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Basement humidity level / percentage ?

Post in 'DIY and General non-hearth advice' started by Drifthopper, Jun 18, 2010.

  1. Drifthopper

    Drifthopper Member

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    Does anyone have any info on what is an “acceptable” level for humidity in a regular house basement?

    What I am asking on “acceptable” , is how high can the humidity be allowed to get before mold and mildew start to form?

    House is a two story colonial, built in 1977, poured concrete basement, about 800 sq. ft. , not finished, glass block windows with center screens that I do not open, washer / dryer / work bench / furnace and wood furnace off to one end under family room, sump pump in far corner…standard, solid basement.

    The guy that sold the house left an old dehumidifier, ( like from 1977 ). Seeing that it was old and having it look like a big energy monster, I would only run it every so often.

    Well, one yr, un-packing Christmas decorations, quite a few items had mold on them, these were tossed.

    So, two summers ago I got a new dehumidifier and a small digital temperature / humidity level reader : Temp on top , Humidity on the bottom.

    Last summer, I was running the dehumidifier on “dry” (there are three settings, normal, dry, and Extra dry” ) Dry is the middle setting.

    My typical humidity level was around 48% to 55% , basically right around 50%. BUT…..the machine was running ALL THE TIME..!!! So even with this new dehumidifier, it still added $15 to 20 bucks every month to the electric bill, spring , summer and early fall.

    This season, 2010 – To save on the electric bill, I have the dehumidifier set on “normal” , it does not run / cycle near as much, but my humidity level is around 60 % to 67% ( 67% is the highest I’ve seen on the digital reader)

    Basement temperature is about the same, 62, 64, sometimes 68, depending on how hot it is outside.


    Now ….i understand that all houses are different and everyone’s situation/set up is different, but there has to be some type of baseline to say what is ok, and what is not.

    Any input (or links for info) is greatly appreciated.

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  2. tiber

    tiber New Member

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    (this should probably be in the DIY forum)...

    I shoot for 70% humidity.

    The actual humidity is about 68% by my super duper humidity thing.

    Check the sump lid. It should have a rubber ring or similar gasket around it. If this is in poor condition, it's a cheap fix. If you live near water such as myself, most of your humidity is coming through the sump even on days when it's not really humid out.
  3. EatenByLimestone

    EatenByLimestone Minister of Fire

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    Why don't you open the windows and get some air in there to help dry things out?

    Matt
  4. SteveKG

    SteveKG Minister of Fire

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    May we assume the dryer is vented to outdoors? I realize this isn't the problem, but I just want to be certain it isn't adding to that problem.

    What is the ambient humidity [what's the normal outdoor humidity around there].

    Could be that if you could circulate outdoor air through the basement and back out, it would help, at least when it isn't cold outdoors. The electricity used by a fan to do so would have to be figured, as in whether it is less than running a dehumidifier, of which it sounds as if you need more than one. Or might.
  5. velvetfoot

    velvetfoot Minister of Fire

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    I've insulated the basement walls and find there is no condensation there. I bet if I insulated the floor there'd be less cool surface for condensation as well. I don't think opening the windows is the answer; it'll just bring in humid, moisture-laden air to a cooler area where it'll make for a higher relative humidity since the cooler air won't hold as much moisture.
  6. jklingel

    jklingel Feeling the Heat

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    The numbers you want are "30 to 50%", according to a chart I read on healthyheating.com. I have also seen those numbers elsewhere. Further, bacterial, viral, allergic rhinitis and asthma, respiratory infections, and ozone production problems actually increase below 30% RH. GL. john
    woodgeek likes this.
  7. velvetfoot

    velvetfoot Minister of Fire

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    I just looked at weather.com .
    It's 87% rel. humidity out there now, and I guess in here too since I don't have air conditioning.
    I think this is a summer time question, not a winter heating thing.
  8. woodburn

    woodburn Member

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    This is a great post. I've also wondered the same thing for my basement. I would think that 55% would be the highest you would want to allow. I too have a humidity meter down there and I keep mine at 50%. I notice once it hits 60% I feel a difference in the air and it gets a bit of a scent. I've never had any issues with mold.

    I'd go back to the "dry" setting. Carefully check the grade around the house and make sure EVERYTHING is pitched away nice. Also, are your gutter leaders long enough? Making sure of things like that might make the machine run much less. I also like Steve's suggestion of checking the dryer vent.

    Good luck!
  9. Jack Straw

    Jack Straw Minister of Fire

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    I believe that you want to keep it under 60%. Letting heat/or the sun in will raise the air temperature. As the temperature rises the relative humidity will go down. You are basically drying out the air. The only time you want to bring outside air in is when the humidity is low. If there is dew on the lawn in the morning you definitely don't want to bring in that air.
  10. Highbeam

    Highbeam Minister of Fire

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    I was told to keep my bedroom's air below 50% to limit the dust mites. In reality we are nearer 60% on average. For a basement, your current 60-67% is just fine and not going to cause mold on the surface if you are getting 60-67% everywhere. My guess is that if you could measure the humidity between an exterior wall and a stack of boxes (where you found mold) you would find much higher RH levels than you do out in the middle of the room or near the dehumidifier. You need air circulation to get all the water out.
  11. Drifthopper

    Drifthopper Member

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    Thanks for the replies ..guys... !!!

    Limestone....when i use to open the small windows in the glass blocks, the humidity seemed higher, i think having it all closed up doesn't allow moisture in......???

    Steve...yes, vented outside. I'm in WNY...close to Lake Erie..summer humidity is always high...60 to 80% , some days higher...

    John....my humidity down there during the buring season is 30 to 40 % ....maybe lower 25%/28% if it's real cold outside for an extended time. (month) , so to get 30% in the summer would be very tough. my dehumidifier would be always running to get 50% .

    Foot.......just askin' a question.....someone out there may have info i could put to use.

    Woodburn...thanks, on my temp/humidity meter, last night it was 68 degrees, with 68 % humidity, and in the upper coner of the digital reader was a little smile face with "comfort" after it, ,,,,, so the meter tells ya a good level, but i don't know how it relates ,/ if it relates to a basement.

    Jack...60% seems good / ok. no smell or odors.

    Highbeam.....one thing i forgot to type in my original post , was i doo run a fan also. just a simple pedistal ocillating fan to keep the air moving, i'll tune this on and off depending on what the outside temps are,,,,high,,,80* plus...fan on , 70* or below..fan off...sometimes i'll just run it during the day....on in the AM , off in the PM. and the boxes with the Christmas decorations are about 1" away from the wall, and up on planks on 2 x 4's off the floor. i figure with the fan goin' and having them away from the wall and off the floor the air will be able to move around 'em.

    Maybe i'll try a drier setting " more dry" just during the night......???.......and just watch the humidity level....
  12. vvvv

    vvvv New Member

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  13. yanksforever

    yanksforever Member

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    30 to 50 is the ideal humidity
  14. Drifthopper

    Drifthopper Member

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    From Blimp's link...........

    "The most important fact is how quickly crawl space relative humidity rises and falls
    with outdoor vapor pressures. When a weather front moves in or out, and outdoor
    vapor pressure rises or falls, humidity in both crawl spaces rises or falls, immediately.


    If moisture had to move through the ground and vapor barriers, these relative
    humidity changes could not possibly occur immediately.

    """""The average relative humidity in the home without the vapor pressure controlled
    device is 70%, """""

    whereas the home with the device averaged 52%. Using a fan only at
    the right time becomes highly beneficial.

    This is winter. In summer, without a ventilator, even with all the foundation vents
    open, the relative humidity will be 80 to 90%. See Reference : Samuelson “Relative
    humidity exceeded 90% for 44 weeks”.



    ................Ok..... copied this from above -

    The average relative humidity in the home without the vapor pressure controlled
    device is 70%,

    ......................So.....is 70% ...Bad..????

    that link is selling a product,,,,but says nothing about what humidity level is bad.
  15. coolidge

    coolidge Member

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    Is your rim joist properly insulated? Is the moisture entering through the concrete floor, or the blocks? Try taping a piece of plastic to the floor and wall to see if moisture is traveling through them, there should be water dropplets on the backside of the plastic after a few days.
  16. semipro

    semipro Minister of Fire

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    We keep ours at 60-65% RH. If it gets any higher than that we start noticing the smell of mold. So, if your goal is preventing mold growth that might the right level.
  17. Laszlo

    Laszlo New Member

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    Aim for between 25% and 60% relative humidity for your living spaces. Mold and mildew is mostly an issue at 70% RH or more, so your dehumidifier's "normal" setting should be fine. However, don't just measure the air in the middle of the basement--the relevant measurement is the RH near potential condensing surfaces, like against cooler walls and floors.

    Don't do this. As velvetfoot stated, letting in moist/warm outside air to a cool basement will raise the humidity in the space and potentially lead to condensation on surfaces and a higher risk of mold. Instead, make sure you tighten up any air leaks with caulk and spray foam to stop moisture-laden air from entering. Pay particular attention to service penetrations such as around pipes and wiring. Properly insulating and air sealing the rim joists can be equivalent to closing off a window-sized leak.

    This is good advice. Use some clear polyethylene, stretched tightly as you can get it and taped down on all sides. Wait a few days, then have a look at what side the water collects on. If the droplets are on the side you can touch, it's moisture from humid air condensing against the cool concrete. If water collects under the poly, then moisture is migrating through the concrete.

    Though building scientists have shown that air leaks move far more moisture than vapor diffusion--according to the DoE, air movement accounts for 98% of water vapor movement! So vapor barriers should be a secondary concern after you do a thorough air sealing.
  18. USMC80

    USMC80 Minister of Fire

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    Try and keep mine between 50-60 %
  19. Joful

    Joful Minister of Fire

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    40 - 60% is generally recommended as ideal, less than 40% indoor being an irritant for most folks (dry sinuses, etc.) and above 60% lending to the possibility of mold / mildew. Most dehumidifiers are actually set to "dry" = 40 - 50% and "normal" = 50 - 60%, as you already noticed.

    Everyone has a different climate, but in climes where dehumidifiers are required, this is usually not a good idea. Outside air is usually where the humidity comes from. Make sure your sump well and perimeter are sealed, and then you can successfully dry the air within.
  20. hearthofgold

    hearthofgold New Member

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  21. hearthofgold

    hearthofgold New Member

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    Drifthopper, have I ever got the answer for you. You could fix the problem by this afternoon, I did, and that was 5 or 6 years ago. Everyone is going to love this!!

    I had a bad humidity problem, and you could smell it when I opened the basement door. I used a dehumidifier for years, and played the empty the water jug game 2 times a day for years, and it drove the electric bill up the old wazoooo.

    Then one day I'm driving home from work and hear an advertisement on the radio, and it was asking if I had a damp moldy basement, bla bla bla, visit our website. So I visit the site, and it shows a vertical piece of duct work, starting a few inches from the floor, and just before the ceiling it exits into a hole in the wall. So the site goes on and on about how great it is, letting you think there is some kind of magic going on, because they weren't saying how it works, but it was going for $1200.00, which you only find out by calling them. So I pull up the reins on my pony, thanked him for his time and didn't get scalped that day.

    Anyway, I'm looking at it and thinking there couldn't be much more than a fan and motor in there. So I hop my pony on down to the local Home Depot, tail and all, and I figured I'd do a little experiment, found a little motor with a 9 bladed plastic fan, already mounted in a piece of 8 or 9 inch duct. Its intended use is for when you have a cold room, you draw air from the warm part of your heating system and direct it to the cooler room. The motor is the same one they used to put in the old record players, totally silent, and I bought a flexible hose. My basement window pains come out by sliding a small piece of tapered metal, which held the glass in place. I replaced it with a piece of plywood and cut a round hole for the duct. Then I attached the hose and made it so it hangs about four inches off the floor. When I tell you that within one hour the smell was totally gone I'm not kidding. My wife and I were almost beside ourselves in disbelief. There is just a pleasant neutral smell, and very comfortable, which is now my man cave. Got my hobby table, exercise equipment, room for jogging, rc cars and helicopters, and I'm the man now.

    The unit they were selling had a humidity sensor to shut it off and on. Big woop. And they wanted hundreds of dollars to install it. My fan runs 24/7/365 and costs $10 per year in electric, that's right ten dollars. I just replaced the motor in 2012. It still ran but would stop for whatever reason, and go again if I pushed the fan, so I figured I got my moneys worth out of it and didn't want any problems, and was probably on it's way out anyway.

    What happens is that in the summer the hot outside air gets in and contacts the cooler surface of the basement walls, hits the dew point, which is when a certain amount of hot hits a certain amount of cool, and moisture forms by condensing, which is called condensation. Never ever open the windows in the summer as it will not dry out anything.

    By having the fan going all the time the air never has a chance to condense. My fan actually changes the basement air two times per hour. You just figure out cubic feet of the basement and how many cubic feet an hour the fan moves. And guess what, if I did have that horrible gas called radon, which rises from the earth in enclosed spaces, it's gone too. So I don't mind having it running all year, especially for ten dollars.

    Guess what, in another article I told about closing all my hot air registers, since I heat with wood, because I was drawing air out of the duct work, and wasting wood, and never knew it until recently, after all the years of using wood, from the early seventies and how much better everything is, using less wood, and more stable temperature. But guess what, where my basement used to be in the fifties it's now in the sixties and it wouldn't be like that without the fan. Man talk about a good deal, this is it.

    Now, I have to admit, there is ever the slightest amount of moisture, but the only way I know this is because if I keep clothes down there for a very long time it does pick up a tiny bit of smell. Big woop, the cave is rocking and rolling.

    I think back then it cost me $86, so it's got to be a bit higher today, but can you say big woop?

    You're gonna love this so spread the word. Any handyman could add this to his services.

    Good luck.
    milleo likes this.
  22. Joful

    Joful Minister of Fire

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    Are these two statements not contradictory? 1) Never let fresh air into your basement, as it carries fresh moisture. 2) My fan actually changes the basement air two times per hour. Not knocking your system, mind you, just questioning the reasoning.

    For every cubic foot of air you pump out of the house, a cubic foot of fresh air must be drawn in to replace it. This comes in thru gaps in windows, doors, soffets, sill plate, etc. Aren't you ignoring the cost of heating loss to this forced draft? This associated cost may be on par with running a dehumidifier.

    I think that what's happening here is that by reducing basement air pressure, you are forcing (mostly) dry air from the first floor into the basement. During heating season or when AC is running in summer, this probably works pretty well. When it's mild outside, and you just have the windows open, this system probably has zero effect, but that may coincide with when it is least necessary.
  23. velvetfoot

    velvetfoot Minister of Fire

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    I just might try that fan thing. Not much to lose, esp. if it would reduce radon levels as well.
    Getting back to original question, this on line dew point calculator seems very useful:
    http://dpcalc.org/
  24. SONOCATIVO

    SONOCATIVO New Member

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    My house was built 1925 in 4 stages by a farmer, the main house was just one large room over a basement, then added a 1/2 story, later added what is the kitchen and another 1/2 story. then 2 additions on either side of the house. The kitchen is actually on a dirt crawl space thats about 18" with ducts and plumbing thats hard to get to.... the rest of the basement is concrete, stone and brick.... any resources they had to build with back then.... Id love to insulate it better, but thinking the only option i have here is to have a new basement dug....Its not as bad with smells, mold, mildew as when I first bought it 3 years ago, had a ton of cleaning to do ( vandalized bank foreclosure ) had to totally gut the place.
  25. sportbikerider78

    sportbikerider78 New Member

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    Great thread.
    I keep my basement on the lower side 60%. I have a block foundation and poured floor.

    Question for you guys. In many places in my basement there is no 'cap' on top of about half the block wall. There is the framing of the home on it, but that does not completely seal off about half of the block.
    What can i use to economically seal off the top of the block (about 4")? I think I get cool air coming up through the block and condensing with warmer air.

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