Is 35% Humidity reasonable in Houston, Texas?

[JPW1]

I work in a building in downtown Houston where it is always comfortable, even with the oppressive Houston heat and humidity. Just out of curiosity, I bought a digital Temperature / Humidity monitor. Naturally, it fluctuates a little, but stays around 75 degrees with 35% Humidity in the office building. I’ve taken it home and noticed that I’m getting typically about 74 – 75 degrees with 45% – 52% Humidity. I think most people will tell you around 50% humidity is acceptable.

I do realize I’m talking about two completely different environments. They are insulated very differently (but I do know my house is sealed pretty well). Also, they are built using different materials, which I suspect is a factor as well.

So my thought is to perhaps invest in a whole-house dehumidifier since I don’t think just about any A/C by itself can get down to 35% humidity, which is where I apparently like it. Is this a reasonable expectation? Do whole-house dehumidifiers perform to this degree in a residential setting? Is there any reason I would not want my humidity level to be this low?

[Shophound]

Your office building likely uses a chilled water system that can run a cooling coil pretty cold in order to dehumidify the air. Since you measured your office suite at 75°F at 35% relative humidity, that's a 46 degree dew point. That means the cooling coil somewhere in your office building is running a surface temperature at least that cold. With a residential system we're doing good if we can get the refrigerant to boil in the evaporator at that temperature (46 degrees), meaning the coil surface itself is likely closer to 50 to 55 degrees, as indicated by the average dew point temperature in your house of ~ 55 degrees.

If you want your house to be less humid than it now is, the choices are to run your a/c system longer, which is costly, install a whole house dehumidifier, which is effective but also involves money, or find out how your house leaks in air from outside, slow that process, and then consider if your a/c then becomes a better dehumidifier, or if you still need a separate dehumidifier. This is also money, but it is also a more comprehensive strategy.

[JPW1]

Thanks! In hot and humid climates like Houston is reasonable to expect that a residential dehumidifer could maintain 35% humidity in a home that is sealed fairly well? To be honest, I've never even seen one of these up close so I have no idea just how performant they are.

[garya505]

35% in a house in humidity would be pretty dang good.

BTW, I wish I could get mine UP to 35%.

[rickboggs]

Personally, I use shophound's first choice.... "run your a/c system longer." I find that sometimes lowering the thermostat setting 1 degree will do it. There are thermostats that will do this for you.

[garya505]

Personally, I use shophound's first choice.... "run your a/c system longer." I find that sometimes lowering the thermostat setting 1 degree will do it. There are thermostats that will do this for you.

Some stats will also lower the blower speed slightly to remove more moisture from the air.

[teddy bear]

Thanks! In hot and humid climates like Houston is reasonable to expect that a residential dehumidifer could maintain 35% humidity in a home that is sealed fairly well? To be honest, I've never even seen one of these up close so I have no idea just how performant they are.

Keep monitoring the %RH in your home and work. 35%RH-50%RH is good target and exceptable. I would expect your home will get much higher humidity during the different weather conditions of the 3 grass growing seasons.
Maintaining <35%RH during the wet coolest time of the year requires more capacity than the normal 50%RH. You should have a fresh air change in 4-5 hours which is reduces the indoor %RH if the outdoor dew point is <45^F, but increases the moisture load with +50^F dew points. Add the .5 lb/hour per adult to the +- of the outside air to determine the dehumidifying load. When the dehumidifying season is over, you may agree with most of us that <50%RH is very acceptable.
Regards TB

[Mr Bill]

35% in a house in humidity would be pretty dang good.

BTW, I wish I could get mine UP to 35%.


Yes it is, but in Houston if you have been here a very long period of time, you would likely develop sinus infections with the humidity that low, how do I know this, just ask me. :grin2:

[Shophound]

Thanks! In hot and humid climates like Houston is reasonable to expect that a residential dehumidifer could maintain 35% humidity in a home that is sealed fairly well? To be honest, I've never even seen one of these up close so I have no idea just how performant they are.

Uncontrolled air movement through a building due to leakage accounts for a considerable portion of the cooling and dehumidifying load of a house in summer. When this leakage is brought under control, the building interior is less subject to variations in humidity when outdoor humidity varies. This shifts the dynamic of humidity removal more toward humidity that is created indoors, such as from people breathing and perspiring, showers, laundry, cooking, pets, plants, etc. It's easier to manage these sources of moisture generation than it is when the building itself is leaking humid air into the interior from outdoors.

One more thing; you many find once you bring your building leakage under control that you don't need 35%RH at 75 degrees to be comfortable. You might find 45% plenty comfortable. It depends on your personal preference, of course.

[garya505]

Yes it is, but in Houston if you have been here a very long period of time, you would likely develop sinus infections with the humidity that low, how do I know this, just ask me. :grin2:

My house is at about 15% right now. I'm not really sure, as my indoor thermometer can only read humidity down to 20%.

[JPW1]

Uncontrolled air movement through a building due to leakage accounts for a considerable portion of the cooling and dehumidifying load of a house in summer. When this leakage is brought under control, the building interior is less subject to variations in humidity when outdoor humidity varies. This shifts the dynamic of humidity removal more toward humidity that is created indoors, such as from people breathing and perspiring, showers, laundry, cooking, pets, plants, etc. It's easier to manage these sources of moisture generation than it is when the building itself is leaking humid air into the interior from outdoors.

One more thing; you many find once you bring your building leakage under control that you don't need 35%RH at 75 degrees to be comfortable. You might find 45% plenty comfortable. It depends on your personal preference, of course.

I sincerely appreciate all the comments, but I'm still searching for an answer to the question. I understand what you have above offering an explanation around the source of how humidity can occur in the home. However, I've had an energy audit completed including that test that they do with the blower door. Ultimately, it was found that the house wasn't perfect, but sealed pretty well and within a good range. The house isn't that old.

So I'm still wondering, "In hot and humid climates like Houston is it reasonable to expect that a residential dehumidifer could maintain 35% humidity in a home that is sealed fairly well during the summer months? Again, I've never even seen one of these up close so I have no idea just how performant they are." Another way I guess the question could be asked a little differently is, "Has anyone ever witnessed a whole-house dehumidifier in a humid location (parts of Texas, Louisiana, Florida, etc.) able to maintain 35% Humidity during the summer months?"

Thanks again for the help.

[Shophound]

Yes, it was my assumption you had not done anything to determine the extent of house leakage. Many have not yet taken that step. Good for you that you have.

Personally, for Houston, while 35%RH at 75 degrees indoor temp may be attainable, do I find that reasonable for a climate like yours? From a standpoint of energy consumption, no. From a standpoint of "I'm not comfortable at any RH higher than 35%", is that really true for you? Are you uncomfortable at 75 degrees, 45% RH? Some people are, and some of these same people can compensate by ceiling fans or a desktop fan blowing on them where they sit. Others will spend more money and energy on knocking 10% more RH from the air.

My own house system has no problem attaining 75 degrees, 45% RH in summer, without a dehumidifier, and I'm north of you in DFW. Not as humid as Houston, at least not as consistently so, but humid enough. I also do not see sharp swings in RH between a/c run cycles, as I've taken steps to check non-controlled air movement through the shell. Therefore when the a/c is between run times, I do not suddenly feel stuffy and warm. If I start edging that way I turn on a ceiling fan over my head and I'm fine. But your mileage may vary.

[JPW1]

From a standpoint of "I'm not comfortable at any RH higher than 35%", is that really true for you? Are you uncomfortable at 75 degrees, 45% RH?

Thanks! To answer your question, I suspect I'm not, that is, I may be comfortable at 75 degrees/45% RH. However, I started this little test only looking at two environments: one where I didn't feel comfortable all the time (my residence) and one where I did feel comfortable all the time (office). So by looking at the data at my office the humidity stays constant at 35% - 36% humidity so I sort of made that my goal. I might fall short, but I thought it made sense to work toward it.

I still probably need some time to think this through some more, but part of my logic at the moment is almost any residence is going to have humidity fluctuations. Doors open and close, and just let it in. So I suspect I can tell the difference between 45% - 53% humidity swings in the house. However, 35% - 45% humidity swings probably feel more tolerable. Again, I need to do some more thinking about this, but right or wrong that is where my head is at the moment.

[Shophound]

Unless you have kids or otherwise a lot of in and out traffic in your house (and even so, nowadays kids stay indoors much more than before), your humidity levels should not fluctuate greatly if the house is reasonably leak resistant.

Do you set your a/c at a higher temp during the day when nobody is home?

About two weeks ago we had a dry air mass move into town. It did not lower the daily high temperature much but it did really drop the outdoor humidity. I had the house closed up while working inside for a good part of the day, and even with breezy conditions and dry air outdoors, the indoor humidity level hovered in the mid-forties. I then opened up the house mid-afternoon to get some "free" ventilation. I expected the indoor humidity to drop like a stone. It did go down, to about 30%, and afterward I thought once I close back up I wonder if it will rebound even close to where it was prior to opening up, at 45%. Within two hours it did just that, which could mean several things. A) I have a fair amount of internal humidity generation going on; B) While the air dried out quickly, all the absorbant materials in the house (sofa, bedding, drapes, walls, clothing, etc) did not dry out nearly as quickly. Therefore when I closed up the house again these materials released moisture into the contained air, raising the RH level up to where it was previously.

Last week we had elevated outdoor humidity levels but not super hot temperatures. So a/c run times were not long at home. Nevertheless interior RH levels never got above 53 - 55%.

Bottom line: I've been working for a few years now to make the structure of my house itself do more of the interior climate modulation than when I first bought the place four years ago. As I've made progress, the heating and cooling plant is relied upon less intensely to manage the indoor climate. This saves me energy and money, and contributes less to stressing the grid when demand is high during peak heating and cooling seasons.

[genduct]

My understanding from ASHRAE is that when there is a diff in RH ( Vapor pressure) that when you open the door it moves at 11 feet per second.
Not the same as the migration of heat, much faster

[Shophound]

My understanding from ASHRAE is that when there is a diff in RH ( Vapor pressure) that when you open the door it moves at 11 feet per second.
Not the same as the migration of heat, much faster

Hmm...I'll have to field measure that next time I get a windless day with elevated humidity levels. Thanks for the head's up! Need to get back into my ASHRAE books some more.

[Mr Bill]

My house is at about 15% right now. I'm not really sure, as my indoor thermometer can only read humidity down to 20%.

Man, I would be having nose bleeds at 15%. :grin2: Been in Houston for 60 years, with an average annual humidity's of 70%......15% would kill most of us. :grin2:

[JPW1]

Thanks for all the comments around this. I think I need to go back and gather more objective data from the two environments, specifically when both environments are put more under stress (high humidity and temperature) and see what the resulting measurements are. Fortunately, I can't do that now since Houston is having incredible weather. I'll be sure to keep this thread alive once I have some more objective data to pass on. Thanks again.