We open our window at night at the temps get down to 70F @20% RH. The temp in the house drops to 75F and the RH is 33%. We close our windows when we go to work.

When mid day comes, outside, the temp is 95F and the RH is 10%. Inside the temp is 76F and RH is 40%. How is this possible?

  • we have no water leaking into the house... with water off... usage is 0.
  • Cement slab.
  • Nobody is home. We have a couple dogs but they can't contribute that much water.
  • There is a temperature difference in the house one side is 74F and another side of the house is 78F.
  • We are waiting on a manual J to be completed, the guy thinks our AC is oversized. But that explains why moisture isn't leaving. But it doesn't explain where the moisture came from to begin with.
  • We had a pressure test done on the house... -0.1 pascals. So we are in good shape there.
  • It seems to me the only way water could get in, if no leaks, is condensation some where. The air handler is in the attic, attic get hot during the day...

In most building calculations... if we recycled 60% or our air and used 40% from outside at 95F at 10% humidity, we would have to add a quart of water every hour to the air to MAINTAIN 40% RH. THIS IS NUTS!!!! So much for the idea that we are breathing air through a leak... as a problem.

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  • In arid climates central air humidifiers are often used to help indoor air be at ideal humidity which is 30-50% are you certain that your new 5 ton system does not have one of these in it?
    – Kris
    Oct 10, 2019 at 3:03

3 Answers 3


I saw a question VERY similar to this answered on This Old House. Here is the link to the video that explained why it happened and how to fix it. Hope it helps.


Haven't you ever wondered why relative humidity is a thing? Why don't they just give water content as grams of water per gram of air? Because warm air holds more water than cool air.

For instance your 95F 10%RH air, when simply cooled to 70F, now is 24% relative humidity even though it has the exact same amount of water. I bet you didn't realize that, or didn't think it could count for anything like that much.

That right there accounts for half your gain.

It could also be coming up through your slab; lay out a large area of plastic sheeting and weigh it down along the edges; then in the morning look to see if you see dampness or condensate.

The rest is human activity.

  • Bathing: even if you presume the exhaust fan takes all the shower moisture, you typically leave a wet bath, shower and sink behind, wet towels on the towel rack, wet wasahcloths, bath mats, etc. That evaporates, of course, and goes where?

  • Cooking: heating food puts a lot of moisture into the air. You might have an exhaust hood, but is it even connected to outside?

  • Washing dishes and leaving them out on the rack to dry. Oh you use a dishwasher: does it have an exhaust fan/pipe to outdoors? I don't think dishwashers have that, so when it heat-dries your dishes, where does that go? Exactly.

  • Your refrigerator's frost-free cycle probably dumps condensate into a pan under the fridge, where it is evaporated by the heat of the condenser.

  • Dry 40%RH air is very hydrophilic and will cheerfully evaporate water out of any open, standing water from toilets to dog dishes.

  • It will do the same to your skin:

Sidebar: Blow some air on your hand. Notice how that spot feels cooler? That is moisture in your skin evaporating, cooling you with the latent heat of vaporization. This is also what wind chill is. "But that would require your skin to be wet, and it's not. How can this work? Likewise, if you're not sweating, why does wind chill matter?" The answer is, skin is moist all the time: you can't turn it off. Sweat is what happens when perspiration fails, i.e. The body needs urgently to cool, but skin moisture isn't able to immediately evaporate because the air is too humid, thin or stagnant. That certainly won't happen in a 40%RH house. That is so dry your skin could crack from drying out.

  • All that to say, your skin is constantly emitting moisture. And quite a lot of it. Dogs don't, but you do.

  • Your breath is also emitting a lot of moisture because water evaporates in your lungs. That's how your breath can fog a mirror. It's used by your body to cool, and this is the only way dogs can cool.

  • Looks like you didn't read the calculation for how much water do you have to add to maintain 40% RH at 75F with outside air at 10% RH at 95F. If you get 2/5 of your air from outside... you would have to add 1 quart of water per hour. The only way to remove the humidity is by opening the windows. So as an AC system should be dehumidifying... this one is not.
    – jdl
    Oct 10, 2019 at 4:33
  • @jdl what does that have to do with my answer? What makes you think I didn't read that? Are you sure that calculator is reliable? Are you sure you're getting that much air from outside? Oct 10, 2019 at 5:40
  • you said "That right there accounts for half your gain." via outside air." ....I am making my point with an extreme case... if I take in 40% of my air from outside, I have to add a quart of water an hour.
    – jdl
    Oct 10, 2019 at 12:49
  • Really? 1 quart of water an hour? Where do you live, the dessert?
    – Gunner
    Oct 10, 2019 at 13:34
  • @Gunner I presume so, since 95F 10RH air is unlikely anywhere else. Air that dry is so hydroplilic it will pick the locks on water tanks... Oct 10, 2019 at 17:24
  1. Hygrometers aren't super accurate +-5% is easily within tolerance on the cheaper ones.
  2. So you go from 75F / RH 33% to 76F / RH 40% and you wonder where the 7% came from?
  3. A pair of dogs could definitely add 7% RH in 4 hours. Hell if you stand near a hygrometer in a house you can increase the reading with the moisture emitted by your body.
  4. Water can come up from your slab via water vapor, water can evaporate from the dog water bowls, water comes out of your sheets/fabrics/etc (anything that could absorb water). Water comes out of your dishwasher after you run it. Water comes out of your shower/taps/etc into water vapor. House plants emit water. Your P-traps are filled with water - this water will be evaporating.
  5. Your house is ~2800 square feet, there are plenty of construction materials that are going to be absorbing or releasing water vapor, depending on how they have been loaded in the past.
  6. Cement is the powder used to make concrete. It is a concrete mixer in case you are wondering.
  7. -0.1 pascals is a pressure, a pressure test typically tells you number of air changes per hour at a given pressure. Passive house likes air changes per hour at 50 Pa. A reading under 0.6 ACH50 meets passive house, standard construction gets 15 ACH50, good is 3.5 ACH50.
  8. You don't really want your RH to be lower than 40%.

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