What is the outdoor humidity?

Something in your house, open water sources, cooking, showers, or humidifier is causing the high humidity.

Michael

Hi Michael,
Thanks for the response. Outside humidity here right now is 50%, and temp is 52 degrees F and clear, which is not unusual for here this time of year. This is a 2 year old 1800 sq.ft rancher with a 2ft, totally dry, heated crawlspace. The ensuite has a 5foot walkin double shower and an excellent exhaust fan that we use religiously. The mirrors never condensate, as long as the exhaust fan is on. Range is a flat top, built-in, with jenn aire countertop exhaust fan. I also have a timer on the main bath exhaust fan that I have programmed to exhaust automatically 7 times in 24 hrs. in 15 minute durations. Yet, every morning. All the windows have condensate on the inside, some worse than others. Any suggestions appreciated.
John

Relative humidity is a comparison of vapor density versus saturation vapor density at a particular temperature.

Put a bit more simply, dew point is 100% relative humidity or the point where the air - at a certain temperature - is no longer able to hold any more moisture. If the air has reached vapor saturation (100% relative humidity), then the air will release moisture which will show up as condensation on any surface that is at a temperature below the dew point temperature of the air.

In order to stop condensation from forming on the surface of a window, you either have to lower the dew point temperature of the air in your home to a level below the dew point temperature of the window surface, or you have to warm up the window surface to a temperature above the dew point temperature of your home, or a combination of both.

Lowering the relative humidity of the air in your home may have absolutely no effect on controlling window condensation or it may completely solve your problem depending on "how" you lower the relative humidity and what affect the "how" has on both the moisture level of your air and the temperature of your windows.

All this because there are two ways to lower relative humidity  first, you can increase the air temperature in your home or second, you can decrease the moisture content of the air in your home.

By increasing the air temperature in your home you will lower the relative humidity but you will not change the dew point  which is based on the amount of water vapor in the air and is not based on the temperature of the air.

So, while the RH is lower with higher air temperature, it may not effect condensation on window surfaces at all  unless the rise in air temperature also causes a corresponding rise in window glass temperature to a level above the dew point temperature.

But by lowering the amount of water vapor or moisture in your air you will also lower the dew point temperature as well. If you lower the dew point temperature sufficiently to drop it below the temperature of the window glass you should have no more condensation issues.

The amount of moisture in the air is typically measured in grams per cubic meter, which is kind of nice for our metric folks but not so nice for our non-metric folks; but the metric version is much easier on the calculator than the English version. However, in the interest of making this stuff easier to understand for all of us non-metric types, I am going to covert back to Fahrenheit rather than use Celsius temperatures in the calculations.

Okay  consider your home at 65 degrees F and with a relative humidity reading of 40%. There are 6.25 grams of water in a cubic meter of air in your home in that particular scenario - which then equates to a dew point temperature of 38 degrees F. So at 38 degrees the air will be at 100% relative humidity or at saturation vapor density.

If your home hygrometer measures the relative humidity in your home at 60% while the temperature of your home is 70 degrees, you will have a dew point temperature of about 51 degrees  meaning that if the temperature of the window surface is below 51 degrees then you will have condensation on that window surface.

Which then begs the question are you seeing surface condensation all over the glass or only at the edges of the glass?

Also, where did you place the tmermometer to get those temperatures? Was the thermometer at the edge of the window or at the center of the glass?

There will be a difference between edge temperature and center-of-glass temperature which can be significant when dealing with condensation issues.

Also, consider that the interior glass temperatures are based on the fact that moving, warmer, indoor air is in contact with the glass at a given time. If the air against the window isn't allowed to circulate, you can have problems as well.

Curtains, shades, other obstructions can cause problems by blocking airflow across the glass  airflow that can have a huge effect on the condition of the window relating to condensation. Also, bay and bow windows can be more prone to condensation  again because of the possibility of decreased airflow over the glass.

And finally, what can happen to the dew point if you keep your home at 70 degrees and you have a 65% relative humidity? Well, for one thing the dew point has jumped up to 57 degrees and anyone with 65% relative humidity in a home at 70 degrees has way too much moisture in their air and they are in serious need of some sort of ventilation system  or at least several good exhaust fans.

Thanks for the informative post oberon.
The condensation is mostly on the bottom 2 or 3 inches of the windows and also up the sides for maybe a foot and a band about ony 1 inch wide. But, only on the fixed windows, NOT on the sliders.
When I checked the temp of the windows, I duct taped the probe to the center of the window about a third of the way up. I really don't believe it is a window problem, but a humidity within the house problem, but I just can't pinpoint where it is.
As I stated before, Humidity stays a constant 50% at 70F during the day within the house regardless of what it is doing outside. But, at night, when the heat setsback to 68F, the humidity spikes to 59%, almost every night.

jgee,

You are welcome.

If you have a gas furnace, has it been checked recently? A faulty gas furnace can pump LOTS of moisture into a home - and potentially carbon monoxide as well. That would be the first place I would check - and I would do it today.

The fact that your RH goes up as temp goes down is normal. Again, RH is based on the ratio of the level of moisture in the air versus temperature.

As long as the moisture level remains unchanged, as temp goes up RH goes down and when temp goes down RH goes up - basic physics - but your change is excessive and is outside of "normal" variation.

To run your numbers, if your home is 70F and you have and RH of 50% then you have 9.2 grams of water per cubic meter in your air and a dew point temp of 49 degrees - as you have noted, that is high.

At night when you drop the temp to 68F and you get RH reading of 59% you have (based on the numbers) an increased moisture content to about 11.5 grams per cubic meter thus raising the dew point to 52.5F - which is very high.

I would check the furnace first and if that is okay then I would begin to address other possible areas that may be pumping moisture into your air.

Hi Oberon,
No fossil fuels in the house at all. Everything is electric, including the fireplace. So, if anything, I should be looking at humidifying the place this time of year, not dehumidifying. That is what has me so puzzled.
Think I am going to get hold of the Hardwood floor guys in the area. They have some pretty sophisticated tools for checking humidity.
Any other suggestions you may have would be welcomed.

Simply, it's not the windows. You need to decrease the humidity in your home. The glass surface just gives the moisture a place to settle. The condensation could be inside your wall as well. Hope this helps, Tim