditioned crawls are considered a construction best practice in the south. Some advocates suggest cooling equipment and duct work can be downsized which would reduce both initial and operating costs https://www.jlconline.com/how-to/insulation/building-a-sealed-crawlspace_o?o=1https://www.jlconline.com/how-to/insulation/building-a-sealed-crawlspace_o?o=1.

I converted the crawl space in my former (coastal VA) home from an open, vented crawl to a sealed, conditioned crawl. I didn't notice a big energy savings. I didn't change out HVAC equipment or duct work, either. I did find the relative humidity in the home and crawl were both reduced and the hardwood floors were more comfortable to walk on in the winter.

Thanks for that info.... when I was in Charleston (late 1970s) I don't think any homes had conditioned crawl spaces.

To make sure I understand; the main idea of installing a vent blowing conditioned air from inside the home with a special floor fan is in order to create a desirable pressure differential between the house and the crawlspace, right? Thereby making it less likely that air would be coming from inside the crawl space into the home? Or is the idea more that you are bringing dry air from inside and displacing the damp air down there? Since the crawl space is somewhat sealed I’m not so sure the positive pressure created there by such a fan wouldn’t simply come back into the house somewhere else rather than being made up in the home by fresh air from outside…the house was recently renovated so it’s not too leaky.

To answer the person who asked what I have done so far to address the issue: first I installed a dehumidifier down there (had to remove a piece of siding to jam it in there.) I’m not sure running that for months on end did much of anything. Especially since during a Hurricane there was enough rain that 2 inches of water flooded the crawl. Thankfully the dehudehumidifier survived that. I did remove it though after that. Elevating it really wasn’t an option given it was already sticking up between the floor joists some. So, recently I have installed gutters and underground pipes that convey the water away from the house to the ditch. I also put in ground water drains around the house as part of that. I think this will help a lot though I’m not sure it will really get the crawl space dry. The level of the lawn around the house is a little higher than the crawl space, so I’m concerned that when the ground is saturated water will still soak its way under there. I’d be happy to hear from anyone who could speak to that. Hydrostatic pressure? Maybe even enough to flood again though I hope not. I do remember seeing water gushing out from a crack in the garage floor during the intense rain from the hurricane…

Thank you very much for all your recent comments everyone. I’m still working on this problem. Currently I can only dream of encapsulating it due to how much that would cost and how terribly inaccessible it is (stopping me from doing it myself). I have thought of digging trenches under there for access so I could encapsulate it, but that’s a big project too. Maybe I could hire out the digging part and then do the encapsulation work myself…and hope it doesn’t rain too hard while the trenches are there…

The overall objective is to control relative humidity in the crawl space. That begins with an elevation of the crawl space floor which is higher than the exterior grade or alternatively, waterproofing the foundation walls; managing storm water away from the home with gutters, downspouts and piping; installing a good quality soil vapor retarder on the crawl floor (ideally with the seams taped and extending up the exterior walls); minimizing infiltration of outdoor air by sealing the rim joist, floor penetrations, and any existing vents; sealing HVAC ductwork, and scavenging moisture by introducing a continuous flow of conditioned air. A 1 cfm/50 sq. ft. flow rate isn't going to pressurize a crawl space. If you can't convince your local code official of that, you can install a simple exhaust vent like a dryer vent--but for goodness sake, don't install a return grill to the living space. Codes for new construction of sealed, conditioned crawl spaces require installation of insulation on the perimeter walls, but for retrofits in my (mixed-humid) climate zone, I find the benefits of perimeter insulation (other than spray foam on the rim joist) are minimal. I've also never found the need to install a dehumidifier on a permanent basis, but they can be helpful for drying out the crawl prior to and immediately after installing the vapor retarder and insulation. Finally, if you aren't able to do everything, do what you can, and increase the air flow as needed to control the R.H.