A vapor barrier protects the warmth and integrity of your home. Below was pasted from the internet. The friend is incorrect in his statement. New homes are very tight due to better barriers and very good sealing windows, but eliminating barriers causes bigger issues. Very tight homes you can leave a window cracked at times. Some newer heating/cooling systems have exchange systems built to pull in fresh air. I personally use poly sheeting as a vapor barrier because it is far superior to faced insulation. Regarding ceilings, check your local building codes, this may vary. But as mentioned below, attics can naturally ventilate as compared with a closed in wall. Through a process called convection, warm air will always move toward colder air, meaning that during the winter the air inside your home is always trying to escape the house and move toward the cold air outside. Warm air also expands, and therefore has the capacity to hold more moisture than cold air -- moisture that is picked up naturally from inside your house. As the air cools, it contracts and loses some of its capacity, so the moisture is given off to cooler surrounding surfaces. The result of all this is that warm, moisture-laden air is constantly on the move from inside to outside. As it leaves the warm interior of the house and enters the walls, attic, and crawl space, it cools off and gives up its moisture, which in turn can saturate the insulation and framing. The more moisture there is in the air and the more differential there is between the inside and outside temperatures, the worse the problem can become -- in the more extreme cases, the insulation is rendered virtually useless and severe rot can take place in structural members. It is the purpose of the vapor barrier to prevent that movement or penetration of moisture from one area to another. Placed between the inside of the house and the insulation, the vapor barrier acts as a physical shield to repel the moisture and keep it from entering enclosed cavities where it can do serious damage. Vapor barriers can be created from a variety of materials. The only requirement is that the material be resistant to moisture, which is indicated by its vapor permeance value, called a "perm" rating. To be an effective vapor barrier, a material needs to have a perm rating of 1.0 or lower. Common vapor barriers used with insulation include plastic sheeting, foil sheeting, and kraft paper that has been treated to achieve the proper perm rating. Vapor Barriers in the Home One of the most important vapor barriers in the home is the one between the wall insulation and the interior side of exterior walls. Because wall cavities are sealed and have no means of allowing accumulated moisture to escape, an effective moisture barrier is crucial in preventing warm, wet air from entering the walls. For wall insulation, foil- or kraft paper-faced insulation is typically used. The insulation is installed between the studs with the vapor barrier facing in toward the house -- the one thing to remember about vapor barriers is that they are always installed between the insulation and heated portion of the house. The "wings" along the edges of the insulation are stapled to the studs to hold the insulation in place -- stapling to the face of the studs instead of the sides creates a more effective vapor barrier. Another alternative for walls -- which is also commonly used in the enclosed cavities of vaulted ceilings where the pressure of warm air trying to escape is even greater -- is to place unfaced insulation into the cavities between framing members and then cover the insulation on the inside with plastic sheeting. The sheeting is stapled to the face of the studs or joists, and overlapped at all seams to prevent moisture penetration. In today's construction, separate vapor barriers are typically not used between the home's interior and a ventilated attic. Most attics are insulated with blown fiberglass, which is relatively porous in comparison to the denser batt insulation used in the walls. The attic also has a continuous flow of air through it that is created by the roof and soffit vents, so moisture from inside the house passes through the insulation and is removed from the attic by the air currents. In areas of high humidity where moisture in the attic could still be a problem, a vapor barrier paint can be applied over the inside ceilings. Another area that typically does not have a vapor barrier is the floor. Most of today's homes are now constructed with subfloors of interlocking tongue-and-groove plywood or other sheet materials -- waterproof resins are used in the making of the sheets, so the subfloor itself becomes an effective vapor barrier that protects the insulation below it. A vapor barrier is used, however over the ground under the house. Typically a layer of 6-mil plastic (1 mil = 1/1000 of an inch) that is laid directly on the dirt floor of the crawl space, the moisture barrier prevents ground moisture from entering the enclosed crawl space and wetting the insulation or the framing.

First, the tuck tape. Look for "sheathing tape". It's usually sold in the box stores, sometimes it's red colored. You could use Tyvek tape as well. The vapor barrier is a bit tougher to diagnose. Your exterior foam, "expanded polystyrene" is usually quite leaky with regards to moisture transmission and is not a strong vapor retarder. "Extruded polystyrene" is a much tighter product, and it could indeed be a vapor barrier, probably a Class II barrier. If the exterior foam is indeed continuous on the walls and well detailed, then it might be best to omit the interior poly and just use the Roxul. Your walls may take in some vapor over time, but the vapor should also transmiss back into the bathroom during drying cycles. The biggest problems occur when you have two barriers and one or both are poorly detailed. Over time moisture gets in but has difficulty getting out. Thus the "sandwich" that you referenced. The "imperfect" in your system is the exterior foam. I'm a proponent of foam, but only if it is the right type and the proper thickness. With your exterior foam probably being a Class II on the exterior side of your wall, your best bet is probably to omit the interior poly and simply use other methods on the interior walls. You can use something like Hydroban in your shower (if the shower is on an exterior wall), it's a waterproof barrier but does allow vapor to transmiss through it. For the remainder of your walls, simply use two coats of latex paint over your interior drywall. Latex paint is classified as a Class III vapor retarder. As such, it'll slow moisture vapor transmission that wants to get into your wall, but if any does, it'll allow your wall cavities to dry out during a drying cycle. It's an imperfect solution for an imperfect situation. I'd be completely confident in my reply were your exterior foam thicker than it is and if I knew it was well-detailed. My concern is the interior side of the foam reaching dew point temperatures in winter and any moisture vapor within the framing cavity condensing on the interior surface of the foam, between the foam and the sheathing. That may not be a problem. But it is a consideration, especially since I tend to go towards the conservative side with an "over the internet diagnosis" such as this one. I'd certainly do more research if you can, you might get better information than what I'm offering. You are addressing moisture generation within the bathroom. That's good. Other stuff: 1) I do sometimes insulate interior walls, Roxul is good for that. 2) A simple yet effective sound deadener behind your washer and dryer is nothing more than a second layer of drywall over the first. 5/8" is better than 1/2" due to the added mass. Beyond that you can go fancier: 2a) sandwich a 1/2" sheet of homasote between the two layers of drywall. 2b) Use RC clips and metal channel between the studs and the drywall. 2c) Use green glue between the two layers of drywall. 2d) If you do research you'll probably see references to mass loaded vinyl, but I usually use that only when doing fully integrated sound deadening room "systems". I think you'd do better (and save money) with just using drywall.

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.

Building scientists who have looked at the long term effectiveness of bubble and foil insulation have been less than impressed whether it is used in floors or on walls. have to have 5" off the foundation - can't afford spray foam...so what do I do? for vapor and insulation? If the floor slit is not filled or overflowing with water, I wonder if it could not be included within the wall--just isolate the bottom plates with XPS and pt wood. Then use a combination of XPS or EPS insulation as tight to the walls as possible, followed by fg batts if necessary to attain the R value you want. Typically, the condensation will occur on the inside foundation wall when water vapour is trapped behind impermeable materials from vapour drive inward in the summer and outward in the winter. That's why foam, which is semi-permeable and unaffected by moisture, is the ideal insulating material. Here is a link that might be useful: Building Science Corp. on Basements