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Old 07-16-2013, 08:53 PM   #1
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Basement humidity and IAQ


What is a good humidity level for a finished basement? Right now mine is hovering around 60-70% at around 61-63 degrees F.

It doesn't feel "Wet" or anything, but would having a basement dehumidifier be a good idea or not? it's a 1,500 sq ft finished basement (completely finished) was looking at one of the larger built in units if it needs one, not a standalone portable unit.. and if so, what brand would you recommend? I've seen Honeywell and aprilaire running $1,000+ depending on the size, but not sure what works better or is recommended...

thanks!

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Old 07-16-2013, 09:15 PM   #2
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Yours is way too high. I have an unfinished basement, and right now it is 70 down there at 57% rh. Normally it is around 52% rh at around 66-70, but since more humid outside, it is a little higher.

I am just stating that because you did not give a actual temp of what it is, just a theoretical temp, which gives others no clue what it is right now, same for the humidity.

Even if my basement is between 58-62 in the Winter, the RH is no higher than 52%, same as the upstairs. I tend to try to keep my basement and upstairs between 47-60%, and temp from 68-72 upstairs, but real temp measures around 75 during Summer, 67 during Winter, downstairs temp stays anywhere from as stated in the beginning of this paragraph.

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Old 07-17-2013, 06:21 AM   #3
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Basement humidity and IAQ


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Originally Posted by gregzoll View Post
Yours is way too high. I have an unfinished basement, and right now it is 70 down there at 57% rh. Normally it is around 52% rh at around 66-70, but since more humid outside, it is a little higher.

I am just stating that because you did not give a actual temp of what it is, just a theoretical temp, which gives others no clue what it is right now, same for the humidity.

Even if my basement is between 58-62 in the Winter, the RH is no higher than 52%, same as the upstairs. I tend to try to keep my basement and upstairs between 47-60%, and temp from 68-72 upstairs, but real temp measures around 75 during Summer, 67 during Winter, downstairs temp stays anywhere from as stated in the beginning of this paragraph.
Right now it's 63% RH at 61 degrees.. this is a newer house, the slab is completely under grade, no walk out or anything... we have a radon system running 24/7 which we were told would "help" some with humidity, it did nothing... under the slab we have some pretty thick plastic sheets (found this out when the radon system was put in) the basement has returns and supplies all over.. right now the supplies are all shut due to the AC running all the time

The weather around here lately has been a LOT of rain, it just recently stopped this last week, but now its really hot and humid out.. but even in the middle of winter the humidity is around the same levels in the basement... upstairs the RH is 55-60% RH at 72 degrees almost always
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Old 07-17-2013, 07:18 AM   #4
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Basement humidity and IAQ


I think that humidity level is a bit high. I keep mine lower than that and my basement isn't finished.

I bought an energy star frigidaire 504dwd. It's been pretty good thus far.
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Old 07-17-2013, 07:25 AM   #5
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Originally Posted by strategery View Post
I think that humidity level is a bit high. I keep mine lower than that and my basement isn't finished.

I bought an energy star frigidaire 504dwd. It's been pretty good thus far.

I already have one of those that has been running almost non-stop to keep it at 55% in one room, but this is an entire basement... it can't seem to keep up with the entire place when everything is open, so was looking at a larger centralized system..

This house was built on clay, we have a high water table, and sumps that run a LOT.. get the slightest rain and they are running about once a minute (2 of them, one in and one outside for the exterior drainage system that dumps into a well to be pumped out to the town storm system)... so humidity probably always was an issue, when we had the basement gutted our concrete walls where always dry, the sumps and the gravel up the walls outside seem to keep the water out...

what would be a good larger system to use? I'm talking "whole home" / "basement / crawl space" types here
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Old 07-17-2013, 07:36 AM   #6
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Basement humidity and IAQ


Is your gutter system functioning pretty well?

I've heard Santa Fe makes a pretty good whole house dehumidifier. It's a bigger upfront investment, but a higher quality machine. It's also better because it's ducted and can dry the whole house.
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Old 07-17-2013, 07:47 AM   #7
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Basement humidity and IAQ


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Is your gutter system functioning pretty well?

I've heard Santa Fe makes a pretty good whole house dehumidifier. It's a bigger upfront investment, but a higher quality machine. It's also better because it's ducted and can dry the whole house.
you know that is the one unknown I have right now with this house... I have no idea where the gutters drain to... the neighbor behind me told me when the house was built they ran the lines into a gravel pit in the yard which would overflow then flood his yard... (they WERE suppose to be connected to a storm water management system that township put in the development, but the builder was too lazy to do it apparently) the lines where apparently moved further away from the house into yet another pit... instead of doing it right to fix that problem apparently..

when I bought the house our sump ran all the time, even in dry periods... turned out the sump pumped out into a HDPE corrugated 4" pipe that was perforated then ran ALONG the house! underground... well it is all gravel down that wall so it would just run back down the wall... I had that corrected and ran to the township connection point in the storm water system like it was suppose to do... now the sumps run very rarely... but the 8 other downspouts on the house I have no idea where they run... need to have them dug up someday and replaced with smooth wall PVC drain line and tie them into the storm water system correctly... so they aren't just dumping into our yard who knows where... I mean this is clay!
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Old 07-17-2013, 08:03 AM   #8
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Basement humidity and IAQ


As long as you have a supply and return for the hvac system, it should be near what the upstairs is. You may need to go around and seal any leaks around the joist bays & sill plates, entrances if a walkout, if the windows are leaking, either cover them with foam board & seal with silicone.

As for insulating around joist & sill bays, I used R-13, because I still wanted some breathing of the basement, others use the Blue Foam board to do theirs, with around 2-4 inches, depending on where they live.

I stopped using a DeHumidifier after we got our central air, and cut a return (used to just let the upstairs returns pull it back up the open stairwell), to help keep the downstairs conditioned.

Right now, my upstairs is jumping between 50% rh @ 71º f, downstairs is at 51% rh 68º f. Thermostat set overnight at 70, during the day 72, due to my wife just had surgery and is home all day, and likes it a little cooler, otherwise I would have it at 73 while at work, and the system only cycles maybe once every hour or hour and a half, due to it does not gain enough heat at the temps we have been having (71-95, feels likes have been around 100-105).

You just need to find the culprits downstairs, fix them, and if no supply or return as I stated before, for the hvac, get them added. Majority of systems are sized with the basement in mind, but some companies never add the ducts, or just add one. I added a vent in the trunk, and took the line that we had in our Kitchen floor, that during our remodel is no longer needed, so using that on the opposite side of the basement in the laundry area. Return is opposite end of the stairwell, about 14x28, which is working out great for us.
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Old 07-17-2013, 08:16 AM   #9
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Basement humidity and IAQ


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Originally Posted by gregzoll View Post
As long as you have a supply and return for the hvac system, it should be near what the upstairs is. You may need to go around and seal any leaks around the joist bays & sill plates, entrances if a walkout, if the windows are leaking, either cover them with foam board & seal with silicone.

As for insulating around joist & sill bays, I used R-13, because I still wanted some breathing of the basement, others use the Blue Foam board to do theirs, with around 2-4 inches, depending on where they live.

I stopped using a DeHumidifier after we got our central air, and cut a return (used to just let the upstairs returns pull it back up the open stairwell), to help keep the downstairs conditioned.

Right now, my upstairs is jumping between 50% rh @ 71º f, downstairs is at 51% rh 68º f. Thermostat set overnight at 70, during the day 72, due to my wife just had surgery and is home all day, and likes it a little cooler, otherwise I would have it at 73 while at work, and the system only cycles maybe once every hour or hour and a half, due to it does not gain enough heat at the temps we have been having (71-95, feels likes have been around 100-105).

You just need to find the culprits downstairs, fix them, and if no supply or return as I stated before, for the hvac, get them added. Majority of systems are sized with the basement in mind, but some companies never add the ducts, or just add one. I added a vent in the trunk, and took the line that we had in our Kitchen floor, that during our remodel is no longer needed, so using that on the opposite side of the basement in the laundry area. Return is opposite end of the stairwell, about 14x28, which is working out great for us.
the house is completely sealed... I did all the rim joists, all the cracks in the slab all that stuff when I bought the house, the whole basement was insulated with XPS foam and then what was left was spray foamed by the contractor where any gaps were left or needed more sealing after being filled with expanding foam.. had a blower door test done before it was done by a certified tester and it was very leaky, had another test done after and it went to the point I needed an HRV/ERV put in...

the duct work was replaced last year and resized when we changed the furnace and AC... manual D and J were done by the HVAC contractor... there are 4 returns in the basement, three in rooms and one in the main room... every room has at least one supply, the main room has 3 supplies due to it's size

radon system was put in took radon from 11 pi/L to 0.9 but didn't help the humidity much at all even though they kept saying it will help some since it takes moisture out from under the slab...etc...etc..

right now with the upstairs AC off.. our humidity starts to go up, and by mid day we are at 65%+ yesterday it hit 70% until the AC went on then that dropped it back to 55% (did this only as a test to see how fast it would rise) the outside humidity was 95% at 90 degrees! it was a humid day...
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Old 07-17-2013, 08:33 AM   #10
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Basement humidity and IAQ


If you do the maths, then both the OP and gregzoll's basements are on the edge where humidity=mould growth is concerned. In both cases, I would suggest some form of dehumidifcation.

60% RH is considered a borderline between no growth and growth of moulds; OK, that's simplified but a good rule of thumb nonetheless. Both of the two basements contain the same amount of actual moisture per pound of air and both are on the limit of what is a bit of a 'loose' border...but the OP's basement has finished elements to it, which may or may not absorb moisture.

But IMO both could benefit from a bit of a continuous "drying out"...
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Old 07-17-2013, 08:37 AM   #11
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If you do the maths, then both the OP and gregzoll's basements are on the edge where humidity=mould growth is concerned. In both cases, I would suggest some form of dehumidifcation.

60% RH is considered a borderline between no growth and growth of moulds; OK, that's simplified but a good rule of thumb nonetheless. Both of the two basements contain the same amount of actual moisture per pound of air and both are on the limit of what is a bit of a 'loose' border...but the OP's basement has finished elements to it, which may or may not absorb moisture.

But IMO both could benefit from a bit of a continuous "drying out"...
what is the math exactly? I thought I use to have a site that told you the "ideal" humidity based on ambient temp and such but I can't find it anymore
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Old 07-17-2013, 09:02 AM   #12
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Basement humidity and IAQ


Ccarlisle, no mold growth in mine. It is going to vary depending on the seasons, but with our basement, we use it for storage of artwork and my wife's Longabeger basket collection.

Between 47 to 60 is where you want the house year round, if you want stuff to last. Any higher or lower, you have issues.

Te temp is also correct for my place, since we also want to keep our electronic items in my househould to last.

Lets concentrate on the OP issue, not try to diagnose a non-existant issue at my place.

As for the OP, with the further info, it is possible that run times on the a/c and thermostat set point could be at play, or the erv is bringing in too much humid air, along with the fresh air intake.

Now if you want to talk problems ccarlise, last week my attic was at 9% rh at 92f.
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Old 07-17-2013, 09:08 AM   #13
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Ccarlisle, no mold growth in mine. It is going to vary depending on the seasons, but with our basement, we use it for storage of artwork and my wife's Longabeger basket collection.

Between 47 to 60 is where you want the house year round, if you want stuff to last. Any higher or lower, you have issues.

Te temp is also correct for my place, since we also want to keep our electronic items in my househould to last.

Lets concentrate on the OP issue, not try to diagnose a non-existant issue at my place.

As for the OP, with the further info, it is possible that run times on the a/c and thermostat set point could be at play, or the erv is bringing in too much humid air, along with the fresh air intake.

Now if you want to talk problems ccarlise, last week my attic was at 9% rh at 92f.
9%? I bet my attic space is in the upper 90% range right now, it's so humid outside that it feels like you walked into a sauna... at least I have solar powered attic fans now (added them last fall) use to get up to 120+ degrees up there, now with them running all day the temp stays at ambient temp outside inside the attic... the soffits / ridge vents just weren't cutting it so figured I'd try them and hey they work great (yes I had all the top plates sealed before this with spray foam so we don't suck air out of the house into the attic space, also sealed around all electrical boxes, etc with the 2 part spray foam)


edit: just looked at the humidity in the attic... 93% at 89 degrees... virtually the same as outside right now... which is 94% at 87 degrees
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Old 07-17-2013, 09:10 AM   #14
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Basement humidity and IAQ


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Originally Posted by BlueBSH

what is the math exactly? I thought I use to have a site that told you the "ideal" humidity based on ambient temp and such but I can't find it anymore
It can vary depending on the structure, what you are using the storage are for (ie basement), and going by skewed info.

I have found that keeping my place between 45-50 in the Winter, and 49-55 upstairs, 55-60 downstairs works great for me. Basement stays cooler in the Summer, house is not muggy upstairs or downstairs, and stuff like electronics fo no wear out, and our artwork and stuff lasts longer.

Some people tend to follow the old achool of thought, or some skewed scientific study with no real world backing. Sort of like the whole Radin argument.
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Old 07-17-2013, 09:20 AM   #15
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I have found that keeping my place between 45-50 in the Winter, and 49-55 upstairs, 55-60 downstairs works great for me. Basement stays cooler in the Summer, house is not muggy upstairs or downstairs, and stuff like electronics fo no wear out, and our artwork and stuff lasts longer.
even for the basement showing the RH levels I have it's never felt "wet" or muggy... guess that is one good thing, now upstairs when it went 60%+ it felt muggy..

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