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Passive radon mitigation in new construction

airfix | Posted in General Questions on

New construction in climate 6a.  I will have unvented roof with cathedral ceilings.  I’m about to lay my under slab radon pipes, which will be buried in gravel below my slab.  I have 3 concrete slabs each as three different levels.

Should I connect them all together and have one vertical going up through my mechanical room chase or should I have them separate?  The problem with separate is that now I have 2 more penetrations in my roof.

The 3 slabs are:
1) Main basement slab
2) Garage slab 6″ below the main basement slab
3) A small main level slab that is for my office and a closet about 10′ above the main slab.

Slab 3 is quite a distance from the chase where slab 1 will vent, however it’s adjacent (and above) to the garage slab 2.  It would be easy to connect slab vent 2 and 3 together.

In order to utilize the stack effect it is my understanding that I need to run the vertical vent through a heated space.  So there is no point running slab 2 and 3 vents through the garage, although the garage will likely be partially heated (about 30F until I heat it up to work in it).

Any suggestions or pointers for me?


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  1. Expert Member
    Dana Dorsett | | #1

    Using a venturi type vent cap can get vent stacks located outside of conditioned space to do the right thing without the heat input. (eg Empire SV04A) A turbine vent pulls even harder when there is sufficient wind to get it to spin, but the bearings will eventually fail- maybe after decades, maybe sooner, but who is maintaining it? A venturi vent has no moving parts, but does need at least some wind to provide additional draw. It doesn't take much wind to hit or exceed stack effect magnitude drives though.

  2. airfix | | #2

    Hmmm, that's a good thought. I'll take a look at those. We're in a canyon and natural heating and cooling of the canyon drives winds up and down the slope.

  3. walta100 | | #3

    Dana would you agree
    1 There is little need to but a separate radon vent under the garage slab? The exception being that your plan is to regularly spend several hours per day in the garage. Say you were going to restore cars in this garage.
    2 The office/closet slab needs a separate system if it represents more than say 10% of the finished space?

    I have a few questions of my own.

    The siphon vent you referenced is made for 4 inch steel pipe. My radon pipe is 3 inch PVC will I need an adaptor of some kind?

    Do you think the siphon vent will work well with 3 inch pipe?

    I am sure my current vent pipe only extends about a foot above the roof. Since I want to draft much like a wood stove should the cap be positioned using the rules for wood stoves.

    How should I seal penetration in the slab like the tub drain in the photo?


  4. Expert Member
    Peter Yost | | #4

    Hi Steve -

    What I don't see in your plan is ensuring you can easily add an inline exhaust fan should your passive radon system not achieve less than 4 picocuries per liter radon levels in occupied space?


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