My apologies for cross-posting, but I asked a question on the Green/Energy-Efficient Building and Construction forum and then realized that, since it was about PEX in concrete, you guys would be a better group to ask this question even though it isn't for radiant heating. Here goes...

Has anyone placed their water supply lines under the basement slab instead of within the framed walls? The house I'm having built is passive solar and earth-sheltered, basically a walk-out basement with a roof on that basement (no floors above). The contractor says it will be easier for him to install the cold and hot water supply pipes (PEX) in the concrete slab rather than within the framed walls. This isn't for radiant heating, simply supply lines to faucets etc. Is this a good idea?

One concern I have is not being able to access the pipes should changes be needed (such as an additional run). I'm not worried about damage to PEX within concrete, I know that's safe for 50-100 years+.

A second concern is heat loss from the hot water supply into the concrete. This isn't an issue during the winter -- the flooring is ceramic tile on the slab so it would be like radiant heating, in that the tap water would take longer to heat up but that heat would be warming up the floor. However in summer I'm afraid the concrete would wick away the heat from the pipe and make the floor warmer when I want it cooler. I could avoid this by placing the PEX in standard foam pipe insulation and then within the concrete -- is that a good idea?

Note that I have 5" of HD Plastispan EPS (R20) under the concrete, so I don't think I should worry about the pipes freezing, despite my eastern-Ontario location (think Minnesota). But are there some disadvantages I've missed? Should I go ahead with this?

If I do, should I insulate both the hot and cold supply lines before putting them in the concrete? Or would it be better to sleeve them in a larger pipe to make them accessible for maintenance? (I'm told I can't put pipe-wrap on them and then sleeve them -- too thick, and too much friction.)

Thanks for you help. I've been thrown this surprise twist with an answer 'needed' tonite as he wants to pour the slab on Thursday!
...Terry

this is done pretty regularly but I do think it is poor practice. I would insulate lines at least, not for home heat/cooling issues, but to keep the tap cool/hot as you wish. the amount of energy we're talking about in terms of heating and cooling is very small, but it can be annoying or require a hotter storage temperature to get the temp you want at the faucet if you don't insulate.

I would keep it out of the slab if you can, that's best. but it's not always possible.

Rob is right,
If you do not insulate at least the hot line it will take longer to get hot water to the tap and waste your energy in the process. One hot leg will not contribute anything to your slab heat, you will need at least 1 foot of pipe for every sq foot of insulated slab.As you have not poured the slab yet you should consider placing pex pipe in for future radiant heat, even if you do not connect it now, some day in the future you may want to. Additional cost for the pipe in floor is minimal.
Dan

Terry,

Maybe I am reading your post the wrong way but I would look hard at the reason for this seeming change in plans.  My interpretation is that you had planned from the beginning to run water supply in framed walls.  Now you want to change because "the contractor says it will be easier" to run it in the slab?  It may very well be easier for the contractor, but that isn't the point.  Given all the possible drawbacks and the fact that you are being rushed in to a decision, I would tell the contractor to stick with the original plan.

I have had contractors do this to me a time or two as well.  Once in a while they do have a better/cheaper idea.  When I get brought a suggestion like this my first question to the contractor is "So if we do it your way, how much are you knocking off the contract price."  That usually ends it right there.  The original plan made sense at one point, no need to change it just because it is easier for someone.  Unintended consequences can pop up years later.

We would run pipes under slab not in slab and yes insulate hot.

I also agree with other posts.

We would run pipes under slab not in slab and yes insulate hot.

I also agree with other posts.

In slab or under you could kill two birds with one stone by putting pex inside 1" poly sleave. Repairable and insulated with adequate airspace from concrete. Quick job, low cost, high value.

It is the standard to bury plastic pipe directly in the soil for water and gas service. The ground below frost line is at a steady temperature protected by everything but backhoe. When buried below a slab, it is protected from everything.

You may insulate the DHW line but that is an option. There is nothing more energy efficient than on-slab construction.

All the worrisome talk over serviceability is old women over their tea.

pipes in outside walls are standard too. I don't think that's good practice either.

I've seen some slabs crack and shift/offset pretty badly - this never causes a problem for embedded pipes (say ones used for radiant heating)? I agree that underneath the slab with sleeves to come through the slab is a different thing.

> When buried below a slab, it is protected from everything.

Except freezing (if the house isn't always heated and the pipe is above any slab insulation). But I suppose in that case, all the lines should be blown out beforehand.

freezing under a house is pretty unlikely. and I've never seen slab damage tubing... if it got that bad, as I think morgan has noted in the past, you've got much larger problems at hand.

Three things:

1. You have insurance.

2. You did not hire the low bidder for your concrete work.

3. If you are taken in by the anti-pex crowd you should use rebar in the slab, as any shear force that would damage reinforced concrete enough to damage embedded PEX in, or below a slab, would make the structure uninhabitable.

A couple of points:

1. The rebar decision should be made by an engineer, who will know what the tolerances are for breakage and slippage in your particular situation. My large 500 sq. ft. slab in earthquake ridden CA had only #4's on 12" centers, but it was not structural - except for the kitchen appliances. Not a lot of metal, but it does take some effort to space and whatnot.
2. You should insulate the cold water pipes as well, as you don't want the cold water picking up heat from the heated floor above.
3. When you sleeve the pipes with PVC, and then insulate with foam, you can change the PEX tubing out, contrary to an earlier worry. In fact, we ran the PVC before the concrete and only ran the PEX - and the insulation - much later. One hint would be to run the Pex without insulation for a few days to get it somewhat straightened out from its initial rolled state, and then pull it and rerun it with the insulation jacket.
4. Do not be taken in the anti-Pex crowd.

Jeff

On demand hot water heater would take care of your heat loss problems of PEX in, or under slab would it not?

not usually. then you'd have to get gas and venting to the water heater, unless you're doing electric. normally it would be much easier just to run an insulated water pipe than a vent pipe.