Here is what I did: - I first tested the concrete floor for moisture evaporation by taping 2ft x 2ft sections of thin plastic (e.g., polyethylene, plastic painters tarp, etc) over the concrete, ensuring a air-tight seal is made around the perimeter of the plastic sheet. Did this in several different areas of the concrete slab. Left the sheets in place for 24 hours or so, and did not observe any moisture evaporation (i.e., non trapped under the plastic). If you have moisture trapped under the plastic, then the following procedure may not be the best solution for your basement. Also, if your basement is prone to flooding, FORGET H/W FLOORS! - After completing/passing the moisture test, painted entire concrete slab with concrete sealer (Behr 1-Part Epoxy Acrylic Concrete & Garage Floor Paint, available at Home Depot). - After sealer has dried for a couple of days, proceeded to lay a foundation of 1 x 4 treated lumber laid out flat (on 4" side) in a 2 x 2 grid that will need to be glued to the floor using ample amounts of construction adhesive. (NOTE: The treated lumber is used just in case any moisture finds its way through the concrete sealer, and the lumber will not rot if this happens. Keep in mind that even if the above moisture test passed, and a sealer was applied, cracks can still appear in the slab over time, and moisture can find its way through these cracks.) The grid will need to accommodate ¾" x 4 x 8 sheets of sub flooring. With the 1 x 4Âs are laid flat, the 2 x 2 grid is stronger than having the standard 16" on-center spacing using for the joist of most residential floor systems. I used the grid layout for additional strength to support items such as a pool table, etc. Be sure to leave at least ¼" gaps (for expansion) between ends of wood sections and around the perimeter of the room and room walls. If your concrete floor is uneven, you will need to either shim the 1 x 4Âs to make them level or use a floor leveling product to make the concrete slab level. I used stacked bricks to hold down boards that were slightly crooked until the constructive adhesive dried (over night). This will help you keep the floor reasonably level, assuming the concrete surface underneath is reasonably level. - Next step is to add your 2nd layer of moisture protection, 6 mil polyethylene sheeting, which can be stapled on top of the 1 x 4 lumber. Ensure you overlap the edges by 4 to 6 inches and that the overlap sections are stapled onto a 1 x 4. - Next step is to add 3/4" sub-flooring. I used the Advantage tongue and groove product sold by Home Depot. This stuff is better than layered plywood and other OSB type products I have seen. It costs a little more, but it simply stands up to moisture better and appears to be stronger than the alternatives. I used 1 ¼ inch decking or galvanized screws to secure the Advantage to the 1 x 4Âs. - On top of the above, I installed Alloc laminated flooring, which is a floating floor that is much better than the stuff sold at Home Depot and Lowes. It is easy to install compared to hardwood floors, and in my opinion is much more durable. Just Google "Alloc" to find a dealer near you. - All of the above may seem extreme, but I feel 100% confident that no moisture is going to make it through the concrete and into the laminated flooring. The floor is very study, does not give, and is quieter than my ¾ inch Brazilian Cherry flooring installed on the main floor above.

High basement humidity. Yes it can do wierd things. Once your get the basement dried out some, don't be surprised if the floors take on a crowned appearence.

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.

In addition, you can use EPS or XPS insulation under the poly; besides reducing the vapour drive, it insulates. Certainly of value in your cold climate. Concrete sealers protect the surface of the floor, but with the exception of crystalline waterproofing, do not impede the moisture drive from below. I used crystalline once on a foundation before platon waterproofing was available in North America.