The intended purpose for laying down plastic is to retard the transmission of soil vapor (which is in almost all soils except in extremely arid areas, and inexorably rising due to changes in barometric pressure) from coming up into the basement space. The fact that you are using a dehu means that you have a perceptible soil vapor problem (at least that's why you are paying the electric co. to run the dehus, which is comparable to running an A/C, in some cases). Note however, that I said retard, not completely prevent the rising of soil vapor into the basement atmosphere. Nothing I know of will completely keep it out. Because we now have a goal of sealing up our houses in the name of energy savings (and have modern, and different, whole-house heating systems) rising soil vapor can get stuck within the building potentially causing damage to the structure or contents or residents, if mold develops. Of course your house, like mine, was built with long-tested systems that did not expect, or require, the house to be as hermetically sealed as we sometimes try for these days. In 1857 the vapor was a-rising just as it is today, but it was exiting the building much more easily. Two other factors: people lived and worked at home and were more likely to be aware of and able to adjust the basement openings during the day, promoting additional passive ventilation to remove the moisture. And they may have highly prized that very same moisture if they stored food in the basement. Of course they also didn't have nifty stuff like cross-linked plastic molecules made into cheap (if you don't take into account any off-stream environmental costs), easily laid-out rolls of plastic to put to use, either. Soil vapor is also coming from the basement's walls as your inspector noted. But blocking that moisture is as hard as, or harder, than blocking the soil vapor. If you had thoughts of converting your basement to modern uses (family room, media room, exercise parlor, etc.) you may be disappointed as this is rarely successful in old houses such as ours. So the remaining issue seems to be: how tight is the house above the basement, and how might that tightness be interfering with the house's natural ventilatory patterns. (I think of it as how the house is "breathing", albeit in a passive way.) And of course, it also depends how much more you may be planning to do, in the name of energy conservation, to further tighten up the house. You have run smack into one of the central dilemmas of old-house care: how do you integrate (without doing damage) modern building and energy standards with a structure that was designed and built to support a completely different system? Solving one problem can easily create a new, perhaps more difficult to manage, unforseen issue. There are many opinions and oxen-to-be-gored in this arena. As a practical matter, if you want to lay down plastic (though I recommend special-purpose materials intended for vapor retardation instead of construction poly) and can seal it well with tape where sheets are overlapped, and along the bottom of the walls, you can susbstantially reduce the amount of rising vapor within the basement cavity. But you should know there are some who posit that doing so will shift this moisture (perhaps with damaging consequences) to adjacent soil spaces, including under and in the immediate, exterior vicinity of your foundation walls. This (according to some) may super-saturate the soil, disturbing the necessary firmness and creating settling or susidence problems. Does it? I surely don't know, but it does create - over time - a very moist soil under the plastic. (You can test this by laying a yard-square scrap of plastic on the dirt floor and see what happens.) Others recommend various systems of forced, or air-pressure augmented passive ventilation to remove the moisture laden air from the basement before it can rise into house interior and replace it with (theoretically) drier exterior air. This method has the advantage of not monkeying around with existing soil mositure states, but it is complicated, and may take personal attention to adjust during each day and/or electricity to run. If you have just purchased this house, I think you should observe the ebb and flow for a while (at least a calendar year), as you think about this. And keep the issue in mind as you contemplate any further sealing and insulation projects. It sounds as though the dehus are keeping things in check, so nothing (except cost of power and fuss of removing condensate) is lost by taking a go-slow approach. And by all means, test for radon if you have it in your area. (Your local or state health department is a resource on this.) An upside of the necessary ventilation to control radon can be a reduction of basement moisture since the gas and at least some of the air vapor are removed at the same time.

Did your main sewage drain back up and spread sewer water over the floor, or did just a water sump fail? If you had sewage (yours or from the mains), you have a bigger problem, than if it was just water. Assuming it was just water (and even then in an urban area it could be storm water drainage which is not benign), then you should do what you can to maintain constant sump drainage, including provding back-up power for emergencies if the flow warrants that. To clean up from a storm water surge, or sewerage back-up, I would be looking to your local authorities for guidance, and perhaps clean-up assistance. Or your homeowner's insurance if you have flood coverage. If you have to do it alone, I would search for info on cleaning on the web. But I am sure you must first disinfect the soil and floor and any immersed wall sections, then run dehus to remove the air moisture and gradually dry out the floor and walls. For maintenance once the storm's/flood's effects are sorted out a dehu (again with power back-up if it's a serious need) running constantly will help. As will covering the dirt floor with a vapor retardant film (not simple poly sheets), and perhaps pouring some concrete over that. Under floor (and within crawl space) ventilation is a subject with contentious viewpoints. You should read about it, a lot, before signing up with any remediation firm. There is info here, including a long list of resources I posted for someone else. If you do a search on my name "housekeeping" you will find it. The simple solution of just putting a vapor retarder below your floor joists may help, but won't get at the root of the problem which is cleaning up after the last flooding, and then drying out and putting a retarder on the floor to block the vapor at the lowest level. I live in a very old building with dirt floors in most of the cellar so I have some experience with these issues, though thankfully never with urban flood contamination as I live in a very rural area. Of course, I can't tell if your health problems are a result of moisture in your basement. They might be, or not. But it's not good to live in a house with a continuously damp basement, nor is it good for the house, either. It's worth some effort, and expense, to get it dealt with. However, be cautious about publicly labeling your house a "sick building", it could make it impossible to sell later on. HTH, Molly~

I've done it successfully with 2" of closed-cell polyurethane spray foam when there is not a serious water penetration problem and the stonework can be made clean and tight. The code required protection of the foam is not fire-rated; it is only a "thermal barrier" to avoid outgasing and smoke from the foam during a fire and is normally required to be 1/2" gypsum wall board. The use of intumescent paint must be shown to prevent the foam from reaching a certain temperature after 15 minutes of exposure to fire. Don't leave foam plastic insulation exposed; it's a killer. Since you are insulating the basement you obviously intend to use it so you need at least a 3" think concrete slab over a vapor retarder membrane. Insulation under it would be a bonus which is usually only required by code in multi-family dwellings. It's OK to lower the sub-surface as long as the bottom of the foundation wall and central piers are not undermined. If you have water infiltration issues use Worthy's link. The temperature of the earth outside may be increased a few degrees by the unheated uninsulated basement but the only part of the earth that must be protected from freezing is directly below the bottom of the foundation wall but the wall should be deep enough to be protected. Worthy is right about opinions.

Since you are installing a cooling system, it suggests that the temperature without the cooling system is warmer than desired. If this is correct, then the best thing you can do is insulate all the surfaces with the highest R-Value insulation that you can afford. The good news is that the floor will have the coolest temperatures of the six surfaces in your cellar, so if you don't insulate the floor, it will have the least thermal impact on your cellar. By way of example, I used our thermal load worksheet to calculate the thermal load of a 10x10x10 cellar assuming R25 in the walls, R30 in the ceiling. 85F peak temp outside the cellar. and 55F desired temp inside the cellar. If I assume uninsulated concrete in the floor, the thermal load of the cellar is 3005 BTUH. If I assume R25 in the floor, the thermal load declines to 2765 BTUH. If you'd like me to plug in your actual cellar dimensions and/or want to add additional factors (eg ducting, heat source inside the cellar, altitude), let me know and I'll be glad to redo the calculation. I hope this helps!