2021 IRC 506.2.3

[Woodrow]

Is everyone enforcing this code? I am in SC and we enforce it. We do slab inspections and packaging has to be on site with proof of the ASTM on it. It seems that we are the only jurisdiction in our state enforcing this. Just curious what every one else is doing.

 

[classicT]

R506.2.3 Vapor Retarder​

A minimum 10-mil (0.010 inch; 0.254 mm) vapor retarder conforming to ASTM E1745 Class A requirements with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where a base course does not exist.

Exception: The vapor retarder is not required for the following:

1. Garages, utility buildings and other unheated accessory structures.
2. For unheated storage rooms having an area of less than 70 square feet (6.5 m2) and carports.
3. Driveways, walks, patios and other flatwork not likely to be enclosed and heated at a later date.
4. Where approved by the building official, based on local site conditions.

I am not in your region, but I can say that within my local area, the BO's (4-local cities) are aligned and have all agreed to apply Exception #4 and allow for vapor retarder to be omitted. That said, our BO's have done so because we had overwhelming support from the local engineering community, and because our native soils are predominately well drained, coarse sands and gravels. We also receive less than 8-in of rainfall a year. Where fine grained soils (silts and clays) are encountered, or where specified as required in a geotechnical report, the vapor retarder is required.

One issue of the 10-mil vapor retarder is the uneven curing of the slab. Moisture loss during curing is only from the exposed top side, which can result in curling or cracking of the surface. This is mostly cosmetic but does present an issue for some homes. Wet curing or a sealer is means to mitigate but comes at a cost. Not using a vapor retarder, particularly where soils retain moisture, can be very problematic given concrete's ability to convey moisture through the slab. I've seen this be a significant issue with glue-down impermeable floor finishes and wood floors. Glue let's go or the wood shrinks and swells.

 

[classicT]

On a separate note...

Where approved by the building official​

​

This is one of those statements we see throughout the IRC/IBC/etc. that I have a real love/hate relationship with. On the first hand, granting the BO the ability to make decisions based upon local factors is hugely important. The local code officials must have some discretion and ability to determine when a code provision is not appropriate or applicable given the nature of the installation.

My issue, where I derive the hate of this notion, is that the BO is seldom asked to make this determination. The design professional/contractor/installer/etc. just makes the assumption that if it is not called on an inspection, it is approved. And when it is called on an inspection, the fight goes right to the BO usually with immense pressure to approve because "it will cost XXXX" to fix. A lame justification, but the BO is often facing pressure from above, lacks time, etc. and is forced to approve even if it shouldn't be.

 

[Energystar]

I see they have changed from 6 mil to 10 in the 2021 code. We are still on 2018. We have been using 6 mil under our slabs and basement floors for 50 years, long before it was required by code. It makes a huge difference in eliminating that dank smell you get in a basement. We have never had issues pouring concrete over it. When it became code, all the other builders complained that it would cause curling issues. That was 20 years ago. No one ever questions it anymore.

 

[bill1952]

If a vapor barrier cause slab problems, why does pouring in metal deck which seems so common not cause the same problems? Certainly, metal deck is as impermeable as 10 mil poly, probably more so after the workers have stamped around on the poly and remesh before trying to lift it to center of slab (of course that's never done.)

 

[Glenn]

FYI: In the 2024 IRC this section will return to how it is written in 2018. 6 mil poly or other approved. There was no opposition to this reversal during the hearings and no public comment to challenge the committee approval.

 

[ICE]

Not realistic. The savings might be $200 on a 2500 sq.ft. slab.

 

[steveray]

Oh good....flip flop codes...

 

[DCole]

Finally the IRC stands up to what is the correct vapor barrier (retarder) for concrete slabs for homes. Then they change due to pressure because of cost. A standard 2,000SQFT basement would be around $250.00 for the 10mil barrier. When a contractor gets called out because of water issues in a basement it will cost more than the @250.00 to repair after the fact. A quality home builder should protect themselves by using the 10mil barrier. The issue of slab curl due to a 10mil barrier has been proven that is is not an issue.

 

[rktect 1]

I did see this since I am currently going through it to make amendments. So, once we adopt the 2021 within my jurisdiction sometime in summer/fall, yes we would be enforcing it.

 

[Woodrow]

I have no issue with the 10mil poly. I posted because the previous BO enforced the 10mil puncture resistant and must meet the ASTM D 1745. This takes the cost from couple hundred dollars to $500 a roll. This jurisdiction is mostly stem wall foundations with compacted sand clay so there is not much of a need for "puncture resistant" vapor barrier.

 

[DCole]

It just makes sense to protect yourself as a contractor. I am a Quality Control/Project Manager and the issues we see with the moisture in the concrete even after several months after placement. We consider it as insurance that we are doing the right thing. If you have ever had to moisture mitigate a concrete floor then most contractors would not even think about it. Even the new adhesives that work up to 99% are not fool proof.