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High humidity. Open cell foam. Conditioned attic. North Texas

JasonJackson | Posted in General Questions on

I’m looking for suggestions on how to fight my humidity issues in the house and conditioned attic space.

I have a 2150 sqft, single story home in North Texas built 9 years ago. We have about 5 1/2″ inches of open cell foam in the attic and exterior walls. I have a 3 year old, 3 ton Lennox 2 stage compressor and 3 & 1/2 ton Lennox air handler located in conditioned attic.

I’ve always noticed it was a little humid in the house but started to monitor it here lately. I’ve been monitoring the humidity levels in the attic as well.

Inside the house, I set my thermostat to 72F, it maintains this temp very well. The AC will cycle on 30-45 min and off 20-30 min depending on the time of day.  Humidity ranges 60%-65% inside the house. In the attic, it ranges 65%-70% and temps range 75F to 82F.

I know building codes have changed regarding conditioned attics since they built my home 9 years ago. Does anyone have any suggestions on how to control my humidity issues without jumping the gun and installing a whole-home dehumidifier?


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  1. Jon_R | | #1

    Better air sealing (using a blower door) will help.

    You can reduce the air flow through your air handler to increase dehumidification. Measure temperature of the air post evaporator coil and lower it as needed (probably around 50F). Make sure the AH fan doesn't run when the AC is off.

    Also install portable dehumidifiers (much less expensive than whole house).

  2. JasonJackson | | #2

    Hey Jon,

    This weekend I did lower the blower speed on the air handler and it has actually helped to dehumidify the air. It's brought it down about 5% both in attic and house.

    I also put a portal dehumidifier in attic, it helps a lot but those are expensive to run all day. If i can find an alternative to a portal dehumidifier that would be great.

    1. raljp | | #10

      ok just got through building home out of Houston tx..2550 sq ft house with tall roofs and open cell foam in walls and top of attic..i have no soffit or ridge vents in living space..all my gas appliances are vented out of non foamed garage roof. this saved me a lot of holes in roof with foamed area..put gas heat and ac in closet in garage to save the expense of 1500.00 to have sealed unit in attic..also saved me a hole in roof for combustion air,,have upper combustion air coming from vented gagage attic and lowered combustion air coming from vented door to ac/heat out of garage..this saved me also a ton and half from having up in attic..10% air comes from outside vent into unit in minimize humidity in attic I bought a danby ddr07 dehumidifier good for 4500 sq ft..cut a hole in sink vent pipe for continuous drain and set at 45% humidity..before this my attic was running from 60 to 70% humidity which is a disaster down attic is at 45% and inside home the same..ive gone from 74 deg in house ac running to 77 deg with 49 % humidity..before it was up to 60% in home..went ahead and set fan speed back up to factory settings to get more air to back of home..i have power ceiling vents in all rooms that can produce humidity..which are turned on anytime for showers and washing clothes..another way of keeping humidity down..our return air was kinda loud in living space so we built baffles and mounted them up in return air where fan motor is sitting..helped air noise immensily to the point I was able to turn the fan speed back up..i'm not a engineer or a all time builder but the ac group liked and ok'd what I did and got some props from builders who were impressed on my research and building in a very respectable time..i don't know if this will help anyone..but its helped in living in a open cell home on the gulf coast..sorry kinda long..raljp

  3. Expert Member
    Dana Dorsett | | #3

    See if the system is set up to provide outside ventilation air whenever the air handler is running. High ventilation rates increase indoor air humidity in much of Texas this time of year, and throttling the ventilation air back ( or drop it to zero) can make a large difference in indoor humidity levels during the cooling season.

    If there are no supply registers in the attic, giving it even a very small supply of conditioned air (50-100 cfm) will bring the dew point of the attic in line with the rest of the house even when it's not enough to appreciably change the attic temperature. That should be enough to not need the dehumidifier in the attic (which raises the attic temperature.)

    Room to room pressure differences are always induced when the air handler is running, which in turn drives unintended ventilation / outdoor air infiltration through randomly located leaks in the house. (Air handler driven infiltration can easily be an order of magnitude higher or more than mere stack effect or wind driven infiltration along the same leak paths. ) Doored-off rooms with supply registers only and no guaranteed-by-design return path will become pressurized whenever the air handler runs. That can usually be rectified by building in a jump duct, such as grilles on partition walls (near the top on one side, near the bottom on the other, for privacy) , or ceiling grilles for pressure relieving ducts located in the attic.

    An Energy Star duct system would have no more than 3 pascals (~0.012" water column) between rooms at all air handler speeds, and all door-open/door-closed variations. Most duct systems are nowhere near that well balanced, especially with rooms that have only supply registers and no guaranteed return path.

  4. Peter Yost | | #4

    Two good BSC resources on this topic:

    As Joe is want to say: no good deed goes unpunished. If y0u build a high performance home in a humid climate, you just increase the shoulder seasons and the need for stand-alone whole house dehumidification. This is a big part of the reason ACCA is working on their Low Load Home (LLH) manual (just released in March of this year, although I have not reviewed it but hope to in the next couple of weeks or so).


  5. JasonJackson | | #5

    BTW, I forgot to mention in my question. I have a supply only fresh air ventilation. Should I close that off and install a whole house ERV?

    1. Jon_R | | #6

      In hot humid weather, it might save $3/day (in dehumidification and cooling).

    2. Expert Member
      Dana Dorsett | | #7

      >"I have a supply only fresh air ventilation. "

      How many cfm?

      Have you tried just running it at a lower duty cycle?

  6. JasonJackson | | #8

    I think it's a 6" duct, unsure of the CFM. I've tried playing with the settings on the Honeywell controller for the damper, but it just opens all the way. Honeywell W8150A1001.

    For now while I'm fighting my humidity issues, I've turned the controller off and shut the damper.

    "Have you tried just running it at a lower duty cycle?"
    Are you asking about the blower speed on my air handler? I did lower the speed and it reduced the humidity by 5% in house and attic.

    1. Expert Member
      Dana Dorsett | | #9

      >"Are you asking about the blower speed on my air handler?"

      I was looking for how often & how long the ventilation fan runs, under the presumption that a " only fresh air ventilation..." is an independent system.

      The Honeywell W8150A1001 is designe to be used when ventilation is integrated into the cooling/heating system ("...set up to provide outside ventilation air whenever the air handler is running..." as I was asking about in response #3.) Is it in fact the air handler, and not a separate fan that's driving the ventilation air?

      If yes, running the air handler at a lower speed also lowers the ventilation rate, and the parasitic latent load that comes in on the ventilation air, which is a good thing. Turning it completely off and closing the damper is still the right thing to do when the outdoor dew points are high.

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