tiling over cement backerboard will prove to be impervious,

Neither tile or backerboard are impervious to water. Water can and will get past the tile, grout and backerboard. If you don't use a surface applied waterproofing material such as Redguard or Kerdi, you will need to have a vapor barrier behind the cement board.

Tile and cement board, as long as there's a vapor barrier behind the cement board, will SHED the water sufficiently to be able to last a long long time. That's actually how MOST showers are constructed today, and have been constructed for years now. What Kerdi brings to the table is that it allows no moisture at all to get past the thinset. Same in the base of the shower. there is no way at all, so long as it's constructed properly, that water will build up in the shower pan, or that screws and nails will rust out, or any of that.

The only time I'd say that the Kerdi (or some other surface applied membrane) is a necessity is if it's a steam shower. THen you need a completely watertight envelope. But other than that, the Kerdi is an upgrade, but not a necessity.

I don't know of anyone cutting corners or anyone who does shoddy work that then goes and spends money on Kerdi.

The only people I know that use Kerdi are actually the higher-end guys.

There's absolutely nothing wrong with a non-topical membrane shower. The only thing I'll caution is that the biggest sin I see these days is people still putting the floor membrane flat on the subfloor and then installing the deck mud preslope on top of the flat membrane. That without a doubt is not only a code violation, but it bad bad bad construction, and it's probably the number one killer of showers these days.

Kerdi versus redgard, I prefer Kerdi. With Redgard I worry about mil thickness, I worry about pin holes...

With Kerdi I have no worries.

As far as a DIY book not having Kerdi or other topical membrane instructions, that's no surprise. They are usually years behind the times.

Here's a fairly detailed instructional thread if you need ideas.

Is Kerdi or Redgard overkill? I'll break it down into a couple of areas:

1) In new construction, thermal envelopes are much tighter today than they were 20 years ago. We have vapor barriers that are integral to the entire house, not just stud bay to stud bay. We have house wraps to help. We have windows and doors that are foamd in instead of gaps stuffed with fiberglass. We have cellulose and foam insulations instead o ffiberglass. Because air changes per hour have been drastically reduced, we have heat recovery ventilators to bring fresh air in the house while conserving BTUs. We soimply have tighter houses.

As houses get tighter, moisture control becomes more important. That's why controlling moisture at the source is more critical. Kitchen vents, bath vents, minimizing moisture driven into and through the wall assemblies. Ths is where topical membranes like Kerdi and Redgard excel.

2) Older construction? Envelopes are leaky. fiberglass insulation that allows air to blow right through. The wind blows outside and you feel the draft around windows and electrical boxes. The thermal envelopes are so leaky that there are significant air changes per hour through the house. This allows any moisture that is created in teh house, or that gets into the wall assemblies, to zip on out. All that air movement, while resulting in high cooling loads int eh summer and high heating loads in the winter, prevents moisture from doing any damage to the house.

So if moisture were to get into a wall assembly, no worries, because depending on the season, it'll dry to either the interior or to the exterior.

So in an older leaky home, moisture control isn't necessarily as critical as it would be in a new tight home, so in an older home a Kerdi shower might be "overkill".

Ditto.

I can't recall anyone ever mentioning heat recovery ventilators (HRV's) in the same breath as membraned showers. Congratulations mongo. You were informative, precise, clear, and you had style. This is the best post ever, on this subject.

Hi Folks - thanks for your thorough answers. Mongo - it was particularly interesting to read about the difference between older homes and new construction. My home was built in the 60's and it has been pretty drafty.

We replaced the fan (which vents to the outside) in the unrenovated bathroom and I've noticed that steam and condensation is taken out pretty well. I would guess then that if we go with a vapor barrier behind the CBU, we would not necessarily need Kerdi based on all of your responses.

Thanks again!

I've done CBU and "mud" showers for years and, properly done, with the appropriate vapour barrier, they are fine. The Schluter Kerdi technique is cutting edge, state-of-the-art as far as a totally waterproof shower goes. One big advantage is that it does dry out far faster as no water ever has the chance to penetrate the grout to the back substrate. Being able to do away with CBU, a preslope, a liner and a vapor barrier offsets the additional cost of the Kerdi and the proprietary drain. Unless my back is to the wall, it's the only kind of shower I do. In addition, I guarantee the Kerdi shower for life. The traditional showers, I guarantee for ten years.

Yes that is well put.

One question I do have is the two part shower drain used in the mud pans is excepted in the Kirdi system? Or does the Kirdi drain have to used?

Schluter /Kerdi Systems use their own proprietary bonded flange drain assemblies. The Kerdi fleece bonds directly to the flange and requires no pathway for weep holes. (and the drain grate is heavy duty and looks better than most...)

I have used both Kerdi and Red Guard in years gone by. Today I use neither and prefer cemenitoius waterproofing like Ardex 8+9 over a liquid and if using a sheet membrane prefer the likes of Laticrete Hydro Ban Sheet Membrane or Noble Company's Noble Seal CIS or TS.

When working with a clamping drain a membrane of 30 to 40 mils thick is required s0 for these installs I would only use Noble Company's NobleSeal CIS or TS.