Personally, my advice to you is not to do this without the permits and inspections that are required by your local building department.

-Roger

Assuming your walls are insulated, you will degrade the R-value of any insulation by retrofit. A 2x4 is a small space.
Not sure why your contractors are telling you to avoid authorities. That leads to questioning credibility or competence.
Have you considered surface run piping? There are aesthetically pleasing options and you avoid as much thermal loss as well as complications penetrating the building envelope.

Legally speaking, there is nothing in the codes that would prevent you from running pipes in an exterior wall.

Technically speaking, in your climate the overall risk of a pipe freezing is slim to none.

Now, having said that there is another point that must be considered; "Murphy's Law", which dictates that wherever something can go wrong, it will go wrong. By example, I will personally match my soldering skills with anyone in the industry, and in some critical situations I have been known to use heavy wall copper and braze the joints but even with those precautions if I were to tell you that I have never had a leak I would be lying to you. Taking this to the next level, it doesn't matter how good the quality of components you select, there is still the off chance that one of the individual pieces you select could have an unforeseen internal defect, which results in a leak. These factors can be especially problematic when you have a shower in an outside wall. In that one in a million chances that a problem occurs if the piping and fixtures are on an inside wall we can generally open the wall from the opposite side to make a repair, however when the pipes and fittings are in an outside wall we are then forced to either open the wall from the shower side, which often means opening a custom tile job or a one piece shower surround, or trying to gain access through the exterior siding of the house, and with my luck, that would probably be a custom brick finish. In any case, just gaining access to the repair can prove to be very costly.

Now in regards to you plumbers reluctance to pull a permit. In the post you stated that you were "Not Super Comfortable" with the idea. In truth you should amend that statement to say that you are "Super Uncomfortable with that idea."

Let us be brutally honest here. In most instances the average homeowner doesn't have a clue as to the code or mechanical technicalities of construction, and for as much as I hate to have to admit it, not all contractors are honorable men/women, but then I guess we could say that about any profession. The codes merely present a set of minimal standards to be expected, but if your not familiar with the codes, how would you know if the contractor fulfills even the minimal standards? Pulling a permit and getting the required follow up inspections merely levels the playing field. On the one hand the inspector is the homeowners advocate to insure the job is done correctly while on the other hand the inspector protects the contractor from unreasonable expectations on the part of the homeowner.

Any time a contractor suggests bypassing the permit and inspection phase of a project, the mere suggestion should raise a red flag. Would you consider buying or selling the house without consulting an attorney? I think not. Then why in heavens name would you consider building a home or making a substantial improvement in an existing home without having the project supervised by a non-biased third party?

Now some will argue that permits and inspections are expensive and they can have the job inspected by an independent "house inspector" cheaper, but keep in mind that the official code inspector has the legal authority to demand the work be done correctly or stop the project, whereas a house inspector can provide little more than a trained second opinion when the problem ends up in court.

Good answer Pup.

In my city - Indianapolis - they have cut the code enforcement department to the bone. I've never seen an inspector, not once. I've seen entire residential structures built without a single physical inspection. Contractors call, they wait the required time, and move on. On smaller jobs like this one nobody would pull a permit. It really is a shame because there is so much really poor work being done.

I recently saw some very expensive five-year-old condos (within sight of the city building inspector offices)where the outside was being rebuilt because of water damage. Among other things, there was no Tyvek (or other building paper) under the siding to protect the OSB. The lumber under the open decks wasn't treated, there wasn't drainage for the semi-enclosed decks, etc. The entire outside was rotten.

Hi again,

Thanks all for the responses! I've seen some really good points, particularly the agreement that I should be actively uncomfortable that it's been suggested not to get a permit and inspection. Good to know I'm not smoking something there. I wasn't sure what to think since multiple plumbers suggested not to do it.

FWIW, we're actually in the City of Austin. My wife and I have never done any sort of remodeling other than painting, tile, and trim before so I can't speak as to whether Austin is staffed well enough to send out an inspector, but I suspect it is.

Regarding access to fix a leak (thanks lazypup!), you are correct that we'd be looking at destroying a custom tile job to fix anything if we had to access from inside the shower. Perhaps it's good that the outside wall has siding which should be "easier" to get into and fix back up? What concerns me though is that we are planning to use Kerdi and thus we likely wouldn't even notice a leak for quite awhile since the waterflow would most likely be heading outside (this is a first floor bathroom/shower.) Hmmm. Would leaving the pipes pressurized with water for a long period of time (a couple weeks?) prior to walling things off be sufficient testing to ensure no leaks? Or would it take running water to do a test? Or does it not matter, as the issue is really failure of a joint over time?

I'm not sure I understand alphonse's point about degrading R-value. Is the point that having pipes in the cavity at all will reduce the space for insulation? Or was the concern that inserting the pipes would move existing insulation quite a bit? If the latter, I don't think it really applies since we'll be opening the walls to the studs and thus have full access to restore insulation. If the former, I'd love any suggestions on how to best re-insulate given that I will have full access to the cavities between the studs, floor to ceiling. Open-cell sprayed foam? Pipe-wrap insulation embedded in fiberglass or cellulose? Or skip the pipe insulation and just go with wall insulation materials? Or something else entirely?

"I'm not sure I understand alphonse's point about degrading R-value."
I'm not a fan of having piping in exterior walls. In new construction, such a thing compromises energy conservation whether in a heating or cooling climate, but detailing can be attended to easily.
In a retrofit, the detailing is up to the installer, and they don't have your best interests as goal #1. As some of the prior posts point out, the uneducated homeowner won't know what's going on. Integrity of the insulating system could be severely impaired. In a heating climate, vapor barrier degrade will lead to rot which often won't be seen until too late.
Were it my home, there would be continuous piping in the stud bay, no joints of any kind. Even a perfect install needs account for repair(s) regardless of pre-commission pressure test.
I would not want to go through the exterior siding as access, but that's preferable to going through tile. Perhaps an exterior access box could be designed, with appropriate weather detailing.

"Were it my home, there would be continuous piping in the stud bay, no joints of any kind. Even a perfect install needs account for repair(s) regardless of pre-commission pressure test."

The failure rate of correctly installed pipe (especially copper) is extremely small.

Over many years millions of joints have been made (even by non-professionals) without a failure rate that remains incredibly low.

"The failure rate of correctly installed pipe (especially copper) is extremely small. "

Acidic water trumps plumbing skill in about 15 years. Murphy still holds sway.

"Acidic water trumps plumbing skill in about 15 years. Murphy still holds sway."

And water that has been cleaned up excessively causes problems also.

That said, that is not a problem relating to joints is it?

You said "Were it my home, there would be continuous piping in the stud bay, no joints of any kind."

And then point to a problem unrelated to joints.

That's right, Brickeyee, were it my home.
Copper is not the only piping material and a sweated joint is not the only method of connection. IME, joints are often a source of trouble. YMMV.

I would have no concerns about pipes in the walls freezing...but I would be careful about what happens in the attic. I would make sure that any run through the attic was under the insulation so it was picking up heat from the interior. I live in a very mild climate where it rarely freezes, but during one cold snap I couldn't get hot water in the kitchen. When I traced the line I found that all of the copper supply that came through the attic was under the insulation except for a couple of feet that were sticking up where the flexible copper line turned to go down into the wall. Fortunately I was able to melt the ice plug in the copper with a hair dryer before the piping cracked. And then I immediately cut and resoldered the line so it was completely buried under the insulation.

You said "Were it my home, there would be continuous piping in the stud bay, no joints of any kind."

Interesting point, but one the other hand, the purpose of placing he lines in the outside wall is to supply water to a shower mixer that is installed in that wall. Regardless of what piping material the customer chooses you still have joints where the supply lines attach to the mixer. If it were not for the mixer in that wall, there would really be no necessity to run any water lines through that wall.

There is another point that none of us have mentioned. Generally, although not always true, the outside wall is the back wall of a shower. Code prohibits installing a shower where the shower head points toward the door of the shower, so in most instances we would have no need to install the mixer in the outside wall.

I would agree that in some regions they have a lot of problems with copper as a result of acidic water, but here again, that is not common to all location and even if the water quality is an issue, if the structure is supplied from a municipal water supply federal mandates require the municipal water supplier to treat the water, therefore the problem is generally only prevalent on private well water supplies.

Far too often the problem of pinholes in copper pipe is not a fault of the water, but rather, it is a fault of the installer because they installed undersized pipes. If the pipe is undersized the velocity of flow through that pipe exceeds the code maximum of 8ft/sec and the pinholes are actually a result of pipe wall erosion caused by excessive velocity. This is especially problamatic in DIY renovations where they upgrade the showers to multiple shower heads, but do not increase the size of the pipes appropriately to handle the additional load.

Many will argue that copper only has a 15 to 20yr lifespan but in my region nearly all the homes that were constructed in the late 40's and early 50's were constructed with type M thinwall copper and 50/50 tin/lead solder, yet we rarely see any problems with that pipe other than the occassional frozen water line or a break that results from some form of mechanical stress. On the other hand, currently the trend is to use Medium wall copper and 95/5 lead free solder, which produces a much harder joint and in some critical locations where gaining access later might prove a problem I often use heavy wall copper and braze the joints, or I run medium wall continuous roll copper to eliminate joints in that area.

Here again, some will argue that PEX would be a better choice, but knowing that rodents of all varieties, I.E. rats, mice, rabbits and Opossums all seem to have a sweet tooth for PEX I am a bit reluctant to use PEX in this application. I can assure you from first hand experience, when a mouse chews a hole through PEX it makes a pinhole in copper seem like childs play.

Cutting to the bottom line, after years in the trade I think Alponse, Brickeye or myself could go on for hours relating the horror stories of leaks in an inaccessible location but statistically speaking, no matter what type of pipe you install, if it is installed correctly the individual homeowners risk of catastrophic failure is very slight.

Thanks again for the continued discussion! In my case, the proposed new sprays are on an exterior wall but do not face the shower opening to the rest of the room. This is possible because the shower is in an exterior corner of the house, and thus only one of the three shower walls is an internal wall.

The plan is to run new supply lines through the attic from the hot water heater & cold inlet for all new sprays, so I'm fairly certain we won't run into issues with the velocity being excessive and causing premature wear on the copper. The plumbing quotes we got all indicated these new lines needed to be bigger than normal due to the desire to have them supply a shower head plus 4 body sprays. No plumber we have talked to for a quote has proposed PEX, which I'm fine with.

"These new lines needed to be bigger than normal due to the desire to have them supply a shower head plus 4 body sprays."

So... A couple of questions:

1. How many GPM is each shower head/body spray?

2. What size is your water heater?

3. What size is your drain?

Q: "How many GPM is each shower head / body spray?"
A: 2.5 gpm shower head, 1.5gpm/spray = 8.5gpm max.

Q: "What size is your water heater?"
A: It's a new Rinnai (sp?) 95 (sorry don't remember the exact model number)

Q: "What size is your drain?"
A: Hadn't given much thought to this so far (and no plumber had mentioned it either.) I can see it's a 2" drain now that we've done demo. Is that large enough?

I live in a colder climate (SE PA) and want to relocate my washer and dryer in my off-kitchen laundry room with hot/cold piping extending from basement to first floor against the exterior wall. I am DIY guy.

A plumber told me it's ok, as long as insulation behind pipes, and better to use 3/4" piping to minimize freeze risk. I'm also planning to wrap the pipes with foam insulation from shutoffs in laundry room to sill.

I'm pretty sure that should do the trick.

inbethlehem-
I see you are newly registered. Some advice: If you are seeking comments on your own issue, you should start your own thread, rather than resurrect someone else's that is 4 years old, and who is doing something else in a different part of the country.