1. Is there a special kind of non-rusting staple that should be used? No need, regular staples are fine.

2. Can I simply staple the poly across the top and then let the board screws secure the rest of the poly or should the whole sheet be stapled? If staples are recommended, shouldn't each staple be sealed to prevent water/vapor flow? The staples really aren't a factor in terms of affecting the poly sheet permeability. There's not a vapor drive in a regular shower like you get in an enclosed steam shower with a true steam generator. So hang the poly, slap a few staples to hold it in place, then cover it with the cement board.

3. Once the hardibacker is in place, should the screws also be sealed? If not, why not? (The Hardibacker instructions only say to tape and seal the seams.) If the screws should be sealed, is thinset enough, or use some other type of sealant? Screws don't need to be sealed. The vapor barrier is behind the face of the hardie so there's no need to seal anything on the face of the board like the screw holes. Any divot where the screw head is will get filled with thinset when you tile.

4. Should the poly be sealed across the tops and sides with some sort of sealant? If not, why not (wouldn�t the water/steam simply go around these unsealed ends? No need. Again, it's not a steam shower where all six sides of the steam shower cube need a tight and integral vapor barrier. We're really just worried about water on the walls getting through the tile/grout, be it through penetration/absorption, through pinholes in the grout, cracks in the grout or tile, etc. In a shower surround I usually hang one sheet of 6-mil plastic and wrap it around the walls, from one wall to the next. Leave it a little slack in the corners so it doesn't get stressed/stretched when you hang the tile backer board.

5. For the area where the poly overlaps the tub flange, again I assume a sealant should be applied between the tub flange and the back surface of the poly. Yes, you can run a bead of sealant or caulk on the flange then bed the poly into the sealant/caulk. That'll hold it secure to the flange. Then cut the excess off with a utility knife.

6. What about the plumbing pipes? Will a good bead of sealant all around be enough, or is there some technique for this area? No worries, again, this isn't a steam shower where you have true vapor drive. The shower arm hole will be covered with an escutcheon plate and it'll be above the splash zone. The trim plate that covers the shower valve usually has a gasket on the back of it, when you install it it seals the trim cover tight to the tile so no water can get through the valve trim and into the wall.

7. EXACTLY what type of sealant should I use? I've seen references to using asphalt tar, silicone caulk, acoustic sealant, Kerdifix�.? Can acoustic sealant be bought at HomeDepot/Lowes/Menards? I usually use a urethane sealant when I'm looking for a sealant. For caulk, usually a silicon-latex will work fine.

8. Should you use the same type of sealant on the poly/tub flange area as you would on the other edges of the poly? Really no need. Just seal the poly to the tub flange to help hold it in place.

9. Here is my major puzzlement: I keep reading that a bead of caulk should be run between the installed poly and the cement board at the bottom of the tub flange. Wouldn't this trap any moisture between the board and the poly?? How can the water run back into the tub if there is a line of caulk blocking the path? On the other hand, if it is left unsealed, how to you prevent water from the shower area getting on the end of the board and wicking up the board? (Or is it irrelevant that the board gets wet because of the poly barrier?) It's a confusing area. You can seal the poly to the tub flange, it's not necessarily a "seal", it's more just tacking it in place so it doesn't slide around. That's really the only thing you have to do at the tub flange, is make sure that the poly is overlapping the tub. After tiling, a lot of folk will caulk the tile to the tub. Some caulk the entire line, some leave a few areas open as "weep holes", some don't caulk it at all.

Whatever moisture gets through the tile and grout then hits the cement board. The poly prevents the insulation and framing from wicking that moisture out of the cement board and into the framing cavity.

What usually happens is that during a non-showering drying cycle, the moisture that got through the tile and grout leaves the wall through the same path it entered...through the grout, the tile, the cracks and crevices, etc, and goes back into free air within the bathroom.

If you do get excessive wetting in the cement board, then yes, liquid droplets can form on the poly. They'll run down the poly, or down through the cement board, and theoretically drain back in to the tub over the tub flange.

Sealing the poly to the cement board deosn't do any good that I see. WEre you to ever get excess amounts of water in there, moisture down there will just wet the cement board and excess moisture will then just drip out the bottom edge of the cement board.

That's why some folk leave weep holes or gaps in the caulk if they caulk the tub-to-tile line, or why some installers don't caulk it at all.

Still, most people want the tile caulked to the tub, and that's fine, I have no reservations about doing that at all.

Some people envision water building up back there...realize that if there were excessive water, it would saturate the cement board, then the grout would appear wet, and the grout would dry with the moisture going back into the bathroom air.

It sounds a lot more serious than it all is. You need a seriously compromised wall to get water collection on the back side of a tiled wall.

10. What about the ceiling � My plan was to leave the existing drywall/greenboard up there (I haven't started demo yet, so I don't know exactly what it is.) Does this need to come down to studs + have poly barrier there also? If not, why not? Wouldn't steam simply absorb into the ceiling and go through to the studs? Again, it's just a shower, not a steam room. If it's in sound condition you can leave the drywall up there and just paint it. The paint film itself can act as a vapor retarder.

11. As for the plastic itself, as long as it is 6 mil, does it matter what "kind" it is? Home Depot only had one type, and it just says "plastic" on the label, which no indication of if it is poly or not. Do I need to find something particular? 6-mil polyethylene, or "PE" sheet plastic. They normally sell 4-mil and 6-mil, use the 6-mil, and it's usualy PE.

12. How far past the edge of the tub/tile area should the poly and hardiboard extend? It usually stops at the edge of the tub under the last column of wall tile. The wall tile hides the transition seam between cement board and drywall.

13. Should I use alkali-resistant tape and thinset at the seam where the hardiboard meets the existing drywall, or regular fiberglass tape and drywall compound? I use paper tape and joint compound with drywall. I use alkali resistant mesh tape and thinset on cement board. Since the transition line will be covered with tile, I use mesh tape and thinset at the cement board/drywall transition seam.

One extra tidbit: If you thinset and tape the seams between the cement board panels on one day and then tile the next, one big mistake people do is they leave too much thinset buildup on the seams, you end up with humps. The seam should be flat. It might even help to tape and thinset the seams as you're tiling the wall.

Mongo, thanks so much for the detailed and quick response!

After much delay, I finally got to the tear down of the tub/shower area and found something unexpected, which has now caused me to question what to do. There was a double layer of drywall on the wall that runs on the long side of the tub (the wall that separates my townhome from my neighbors). Here are the layers going from inside to outside:

a) the inside layer of drywall had the tile & mortar attached and ran from ceiling to tub flange.

b) the one behind it (attached to the studs) was x-type fire board and ran ceiling to floor even BEHIND the tub.

c) the next was studs and faced 3-1/2 inch pink fiberglass. (I have removed the facing so as not to have a double vapor barrier sandwich)

d) the last was more drywall (the outside wall). I know from when they constructed the place I saw that behind this layer is a couple inches of dead space to separate the units. The only places our units touch are the roof and the outside siding (supposedly).

On my original plan as discussed above, I was going to attach poly directly to the studs and the tub flange, then hardiebacker, then tile. But I can't do that if the out layer of drywall runs behind the tub. I'm now leaning towards using Kerdi instead of the poly to avoid the issue.

So here are my new questions:

1) Since this is a townhome, I'm not sure if the purpose of the double layer was firewall or sound proofing or both. I don't know what code for townhome construction is, but I want to duplicate what was previously there. My fear would be that if I simply put up poly and hardi and skipped the outside layer, and later there was ever a fire, I wouldn't be covered by insurance because they would say that I removed the firewall.

However, I'm not sure that the purpose of the outer layer really is a firewall because it was NOT touching the ceiling or the floor (about 1/2 in gap on bottom and 3/4 gap at ceiling), and it was not taped or attached in any way to the abutting pieces of drywall that go into the adjacent rooms.

So to replace this layer, I will get x-type drywall and run it floor-to-ceiling.

1a) should I keep the original gaps in the drywall layer, or butt them to all edges, or seal with the fireproof red caulk?

1b) which means if I put poly behind it, there is no way to attach the poly to the tub flange and any water that gets through the hardi/tile layer will go directly into the drywall, so that seems to make no sense to do. But if I put the poly BETWEEN the outer drywall layer and hardi (so it would be from outside-to inside in this order: outerdrywall/studs+insulation/x-type drywall floor to ceiling/poly/hardi/thinset/tile), then does this create moisture issues with the drywall layers under the plastic and condensation on the poly since this is kind of an outside wall in Chicago climate? I would want unfaced fiberglass, right?

1c) if I use Kerdi instead, and I attach the drywall + hardi to the studs, I should still use unfaced fiberglass? (so it would be from outside-to inside in this order: outer drywall/studs+unfaced insulation/x-type drywall floor to ceiling/hardi/thinset/kerdi/thinset/tile)

1d) for the x-type drywall that goes behind the tub (assuming the poly goes between this layer and the hardi, or I use kerdi), what if anything should I do about the area directly behind the tub (the 15 inches from floor to tub flange)? There would not be any vapor barrier at all in this area. When I removed that portion, the old stuff (11 yr old) was a bit moldy, but not too bad. Should I use faced fiberglass only for this portion? Should I put a second piece of poly that covers only that portion? Any other suggestions?

2) For the Hardi, I was planning to use the 1/4 inch thickness with 1/4 inch furring strips on studs to bring it level with the existing drywall, but I have read conflicting information on whether 1/4 hardi can be used on walls due to support + flexing issues on 16 in wide studs. The hardi instructions say it should be screwed every 8 inches.

2a) For the wall discussed above, since there would be additional support provided by the drywall behind it, I assume 1/4 thick is ok.

2b) For the other walls where it goes direxctly onthe studs, do I need to add additional studs/supports if I use the 1/4 in? What if I use 1/2 in?

2c) if I use the "1/2 in" hardi (which is really only 3/8 thick), then it doesn't match up with the drywall by 1/8 in. I have read about other people using 1/8 peg board cut into furring strips to even this out. Is it wise to put that type of material in the wall cavity in a constantly humid area? How is this unevenness usually compensated for? Would extra joint compound make an obvious transition?

3) On tub/showers where the tile does not go to ceiling, do you normally use hardi only for the tiled portion and green board for the untiled portion, or is it ok to use backer board all the way to the ceiling (my preference)?

3a)If I use POLY as my barrier:

3ai) If I choose to use hardi all the way up to ceiling, is it ok to skim coat the untiled area with my joint compound, sand and paint?

3aii) Does the hardi need any primer? Is there anything else to consider if I want to do this?

3aiii) Also, will I have problems matching the paint texture on the skimcoated hardi to that on the drywall?

3b) If I use KERDI:

3bi) If I choose to use hardi all the way up to ceiling, would I only use Kerdi up to the tile line or also up to the ceiling?

3bii) If kerdi goes all the way up, can I skim coat the untiled Kerdi and then paint?

3c) From an aesthetics point, for tub/shower combos, how often do you see tile all the way to ceilings on your projects? Would this look odd since it is not a walk-in shower?

I'm afraid to proceed because I don't want to screw this up. Thanks so much for your advice.

The two layers of drywall between your unit and the unit next door is to slow the spread of fire from one unit to the next. i think it is interesting that you found drywall behind the tub because this is in a way thats the way that i install. I hang green board down on to the top of the tub flange. then before i install the hardibacker board i use vicor as a waterproofing material. it is a ruberized material which seals any penitrations so that none of the screws that hold the backer board to the walls can allow any water to pass. vicor is not cheep. this product can cost you between 60 to 100 dollars to install but if it is your home or your reputation for doing work that will last a life time then it is worth it. I am a carpenter and a builder and i have ripped out many a bathroom and have seen many different kinds of water damage and many types of attempts to prevent water from reaching into the structure to cause damage and i truly believe that if all the penetrations through to the frame are sealed you are not going to see the rot and mold and the dryrot damage. grout is porous and therefor is not waterproof. motor and thin set do not hold out water. yes water that has had the chance to wick behind the tile due to the capillary effect will be drawn back into the air space once the moisture level in that room has decreased, water that has made it all the way to frame it will soak into the wood. once water has established a pathway all the way to the frame it will continue to do so and it will get worse. the vicor prevents any water from following any penetrations into the frame. Working with this material is not easy, once it touches itself it is all over for that piece. also the wax paper that is the backing for vicor left to pile up on the floor creates a very dangerous slip hazard. I use the 9" wide roll and I measure and cut pieces ahead of time, only roll back 3 or 4" of backing then i start on one side attaching the material to the wall then as i proceed i pull more of the paper off and make sure i follow level lines which i have already drawn on the green board. try to smooth from the middle out and do my best to keep out wrinkles. good luck! ive gotten quite good at this now that ive done maybe 20 tubs to be tiled over the years. the contractor who i developed this process and technique with says this is how he is able to sleep at night. in other words he knows his customers are getting the very best protection to prevent the kind of mess which we pulled out!