High indoor humidity levels in Chicago Area


I am seeking help. I live in the chicago area, and todays temps are about 12 degrees high, and a overnight temp of 3. That bieng said, my indoor temp is currently 68 and relative humidity is 43%. I am a Illinois licensed plumber and 100% positive there are no plumbing leaks, no known roof leaks, no fish tanks, no tropical plants. All bathrooms vent to exterior soffit via metal pipe, ,and have a 200 cfm fan bathrooms with showers. I have batteling humidity for 5 years to no avail.
Here’s what I am wondering: I have a single story ranch with a 4\12 pitch. There is a section of the house that is a “great room”. This room is 20’ x 14’ which peaks at 13’ tall and tapers to 8’. I currently have 4 mushroom vents on the back of the house with 24" soffit vents. I removed the soffit vents and added plastic air vents stappled to 1x2’s. I then fished the 12’ sections up the vaulted ceiling with a contraption I made with PVC pipe. This pushed the insulation down and allowed me to push the vents to the Ridge.
Sorry so much info! I am wondering if i would be better suited for a ridge vent vs the mushrooms to allow better air flow. The front of the room (13’ long) is all porch and does not have air intakes of any kind. I also went around the whole attic (2500 sq ft house) and installed air baffels in every bay. All this was done 2 years ago.
On cold days like today, I have ice on the underside of the sheating. In the morning, I will wake up to a few water stains on the vaulted ceiling. No other water spots elsewhere. The flat part of the attic has a R-60, and the vaulted has a batt stlye R-38.
Thanks in advance for any help!

The short answer… You have moisture entering your attic. It needs to be exiting, which you know its not. You need to find the source.

Every time I come across a situation where the vents go to the soffit or into the attic space I find mold. I suspect the vents going to the soffit are the culprit. And a lack of ridgevent isn’t helping.

Thanks for the reply. I failed to mention that the bathroom fans have a “termination kit” that exhausts them on the underside of the soffit. Is it possable that the day to day humdity from 3 people living in the house is rising to the attic and not escaping do to lack of ventaltion???

Ridge vent the whole thing first. The mushroom vents only vent the individual rafter bays.
Gable vents might be in order if you have consistent heavy snows that cover the roof ridge.
Add some venting to the front eave.
Any vapor barrier?

Thanks again!
There is a vapor barrior in the batt insulation. The whole attic has that, then there is blowin in on the flat part of the house as well as the front side of the vaulted cieling. The “front” side follows roof line but has bit of accesable area in attic. The back side is rockd to the 2x12 rafters.
I also failed to mention that the peak of the inside vault is not the true vault of the roof. There is a 2’ area in the attic i can see but not acess under the center beam on roof line. In that “open space”, there is where the mushroom vents are.
I am completely out of ideas, and have resorted to the thought that there is not enough air flow in the attic?

When the fans blow out the soffit, do they not just blow next to the other soffit panels? The soffit is the intake as you already know.

Could you describe your intake/soffit more?

I theory the air could go right back up. I cold weather, I can see the condensing air from the fan. It blows out past the overhang and then quickly blows away. I suppose a solution would be to exit through roof an flash as needed?
The soffits are aluminum 20" pannels with air intake “grills” every 4 feet. Each panel interlocks and are 1’ long. I just went up in the attic, and the temp is 31.1 degrees. The outside temp is currently 16. There is a layer of frost on the sheathing.


Out the roof would be best Ben.

You are on the right track. I am not as good at explaining things so hopefully someone will jump in…

You need more intake. Every panel should be a venting one. And ridgevent would be a better. Search on here for EdTheRoofer.

As you know (by the frost) You have moisture entering your attic and it is not escaping. More intake and more exhaust is needed.

Is the whole deck covered in frost?

You already know all the answers, so i am just going to confirm it for you.

There needs to be ventilation all around across the bottem.(yes, your poarch if its roof enters your main house roof) And for it to leave across the top somewhere. ridge-venting, off-ridge-venting,
gable-venting, power-venting.

No insulation can be blocking it.

If there is a way for the Air/moisture to escape through the top and enter through the bottem, then i think it is insulation blockage. (at the soffits, usually when involving cathedral ceiling.

If all pasage ways are really clear,
then those bathroom exhaust pipes exiting the soffit are not really exiting.
Soffit is the intake area…

Do all these things or quit causing heat in the house.
You know all the answers. Re-look them again.

Hi guys,

I am 100% certain that all passage ways are open with baffles in all of them as I did it my self. The vaulted ceiling had the insulation blocking air flow, but I moved that. I did this by removing the soffit panels on the back of the house, cutting “long” insulation, and the fishing the baffles up the vault which were stapled to 1 x 2 's. I can see them from the attic, and they are all between the plywood and the insulation.
I am just concerned that there is not enough exhaust on the vault. There are about 15 bays that are 16" wide. As I stated, there are 4 mushroom vents for that area, but I wonder if the vents only vent the individual rafter bay?
In regards to the fart fan the only option is to cut open the ceiling and poke through roof. I could alSO rig up something with PVC pipe to divert the air to ground level. I feel that the air is not going there due to the fact I can see it in the cold weather? Don’t really know
How much is enough make up air for a soffit. I will walk the parameter of the house and get a total.

I don’t know if it matters, but there are 39 of the 20" x 12" aluminum soffit intakes. They are all free with no wood under them!
There are 10 of the mushroom vents on the roof top. That total includes the 4 on the vault, and 2 on the attached garage that is 2 1\2 car. There is also a non working 1200 cfm fan that is a close to the vault a I could get it. It is now burned out. It has a humistat, which would slightly help the cause in winters past.


Thats the first that caught my eye. I’m sure there is better.

More soffit vent, more ridge vent, and move the fan vent.

Last time. :slight_smile:

Goggle proper roof ventilation if you aren’t sure!

I see similar problems frequently. As you stated, you find it more over the heat ducts than other locations. You have three choices, more ventilation in that area, less heat loss, or combination of both. The soffit vents aren’t pulling enough makeup air and the heated air exhausts. The rafters make for nice open duct channels.

I didn’t read all the replies, sorry.

How many people in your family? If you have alot of cooking and showeres going on that will increase your humidity level.

How high is your humidistat set on your furnace? That’s too obvious and I am guessing you probably already checked it.

Those bath vents vented to the soffit might be a problem. Are they terminated with a proper baffled soffit vent, or are they left exposed?

Do you have a heater and or heating duct work in the attic that could condense?

Attic ventilation is a science that is unique to the architecure of each building. Could you post a photo? Genrally on a hip roof, I find the ridge line to be too short for ridge vent, but not always.

Oh one more thing, do you let the bathroom fans run for 20 minutes after each shower?

Solar fans are also a great alternative when I find tricky attic humidity problems.