The most expensive but best insulating process is closed cell spray foam. For a large space, with ten foot ceilings it might be no more costly than board insulation. XPS is more labour intensive. For NYC, I'd use t&g 4'x8' x1" boards; if you can't find those, then use 2'x8'x1" t&g. Seal all the joints with building tape--Tuck and Tyvek and two popular brands. I've not used sealers, but a building writer whose information I've always found to be sound, swears by Drylok. (I assume this is not a rubble or brick foundation.) The advantage of steel framing is it's neater to put up (no sawdust) and there are no subsequent nail or screw pops. However, as it's sensitive to moisture, I'd have the baseplate resting on strips of XPS not only to create a thermal break but to keep it away from any minor floods that may occur. Since one inch of XPS is R 5, I always add high density fg batts, adding another R 15. No vapour barrier is necessary, though if this is a permit job, the inspector may insist on one. It wont' hurt anything. (See Building Science links in my first post.) After two years, a number of tiles cracked at pressure points on a brand new home I built. And that's with the tile on top of at least 10" of reinforced concrete and power tamped gravel. Since then, I figure it's a lot cheaper in the long run to use an uncoupling substrate such as Ditra. (If you dont', be sure to keep a lot of extra tiles just in case.) Ditra and cement add maybe 3/8" extra to the flooring height. NYC labour. My condolences!

I just realized that we will be installing a preformed niche or recessed shelf in one of the shower walls. Won't that compromise the vapor barrier? Is there a way to handle that. I did install the poly in horizontal strips but I'm not sure I can easily place another strip behind the shelf and won't that inhibit water from traveling down as it will run into the shelf? What about screwing grab bars, etc, into the tile, won't those also compromise the barrier? I was kind of wondering how the staples and hardie backer screws would affect the barrier too since they puncture the plastic as well, we had a hard time keeping the staples from tearing when it was a big sheet, that's why we went to strips and overlapped them. After reading some more posts, it sounds like I do not need to install a water proofing barrier since I have the vapor barrier in behind the board. It also sounds like I need to use a penetrating grout sealer, not a topical on walls and floors and that Aqua Mix will work AND is low VOC. So now my questions are: 1) how to insert a wall shelf with vapor barrier and hardie already in 2) can items be screwed through the wall into the barrier without causing problems? 3) is it ok to use a steam mop on the sealed caulk floors? 4) is Aqua Mix Seal and Finish the same thing as the Gold version? 5) is the Aqua stuff for sealing both wall and floor grout on both ceramic and slate floors? 6) do I need ANYTHING else to seal or finish the walls or floors? Thank you so much!!!! Now I have to go paint and install wood floors or we will never be done with this. kj

Everything always has a bit of natural "drying" ability. Spill a little water on your floor and it'll evaporate. Set up a slow drip and if the rate of accumulation outpaces the rate of evaporation you'll end up with water damage. Same with moisture in house walls. Moisture vapor can be driven into wall assemblies, mostly via air movement. Some insulations are better at restricting air movement than others. Foam and dense-packed cellulose are great air infiltration barriers, fiberglass is not so good. When moisture vapor does get into walls, some wall assemblies can "hold" the moisture better than others. Cellulose can hold moisture fairly well, fiberglass insulation not so well. Same with liquid moisture. Liquid moisture can get into walls via a leak in the plumbing, cracked grout, a roof leak, etc. When moisture gets into an assembly, it needs a way to escape. It usually escapes via the same path it entered. Moisture is accumulated during a wetting cycle, and given up during a drying cycle. If the wetting exceeds the drying, then over time you get water damage. Rot, mold, etc. Building science generally recognizes the idea of wetting and drying cycles. It recognizes that some materials hold moisture better than others. It recognizes that moisture may get into assemblies. Because of that, we also want to provide a back door through which it can escape in case it can't exit through the way which it entered. That's why building code prohibits two vapor barriers in a wall assembly. Moisture that gets in could be trapped between the two barriers, creating what's referred to as a "moisture sandwich." There's a way in but not enough of a way out. I certainly understand there are all sorts of "but what if..." and "but how could it..." on this subject. But that's the short long answer. The long long answer is really long...and evolving. Just like building science.

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.