Efficiency of cooling

[Tom7227]

How come the humidity in the house doesn’t fluctuate in sync with the outside when the AC is on? I’ve had the AC on for the past couple of days and right now the humidity in the house is 62% and outside it’s only 46%. It seems that during the night when the AC cranks less often the humidity in the house goes up even though there’s nothing happening like showers or cooking that should cause it to rise.

I also was wondering if there’s any way to calculate the efficiency of the house by combining the amount of energy spent heating the home - natural gas with a 95% efficient furnace - and the square footage of the house. I suspect you’d have to figure in degree days or some factor for outside conditions. It just seems that I ought to be able to come up with a formula that says for instance that if it costs me X cents per hour or therm of gas to heat a 2200 square foot home that I am in Z category and need to consider some home energy improvements. I suppose I could spring for one of those big deal tests where they use a blower to check for leaks and thermal imaging to spot them, but I would like to know if there’s a straight math method.

Anyone have any experience at this sort of thing?

[BobT]

Are you using the same instrument to measure your humidity both inside and outside?

Also, relative humidity is affected by air temperature. All else equal, colder air can't retain as much water vapor. Air that has 50% relative humidity at 50 F. will be at a lower relative humidity at 70 F. even though the amount of water vapor overall hasn't changed.

This is evident when we hear things like dew point. The higher the relative humidity the higher the dew point temperature becomes.

I'm not an expert so some of my descriptions and terminology may not be entirely accurate but I believe the concept is sound.

[Smoker]

It sounds like you have to big of an air conditioner. If it is to big you are going to cool the house down to fast before it has a chance to remove the humidity. It should end up being about 600sf per ton of air conditioning. It will depend if you are in the shade or in the sun also.

[Surface Tension]

What type air conditioner do you have?

Could it be at night when outdoor temps are cool your air conditioners compressor is not kicking in, rather is is just blowing the cool air(fan mode)into you home. That air would have a ton of moister in it.

Also do you have an air exchanger and if so what type.

[Tom7227]

The AC unit is a Goodman and it around 20 years old. I do not have an air exchanger. The furnace is about 4 years old and when it was put in I just had them reinstall the old A coil. I keep the furnace fan on all the time when either the heat or AC is on. It has a dual speed fan that switches to some low draw mode unless the heat or ac are actively working.