GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Humidity data after CCSF in attic

whnh | Posted in General Questions on

Hi all,

Due to a pre-existing air handler in the attic, I bit the bullet and spray foamed the attic ceiling/gables to make this a conditioned space. I had a good month of data after the insulation and with no HVAC on in the house (due to other work going on) and the attic temperature was consistently around 80 (not far off the second floor temp) with RH around 50%. 

On Monday we turned the HVAC system on set to 68F. The house, including the attic, quickly cooled down but I am now seeing attic relative humidity in the range of 65-75%. Granted it is extremely hot and humid outside right now so I’m not sure that this is all that representative. I know the foam will keep the moisture from the roof sheathing but should I be concerned about these humidity levels? The dew point in the attic over the whole month has been much more consistent and didn’t trend up with the HVAC on which I think means moisture in the air is roughly the same and the RH is going up because the temperature has dropped…but I’m not sure if it’s the total moisture or the RH that I should be focused on.

The data is attached and red arrow shows when the HVAC went on.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. aunsafe2015 | | #1

    I'm no expert, but the thing I immediately notice is that it's dropping down to 61 degrees in your attic despite the fact that it's "extremely hot and humid outside right now"? Given that humidity is relative to temperature, it doesn't seem a huge shock that dropping way down to 61 degrees would yield a corresponding spike in relatively humidity.

    As to whether the humidity levels are a problem, and how to fix them, hopefully others can chime in, but my first thought would be to consider supplying less air from the a/c to the attic.

    1. whnh | | #2

      I agree that that is what's happening. I don't have a supply or return register in the attic but the conditioned air is clearly making its way into the attic which isn't surprising. I just don't know if that RH number should be a cause for concern

      1. aunsafe2015 | | #3

        I'm not knowledgeable enough to say whether it's cause for concern or not. If your attic is dropping to 61 degrees, even on hot/humid days, it would seem that the ductwork in your attic is probably pretty leaky. Could have somebody come out and test your ductwork.

        Another option would be to put a dehumidifier up there and keep it at a relatively high setting, something like 60%.

      2. brendanalbano | | #5

        My understanding is that now that you’ve converted your attic into a conditioned attic, you want to condition it, just like any other room in your house! This could mean adding a supply and return to the attic. Hopefully an hvac expert can chime in.

        1. aunsafe2015 | | #6

          But I don't think anybody would think it's good or necessary to be cooling it all the way down into the low-60s in the middle of the summer. I think that's the general source of the OP's concern.

  2. user-5946022 | | #4

    How about buying a bucket of duct seal, and going up into the attic to seal the HVAC unit and it's ducts? This would theoretically reduce the amount of cooled air leaking into the attic, thus reducing the cooling of the attic and increasing the dewpoint, and (I think??) reducing the relative humidity.

  3. user-2310254 | | #7

    Have you looked around to make sure there are no "extra" supplies serving the attic. My recollection is that as little as 50 CFM per thousand square feet is required to manage moisture. It would be worthwhile to poke around to how so much cooling is getting into that space.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |