Crawl Space Negative Pressure

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Old 12-24-16, 08:36 PM
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Crawl Space Negative Pressure

I ran a short term radon test on my first floor prior to encapsulating my crawl space and got a value of 1.3.
I recently ran one IN my crawlspace after encapsulating it and got a 10.4.
Encapsulating is a big change but so is summer vs winter for the test.
I am going to run another one on my first floor shortly but I decided to try a passive radon vent and then potentially add a fan into the pipe.
I shoved some 4 inch pipe around under my vapor barrier to add some air tunnels (dirt floor) and then ran a pipe through the vapor barrier and out through the previously sealed exterior crawlspace access door (cut a 4inch hole). I was hoping air would go OUT the pipe but instead it's coming into the pipe pretty significantly - it will hold a piece of paper against the pipe outside. Safe to say my crawlspace has negative pressure.

I can still install the radon fan and basically push through the pressure but is there anything else I should be considering?

I feel like I did a really good job sealing the vapor barrier but I have no doubt it's not 100% air sealed as this obviously shows. Should I address that part? Although it will be somewhat impossible as I have a sump that takes drains from my air handler and dehumidifier so those will always be an air leak to the inside.

Just looking for any thoughts. Thanks
 
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  #2  
Old 12-25-16, 04:23 PM
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Your passive radon vent should go through the warm house all the way up above the highest ceiling and vent to the outside. With the vent pipe filled with warm air the resulting atmospheric pressure at the bottom will be less than what is inside the crawlspace, thus you will get an upward air flow.

The crawlspace pressure is negative with respect to outside, but we need to think of it the other way around. It is the outside pressure that is more positive.

Do you have a path where you can extend your pipe up through the roof?

Bud
 
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Old 12-25-16, 04:55 PM
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Unfortunately not for a 4 inch pipe. Doing a remodel of an area this spring/summer where I might be able to work a fake wall in but can't guarantee it.
Hoping my first floor test comes back below 2 so I won't feel like it's quite as urgent but skeptical.

Do passive vents work very well anyways? I know with an active vent you use one of those gauges to confirm suction - would a gauge on a passive vent show the same (or any) suction?

As for the negative crawlspace or positive outdoor pressure - normal? Or is there something I need to do differently there? The only vent fan that gets used often is in the master shower and it's a 100cfm.
 
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Old 12-25-16, 05:59 PM
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Radon systems are primarily about that negative pressure, thus they seal as much of the floor as they can and then pull the space below down to a negative pressure ranging from 1.5" to 4" of water column. Converting that to Pascals the range would be roughly 300 to 1,000 pascals. A passive radon system would at best generate about 4 or 5 Pascals but more typically in the 1 to 2 range. Now, that is in addition to the existing negative pressure already in the basement so it would be moving the air in the right direction, but it is very limited in its capacity. The nice part is, when it is enough it is free.

And yes, a negative pressure in the crawlspace with respect to outside is normal during the winter, it's called stack effect.

How much exhaust fans affect the house pressure is a function of several variables, and they do but rarely run 24/7. If you were to use an exhaust only system to provide a required air exchange then how it interacts with Radon should be evaluated.

Bud
 
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Old 12-25-16, 09:16 PM
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Thanks a lot for that info. Really helpful.
 
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