One of my concerns about putting a vapor barrier down on the dirt floors in my old house's cellar has to do with the risk of creating a super-saturated zone right at the foot of my dry-laid walls (some rubble, some much more carefully selected and laid, but all w/o more than chinking mortar.) The vapor barrier, in my view, is there to stop the inexorable rising of water vapor from the soil. (It rises due to constantly changing atmospheric pressures that no one can do anything about.) Blocking it at the cellar-floor surface level won't stop it rising, but it could channel it all outward to the edges of the barrier. If the barrier is tightly sealed to walls (as it ought to be) then where does all that accumulated water vapor go? It seems to me it will create a lot of sogginess in the soil at a very bad place: where the foundation of my house rests on what I assume was undisturbed soil at the time of construction c 1845-ish. I think it's important to differentiate between free water that comes in during periods of high soil water or the result of poor local drainage from roofs, flooding, etc, and moisture that comes from rising soil moisture vapor. The free water part may be containable, managable, or mitigated, but soil moisture vapor is probably not something you can stop. Moisture arising from the soil in very old houses may be a natural phenomenon that we have lately defined as a problem, or that has become problematic because of material changes we have made to our buildings by installing central heat, insulation, double-glazing and even running water. It may also be that our expectations have changed to cause something previously deemed "normal" to be seen as a problem in need of action to eliminate. Before there was electricity and air-conditioning technology I doubt that people were much pre-occupied with removal of air-borne humidity by dehumidification. But the fact that we can do it, may now make us feel that we have to do it. I also disagree with the notion that keeping cellar rooms closed up during the warm months is the best course of action. I do see the logic that keeping warm, summer, moisture-laden air away from the soil-cool stones of a foundation avoids condensation on the walls. But I am less concerned with condensation coming from warm-air atmospheric moisture, than with venting soil moisture vapor to the outside of my building envelope. Old people around here who lived in old houses like mine have told me of the careful attention they paid to opening up the cellar and airing it out after the winter. Since most of them lived and worked on the farms they were at home to attend to this and paid close attention to the direction of the wind so to maximise the flow-through pathways by opening different windows, doors, etc. I practise this as much as possible and it results in a dryer cellar, even though I still get some condensation on the walls, especially in the early Spring. In the years when I can pay the most attention to active cellar ventilation, my cellar - and consequently the rest of my first floor - stay the driest. But it often takes multiple changes during the course of a day, and I have to have my whole outdoor cellar stairway open all the time (during the day, it's generally closed at night to avoid dew-laden night air), which many people couldn't or wouldn't do out of fear of intruders. My cellar walls show evidence of multiple layers of white-wash (quick-lime and water). This is an effective preventive, curative and aesthetic disguise for dampish stones. It does nothing to reduce humidity in the air (adds to it for a few days as it dries off), but it makes the cellar seem less grungy. I struggle with this issue, a good deal, because I think a lot of the best-advice from old house "experts" may not be the best for the building's long-term survivabality. Info dump: My house is located in northern, NY. Foundation as described above, otherwise timberframed, two-story, wooden structure with its original plaster walls intact. Wall cavities are mostly filled with two separate layers of backplastering on wooden lath. Two layers of 5/4" X 10" T&G sheathing with clapboards nailed on top. Slate roof. Still no central heat, though now only two modern, more-effective stoves rather the original 4-6. Electrified (but not in every room) just before WW II; minimal indoor plumbing installed in 1960. In short, a oddly long-term survival of 19th technology, which I am slow to alter. If you're going to put vapor barrier down, and going to the trouble of sealing it properly, you should use material specifically intended for the purpose, not construction poly. I haven't solved this problem to my satisfaction, yet. Lately, I have been thinking that I should undertake a more active course of observation (recording indoor and outdoor temps, humidity and wind direction and speed) along with a program of trial openings and closing to gather better information to form a more thought-out moisture vapor reduction program. (I expect this was all empirically known a hundred years ago, but even old timers now have forgotten the details, alas.) This summer has been dryer than normal so perhaps this would be a propitious time to start. There is a guy, an engineer, (located in eastern MA, I think) who owns an old house and he has basically sealed up his entire cellar walls and floors from within. I'm drawing a blank on his name, but a frequent poster, Worthy, often posts links to his work so if you search for Worthy's posts on this forum you should turn up the link. I have read this guy's writing on his website, but remain unconvinced that it's what I should do with my old building. I don't think he's a crank, or anything - and he may be right - but what he does is essentially non-reversible. That irreversability is what's stopping me. L.

Use a Class II vapour barrier. ( See link.) Since you have the wall exposed, pay special attention to air leaks. Use single component foam around any windows. (Great Stuff etc.) Here is a link that might be useful: Vapor Control Layer Recommendations

Moisture is generally driven into wall systems by moving from the warm side of the wall to the cooler side of the wall. In northern or heating climates, vapor barriers are put on the interior side of the wall, normally right behind the drywall. The goal is to prevent interior moisture vapor from entering into the stud cavity and in cold weather, condensing within the wall. In northern climes housewraps, such as Tyvek, which are designed to shed liquid moisture but allow moisture vapor to pass through, are commonly installed on the exterior of the wall, OVER the sheathing, but UNDER the siding. Tyvek allows moisture tha has gotten into the wall to dry to the outside, yet it prevents liquid moisture (wind-driven rain, solar driven moisture) that gets behind the siding from getting into the wall system. In southern or cooling climates, the vapor barrier is normally put on the exterior side of the house. This prevents outside warm humid air from being driven into the wall system toward the interior cooler air conditioned space. WEre humid air allowed into the wall system, as it cooled due to the cool indoor air conditioned air, moisture vapor could condense within the wall's stud cavities. Or something like that! In Florida, when plywood sheathing (or something like T111 siding which it sounds like you are using) is being used on the outside of the wall, an additional vapor retarder/barrier is generally not required. Some southern builders will still use Tyvek/Typar or some other type of housewrap between the framing and the siding, but remember, these housewraps are NOT vapor barriers. They are vapor permeable, but will still shed water droplets. Mongo

Ding! We have a winner. Just make sure you purchase the moisture resistant seam tape (aka Red Duct tape) to tape the seams together. They come in rolls of 75 feet. That means you will have to do a bit of measuring (diagrams are REALLY HELP FULL) before you purchase the duct tape. The seams will need to be overlapped by 6" - 8" and then taped the entire length of the seam. That means you have to have a decent understanding of how many runs of sheeting you will need for the rooms you are working with...and how many LINEAR feet you will need for the tape. I know it shows tape on the edges...I don't trust them. That's the reason why this item is MUCH more expensive than the basic stuff WITHOUT the tape. But sometimes we can't be choosy if we only have a few shops to choose from. You are on the right track.