No, you still want the vapor barrier on those two walls. The problem would come if there was already a vapor barrier inside of the insulation, so that the vapor barriers would be back to back. You would STILL install YOUR vapor barrier, but only after cutting slits in the existing one.

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.

Yup, you should put a vapor barrier down on the dirt floor of the crawl space. You might even want to put one down on the floor of the full basement and pour a second layer of concrete there. Some people use poly sheets, but there are better more long-lasting, and less permeable products specifically for VB. The biggest deal about installing a VB is sealing the seams and then sealing it to the inside of the foundation. Since you have a poured foundation, the sealing shouldn't be too hard to. You asked about whether it was a good idea to add VB to an older house. The answer is yes, because nowadays we go to a lot of trouble (in the name of energy savings) to tighten up our houses so moisture vapor that rises from the soil has nowhere to go. That's when you see sticky doors, musty smells etc. It won't hurt the soil or damage the house to install the VB. It's one of those things that yields a good, pretty immediate, payback in improvement in the living atmosphere. You asked about plaster cracks in another post: simple answer, don't caulk (or spray foam) the ones in the walls or ceilings because they really won't stay invisible for long. If you haven't the time to fix them now with joint compound, or the taping/replastering technique you read about, just go ahead and paint over them if you're using latex paint. They will still show, and probably drive you nuts and you will have to repaint after you fix them later, but at least you'll feel as if something got done. Repair of plaster cracks can eat up a lot of time, but it is not hard to do. Some can simply be filled with joint compound and feathered out. Some need to be cleaned out and then refilled. Some will reappear, no matter what. The reappearing ones need to be marked with a straight-edge drawn line and watched as they may indicate some structural issues. It would be fine to use caulk (with backer rods if necessary, in wide cracks) in the cupboard. Another option is to cover the gaps with additonal trim pieces. But before you plan on storing linens figure out why it's smelling dirty and correct that! Spray foam is very hard to make look good it always looks like some bulbous, scary, fungus, even when painted. I have used it, though in desperation. If you search here for vapor barriers/ or vapor retarders you will find a link I posted awhile ago to a manufacturer of a good one. Molly

The perm rating for exterior-grade plywood is what it is despite any physical defects typical in the material. While it has a low perm rating, it's a poor candidate to install under sheetrock as a vapor barrier. In my market, 1/2" CDX plywood costs roughly $.55/SF compared with 6 mil polyethylene sheeting which costs $.05/SF .... @Charles Ross, looking back at the picture I posted the plywood has a perm rating of .70 and the polyethylene sheeting has .06 --- the picture states anything with a rating less than 1 qualifies as a 'vapor barrier'. So the question of which is better I'm not really arguing, just expanding the conversation to show that there are different materials to use as a vapor barrier as well as the location of which to install it (climate concerns). With that said, what is better in terms of the rating regardless of cost or installation problems or concerns? Here in the south they not only put up plywood but then also wrap the house, usually in Tyvek. But the home I live in was built in the late 70's has either asphalt coated paper or some sort of aluminum coated paper. Another important difference is in my area the product is installed on the exterior of the house. I think a wrap would be a harder install on interior walls. I think if it were me I would choose the 'Vapor barrier paint or primer' for homes in the north where vapor barrier should be installed internally. The rating of the 'vapor barrier paint or primer' according to the picture I posted a few clicks above was rated at 0.45 which is almost half of plywood... but much higher than polyethylene at .06. Due to the ease in which the 'vapor barrier paint or primer' can be applied, the higher rating and no interference that the ply wood make due to the thickness of it, 'cracks and crevice' problem I mentioned... But the question is as someone who doesn't do this sort of thing... what is the difference of the rating between these different materials.... short of costs, installation methods or problems with such. Is a rating closer to 1 better than a rating closer to 0? That primarily is the point to everything I posted, it wasn't a venture off into the weeds. (What's the best in terms of the rating?)