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Conditioned attic “venting”

E247 | Posted in General Questions on

I’m building a house in Climate Zone 2A (Hot/Humid) and it will have open cell spray foam on the walls and roof deck.  In order to control the humidity levels in the attic, it is suggested that a supply duct from the HVAC be installed in the attic.  Alternatively, air can be supplied to the attic by a fan blowing air from the occupiable space into the attic.  The physical installation of the fan blowing air into the attic seems straightforward, but my question is about the location the air intake in the “occupiable” space and the location of the exhaust into the attic.   Where would the best place to pull air from (closet, living room, etc.)?  Does it matter where the air is exhausted into the attic (near the highest point on the roof, opposite side of the house from the HVAC return, etc.).

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    E247,
    First of all, can you tell us your name? (I'm Martin.)

    The issue of high humidity in unvented conditioned attics insulated with open-cell spray foam is complicated. There are no simple answers to many questions on how to handle this type of humidity problem. In general, I advise builders and homeowners to avoid the use of open-cell spray foam for this type of attic -- closed-cell spray foam is safer.

    For a thorough discussion of the issue, see "High Humidity in Unvented Conditioned Attics."

  2. E247 | | #2

    I've read that article (and many others) and my question is more about supplying air from the conditioned space to the attic. Is there preferred place to draw air from and supply air to or is a supply register from the HVAC system the best option?

  3. Steve Knapp CZ 3A Georgia | | #3

    I had my HVAC installer install a supply delivering about 40 CFM when the system is operating. (But my attic is fairly small. What did your designer recommend.) He also wanted to install a return from the third story living space to the attic. But I am delaying that step until I have more data on conditions in the attic.

    To collect that data, I installed a SensorPush sensor near the attic peak. Long-term I may install a vapor diffusion port or mechanical dehumidification if humidity levels in the attic become worrisome.

    1. E247 | | #4

      The HVAC design called for a 50 cfm supply vent in the attic, but it was missed during the installation of the ducts. I can have them come back and install the supply vent, but I think it would be easier to use a fan to supply air from the house to the attic. My main question was where is the best place to draw the air from and if it makes sense to exhaust this air near the peak of the roof since that's where the humid air tends to collect.

  4. Steve Knapp CZ 3A Georgia | | #5

    It is my understanding that what you are doing is returning air to the attic. A builder I know always puts in a return to allow air from the main living space to mix with the air in the conditioned attic. He does not install a fan. Instead, I believe he is relying on the stack effect. I'm not sure it really matters where you install the return as long as its not in a bathroom, kitchen or high-moisture environment.

    That said, I would suggest monitoring the temperature and humidity levels in your attic.

  5. Jon R | | #6

    Exhaust as high as possible and sufficient face velocity for good mixing makes sense to me. I wouldn't put the intake right at the ceiling (the air there has more moisture).

    While code provides several options, I'm sure they aren't exactly equivalent in effectiveness or energy loss. I expect that hvac air is most effective.

    I like Steve's idea of optimizing based on measurement (preferably at the wood sheathing).

  6. GBA Editor
    Martin Holladay | | #7

    E247,
    Don't forget the fire safety issue -- building codes forbid exposed spray foam in an attic if there is communication between the attic air and the air in the conditioned space below. For more information, see "High Humidity in Unvented Conditioned Attics."

    In that article, I wrote:

    "Because spray foam is a fire hazard, this approach [leaving spray foam exposed in an attic] only works if you don’t have any HVAC ducts capable of bringing smoke from the attic into the conditioned space below. Lstiburek wrote, 'The assumption behind the approval of the use of intumescent coatings is that air from the attic does not communicate with the rest of the building. … None of the International Code Council Evaluation Service (ICC-ES) Evaluation Reports for spray foam insulations allow this type of application if there is "real" air change or communication with the "occupied space." Unless, the spray foam is covered with gypsum board. Cover the spray foam with gypsum board? Not going to happen.'

    "Return air ducts in this type of attic are technically a code violation, while supply air ducts are in a kind of legal gray area."

  7. E247 | | #8

    In the comment section of the article "High Humidity in Unvented Conditioned Attics", Joe Lstiburek says (see comment #18):

    "Apparently there are no smoke issues or fire issues when ductwork is leaky and ceilings are leaky to poor workmanship. But when we have tight ducts with a smaller amount of supply air than was present with leaky ducts there is a problem? Welcome to code world. Now, in code world supplying air to the attic is ok - returning it is not ok. Exhausting air from the attic to the outside resulting in a negative pressure in the attic causing air to be pulled from the house into the attic is also ok. So my code changes are ok in code world. In the real world they result in a safer building and one that is free from moisture problems. "

    So, supplying air to the attic is ok according to the code. What difference does it make if the air is supplied by a HVAC supply vent or if a fan blows air into the attic space? In either case, there is a supply to the attic, but not a return.

    1. Jon R | | #9

      a) the HVAC probably doesn't run all the time - so it provides fewer CF/day (zero when not in use)
      b) in summer with AC, the HVAC air has lower absolute humidity - so less moisture/CF

      IMO, the most reliable warm weather solution is a dehumidifier (with moisture monitoring to set the humidistat and to alert you when the dehumidifier fails).

  8. E247 | | #10

    I do have a stand alone dehumidifier that is not connected to the HVAC duct. Would it be a better option to divert some of the air from the dehumidifier exhaust into the attic space?

    1. Jon R | | #11

      At the same CFM, it would provide more drying than interior air. But this may be more drying than is needed.

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