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Humidity in unvented attic (but we have closed-cell foam insulation)?

jeasto | Posted in General Questions on

Hi everyone,

We have been living in our  air-tight house (.44ACH50) for a couple months now. We live in climate zone 5.

We have an unvented conditioned attic space that we use for storage. Our ERV and the indoor component of our Fujitsu unit live up there, too. We have closed-cell foam sprayed under the roof sheathing, and then there is blown insulation under that (for a total of R-93)

We have noticed that the attic’s RH tends to stay 5 to 10 percent higher than the rest of the house. This wasn’t that big of a concern in the summer. The attic stayed between around 50 to 55 RH and the rest of the house was usually around 45 to 50 RH (we tried to keep it toward the low end).

However, it’s starting to get cold at night, and I have read that best practice for airtight houses is to keep indoor RH 40% when it’s 40 degrees and 30% when it’s 30 degrees.

The attic was at 51% RH, and the outdoor temps have been 60s in the day, 50s to 40s at night. We attempted to get the attic humidity down for the past two days. Using a combination of dehumidifiers and the “dry” mode on the Fujitsu, we got the main part of the house down to 35 RH, but the attic would not drop below 46. (We kept the attic stairs open, hoping for air exchange.) Then, overnight, we decided to turn on the fan option on the Fujitsu, thinking that would circulate the air in the house and even things out. It only budged about 1%.

We have a supply vent in the attic but not a return.

From what I can tell reading on GBA, elevated humidity in the attic is usually due to closed-cell spray foam, not open cell. So what’s going on here?

And should we be concerned? It is possible this elevated moisture is due to construction moisture and will eventually resolve itself? Is it possible the blown insulation is doing something? Do I need to be concerned if we can never get the attic below 45% RH, even in the dead of winter? Should we install a return grille in the attic (apparently this can be a code violation)?

I am just a homeowner. Not a builder. We are trying to be hypervigilant about potential moisture problems.

Thanks in advance!

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  1. Expert Member
    Michael Maines | | #1

    What are the temperatures in your attic and living spaces?

    1. jeasto | | #2

      The main living area hovers around 73 degrees. In general (before we started trying to get the humidity down), the attic was a few degrees cooler than the main living space.

      After running the fan mode, both the attic and the living area were about 73 degrees.

      When we have the "dry mode" of the Fujitsu on (which prioritizes dehumidifying but also will eventually cool the air), the attic will drop to about 70 degrees. We only keep that on for about 2 hours at a time, which is about when the dehumidifying effects top and the house is on the verge of becoming too cold.

      1. Expert Member
        Michael Maines | | #5

        The reason I ask, which you may or may not already know, is that since the attic shares air with the living area, the two spaces actually have the same amount of moisture. 73°F air at 45% RH will be about 50% RH at 70°F and 60% at 65°F.

        1. jeasto | | #6

          I ~sort of~ know that. What I don't understand is why advice tells you to have a certain RH depending on the outside air. If what is happening is simply that the attic is cooler, so the moisture level is the same but the RH is higher in the attic . . . how does that square with the advice that you should be at 40% RH when the outside air is 40 degrees F?

          I think it has to do with dew point, but I actually don't understand what that means re: moisture where you don't want it.

          What I really just want to know is . . . how do I know if my humidity level is at a good level / if I'm at risk? It seems like everything is a moving target. I've been told that things should settle down once the construction moisture has dried out, but that can take a while and I should pay attention to what is happening.

  2. Expert Member
    BILL WICHERS | | #3

    Open cell spray foam can be an issue with moisture, closed cell is OK.

    You're looking at a 5-10% difference in humidity levels. You might not even be seeing what you think you're seeing, since humidity is notoriously difficult to measure accurately. It's entirely possible that if you're using two seperate sensors, they might differ from each other by more than 10%, making comparisons between the two not particuarly useful. If you use the SAME meter, and allow a few hours for it to stabize in each location before taking a measurement, that would be more reliable. Never assume that two measuring devices are actually accurate if they haven't been calibrated -- especially when measuring very small differences --, which almost no devices targeted at the residential market are.

    Assuming you ARE getting reliable measurements, I wouldn't worry too much here unless you're having condensation problems. Humid air tends to rise, so it's normal for conditioned attics to be slightly more humid than the rest of the house. More airflow through the attic would help to equalize things, which might require adding a return register up there so that air can circulate and not just "pool up" and pressurize the attic space slightly.

    BTW, is your blown insulation held up against the spray foam under the roof? If it is, no probelm. If it isn't, and the loose fill is on the attic floor, than the R values don't really add together. That's something that has come on in the forums on GBA in the past.


    1. jeasto | | #4

      Thank you, Bill. That makes sense. So you don't think I have to worry about the roof deck part getting moist and if there is a problem, we will know by examining the inside of the attic? I guess I was worried something could be going on beyond the insulation, where I can't see.

      We have two brands of meters, but the one we have in the attic is also one we have downstairs, and it shows a difference between the two.

      These two are supposed to be accurate within ~2 to 3% (they were relatively expensive and can connect to our phone). We also have some cheaper ones downstairs in other rooms (Accurite brand) that are also supposed to be accurate within ~2 to 3%. We have noticed that these seem to underestimate humidity by 2 to 3 percent and the expensive ones seem to overestimate humidity by 2 to 3 percent (if the manufacturer can be trusted). We felt that somewhere in the middle of the two readings might be most accurate.

      Yes, our blown insulation is held up against the spray foam under the roof.

      1. Expert Member
        BILL WICHERS | | #7

        Two meters from the same company doesn't necassarily mean they will read the same. Calibration is the act of setting the meters so that they agree with some reference standard. Some meters will let you manually calibrate them, so you could at least make them agree with each other to allow you to make relative measurements (compare one to another), even if the absolute reading might not be completely accurate. A real lab-type humidity meter can be several hundred dollars. I don't usually trust the little consumer-grade meters for anything needing high accuracy, because I've seen too many of them that aren't very accurate, despite the claims of the manufacturers.

        I don't think you really have any real issue in your attic though. As Michael correctly points out, relative humidity depends on the temperature, since it is a measurement of how much water is in the air compared to how much water the air can hold AT A SPECIFIC TEMPERATURE. Warmer air can hold more moisture, so a given volume of air (such as the enclosed volume of your attic), will show a higher %RH when it's warmer, even though the absolute amount of moisture in the air hasn't changed. The same goes for the cooler attic vs warmer house as Michael pointed out.

        Your measurements are within the error band that can be expected for humidity measurements, which means they are "the same" as far as statistical significance. Slight temperature differences can explain small variations between the humidity level in the house and attic. As long as you don't have condensation problems, you really don't have anything to worry about here I think, although you could add a return for more airflow if you want to see things a little closer.


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