We have a two year-old, two-story town house with:

  • fiberglass insulation in the walls;
  • a Tyvek vapor barrier under foam board on the exterior walls;
  • blown cellulose insulation in the attic;
  • stucco outside;
  • and a refrigeration cycle air conditioning.

We live in Phoenix, AZ (desert climate) and our indoor humidity levels are consistently 20% to 35% higher than outside. During the summer we consistently have humidity levels in excess of 80% (not 70%), even with the damper fully open. It has been so humid inside that the tile floors had sweat on them.

We have no indoor plants or water features. Our dryer vent is operational, and our exhaust fans from the bathrooms are vented externally through the roof. There are no unusual ceiling vaults and we do have ceiling fans to stir the air. Ours is an end unit, so we have one shared wall with a neighboring town house, which has no vapor barrier.

We have had the builder heavily involved for warranty work to determine the root cause of the humidity sources. We have tried:

  • Slowing the AC blower motor down to allow the water in air to condense;
  • Looking for water leaks with an infrared camera (FLIR);
  • Looking for blockages in the vents in the eaves;
  • Calibrating the thermostat (Ecobee);
  • Balancing the air flow by adjusting the vents inside with flow meter hood;
  • Adjusting the air conditioner refrigerant cycle pressures;
  • Fully opening the damper to let in dry air;

    - Increasing the exhaust fan to run for 30 min in a 60 min cycle.

The problem hasn't gone away and we're not sure what to do next. Is there anything else we can try before we resort to installing a whole house dehumidifier, or adding additional attic ventilation?

What are your thoughts on the RH trend over time?

Indoor Temp and Humidity over time

Aug 1, 2018 indoor temp and RH

Also, I did a rough calculation that the home needs to express ~350 pints of water per day (ASHRAE) psychometric chart corrected for altitude, but the HVAC company measured the AC system is operating properly and measured the AC will remove 77 pints of water per day. I read that whole home dehumidifiers typically remove ~100 pints per day. What are your recommendations on how to best ventilate the attic while controlling cost?

Historical Indoor RH and Temp. Including limited outdoor data

  • 2
    $\begingroup$ Refrig AC tends to dehumidify the air inside, so you issue is unusual. Do you have indoor plants & if so, is it like a jungle inside? No indoor water feature? How about the exhaust from the clothes drying? Has the house been so well sealed that there is minimal flow through air from outside causing humidity to accumulate? Are the ceilings higher than usual, causing stagnant air pockets above? With the humidity outside, have you measured it or just taken the number provided by the weather service for your area? Maybe you live in high humidity area pocket, with lush gardens & well watered lawns. $\endgroup$
    – Fred
    Commented Nov 11, 2017 at 22:50
  • 2
    $\begingroup$ I'm thinking it may be a ventilation issue - not enough outside air flushing the inside air. Two more ideas though, being a townhouse, could be issue be due to either or both of your neighbors? The other idea is that the higher the air temperature, the more water vapor it can hold & thus the more humid the air can become. Maybe your AC air temperature needs to be lowered &/or shading placed over windows, particularly large ones. $\endgroup$
    – Fred
    Commented Nov 12, 2017 at 7:27
  • 4
    $\begingroup$ Refrigeration-type air chillers continually reduce the relative humidity of the air cycling through them- which means that your humidity should be decreasing not increasing, even if your system is drawing no outside air at all into the process stream. But that moisture (called "condensate", which drips off the cold coils) has to be plumbed somewhere to a drain. If the condensate drain is blocked, it may be running out into the house somewhere, preventing the humidity from dropping. Have the tech also check that the condensate drain is not leaking into the structure. $\endgroup$ Commented Nov 13, 2017 at 2:17
  • 2
    $\begingroup$ I've tried to consolidate all these important additional details into the question; now if folks could start writing up their suggested solutions as answers, we could have a pretty good Q&A on our hands. ;) $\endgroup$
    – Air
    Commented Nov 13, 2017 at 16:51
  • 3
    $\begingroup$ Among other odds and ends, I'm an HVAC tech, but its hasn't been my day job for over 15 years. Since you have a condensate drain, the first order of business is to measure how much condesate is produced in a day and compare that to the unit's specs. In AZ, they probably aren't using air handlers designed for high moisture removal. Second, report the actual wet bulb temperature that you have in your house, and the outside temps and inside temps as well. Third, 70% RH isn't a bad thing, but it might feel weird in AZ. But you should be able to get lower. $\endgroup$
    – Phil Sweet
    Commented Nov 15, 2017 at 1:00

4 Answers 4


Sounds like you have covered your bases. The only thing I can recommend is gathering data and considering some scenerios to evaluate that data against. I am not an HVAC guy but can give you the 1000ft engineering perspective.

Any of these trends would be helpful in debuggig the situation:

  1. Relative humidity and temperature at a discharge of the HVAC system trended over time.

  2. Condensate discharge from the air conditioner into a bucket, gallons per day.

  3. Relative humidity and temperature in each room trended over time.

  4. Surrounding water table level

  5. Condensation locations and times of day

Possible scenerios in order of likelyness:

  1. A portion of the condensate being removed from the air is not being drained off and is being revaporated. Either due to installation issue, mfg defect, or possibly a very frequent cycle time.

  2. To small of airflow is being cooled then mixed with the main air stream. When a small stream of air is cooled very cold, most all the water is removed from this stream. However, no water is removed from the larger main stream it mixes with. This can result in the mixed stream being 100% humidity (think fog from dry ice). To mimimize humidity, fresh air from outside should be introduced upstream of the air conditioner before being distributed to the rest of the house.

  3. Moisture wiking through concrete. May be exacerbated by irrigation or a high water table.

  4. Unnoticed moisture producing item. A fridge with bad defrost settings and an icemaker.


While your AC is running, what is it's typical duty cycle? In other words, how long does it run during each cycle (i.e. 5 minutes on, 15 minutes off) to keep things cooled to your set point? If it doesn't run very long at all to keep things cool each time it runs, you may have a vastly over sized AC unit that isn't running long enough to actually remove the moisture in the air before the temperature gets to the set point. An ideal AC (or heating system) should be the most efficient and do the best job when it runs close to 100% of the time in the worst-case design scenario. Bigger isn't necessarily better when it comes to AC and furnaces.

If this (short cycling) could be the problem, you should have your AC contractor run a load calculation on your town house to verify that your AC unit is sized properly for your specific unit. Depending on the quality of the contractor doing the install, they may have just slapped in whatever unit they may have on hand, or just put in a big one because you're in a hot environment and figured that they could charge you (or the builder) for a larger unit than necessary.

The other thing to check is the location of your thermostat. Make sure it's not causing short cycling of your AC unit by being right in the blast of the vents. Ideally you'd want it to be away from any direct air movement from the vents so it's more accurately measuring the overall room temperature and not just the air right around the vents.


If I understood correctly you have one year old house. Do your neighbours who don't have your problem also have one year old houses? Once I have heard of people having similar problems with humidity due to moving in before construction materials had time to dry so humidity was stored in concrete floors and it cused problems. This link could help explain better humidity in new house. If this is your case maybe it could be solved using industrial dehumidifiers for some period of time, maybe there are companies who deal with such requests and can give you better answers.

  • $\begingroup$ The home is going on 2 years old now, but the humidity issue still exists. Do you have any other thoughts? $\endgroup$
    – user13416
    Commented Aug 11, 2018 at 17:24

Your relative humidity (RH) of 70% doesn't seem that high. If you really want to decrease it, then you need to increase your ventilation flow, since your living and outside air both contribute to a high humidity. Your hot showers, washing, cooking, breathing, sweating, adds a substantial amount of water vapor to the inside atmosphere (some estimates put this at 20 lb per day for a typical household, which means you would reach 100% RH after a few days if your house was a sealed plastic bag). At best, all of this air with high humidity is replaced via ventilation with outside air, or condensed out when passing through the HVAC cooling coils . Now, that outside air at, say, an average temperature of 95 °F and 25% RH will have a RH of around 55% at an inside temperature 75 °F. So at best case, your air will have a 55% RH. Now adjust for the fact that your ventilation and heat pump/AC unit throughput is normal and not very high, and your 70% RH is to be expected.

  • 2
    $\begingroup$ While true, clearly RH is not the concern. The problem is that whatever humidity is in the input air is significant enough to cause condensation and sweating. $\endgroup$ Commented Nov 15, 2017 at 12:17
  • $\begingroup$ The sweating and condensation means that the RH is 100% at those place - ie you ve reached the dew point. Does this occur in the most-likely location of the basement, or everywhere equally? Again, more ventilation and/or circulation through the AC would fix this. What's the current flow rate through the unit? $\endgroup$
    – user166624
    Commented Nov 15, 2017 at 18:50
  • $\begingroup$ The thing is, our neighbors do not have humidity issues or high electric bills. In Arizona some homes have evaporative coolers and those systems use higher air velocities to remove heat. The manufacturer rep (Carrier) actually lowered the blower speed to allow the airflow to cool enough for the water in the air to condense. We have had 3 different HVAC companies out to look at the system, as well as the manufacturer. All to no avail. The consensus is the system is sized appropriately and is functioning properly, yet the problem still exists. $\endgroup$
    – user13416
    Commented Nov 16, 2017 at 1:13
  • $\begingroup$ Just painting with a broad brush... more water is going into the air than is coming out. I’m think I need to strongly suggest a whole house dehumidifier needs to be installed. A dehumidifiers basically a small A/C system to chill the air causing the water to be squeezed out. $\endgroup$
    – user13416
    Commented Nov 16, 2017 at 1:16
  • $\begingroup$ ASHRAE 2013 recommends max RH AT 65%. EPA states min 30 max 60% RH. From CDC. bit.ly/2OnSpc0 $\endgroup$
    – user13416
    Commented Aug 11, 2018 at 17:33

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