Sub Slab PRESSURIZATION
  • Ed Smith
    10
    Good Morning All.
    A recent mitigation of a standard block wall/concrete slab foundation has turned into one of those 1 in a 100 head scratchers. Everything is tight and sealed. Have mitigated others in the same neighborhood. Initial radon level was 28 in the basement. First vent, which was developed in a boney gravel, yielded 1.1” WC on the U-tube manometer using a Fantech HP190. Did have marginal vacuum at one end of the basement. That produced a reduction level of 13. We have subsequently put in 2 more vents, with very good sub slab vacuum throughout, dropping the U-tube reading to .8”WC but only dropping the radon to 10 pCi/L.
    Now to my question:
    I reversed the fan and pressurized the sub slab and the radon level is now below 3.
    The client is grateful, happy and wants me out of there.
    What are the downsides to a pressurization? I am in the Northeast.
    Thank you.
    Ed
    Ed Smith Options
  • Duane
    1
    seems like a good idea, how much back pressure is that going to put on the fan and will the fan be able to sustain that for the long haul?
    Duane Options
  • Robert Burns
    31
    I reversed the fans on a difficult home with positive results on lowering the radon level but changed them back because of the fear of freezing plumbing under the floor during the winter.
    It is rare but I have added suction to the block wall on more than one house when the subslab system didn't reduce the level below 4. It is important that the top row of blocks has been poured full of concrete or sealed in some other way.
    Robert Burns Options
  • Dick Kornbluth
    26
    Besides a cold slab, a likely problem will be condensation on the surface of the interior pipes during cold weather. Depending on the relative humidity in the basement, it could be extensive, and would require installing pipe insulation on all interior pipes, especially horizontal runs. The other potential problem is debris, such as leaves, falling into the vent unless you have a vent cap.
    Dick Kornbluth Options
  • Dan Pickett
    2
    I am in Virginia but deal with lots of difficult mitigations in my area. If you have the entire slab under a negative pressure confirmed by communication testing it is likely that the remaining source is not under the slab. if you happen to have a garage slab or other large slab on the main living level in some cases the radon trapped under that slab can be coming into the hollow block walls. If that is the case a suction point through the block wall into the gravel bed under the elevated slab can do the trick. I have had a similar situation and found very high levels coming into the basement around a Well line. Hopefully this will help.
    Dan Pickett Options
  • Bob Wood
    95
    Did you try sucking on the block wall, had one a while ago could not get it despite slab being all under negative pressure. Grab sampled in the block and found 100 pCi/l in wall. Sucked on block all 4 sides down under 2
    Bob Wood Options
  • Henri Boyea
    107
    Agree with Dan. We tap walls, upper slabs 4-5 times/yr. Dick makes good points about negative effects. Big question is: Where are you "pushing" the radon to?? Did you do follow-up test UPSTAIRS?
    Henri Boyea Options
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