Wood floor framing over a slab

Dear NACHI Members, I looked at a home recently that was a converted shop. I had a number of concerns about how the remodel had been done and they are all inter related. I wasn’t sure how it would all come together, so I referred to an engineer. I’m curious about what other professionals think about it. Here are the issues:

  1. The original shop had a concrete slab floor with radiant heat;
  2. The tubing for the radiant heat was polybutylene;
  3. The new floor was framed with I-joists about 1" off the slab;
  4. There was no vapor barrier between the slab and the framing;
  5. There is no ventilation for the space between the slab and the floor- all the heat would be lost if there were;
  6. There is a creek in the back yard about 6’ below the slab so the water table is near;
    *]The waste lines run in the space between the slab and the floor and exit above grade.
    This looks like a very small, unventilated crawlspace to me without the vapor retarder normally found in a conditioned crawl. There are a lot of “ifs” here. If the radiant heat keeps the crawl warm enough to prevent condensation, if the waste lines don’t leak, if the PB pipe doesn’t leak, if the slab is on a free draining surface like crushed rock, if, if ,if… I’m not sure what to think about this one. Any help would be appreciated. Greg Brainerd Livingston, MT

Hey Greg, I can see why you might have a lot of if’s and concerns on this one but if it helps I will share my opinion. First question is, is the radiant heating still used? If so, then I personally see no need for a vapor barrier between the concrete slab and the new framed floor. I could see this being a bigger problem if there were one present. So a “screed” floor was built, only it was left an inch off the concrete. This is a plus because it doesnt create a lot of cavities, this space is good for circulation of air. This leads to your ventilation question. Now here’s mine. Are there any “registers” in the subfloor of this wood framed structure? Perhaps this could be a simple solution to allow and allot some air in and out of that space. How long has this floor been in place and if it hasn’t caused any issues in the past, then perhaps you shouldn’t let your if’s run too far. Just share your opinions and make note of it. And the waste lines, are these running through the I-joist webbing or are they only 1 inch diameter pipe literally “between” the floor and slab, I’m not picturing that one. Many of these situations may have to be looked at as “grandfathered” and if not causing current issues or signs of problems then again, just share your thoughts and opinions don’t make it seem so dramatic if it doesn’t have to be.

Depending on the age of the slab there may be a vapor barrier under the slab that would reduce ground moisture from comming up through the slab.

I would also hope there is foam board under the slab as well due to the radiant heat.

I would report that you could not confirm the presence of a vapor barrier. Without a vapor barrier the wood framing could be eposed to higher moisture…etc. etc

I would report the PB as you normally would with PB.

Hi Jose,
Thanks for the response.

There are no registers in the floor to circulate air through the under floor area. That would be a good idea- it would keep the moisture from accumulating. Heat is still provided by the radiant in slab system so the registers wouls increase the efficiency of the heating.

The remodel has just been completed, so I don’t have a history to work with.

The waste lines run parallel to the joists for the most part- the toilets are all lined up so the larger lines don’t need to go through the web. The sinks and tubs are offset and go through the web- I think. There’s no access to see. The building drain exits the rim about 8" off the ground and makes an immediate 90 into the ground.

Were you able to verify the existence of a trap for the tub?

Hi Ken,

I couldn’t verify a trap for the tub. The only access to the under floor area is a 2’x3’ hatch where the manifolds are connected to the PB pipe in the slab.

Greg Brainerd

I’m surprised it passed inspection then. At least here, there has to be access to the plumbing. I would recommend having the buyer pull the permit and inspections, and verify the C of O was issued.

Hi Ken,
In other places, that would be true. Montana is still a part of the wild west. The only things out here that get regularly inspected are the electrical and the septic.

I think the reason for that is that if you have a bad electrical system and your home catches fire, it costs the county money to send the fire truck. With other components- it’s your house, do what you want. We’re supposed to build to code but not everybody does. If the septic is bad, you pollute the groundwater. People have been killed in water disputes in this part of the world.

Part of the reason people love it here is that the state doesn’t mess with you too much. We didn’t have a speed limit until a couple of years ago and there was no open container law until last year. If you got busted for drunk driving, it was off to prison for you. It’s a do as you please state, but if you cause somebody else a problem, the state will come down on you like a ton of bricks.

Montana enforces the 2006 International Residential Code statewide.

It is a plumbing code violation to not have access to traps below showers or baths if they use slip-joint connectors. If the traps are solvent welded, no access panels are required.

Plumbing code requires that any pipe…waste…soil…water supply…that is installed outside the building’s thermal envelope must be insulated, heated or both. So the building drain where it exists the structure is required to be insulated.

IRC plumbing code requires a cleanout on the main building drain within 3 feet of the foundation. This required cleanout can be inside or outside. You did not mention one. It is required to be there.

Otherwise, I would presume a vapor retarder under the slab. They are normally installed under the slab when radiant heat is installed.

Ventilation is not otherwise required between the slab and the walking surface of the floor above no more than you would need to ventilate a heated basement.

The key word here is “enforce”. Like I said, we are supposed to build to code, but a geographically large area, a population that does not like interference and a small number of inspectors combine to make an environment where code enforcement is minimal.

Montana enforces the 2006 International Residential Code statewide.

I’m looking for a link listing state by state what code each of the states uses.