How to Dry Out a Crawl Space & Keep it Dry
This article describes ten steps needed to get into, inspect, clean, and then dry out a building crawl space and keep it dry.
We add advice on how to keep the crawl space dry and clean so that this process doesn't have to be repeated.
This step by 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.
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Damp or wet crawl spaces or basements are often a source of health and structural problems in buildings. Wet areas beneath the occupied space invite mold contamination, insect attack, and structural rot and may also contribute to bacterial hazards. Keeping these spaces dry and clean is not difficult if we address the steps needed in the right order.
[Click to enlarge any image]
The crawl space shown in our page top photo was in our opinion not a readily accessible area because of flooding. This decision is made by the inspector on the scene, not by anyone else. The crawl space shown at left was tight and so junk-filled it could not be entered either.
We break down a thorough crawl space dryout and cleanup process into these steps - presented here in order as a series of crawl space dryout and cleanup and waterproofing articles:
CRAWL SPACE REINSPECTION: Inspect the crawl space periodically to make sure your crawlspace dryout measures have been effective. How often do you need to inspect the area? It depends ... on site conditions and building history. At least once a year you should look for any new leaks such as a leaky plumbing drain or an outside water entry problem. If you have been having trouble keeping water out of the crawl area, you should check more often until your confidence is restored.
Watch out: for steps 1-7 above, in some conditions, dust containment, negative air, and more protective gear or help from professionals may be needed.
Also see MOLD CLEANUP - MISTAKES to AVOID for a master list of the principal ways that people foul up mold cleanup projects.
Hazards in some crawl spaces include breathing moldy or unsanitary air, getting poked by a rusty nail, stirring up a hornets nest, getting shocked or electrocuted by unsafe wiring while crawling over wet ground, crawling through unsanitary water from burst waste piping, kneeling in unsafe pesticide chemicals left by an ignoramus, and the occasional spider, rodent, snake, or even trapped raccoon.
Wear appropriate protective clothing, use a good light, and don't work alone.
Take a thorough look in all areas the crawl space itself for water and dampness and for unsafe or unhealthy conditions such as
Continue reading at CRAWL SPACE DRY-OUT PROCEDURE or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
Or see CRAWL SPACE DEHUMIDIFICATION
Or see CRAWL SPACE WATER ENTRY STOP
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(Sept 21, 2011) Mrs. Smith said: In regards to this subject , I found this article to be extraordinary. Very comprehensive. Thank you.
(Nov 3, 2011) Jules said: LOTS of good information, Well done
(Feb 23, 2012) Elizabeth said: Thank you so much for all this information!!!
(Mar 12, 2012) Derek said: Great information!
Thank you for the nice remarks. We also welcome questions, criticism, content suggestions. Daniel.
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
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
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.
While this article did address the dehumidifier to a large extent, it really did not give instruction on how to rapidly dry out the crawl space after the water issues had been resolved so that the other remediation steps could be taken. More instruction on that would be extremely useful. - C. Brown
Thanks for the comment C. Brown. In response to your well-put query, I have added material at
CRAWL SPACE DRY-OUT PROCEDURE - how to dry out the crawl space rapidly after standing water has been removed, and thus to try to reduce further moisture and water damage to the rest of the building (of course effects may already have taken a toll so a corollary rule will be how to inspect the building for hidden moisture related trouble that originated in the crawlspace) we take the following steps:
By this interpretation we use multiple fans to increase air circulation in the crawl area, thus picking up moisture rapidly, combined with one or more fans exhausting - blowing the moist air outdoors.
Beware that in hot humid weather, bringing outdoor humid air into a crawl area can make for new condensation issues. Some have experimented with a humidistat that changed direction of airflow depending on which air (inside or outside) was the less humid.
If we cannot exhaust our stirred-up air outside, that may be ok - if we run one or multiple dehumidifiers whose condensate is collected and taken by gravity or pump to a drain. I've seen very good dryout success using the combination of extra circulating fans and a constant-running dehumidifier. The additional fans significantly increase the efficacy of the dehumidifier, increasing the rate at which it removes water from crawl space air.
Following an initial dryout, if it was not already addressed by implication in your question, we must make darn sure we've stopped water from entering the crawl area. This means making sure that roof drainage is away from the building, that there are no other leaks into the crawl area (such as from plumbing, or even a nearby spring), and that we have adequate moisture barrier (6 mil poly) on the crawl space (dirt?) floor. Articles above include addressing the crawl space ground cover.
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
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.
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.
Our home flooded due to toilet break. 3 weeks water ran totaling 130,500 gallons of water. A company mediated our home, neglecting the crawl. I went under and dug down 6" the dirt was damp. How do we dry this out? - Ronald 7/22/12
Ronald I'd suggest starting crawl space dryout with a review of the suggestions in the article above - that's my best shot. When you've got the in crawl surfaces dry you'll want to take a look at the additional topics (see crawl space links near the top left of this page) such as how to put down a plastic moisture barrier to stop soil pumping moisture into the building.
I just bought a house that was mostly gutted when I got it. Neighbors tell of roof leaks and mold, though what was left on interior walls had no evidence of mold. Still, I finished the demo, removing everything down to stud walls and floor joists. Much of the subfloor (old chipboard and thin OSB - house was built in 1980) was rotted, to varying degrees.
There was much mold-like substance on the floor joists - (I've been all over your website trying to determine the type) but they include the black cosmetic mold, brown fuzzy mold (not the hair-looking stuff, but sort of looked like spun cotton candy), white stuff, a little bit of yellow stuff. I'm using the folded rag method to clean with water and anti-bacterial soap, with a little bit of bleach for good measure. It seems to be working well.
My builder is now putting down new Advantek subfloor, as I'm finishing the cleaning. I'm also cleaning out the crawlspace as recommended, removing all the debris, the old poly, all insulation, etc. There is water encroachment during rains from one corner of the house where the gutters aren't working and the ground slopes towards the house.
Obviously, those last two things must be corrected immediately. The crawlspace had a few areas of standing water after the last significant rain. It had dissipated by the time I started the cleanup down there. However, under the plastic, some of the ground (red clay, here in NC) is pretty wet. Where I had removed the plastic, it has started to dry out pretty quickly. (three days?)
So, is it okay to let my builder complete the subfloor installation while I continue to clean the crawlspace, or would I be better served to have him wait to complete until I'm done? Should I leave the plastic off for a period of time to let the ground dry out before replacing it? also, there are a few small visible roots under there that themselves look moldy - black and spidery.
What of that? The ground is uneven and I'll smooth it out to eliminate the low spots, but I'm uncertain as to what is the best next step. - Charlene Blevins 10/21/2012
It sounds as if your approach is reasonable, especially with the attention to outside sources of water entry;
If you address the sources of water entry, clean and dry the crawl area, and put down gravel and poly those are good steps; but if the area is going to be chronically wet I still expect trouble; I'd be adding an interior drain similar to what Walden is considering, along with perhaps dehumidification.
I'm not too concerned about soil roots & stuff that will be dried and covered by gravel and poly.
hey first off great site, very informative.
I have a 450 sq.ft. crawl space accessed directly from a living space basement. It is clean and dry with gravel and stuff all over the place. I just bought the place and I was thinking maybe I should just have someone put french drain system around the perimeter there and put a 20 mill poly, a sump pump and just pour concrete all over (saw that online and thought is a good idea)and last spray foam the walls and joists. My question is how much roughly would all that cost me and do you think it is a great idea to do so.
thank you so much for your advise. - Walden 11/1/2012
The cost to install an interior perimeter drain in your crawl space depends mostly on
- the area covered
- the ease of access - entry and headroom
- the condition of the surface - e.g. if it is not level or doesn't slope to a drain location
- ease of disposition - to where water has to be pumped
- extent of wiring done
- need for backup power
Figure anywhere from $500 to $5000 for an area I know nothing about
I think I would first look for the history of water entry, signs of moisture trouble, ease of fixing outside sources of water entry, and other crawl space moisture sources and weigh those against the cost for this insurance..
Thanks for all the info, very helpful.
Question. What about a crawl space with in floor radiant heat? Built in the 1940s in Flagstaff, AZ. So very cold winters and mild summers. The crawl space is damp and I have found rotting wood at the foundation. I can do the above repairs as noted, but does the in-floor radiant heat change any suggestions with regards to venting or vapor barrier. Also, I have read elsewhere that just a vapor barrier on the ground, not necessarily up the foundation walls, will stop 80% of the moisture. Is this adequate? Thanks for your help.
- Robert 11/4/2012
Robert, it's an interesting question that I'll think about further, but my initial thought is that presuming you're talking about tubular radiant heat - tubes stapled up under the subfloor over a crawl area, I don't think that material's presence changes our normal recommendations for drying out a crawl space
- remove the sources of water entry
- put down poly
- inspect and clean if moldy
- convert to a heated, conditioned space
You are right that in your climate, most of the benefit of poly will be from what you place on the ground, though in winter there might be condensation on the interior surface of the block foundation around the crawl space - moisture you can keep out of the crawl area by running poly up the walls.
Hurricane Sandy hit my old Sears House (1930's)in Ocean City, NJ so the water was 1 foot deep on the first floor (the floor being 3 feet off the ground!) Sometime within the last 25 years the water was deep enough to get the insulation wet and it was removed at that time and not replaced (The house is not heated in the winter).
What should I do in the crawl space area? If I would encapsulate the crawl space, it would be like a swimming pool next time it floods. Ideas on a good solution? Is a spray insulating foam worthwhile for protecting flooring from underneath? Thanks!
Rick - 11/12/2012
I have also a home in Ocean City, NJ that got hit with Sandy. I have just pulled out the mostly wet insulation. I got water damaged in my first floor from the water seeping up from the craw space. Electric wires are down now and I am worried about structural damage.
Can anyone recommend someone in South Jersey who could do work of reinforcing existing floor supports?
These past week I am worried that those who are hired to do work are being extremely careless and doing other damage. THanks - Roberta 11/13/2012
Closed-cell foam products are somewhat resistant to wetting from flooding or other water intrusion in a building crawl space or anywhere else, but if a building area is actually inundated with floodwaters again after such an installation, I'd be concerned about the difficulty and cost of disinfecting or addressing the risks of sewage-contaminated floodwaters.
For this reason, just taking up some closed cell foam board can be problematic - how will you clean the space that was soaked with sewage waters between the foam boards and framing or subfloor above?
Talk with spray foam insulation contractors in your area about the water resistance of sprayed-in-place closed cell foam insulation. That product actually adheres to wood surfaces, a step that may resist sewage-contaminated water from entering the space between the insulation itself and wood surfaces that otherwise would need cleaning.
In addition, sealing the exposed wood and interior foundation surfaces with a sanitizing or fungicidal sealant (after they are thoroughly dry) will also reduce the moisture uptake (and sewage-contaminated water uptake) of those surfaces in future flooding, making surface cleaning and area dryout a bit faster after the next flood.
Frankly, if the home is likely to be flooded to a depth that submerges the first floor or higher, no crawl space encapsulation is going to completely protect the building; if you cannot afford to raise the building on a taller, flood-damage-resistant foundation or pier system, I'd be troubled about the prospect of recurrent, perhaps even increasingly frequent significant cleaning and repair costs from future area flooding.
I recently moved to an 800 square foot single story historic house in Pacific Grove. The home has a very shallow crawl space (as little as 6" near the perimeter- the foundation is only 12" deep) and the crawl space was covered with a moisture barrier when I bought the home. It has a solid concrete wall bisecting the underneath of the house (the long way).
I had termites and was advised by the inspector that the house is very damp underneath. They recommended removal of the moisture barrier and installation of fans to dry out the space. I have been having symptoms like I get when I am exposed to dust (dry, red nose) and feel very uncomfortable ever since the fans went in. This is driving me absolutely nuts.
The crawl space is wetter now than before (it has rained). I can't put in a french drain because there is only about 6" clearance between my house's crawl boxes and the fence, also it is on top of the sewer lateral.
I live on a hill in an area known to have underground springs but nobody knows exactly where they run.
I think they should remove the fans and put the moisture barrier back.
Do you agree? Is there anything else I can do? Perimeter moisture barrier?
Thank you. There is lots of mold down here and I am concerned that just putting the moisture barrier back will cause mold or rot. - S.A. 2/7/2013
Sounds as if you got some advice that was good in intent but not competent.
I do on occasion recommend adding a fan to increase air movement in a crawl area or basement where a dehumidifier is at work, as that will increase the rate at which the dehumidifier can dry out the area. But just blowing air around in a wet moldy crawl space seems like a bad idea.
Removing the moisture barrier from the crawl floor and blowing fans turns the crawl space into a moisture pump, moving moisture from soil into the crawl space air. Perhaps if the fans blew OUT of the crawl space that might have been better, but the proper approach is to find and fix sources of water entry, seal (poly is ok) the floor, and dehumidify the area.
Watch out: there is an added risk of blowing pesticide contaminated dust and debris into the home if the applicator used a surface spray - something that's not usually done for termites. Usually for termites the pest control officer places a termiticide in the soil around the home; but in a home with a dirt crawl space they may be unable to take that approach because of the risk of chemicals surfacing in the crawl area and entering the living area - making occupants sick. So I'm not sure what has been done about your termite issue but that too needs expert review.
I suspect that dust, possibly allergens (insect fragments, mold, even soil particles, potentially other particles) have been stirred up; and if the fans were not blowing out of the crawl area, it may have been pressurized by the air movement; if that's the case, the arrangement may have increased the movement of particles up into the living area from the crawl space.
More likely you need to stop the fans, find and fix outdoor water sources like roof or surface runoff spilling by the foundation, put the poly back down to stop pumping water into the crawl area, and after the mold problem has been evaluated and most likely removed (cleaning the wood surfaces, tossing out insulation), then you might get a fan and dehumidifier back at work to keep the area dry.
(Nov 18, 2012) Wood foundation, crawlspace, ven said:
Hi, I have a home with a wood foundation that is roughly 25 yrs old. I'm sorry I haven't done this earlier, but I want to make things right down there.
The home is located in Central Wisconsin.
What would you suggest for:
1- Venting (if any is needed)
2- Encapsulation...does 6 mil do the trick or should I be doing more?
3- Sidewall insulation vs floor joist insulation
4- Dehumidifier (what size do I need for a 1500 sq foot crawlspace?
Thanks in advance.
Before encapsulating or venting, I'd inspect the condition of the wood foundation, as if foundation repairs are needed that's when you'd also install appropriate footing drains and waterproofing membranes outside the structure.
(Dec 3, 2012) Jeff said:
I bought a one story house this Summer where the basement is half finished and in the half unfinished area, there is an uncovered access to a crawl space that is elevated with plastic sheeting covering the gravel floor. There are two screened "grate-like" vents to the outside of the house at the foundation. I believe since this is uncovered at the access, I obviously have a problem in that regard with temperature in the home.
The home is drafty at times and very chilly in the basement. There is side wall and floor joist insulation above. I have a dehumidifier in the unfinished half of the basement but not up in the crawl space. Should I block out those vents to the outside and create an access door to this crawl space? What do you think from the sound of it?
I would close the crawl space vents,
Inspect for and fix any leaks into the crawl space from roof drainage or surface runoff
Inspect for rot, insect damage, mold and clean, fix, treat if needed
Then install a dehumidifier in that space - one that automatically drains to a suitable location;
If you need to speed the initial dryout, try adding a fan or two to circulate air in the crawl area.
(Feb 5, 2014) Ken Forrester said:
I inspected an older home yesterday here in Richmond VA. It had a partial basement opening up to a crawlspace. The crawlspace was covered with builders paper. No taped seams. Do you think I should just report that as substandard or also as a cellulose material conducive to wood destroying organisms? Any thoughts?
Hi Ken, nice to hear from someone in my home town.
What was covered? The crawl space floor or the joist undersides;
Paper is not a great moisture barrier if that's what's on the floor, and it may be inviting a mold contamination issue more than an insect pest problem.
The un-taped seams is a secondary issue. However in general, when placing a dirt floor moisture barrier, if the material segments are not overlapped about 24" we want to see the seams sealed.
(Apr 30, 2014) Adan said:
I recently started the labor intensive project of installing drain tile in my crawl space and I ran into a small problem that I cannot find the answer to. After I started digging the trench around the foundation I noticed that the footer is stair stepped. I’m guessing this was done because my house is on a slope. I’m not exactly sure what to do now because of this. Do I continue dig down to the footer? Or should I attempt to keep the trench the same depth around the foundation and abandon digging down to the footer? I hope my question makes sense.
Adan if your drain system is picking up water and carrying it to daylight or to a sump system it simply needs to slope to its destination, typically at 1/8" to 1/4" per foot in the crawl space floor. You don't need to step it down to a deeper footing.
I'd watch with care that your system doesn't remove soil via digging or by pumping muddy water in a way that ultimately undermines the footings themselves.
(May 1, 2014) Adan said:
Thanks for the help with my dilemma! And the quick response. I will be getting back to my project tomorrow....all the rain we've had lately has slowed me down. Thanks again, I really appreciate the advise.
(May 22, 2014) Adan said:
I finally had the final inspection done on the drain tile I installed and it passed with flying colors. The inspector even commented on the job well done. We had a few down pours last week and I was able to see the drain in action and all worked well.....no water in the crawl space! :)
I just wanted to give you the update and say thanks again for the advise. I really did not know how to proceed until I got your help.
Thanks for the good news and the update Adan
(May 1, 2014) Anonymous said:
To keep the soil/muddy water out of my sump pit I plan to line and back fill my trench with pea gravel. Would you recommend the same?
Anon: pea gravel may be a bit too fine. I prefer #4 crushed washed stone.
(July 11, 2014) Brad (July 11 2014) said:
Two years ago our snowmobile club built a cabin for our club members and guest to use as a day lodge/warming hut. The cabin has a full basement with a dirt floor and is 30'x50' 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 and 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 the 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 problem 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.
I've made suggestions in the article above - basically you direct roof and surface runoff away from the building and put down plastic over the soil under the building.
As the building is not occupied nor heated full time I can't suggest converting the basement area to conditioned, heated space;
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