Results 1 to 6 of 6
  1. #1
    Join Date
    Sep 2009
    Posts
    14

    First of many Q's: humidity control and crawlspace

    Hi there,

    We built our home 10 years ago. My wife started getting sick a few years into our living there, and it took us a while but we figured out that it was likely mold that was causing her grief. We've since moved out of the house and she's much, much better now. So we're at the point now where we're fixing the house. We had our crawlspace flooded before we moved in 10 years ago, so that's one source of mold as there was a good deal of debris there that molded and counts were very high there. Also we had a few other water intrusions that we have addressed as well.

    So, first question: we live on the coast of California, and as such, outside humidity can range from 90+% to 25% depending on the day. During the summer, like today, it ranged from 90% this morning, to 50% right now (mid-day) and will probably drop a bit more before the fog comes in again tonight. I'm wondering what is the best thing to do in the crawlspace to keep the mold to a minimum - seal the vents, or add fans to keep air moving through. I can seemingly argue both ways, but I'm not sure if moving humid air is better than stale, more dry air.

    Our IH is recommending against a vapor barrier on the floor of the crawlspace, however, due to it trapping moisture below it and contributing to more growth. He's suggesting fans to keep air moving through the crawlspace.

    Any thoughts? I apologize if this is an elementary question, I can see why one would seal crawlspaces when humidity is always high, and leave them open when humidity is always low, but we fluctuate so much I'm not sure what is better in this case...

    I have many more questions including whole house dehumidification, vapor barriers in attics, and air purification...but I'll research them a bit more before asking. Thanks!

  2. #2
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by jeffinca View Post
    Hi there,

    We built our home 10 years ago. My wife started getting sick a few years into our living there, and it took us a while but we figured out that it was likely mold that was causing her grief. We've since moved out of the house and she's much, much better now. So we're at the point now where we're fixing the house. We had our crawlspace flooded before we moved in 10 years ago, so that's one source of mold as there was a good deal of debris there that molded and counts were very high there. Also we had a few other water intrusions that we have addressed as well.

    So, first question: we live on the coast of California, and as such, outside humidity can range from 90+% to 25% depending on the day. During the summer, like today, it ranged from 90% this morning, to 50% right now (mid-day) and will probably drop a bit more before the fog comes in again tonight. I'm wondering what is the best thing to do in the crawlspace to keep the mold to a minimum - seal the vents, or add fans to keep air moving through. I can seemingly argue both ways, but I'm not sure if moving humid air is better than stale, more dry air.

    Our IH is recommending against a vapor barrier on the floor of the crawlspace, however, due to it trapping moisture below it and contributing to more growth. He's suggesting fans to keep air moving through the crawlspace.

    Any thoughts? I apologize if this is an elementary question, I can see why one would seal crawlspaces when humidity is always high, and leave them open when humidity is always low, but we fluctuate so much I'm not sure what is better in this case...

    I have many more questions including whole house dehumidification, vapor barriers in attics, and air purification...but I'll research them a bit more before asking. Thanks!
    Jeff,

    If you read through this discussion I think you will find many of the answers you are looking for.

    http://hvac-talk.com/vbb/showthread.php?t=347672

  3. #3
    Join Date
    Mar 2005
    Posts
    997
    Think you should fire your IH because of his old school thinking and most charge to much and take un-necessary test. VB is a must to keep the moisture from getting into your crawl. Close vents, insulate. Keep an eye on the RH only then if high get a dehumidifier.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,446
    The idea of a vapor barrier is eliminate moisture from the soil evaporating into the crawlspace. Yes, the soil will be wet under the plastic. Overlap the plastic and seal to the walls. Wet soil without air does not grow mold. If your outside air is wet for more than 10 hours at a time, close the outside vents and maintain <55%RH in the crawlspace. This will require a good commercial dehumidifier like the Santa Fes.
    Fresh air ventilation is the other critical issue. You should have an air change of fresh filtered air every 4-6 hours when the home is occupied. You must all so maintain <55%RH humidity throughout your home. Providing air fresh when the outside dew points are high will also require some dehumidification during wet cool weather. There are whole house ventilating dehumidifier that are able to provide filtered fresh air throughout the home and maintain <50RH throughout the home. Check out units like the Ultra-Aire or Honeywell. There are other brands also. A good heating/cooling contractor can deal with this.
    Fresh air change, good air filtering, and <50%RH are the basics of IAQ.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,347
    Quote Originally Posted by jeffinca View Post
    Hi there,

    We built our home 10 years ago. My wife started getting sick a few years into our living there, and it took us a while but we figured out that it was likely mold that was causing her grief. We've since moved out of the house and she's much, much better now. So we're at the point now where we're fixing the house. We had our crawlspace flooded before we moved in 10 years ago, so that's one source of mold as there was a good deal of debris there that molded and counts were very high there. Also we had a few other water intrusions that we have addressed as well.

    So, first question: we live on the coast of California, and as such, outside humidity can range from 90+% to 25% depending on the day. During the summer, like today, it ranged from 90% this morning, to 50% right now (mid-day) and will probably drop a bit more before the fog comes in again tonight. I'm wondering what is the best thing to do in the crawlspace to keep the mold to a minimum - seal the vents, or add fans to keep air moving through. I can seemingly argue both ways, but I'm not sure if moving humid air is better than stale, more dry air.

    Our IH is recommending against a vapor barrier on the floor of the crawlspace, however, due to it trapping moisture below it and contributing to more growth. He's suggesting fans to keep air moving through the crawlspace.

    Any thoughts? I apologize if this is an elementary question, I can see why one would seal crawlspaces when humidity is always high, and leave them open when humidity is always low, but we fluctuate so much I'm not sure what is better in this case...

    I have many more questions including whole house dehumidification, vapor barriers in attics, and air purification...but I'll research them a bit more before asking. Thanks!
    I hope you've removed the debris from your crawl space. Mold needs food. Debris = food.

    Dr. Joe has an alliteration pertaining to indoor air quality problems...the "Four P's"; People, Pollutant, Path, and Pressure. The pollutant needs a path to a person via pressure. Damp soil in your crawl space has vapor pressure, which will send moisture into the air in the crawl space during the daily swings in relative humidity (since the crawl space is ventilated with outdoor air). This can drive moisture into debris and construction materials in the crawl space, creating an environment favorable for mold growth. The path to the person (you and your wife) for the pollutant (mold spores) is via not only the vapor pressure from the soil, but likely a pressure difference between the crawl space and the interior of your home, drawing air into the house from the crawl space.

    Long story short, I agree that the IH is off-track and I would also go with a vapor barrier over the crawl space soil, along with sealing it off. A dehumidifier may be necessary if the crawl space moisture levels still run higher than 55%.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
    Join Date
    Sep 2009
    Posts
    14
    Thanks guys for the feedback. It's much appreciated.

    We had originally planned on doing a "CleanSpace" install, an installed vapor barrier that essentially envelopes the crawlspace. We decided not to due to cost, and concerns about longevity (it was about $10k).

    My question that I'm having a hard time with is are we concerned with keeping the RH% in the crawlspace *always* under 55%, even when outside humidity is approaching 100%? If so, then I see how sealing the crawlspace and dehumidifying would accomplish that (but at quite a big financial cost). Or, is it reasonable to just maintain air movement in the crawlspace via a few push/pull fans, and do our best to seal the crawlspace from the living space?

    As I said, the humidity swings here on the coast are pretty big. The soil under the crawlspace appears dry but on any given day is a few % higher than outside humidity.

    Thanks again, I continue to read and learn...

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event