Author Topic: Help with humidity/mold problem (old house)  (Read 720 times)

MeHg11

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Help with humidity/mold problem (old house)
« on: March 03, 2021, 06:43:44 PM »
I'm in FL and have an 85 year old home with a somewhat poorly ventilated, small crawl space. Recently had the inside of the home tested for mold and levels were off the chart high. Humidity inside was >60% in some rooms.

I have a large (4 ton) relatively new (~3 yrs) AC unit which is probably oversized as it's cooling only about 1800 sq ft. I have 2 mini splits in rooms farthest from the AC unit. I say oversized as I've been told the problem is that the unit is cooling too fast and not removing the humidity. I just installed a Nest thermostat and the unit turned on/off 19 times in 24 hours - this seems excessive to me, but I'm no expert.

To make things worse the humidity levels from the AC supply ducts in the floor were incredibly high - almost 80% after the unit had shut off and the blower was still going. I was told that the flexible ducts could also possibly be compromised (perhaps a hole in them and thus exposed to outside air?).  Another problem is that the flexible duct sits right on the ground which I'm sure is not ideal given the moisture it's likely exposed to. The ventilation of the crawl space is not ideal either.

My question is this - should I go for a smaller AC unit and fix the problems with the ducts/ventilation and perhaps install some sort of barrier on the ground to block moisture? Or should I just bite the bullet and remove the unit and the ducts and just install splits in all rooms (4 heads total)? I've had conflicting advice from different AC contractors and it's very frustrating. I'm not so concerned about the cost - really just want someone to give me sound advice so I can get the problem fixed. 

MeHg11

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Help with humidity/mold problem (old house)
« Reply #1 on: March 03, 2021, 06:57:33 PM »
[MOD NOTE: Duplicate topics merged.]
« Last Edit: March 05, 2021, 09:10:31 AM by arebelspy »

Fishindude

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Re: Help with humidity/mold problem (old house)
« Reply #2 on: March 04, 2021, 07:15:29 AM »
Keeping some air moving will be the biggest help.
For a quick and dirty fix, try running ceiling fans 24/7, or putting some oscillating fans on timers to kick on several times daily.

lthenderson

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Re: Help with humidity/mold problem (old house)
« Reply #3 on: March 04, 2021, 08:04:29 AM »
I think perhaps the cheapest solution might be to invest in a large dehumidifier and run it for awhile to see if you can get your humidity down to more reasonable levels. I would hate to just replace a 3 year old system with a smaller system simply to remove more humidity.

norajean

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Re: Help with humidity/mold problem (old house)
« Reply #4 on: March 04, 2021, 08:08:02 AM »
You may have multiple problems to solve.  First, determine the correct size of AC unit by having an expert do a manual J calculation.  You may or may not have the correct size as it depends on a lot more than square footage (for example, I need five tons for my living room and loft alone). 

On the hottest day of the year, the AC should run almost all the time and be able to maintain the desired temperature.    On less hot days it will run less and dehumidify less.  Number of times on/off is not important; total run time per hour is.

The ducts should all be sealed and no outside air being pulled in.  Even so, you will get outside air coming in to the house via windows, etc. 

80% suggests you have a crawl space ventilation problem, assuming ambient outdoor humidity is lower.

If all else fails you can try a dehumidifier.  They are noisy and need to be plumbed to a drain but they work.

If you have had these conditions for some time, you may have mold in the house (as you indicated was measured). It may be growing in the crawl space, wall cavities, ducts and elsewhere. 

Paper Chaser

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Re: Help with humidity/mold problem (old house)
« Reply #5 on: March 04, 2021, 09:39:07 AM »
Encapsulating the crawlspace would eliminate any moisture intrusion from under the house, and it sounds like some of the duct work in the crawl could be modified to condition that space pretty easily. That cuts down on humidity coming in (either from the soil below the crawl or any outdoor air coming into the space through vents), while also increasing the cubic footage within your HVAC envelope, making your AC unit less "over sized". It should also improve air quality and make your home more energy efficient.

EricEng

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Re: Help with humidity/mold problem (old house)
« Reply #6 on: March 04, 2021, 12:46:26 PM »
I'm in FL and have an 85 year old home with a somewhat poorly ventilated, small crawl space. Recently had the inside of the home tested for mold and levels were off the chart high. Humidity inside was >60% in some rooms.
60+% isn't great, but it is not awful either.  Happy range is 40-60%.  Florida is always super humid (was just visiting in laws there), so you are always going to have humidity creeping in.
I have a large (4 ton) relatively new (~3 yrs) AC unit which is probably oversized as it's cooling only about 1800 sq ft. I have 2 mini splits in rooms farthest from the AC unit. I say oversized as I've been told the problem is that the unit is cooling too fast and not removing the humidity. I just installed a Nest thermostat and the unit turned on/off 19 times in 24 hours - this seems excessive to me, but I'm no expert.
This is actually really good for an 85 year old that is usually poorly insulted.  It will depend on differnce of outside temp and inside as well.  Running 2-3 times an hour during heat of the day is common.  Expect it to run about 15 mins on nice warm days and possibly non stop on 100+ degree days.
To make things worse the humidity levels from the AC supply ducts in the floor were incredibly high - almost 80% after the unit had shut off and the blower was still going. I was told that the flexible ducts could also possibly be compromised (perhaps a hole in them and thus exposed to outside air?).  Another problem is that the flexible duct sits right on the ground which I'm sure is not ideal given the moisture it's likely exposed to. The ventilation of the crawl space is not ideal either.
Again, not that unusual.  Humidity is modified by temp.  You can have the exact same amount of moisture, but lower the temp and the humidity % will be much higher because the air can't hold as much moisture.  In a humid climate I would expect the cold air (usually 40-50 degrees) from an AC to report a higher humidity than your ambient room, but once that air reaches ambient temp the humidity % should be same or lower.

Given your crawl space is in Florida, most of the year you probably can't get it lower than 80% because that is just what it is outside.
My question is this - should I go for a smaller AC unit and fix the problems with the ducts/ventilation and perhaps install some sort of barrier on the ground to block moisture? Or should I just bite the bullet and remove the unit and the ducts and just install splits in all rooms (4 heads total)? I've had conflicting advice from different AC contractors and it's very frustrating. I'm not so concerned about the cost - really just want someone to give me sound advice so I can get the problem fixed.
I don't think replacing your AC unit will magically fix humidity issues, more likely they are trying to make money off you.  Maybe just look at some dehumidifier units? Look at possible insulation issues too.

MrTurtle

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Re: Help with humidity/mold problem (old house)
« Reply #7 on: March 04, 2021, 05:51:09 PM »
Yes, your unit is short-cycling because it is oversized.  In Florida, I had a 3-ton unit for 1900 square feet.  You have 4 tons PLUS the capacity of the mini splits for 1800.

Also, check for leaks in the ducts.  A leak in the duct means you're taking air from inside the house and putting it outside.  Whatever air you put outside gets replaced by unconditioned, hot, humid Florida outside air.  I had a mold problem when I lived in Florida that was directly caused by a gaping hole in the supply duct.  If you have mold in only one place, close to a door or window, that's likely the problem.

I've never seen underfloor air conditioning before...is it normal to put ducts directly on the ground?  Seems like a half-assed installation that exposes the ducts to lots of nasty things.  I would think it should be suspended off the ground, or at least have something between the duct and the dirt.

Your mini splits are in the rooms furthest from the unit.  Is that because of low airflow in those rooms?  If that's the case, you might have a correctly sized main unit and the air just isn't making it to those rooms.  Then fixing the problems with your ductwork might solve all your problems while eliminating the need for the mini-splits.


Endo1030

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Re: Help with humidity/mold problem (old house)
« Reply #8 on: March 04, 2021, 08:07:18 PM »
I just installed a Nest thermostat and the unit turned on/off 19 times in 24 hours - this seems excessive to me, but I'm no expert.

To make things worse the humidity levels from the AC supply ducts in the floor were incredibly high - almost 80% after the unit had shut off and the blower was still going.

I'm not really an expert on HVAC, but there is an option on nest thermostats called "Airwave" that I believe should be disabled.  It runs the fan on your hvac unit for a certain amount of time after the condenser shuts off.  This exacerbates humidity issues because while the condenser is running, condensation forms on the cold coils, this is the result of dehumidifying the air in your house.  If the condenser turns off, the coils warm back to ambient temperature, and if the system continues to move air across them, then a portion of that condensation will evaporate again back into the air you just dehumidified. 

Logically it makes sense to me.  I have a nest and I disabled the function.

MeHg11

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Re: Help with humidity/mold problem (old house)
« Reply #9 on: March 04, 2021, 08:15:47 PM »
House is actually a little over 2000 sq ft and the splits are cooling a bedroom (~130 sq ft) and a laundry room/office/bathroom (~180 sq ft). I do suspect the ducts are somehow compromised.

Yeah I dont think it's normal to put ducts on the ground either. Had terrible experiences with AC companies who just wanted to sell me the largest unit. It's been a debacle since it was first installed (that's a whole other story). But putting it on the ground was lazy and indeed half-assed.

As for the splits - yes, these were installed because essentially no air reached the back rooms despite the fact that duct work goes to these rooms. Could be partially due to several 90 degree bends in the duct work but again the AC companies (3 separate companies!) I dealt with never could figure this out.

MeHg11

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Re: Help with humidity/mold problem (old house)
« Reply #10 on: March 04, 2021, 08:27:40 PM »
Really appreciate the insight and suggestions. I'm running a dehu nonstop and moving it to different rooms to see if that helps. The Airwave unit on my Nest is currently "on" so I'll have to see if that makes a difference. I just figured out how to download data from Google from my Nest account and humidity has been fluctuating. I just don't know if its connected to the unit going on/off.

I'm going to ask another AC company about an in-line dehumidifier. Not familiar with them though.

I have tried to increase the ventilation around the outside of the house and have dug french drains to move water away from the foundation.  I've also though about putting down a vapor barrier (visquine (sp?) sheet insulation). I've heard conflicting advice about this. 

I will get under the house and check on the duct work. Thinking of having it attached to the floor joists to see if that helps the situation. 

Paper Chaser

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Re: Help with humidity/mold problem (old house)
« Reply #11 on: March 05, 2021, 05:37:11 AM »
Increasing ventilation is probably just letting more of that sticky Florida air into your cool, damp Crawlspace where it can condense on pipes, ducts, framing, etc and then move into your living space above. Modern building codes in many places don't allow for Crawlspace vents for this reason. A vapor barrier just laying on the ground won't be enough because the moisture will still enter the crawl around the edges of the vapor barrier and through the foundation vents.

The current thinking when it comes to building science and crawl spaces is to completely seal the crawl from the exterior, and then condition it somehow. This provides extra insulation, extends the life of anything in the Crawlspace like ductwork or plumbing, and makes it much easier for your home's HVAC to regulate temperature and humidity in your living space.

Roots&Wings

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Re: Help with humidity/mold problem (old house)
« Reply #12 on: March 05, 2021, 06:34:10 AM »
^ Yes, you can check out Building Science's free guidance articles for crawlspaces for hot/humid climates: https://www.buildingscience.com/document-search?term=&field_doc_topic_tid=All&type%5B%5D=3
Or check if your library has the book.

mozar

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Re: Help with humidity/mold problem (old house)
« Reply #13 on: March 05, 2021, 08:39:06 AM »
I'm considering a whole house fan and more ceiling fans for my house. My understanding is that moving air makes it hard for mold spores to settle. But in Florida that might not work if you don't have an attic though.

lthenderson

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Re: Help with humidity/mold problem (old house)
« Reply #14 on: March 05, 2021, 12:26:39 PM »
I'm considering a whole house fan and more ceiling fans for my house. My understanding is that moving air makes it hard for mold spores to settle.

I don't think I buy that explanation. My ceiling fan runs non-stop 365 days a year and there is still plenty of dust on objects and the fan blades that settle that I don't think mold would have any problems unless you turn your home into an efficient wind tunnel of air with very high wind speeds.  I think what fans do is distribute air throughout the house so if you have say a wet spot in corner A producing high humidity, it will distribute the humid air in that air with dryer air from the surrounding area and reduce the overall humidity.  I can see this reducing overall humidity in spots inside a house but not the overall humidity by much. I still think a more efficient way to rid a house of humidity is with a dehumidifier or simply prevent it from getting inside in the first place.

MrTurtle

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Re: Help with humidity/mold problem (old house)
« Reply #15 on: March 05, 2021, 04:57:43 PM »
I'm going to ask another AC company about an in-line dehumidifier. Not familiar with them though.

Don't do that.  It would be a dead giveaway that you know nothing about air conditioning and they might take advantage of that.  Dehumidification works by cooling the air down to 55 degrees so water condenses out of it then, if you don't also want cooling, heating the air back up with a reheat coil.  So, your cooling coil is the "in-line dehumidifier" but you don't have a reheat coil because that would be way too much for residential air conditioning.

I will get under the house and check on the duct work. Thinking of having it attached to the floor joists to see if that helps the situation.

I have high hopes for this because flex duct is usually installed wrong so you likely have lots of room for improvement.  Check for leaks, kinks, unnecessary turns and tight elbows, anything that makes it hard for air to get to the rooms far from the unit.  It would be great if you can solve your AC problem just by fixing the duct routing because that's way cheaper than replacing the unit.  Remember to check the airflow at the far-away rooms before you change anything so you can see how it worked.  Your goal is to cool the entire house on the one 4-ton unit so you can ditch the mini-splits.

ChpBstrd

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Re: Help with humidity/mold problem (old house)
« Reply #16 on: March 05, 2021, 08:04:19 PM »
It sounds like you should do all the above, and you're on the right track.

FWIW, I just tracked down the source of a chronic humidity / mold problem centered on one room. I narrowed it down by moving a cheap hydrometer around the house, and the humidity seemed to be highest (on humid days) near the floor in one bedroom, where there was a small draft near the floor near one wall. Investigation underneath the house revealed a completely rotted-out subfloor and outside joist! Crawl space air was coming in under the quarter round. Big project.

So if you have floor rot, look for an air leak.