At a Loss (High Humidity )
Hi All – New member here. Trying to help my sister. She built a modular home in southern NY almost 3 years ago. House has always been humid, but I just figured it was due to construction drying, and also because she has no ductwork/ventilation system. ACH50 = 2.6. I was there one recent morning and the windows had light condensation, which she tells me is frequent. So I’m getting concerned.
The house is kept at 67F and 47-50% RH. I did a consult with a Building Performance specialist who is very knowledgeable about the ventilation of tight new homes, and his opinion was that ventilation would obviously help, but that it was not the primary cause. Specifically, he said the house should not be that humid given the dryness of the outside air right now in NY during Dec/Jan. I had her open the windows a few times for an hour which can drop the humidity to 40 RH, but it seems to bounce back quickly. He felt there must be some “source” and advised me to investigate the basement. It is uninsulated/half above grade and generally 50F/70RH. If you do the math that air is actually much drier than the main floor. Also, just did a 24-hour test with plastic taped to below grade concrete floor and walls, and it was bone dry.
I’m at all loss. Any food for thought would be greatly appreciated. Thank You!! Joe
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Replies
Joe,
From your description I'm not convinced there is any other source of high indoor humidity except the ordinary day to day activities. The problem is there is presently no mechanism to remove it. Even at 2.6 ACH, most of the time there will be very little air exchange with the outside. The house needs some form of ventilation, either natural or mechanical. I would have your sister run the bathroom fan constantly for several days to see what effect that has. If it reduces the indoor RH, she can either run the fan on a timer at a rate that keeps it down, install a balanced ventilation system, or use a dehumidifier. It shouldn't take much. 50% RH at 67F really isn't very humid at all.
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"50% RH at 67F really isn't very humid at all."
Agree it does appear bad in absolute terms. But the possible issue is that her indoor air contains about 50% more moisture than the ambient outside air. My understanding is that maintaining that much more humidity this time of year vs the outside is very difficult from just day to day living.
Joe,
I don't see why the low outside air having less absolute humidity matters at all? The indoor RH affects the dew point, meaning more condensation on cold objects like windows, and also that wall and roof assemblies are more likely to accumulate moisture than if it was lower, but the outside air having less moisture aids drying.
Hi Malcom,
Agree the indoor vs outdoor humidity does not matter per se; but was thinking it's an indicator of a moisture source or issue in the house
Joe,
I may be wrong about there not being one discrete source of indoor moisture as your specialist suggested, but it doesn't surprise me at all that a house with no ventilation, kept at 67F, with several occupants doing normal everyday activities, stays around 50RH.
Malcom,
You very well may be correct. Been looking at installing a Santa Fe WHD with a fresh air vent. Can cut in a few registers. That thing seems like it solves a lot of problems right there: can run the fan 24/7 for circulation, Merv 13 filtration, fresh air, and the ability to dehu during the spring, summer and fall.
Is there something like an unvented fireplace in the house? Also check the vents from any other fuel burners, make sure the exhaust is not dumping into the house.
Unvented heaters are a big source of moisture along with a lot of other nasty stuff.
Sometimes also people vent dryers indoors, this can be a big source of moisture.
Thanks Akos.
Yes I checked the dryer and bath exhaust connections to the outside and they are fine. No unvented fireplace or anything like that.
I would consider 47-50% RH during heating season to be ideal.
I would focus on the other symptom, which is condensation on the windows, which I agree is not good. My guess is the issue there is that the inside surface of the windows is quite cold. I don't know of a fix for that other than replacing the windows though.
Yes that's probably I big part of it....hearing these Andersen Builders grade windows have very low R value at the edges.
I have often seen these windows sealed by jamming fiberglass insulation around the edges. Since this doesn't provide an air seal, you can get a lot of air infiltration around the edge of the windows which keeps the window frames very cold. Air sealing it a bit better might fix the condensation.
An ACH50 of 2.6 isn't that low, it's entirely possible that the windows weren't well sealed, which would explain why the interior would be cold.
Depending on how they're trimmed it might not be that big a job to remove the interior trim and check the sealing.
Maybe one of those storm window covers? Alpen makes some for retrofitting historic windows. They call them Winsert, it is a secondary removable window pane sort of like a storm cover.
Damm that might just work! Thanks Matt
Here is a cheaper interior storm that I have used ...
https://www.windowinserts.com/
There is no "right answer" as to the ideal winter-time, cold climate RH, but I do think 50% is a bit high for southern NY (are you zone 6?).
Joe Lstiburek, in the comments of a youtube video titled "Is 50% Relative Humidity The Right Amount?" says:
"In cold climates during winter months I typically recommend 30 percent. In hot humid climates I recommend 60 percent. Note that this is for residential/commercial occupancy."
Lots of other sources seem to suggest somewhere around 30% and up to 40% (cold climate).
In any case, most people can't afford to replace windows to deal with a condensation problem, so lowering it at least a tad is probably a good idea.
More details would help:
What is the heating system?
Are there functioning bath and kitchen exhaust fans?
Gas stove?
Lots of plants?
How many occupants and what is the building square footage?
Is the soil clay heavy and/or with pooling water near the foundation?
Lots of showers and cooking?
The idea that: "ventilation would obviously help, but that it [is] not the primary cause ... there must be some 'source'" strikes me as akin to saying: "more insulation would help, but it is not the primary cause of being cold; the too small of heating system is."
In other words, it's all about the balance of moisture produced vs moisture expelled. Moisture is always generated and it must always be expelled. It's really a question of whether the amount of moisture being produced is due to some undesirable or preventable source, or whether it is due to simple living. If the latter, more ventilation is absolutely the solution. (i suppose a dehumidifier works too, but having some ventilation is good for other reasons too).
Your note that the humidity seems to bounce back after closing the windows is not too surprising, assuming there is plenty of hygroscopic materials in the house (e.g. wood, drywall). They act like hygric inertia. If you search high and low for extraneous sources and come up dry, I would try increasing ventilation, even if by temporary means (running all exhaust fans, cracking a window) for an extended period of time and seeing what happens.
At 40% RH I wake up with a sore throat in the morning.
Those recommendations don't make any sense, except as minimums. Is 50% RH in any way unhealthy? In heating climates, usually the struggle is getting the humidity high enough that it's not uncomfortable.
you don't live in a cold climate. I wouldn't be giving out advice to people living in cold climates to run 50%rh when you don't know any better. Look it up. Sorry about your scratchy throat.
By "southern New York" I assume the poster means White Plains or nearby, where the design temperature is 18F. My design temperature is 22F, not that different. Current temperature here is 5F warmer than what Weather.com is telling me for White Plains. I have three humidity sensors in my house, right now they're reading 45, 49 and 50. I have no issues that I'm aware of with winter humidity or condensation.
Perhaps the poster is closer to your design temp than mine, I'm not sure. And certainly there would be a sliding scale depending on where exactly you are.
My point is that all the true experts, so far as I can tell, state that running up around 50% in cold climates has a higher statistical risk. This doesn't mean no one should ever run 50% or that no structure can handle it. It just means that given the reality of our building stock, there is slightly higher risk when running that sort of RH in cold climates.
So as general advice for the general populace, without more context as to the specifics of the building envelope, I don't think the advice should be that 50% RH is ideal if you are in a sufficiently cold climate. Whether the poster is in a 'sufficiently cold climate' I guess I don't know, but for my tastes, a would run it a tad lower.
As an aside, in the youtube video I mentioned above, Joe states that he thinks the newest research doesn't support 50%rh being the "most healthy" and that it can in fact be lower. But I certainly won't claim to be able to make statements of fact or even opinion on that medical topic. Most likely, it is a matter of tradeoffs.
I think the problem is more the risk of condensation in very cold climates on various condensing surfaces in your assemblies. In general, I see most of the testing and analysis for the safe assemblies tested with an assumed indoor humidity closer to the 30% mark. With a high indoor humidity I think you would get riskier assemblies, but I’m sure there are a lot of other confounding variables to account for as well, ie quality of install will probably be a bigger factor.
Thanks everyone. There is nothing unusual about the house. No plants, no gas stove, no drainage issues (in fact we had an epic drought last fall). How water baseboard heat.
There are no comfort issues in the house; this is more an issue of protecting her assemblies. Just for the record, here is more detail on how the issue was explained to me.
Outdoor conditions recently have been at 45F/50RH = 22 grains per pound (gpp) of water
Indoor = 66/50 = 48 gpp. Even with the lack of a ventilation system, it's still getting an air change approximately every 7 hours based on a very rough estimate of ACH (natural). Plus, she has been opening windows for about 30 minutes daily. I'm told for the interior air to consistently have 25 gpp of extra water vs outdoor this time of year is very odd....and even if they were showering and not using exhaust that it would still be odd.
I have a basement pro who is BPI certified coming out tomorrow to see if he sees anything unusual in basement.
Looking at weather.com, right now in White Plains it's 34F and 49% RH.
Let's say the house is 2000 sf, average floor height of 9 feet so a volume of 18,000 cubic feet. ACH50 is 2.6, let's say ACH natural is 1/20 of that or .13 ACH. So that's 2340 cubic feet of ventilation per hour, or 39 CFM.
If it's 67F/47% inside and you're exchanging 39 CFM with 34F/49%, you're losing 0.8 pints per hour of humidity. The rule of thumb is each occupant of the house generates 0.2 pints per hour from their activity. So if you have four occupants that would sound about right. If you don't have that many occupants, either the infiltration isn't that high or more humidity is being generated inside the house.
Didn't see your post #21.
The formula for pints per hour is 0.68 times CFM times change in grains per pound divided by 1000. So 25*39*0.68/1000= 0.67 pints per hour.
DC - thanks this is helpful. I dropped the most accurate numbers I have into your calc and came up with .44 pints lost per hour. There are generally 2-3 people in the house (plus 2 cats LOL). The numbers don't account for every single thing like bath vents and open windows, but it is helping to illustrate that with the CFMs In/Out so close, maybe it is just a ventilation issue and 60 CFMs from an ERV or Santa Fe would easily drop it this time of year. Correct?
12,500 Cubic Feet
2.48 ach50
0.124 ach nat @ 1/20 (estimate)
26 CFM leakage
25 Average Diff in GPP between outside/inside
0.44 Pints Lost per Hour
.4-.6 Pints Generated per Hour