Basement Exhaust Ventilation
I am considering installing an inline exhaust fan inside of a 4″ duct running from near the basement floor up the wall and out through the solid concrete wall (above grade) or through the rim joist. The concrete walls are 8′ high on all sides of the basement, but the grade is lower in the front which would allow going through the concrete wall. The basement is conditioned space and a dehumidifier is used. During the spring and fall, when the HVAC is not running as much, I notice a musty smell that is not present during the heating and cooling seasons. I’ve considered air exchange units, but shy away from them as they would require a larger hole through the wall. Since I would be core drilling through concrete and brick veneer or through the rim joist and brick veneer, I would like to limit the size of this hole.
My question is, would this exhaust-only system create a negative pressure in the basement and possibly cause radon levels to increase? The house was built in 2004, brick veneer over wood framing with a poured concrete basement. I would not consider the house to be extremely air tight.
There are two wood framed awning type windows just above grade on one end of the basement. I have considered changing these windows to vinyl sliding windows and keeping them cracked open instead of installing the exhaust fan. Since the awning windows are close to the grade, I believe these windows may rot from rain splash if I leave them cracked open.
Thank you in advance for any advice or suggestions.
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What you're planning here is very similar to an actual active radon mitigation system. The main differences are that a radon mitigation system generally uses 3" or 4" PVC pipe as a "duct", and such a system usually draws from an underground perimeter drain (perforated corrugated pipe or similar) under the slab. Radon systems often also bring down indoor moisture levels a bit by maintaining negative pressure in the ground underneath the home. If you're worried about radon levels, you might try installing your fan system the same was as an actual radon mitigation system would be installed, then you're covered.
BTW, even if you don't want to core through your basement slab to get to a drain pipe (and you have to seal anything connected to that pipe if you do), you might still find one of the radon fans made to be connected to PVC pipe would be a good "inline fan" for your application. If you are just drawing from the basement, I'd have two concerns:
1- You may increase radon levels. Radon levels tend to be higher when the basement is under negative pressure, since the negative pressure helps to draw the gas inside.
2- You'll be pulling in outdoor air as "makeup air", which might just bring in humidity from a different source.
Bill
Except that instead of pulling the under-slap gases through a pipe and venting them outdoors, you're pulling them into the living space. If there is radon in the soil this would increase the concentration in the living space.
The quick answer is that yes, forced ventilation can draw radon out of the soil and into your basement. And no, ventilation is probably not the answer to excessive humidity.
You need to figure out where he humidity is coming from. If dampness or even liquid water is seeping through the walls, ventilation isn't going to dry that out, you need to stop it at the source. If warm humid air is coming into the basement from either the outdoors or the rest of the house, ventilation is just going to draw in more of it and make the problem worse.
Oh, and if you're not measuring you're just guessing. You need to measure basement and outdoor temperature and humidity at a minimum.
The current temperature and humidity inside the basement is 71 degrees and 48% humidity with a dehumidifier running. Outdoor readings are 49 degrees and 80% humidity. The HVAC is not running.
The fan I was considering can be adjusted from about 50 to 125 CFM. The basement is 2100 SF with 9' height slab to plywood flooring. The fan could be set on a timer to run intermittingly. Would this size fan likely cause negative pressure and increase radon levels?
Thank you.
The interior air has a dew point of 50F. The exterior air has a lower dew point, so exchange it will shed some moisture. At 125 cfm it would be about 7 pints per day, which is pretty modest. I think you're better off using the dehumidifier. Which seems to be doing a fine job if you've got 48% indoor humidity.
If there is radon in the soil, depressurizing the basement will cause more of it to flow into the living space.