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Community and Q&A

Moisture in Attic – Dew Point

ChuckJupiterFL | Posted in General Questions on

I have made several improvements to the building envelope of my house that was built in 1985. I have added a metal roof, changed out and replaced windows and sliders with hurricane impact glass. I have likewise changed out all doors with hurricane impact doors. Finally, I added a new Trane 4 ton 21 seer variable speed a/c unit. When I had the a/c replaced, I also had all of the duct work replaced. My wife likes to keep the temperature at 75 degrees during the day and 71 degrees at night. 

Last summer I did not notice any problems. This summer has been hotter than any summer I have experienced, just like the rest of the country. Humidity is high and the dew point is higher than past years. I feel confident that I do not have any leaks in my roof and there are no pipes leaking but certain walls and ceilings in my house are holding excessive moisture. 

 When I ruled out roof and/or pipe leaks, I started looking at other potential causes. I noticed that my a/c contractor did a horrible job on the installation of the new duct work. Most all of the joints appear to be not properly sealed with moisture collecting around the joints. I also noticed that my insulation on the floors of the attic is old, insufficient and completely missing in some areas. Also, I suspect there is a lot of leakage through the penetrations of the attic floor.

A professional roofer recommended an attic fan, but based upon my research, I think this will make my problem worse. I think my problem relates to condensation and I am having trouble finding the appropriate person to address this issue. 

Any comments on fixes or recommendations of who to call in the Jupiter Florida area  (close to West Palm Beach) to help address this problem will be greatly appreciated.

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  1. walta100 | | #1

    Let me start by saying it is a silly idea to put your HVAC equipment in your attic. Yes, I know everybody in the south does it. Like my mother said just because other people do something does not make it a good idea. I understand I might as well go yell at the clouds because this dumb idea is not going away.

    Seems to me since undoing the poor choice of locating the HVAC in the attic is off the table the only option left is to condition the attic if you insist on keep home at a lower temp than the outdoor dew point and that sound like it is nonnegotiable.

    You will need to fully condition your attic and the only viable option for insulation is applying spray foam to the roof inside the attic and installing HVAC registers in the attic to keep the attic at more or less the same temp and the rest of your home.

    Note you should remove the old insulation from the attic floor as now it is working against your new goal of keeping the attic at the same temp as the house.

    Note spray foam is risky in that it is complex chemical reaction being performed in uncontrolled conditions by cowboys. Spray foam is the most expensive and least green way to achieve R value.


    1. ChuckJupiterFL | | #2

      Thanks Walta for your response. The A/C system is in the garage and outside. The duct work that delivers the cold air runs through the attic.

  2. walta100 | | #3

    Having the equipment in the garage is both better and worse.

    The equipment and ducts are still outside of the conditioned space but likely cooler than the attic. The garage is almost certainly on one end of the house so the ducts must be very long.

    I would try to find a contractor that owns a piece of test equipment called a "Duct Blaster" have them test and seal the leaks in your ductwork then incase the ducts in close cell spay foam and bury the foam under a foot of fluffy insulation.


  3. user-5946022 | | #4

    Walta is correct in that running ductwork through a space that is at the extreme opposite of the conditioned air going through those ducts is not ideal. However, 100's of thousand houses have this condition. If you can address this condition it is ideal, but if you address it and they do a poor job, you have a bigger problem.

    Humidity rises. So it goes from in your living space through any air leak and into your attic towards the ridge.
    1. Do you have a ridge vent? This will help immensely. If not, tell us how the attic is vented.
    2. You likely have ALOT of air leaks between your living space and the attic. Fortunately it is easy to address. Seal every penetration. At every j box for a light fixture, etc, move the insulation and caulk all around the gap between the drywall and the box. Install putty pads over the box and around all wires to seal that up. At all duct boots, spray foam around the perimeter between the boot and the drywall. At all penetrations in the attic where wires or pipes come up through the top plate, spray foam in the holes. Seal any can lights. Seal attic access.
    3. There is also a high probability your AC is oversized and thus cycles and thus does not remove humidity sufficiently from your living space. Get some cheap sensors and determine the interior humidity of your house (also get some for your attic and put them in various places include near the ridge). Get a dehumidifier and try to keep the living space humidity at 50-55%. This will also make your house much more comfortable and possibly let you raise your temp with the same level of comfort. If your living space humidity is already at 55% or below, then consider putting a dehumidifier in your attic while you work on items 1 & 2 above.

    What you should NOT do:
    a. Attic fan. That will just pull more conditioned air from inside your house into the attic. It is unlikely to solve anything.
    b. Seal the duct in attic - eventually you will want to do this. However until you resolve the humidity problem, the ducts leaking a bit of cold conditioned air into the attic is helping you - colder air has less moisture in it. Once you solve the problem definitely seal the ducts.

  4. ChuckJupiterFL | | #5

    Thank you User-5946002 and Walta. On further investigation, I believe the duct work in the attic has some serious leaks going on. The dew point here in Florida has been running between 78 to 80 degrees lately. Can the cold leaking air from the ducts in the attic which temp is below the dew point be causing the excessive moisture in my walls? I bought several devices to monitor the temp/humidity and put one in the interior of the house, one outside and 2 in the attic. Interestingly, in the morning hours the attic is about 10 degrees cooler than the outside air which I thought was odd. At night, we turn the temp down to 72 degrees. The attic temp did not equal the outside temp until 2:30 pm and slightly exceeded the outside temp until 6:30 pm which thereafter started seeing temps drop below the outside temps. It seems that I must be having a bunch of a/c ducts leaks which are conditioning my attic spac (along with any leaks from the interior of my house into the attic space). Most of the duct joints do not have any mastic but rather just the metal tape. The ones that I could get to were sweating pretty bad.

  5. Expert Member
    BILL WICHERS | | #6

    Foil duct tape is better than the fabric-type stuff, but not as good as properly applied mastic (emphasis on "properly applied" here, as mastic is often applied in a very thin paint-like layer that isn't really effective). Mastic is supposed to be "nickle thick", so it should form a sort of sheet-like seal, not just a painted-on smear of a coating. Be sure to seal both supply AND return lines.

    You also want to INSULATE those ducts in the attic. That's important too.

    My guess here is that your A/C system is drawing down the temperature in the attic during it's heavier nighttime run cycles, and that's why you're seeing the lower than expected attic temperatures early in the morning. Depressurizing the attic with an exhaust fan is not likely to improve things, and may cause other issues. Fix your ductwork first, then look into attic ventilation if needed, trying to stick with a passive (ridge and soffit vents) system. Ideally you want a little more "intake" (soffit) venting then "outlet" (ridge) venting so that you don't depressurize your attic, which would worsen any/all air leaks between the living and attic spaces.


  6. ChuckJupiterFL | | #7

    Thanks Bill. I lost the ridge venting when they installed the metal roofing. I was surprised to find out that around here they do not install them and it is not required by code. I can have a ventilated ridge cap system installed for about $2,500. I have been waiting for someone to diagnose the problem before I start spending money on potential fixes, that could even make the situation worse. I have someone coming Tuesday to do pressurized testing and a complete review of my a/c system on Tuesday.

    Would love to hear further input from you as to whether you think a ventilated roof cap would help the issue. I spoke to a couple of roofers and they really do not have anything to say but that is just the way we do it. One roofer said it will reduce the strength of the roof during a hurricane and will be a place for water infiltration during a heavy storm / hurricane situation. Do not know whether that is correct or not.

  7. ChuckJupiterFL | | #8

    Bill, by the way, in reference to the mastic, they applied it in very few spaces and very sparingly. I would not be surprised to find that they did not apply any mastic to the inner sleeve. I am actually going to remove one of them tonight to see how they attempted to seal the inner sleeve. Also, in reference to the supply ducts and returns, they did not apply any mastic to them in the interior of the house and I found many punctures and areas for leaks in them. I am just about done sealing them up myself.

  8. walta100 | | #9

    “I would not be surprised to find that they did not apply any mastic to the inner sleeve.”

    Until I read this statement, I had envisioned rectangular steel duct work. Now it seems likely that is wishful thinking and your attic is full of 38-year-old plastic flexible ductwork? If that is the case the plastic is likely due for a replacement or upgrade to steel.


  9. user-5946022 | | #10

    You wrote in your first post that your primary concern is humidity/moisture. In post #5 it sounds like your primary concern is temperature in the attic. Temperature alone is not much of a concern. Moisture (high humidity) can cause numerous other problems so it is a concern. Can you please confirm your concern?
    1. Do you have a ridge vent? In #7 you noted you do not. A ridge vent will help moisture escape your attic. How is the upper part of your attic vented? If you have a vented attic with soffit vents but no top vent at all I'm not surprised you have moisture issues.
    2. Do you have leaks between your living space and your vented attic? Have you had a blower door test done? If so what was the result?
    3. Have you measured the actual humidity in the attic? What are the measurements?
    4. Don't seal the ducts until you get the humidity under control. The leaky ducts are helping.

    5. It is totally normal for your attic temperature (and humidity) to fluctuate. Also a metal roof, if it is reflective, will help keep your attic cooler than a shingle roof.
    6. You posted in your first post that you had the ducts replaced when the AC was replaced. You in #8 you referenced an "inner sleeve" of your ducts. Did you have your ducts replaced with flex, that have an inner sleeve and outer insulation? If so, those don't get mastic. The hard metal ducts are what get mastic. Regardless of whether your ducts are flexible or hard, there should not be punctures in them. Whoever installed your new ducts ought to be responsible for fixing this, as the code requires them to test the DUCTS for leakage and meet a particular threshold. What you describe does not sound as if it would pass.

  10. ChuckJupiterFL | | #11

    I have excessive moisture in various areas of my drywall. I have no roof leaks or pipes leaking. After confirming there are no roof or pipes leaks, I was left with the conclusion that the attic conditions must be causing the problem.

    I have much info since my initial post. I have a large 20 foot interior wall in the center of my house that is centered directly underneath the ridge cap. This all began when I started smelling a musty smell coming from this wall. It was collecting a large amount of moisture where it started to smell musty. My initial logical reaction was the ridge cap must be leaking. I had a roofer pull the top of the metal ridge cap off. The ridge cap was covered and sealed with the underlying peal and stick with zero signs of any water making its way to the peal and stick. Additional caulking was applied to make it impossible for any water to get past the metal ridge cap.

    I have been keeping the moisture in this wall under control by running a dehumidifier right next to it. If I turn the dehumidifier off, the moisture in the wall comes back fairly quickly despite the fact that there is no rain. There are no water pipes near this wall. Most of the water plumbing is in the foundation. There is only a short run of plumbing in the attic which is not near the wall. I confirmed this small area of piping in the attic is not leaking. None of the walls near this pipe have any moisture issues.

    My initial concern was I must have extreme heat in the attic that is trapped because of the loss of the ventilated ridge cap that occurred when the metal roof was installed. A roofer suggested I install a solar powered fan to help circulate the air. Before making that change, I started monitoring my attic temperatures. I have two temperature devices which also measures the humidity in the attic that I try to record the data every half hour. To my surprise, I do not have an overly hot attic. The white metal roof is doing its job of keeping it cooler. Generally speaking, the temp of the attic is about 8 degrees lower than the outside temp in the morning which I thought was odd (leaking duct work cooling down attic?). In early afternoon when the temps get close to their peak, the attic temp matches the outside temp. By 5 or 6 pm, the attic temp will be around 6 to 8 degrees warmer than the outside temp and thereafter will drop back down below the outside temp before I go to bed. During this testing period, I have been running my thermostat at 77 degrees during the day and 72 at night. The humidity readings have no pattern like the temp readings. For the most part, they are pretty close to the outside readings but at times can exceed the outside humidity readings by 10 but seem to drop at night and can be 10 to 15 times lower in the evening hours. During these readings, the humidity outside has been typically running in the mid to high 80s.

    I also purchased a thermal imaging gun which in the heat of the day showed my attic temperatures ranging from 90 to 109 degrees. I went to my neighbors house at the same time which is a cookie cutter copy of mine with also a recent new white metal roof, which showed a temperature range of 95 to 118.

    I am definitely not qualified to use the thermal imaging gun but it appears to show some significant cool spots in my ceiling. One significant run goes right along where the duct would run to feed one of the supply lines. There are no entry penetrations for a leak in my roof at this location suggesting a major duct line leak.

    The original metal duct work in the attic was fully replaced with insulated flexible duct work in 2021 when the a/c replacement occurred. As a back drop, the entire a/c system and the duct work was done in one day by 2 people. I am told that this should have taken much more time to do such a large job, so quality of work was probably lacking. In reference to the interior, several returns were added per code. I checked all of the interior vents and supply vents, none of them were sealed as required by standard practice with many places for obvious leaks. I am almost done sealing all of the potential areas of leaks for these return and supply ducts from the interior of my house.

    In the attic, there looks to be many things that were done wrong with the installation of the duct work. I am no expert of course. In attaching the flexible duct work to the boxes, my research shows that the inner sleeve should first be secured with basically a huge zip tie, with mastic next applied to seal it and left to dry. Next, the insulated part of the flexible duct is pulled over with mastic applied all around it. With mine, the duct work is connected with the metal tape. The insulated part of the flexible duct does not seemed to be flush with the box. The gap between the box and the flexible duct just feels like the tape. It is sweating a bunch in this area. I have seen temp readings in these joints as low as 70 degrees with the insulated duct registering a consistent 85 degree temp. Right after I post this, I am going to cut the tape at the joint to see how it was attached underneath.

    I think it is safe to assume that I have the typical leaks between my interior space into the attic. These areas of leaks existed since the house was built in 1985. What has changed, is the addition of the new roof and a/c system. I have also changed out all windows, sliding doors, and exterior doors with hurricane impact.

    The attic insulation is the original insulation that was installed in 1985. There are small areas here in there were coverage is missing that likely occurred from the installation of the new flexible duct. I suspect that I probably should upgrade the insulation as one my my potential fixes to this problem.

    I have someone coming Tuesday to do a complete review of my building envelope and a/c system. He is going to do a blower test. Because it is so hot here in Florida, anyone doing any kind of a/c work is very busy. It was the quickest I could find anyone to come out and investigate.

    My unqualified opinion right now is this problem is being caused by the existence of many duct leaks in the attic, with a contributing factor of leaks from my interior space into the attic. Hoping to get my answers next week from the upcoming blower test and inspection.

    Any questions and your comments are so greatly appreciated.

  11. ChuckJupiterFL | | #12

    I made a small cut at the seam of one of the duct work connections in the attic. The inner sleeve was attached to the collar with a zip tie, but no mastic was applied around the inner sleeve as required by standard practice.

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