Basically two questions here.

1) what type of stone base do you use for under a basement slab? I'm thinking in terms of ensuring it is stable but also has good enough drainage so that the insulation doesn't soak up any moisture (and can release anything that it might soak up). I was thinking something like AASHTO #67 (3/4 INCH), as a clean stone w/o dust would be best, but wanted to see what everyone else thought.

2) for a driveway that may potentially be asphalted later, but will be just stone at first. I'm planning a 2a for the top, but what is a good (cost-effective) base for underneath? I've had 3 different contractors propose 3 different bases (shale, 3a, 2a).

Thanks.

Re #2 - I put down fabric and then 2A. Or consider crushed asphalt. I agree with something clean for #1.

Thanks. Regarding the crushed asphalt, would you put a base down under that? or just put that direct on the dirt?

I'd put fabric on the dirt. Above that, I don't know - I'd guess one can just use 4-6" of crushed asphalt (ie, no gravel base).

Don't know what area you are in but radon prevention is a good thing, especially if people are going to be in your basement any amount of time.

Sounds like you are doing close to what the code calls out.

The nice thing about the gravel is that it is self compacting.

Here is the code.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_appf_sec003.htm

Thanks for the link. What barrier is needed to block radon? is 6 mil poly good enough? I'm assuming even if you do have a barrier, that the code will still likely require a vent pipe, correct?

And I know it's been beaten to death, but what is the 'better' stack order for insulation and vapor barrier? I've read the building science article that says you always want the insulation under the barrier, but it sounded like that was mainly "just in case" you get rain or otherwise get water down between the cracks of the insulation. If the slab is poured after the house is under roof, and the cracks are sealed with proper tape, would it be the "ideal" setup to have the barrier under the insulation? or is it still better to do barrier above? I am leaning towards barrier on top, and letting the stone ensure proper drainage, but figured I'd pose the question anyway.

and I'm in southcentral PA

I would be suspicious of using anything asphaltic if there was a possibility that hydrocarbon vapors might have access to the living spaces.

ICFHybrid, I would not use that asphalt stuff for the basement. that was related to the driveway portion of my question. Sorry for any confusion.

My understanding is that the only gravels that are reliably self-compacting are round stones (like pea gravel). Nothing crushed.

If you have a gravel driveway and don't use fabric, then you are counting on the compacted fines to prevent clay from pumping up into the gravel and ruining the support.

I am surprised you are not required to do a radon system from looking at the map for your location.

6 mil plastic is what is normally put down. If it is not required for you then there are many options. You can put the plastic down and stone down and then test for Radon in your house.

If it failed you could bore a hole in the cement and add the tee. You could also add the tee before you pour and cap it at floor level, then if you fail you run the pipe out the roof.

Radon code requires you to put down stone, then plastic, then insulation, and then cement.

Good luck.

Posted By newbostonconst on 15 Mar 2016 10:54 AM
I am surprised you are not required to do a radon system from looking at the map for your location.

6 mil plastic is what is normally put down. If it is not required for you then there are many options. You can put the plastic down and stone down and then test for Radon in your house.

If it failed you could bore a hole in the cement and add the tee. You could also add the tee before you pour and cap it at floor level, then if you fail you run the pipe out the roof.

Radon code requires you to put down stone, then plastic, then insulation, and then cement.

Good luck.

Interesting. Radon system may be required and I'll make sure to discuss that with my contractor. I'm merely interested in knowing my options and what is "correct" so if he tries to tell me something different, I can actually have a conversation about it instead of blindly trusting what he says.

That said, do you have a link to where the radon code states that plastic goes under insulation? In your previous link it didn't mention anything about insulation and most information I find doesn't mention insulation at all. Thanks!

The insulation is usually specced as part of the energy code, and then usually only when the space or slab is heated. Going to spend any time down in this basement? Insulation under the slab is a good idea and really hard to put in after the pour... Depending on the installation, taped seams at all the insulation panel edges can also qualify as the barrier...

The basement will be it's own zone (so I can heat it or not, at will) and won't be used right away. however, I have every intention of using it down the road, so I definitely want to do the slab correctly as it's built. the seams on the insulation will be taped regardless, but I still think I'd need (or at least want) a real moisture barrier as well.

yes you do need a moisture barrier.

yes you do need a moisture barrier.

I recommend that my customers use Form a Drain for footings their set up is ideal for capturing radon .

Posted By berky on 15 Mar 2016 02:48 PM
That said, do you have a link to where the radon code states that plastic goes under insulation? In your previous link it didn't mention anything about insulation and most information I find doesn't mention insulation at all. Thanks!

From memory; code states stone must be in contact with ground and that plastic must be in contact with stone, so the only place you can put insulation is on top of the plastic.

Not sure about local codes but THE BARRIER will act as moisture, radon and insulation all in one roll out product.

I see no references to "must be in contact with". It would require "approval" and two layers of foam to meet the overlap spec.

R506.2.3 Vapor retarder.
A 6-mil (0.006 inch; 152 µm) polyethylene or approved vapor retarder 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 no base course exists.

Posted By jonr on 18 Mar 2016 12:44 PM
I see no references to "must be in contact with". It would require "approval" and two layers of foam to meet the overlap spec.

R506.2.3 Vapor retarder.
A 6-mil (0.006 inch; 152 µm) polyethylene or approved vapor retarder 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 no base course exists.

I read it as saying rock has to touch ground and plastic has to touch rock.

AF103.2 Subfloor preparation. A layer of gas-permeable material shall be placed under all concrete slabs and other floor systems that directly contact the ground and are within the walls of the living spaces of the building, to facilitate future installation of a subslab depressurization system, if needed. The gas-permeable layer shall consist of one of the following: 1. A uniform layer of clean aggregate, a minimum of 4 inches (102 mm) thick. The aggregate shall consist of material that will pass through a 2-inch (51 mm) sieve and be retained by a 1/4-inch (6.4 mm) sieve. 2. A uniform layer of sand (native or fill), a minimum of 4 inches (102 mm) thick, overlain by a layer or strips of geotextile drainage matting designed to allow the lateral flow of soil gases. 3. Other materials, systems or floor designs with demonstrated capability to permit depressurization across the entire subfloor area.

AF103.3 Soil-gas-retarder. A minimum 6-mil (0.15 mm) [or 3-mil (0.075 mm) cross-laminated] polyethylene or equivalent flexible sheeting material shall be placed on top of the gas-permeable layer prior to casting the slab or placing the floor assembly to serve as a soil-gas-retarder by bridging any cracks that develop in the slab or floor assembly, and to prevent concrete from entering the void spaces in the aggregate base material. The sheeting shall cover the entire floor area with separate sections of sheeting lapped at least 12 inches (305 mm). The sheeting shall fit closely around any pipe, wire or other penetrations of the material. All punctures or tears in the material shall be sealed or covered with additional sheeting.