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Relative Humidity Meter

I had a cust this past week who had condensation on several windows. This was a 4 thousand SF plus home...4 years old...flash and batt...geothermal...steam humidifiers and his monitors said he was at 20%. My Extech meter said he was at 34%. Regardless of whose meter was correct the windows were telling us that the RH was a bit too high. The outside temp was 38. He wants to purchase a RH meter that is very accurate so I'm hoping you pros can give me some advice.


  • Buying a meter - Spend 130 for a HOBO Data Logger. Remember the issue is not the humidity, but the dew point in condensation issues. Especially near the windows. Set the logger at one window or another. See what you get. See if you get different readings at different windows. In this case the condensation is near the window not in the middle of the room.
  • The sort of variation you describe is not surprising for RH. Consumer grade meters aren't usually rated, and even those that are, I don't trust.

    But even if the true RH was as high as, say 40%, at 38F ODT, even single pane glass shouldn't sweat. More likely, those windows were either exposed to a direct moisture source (shower, cooking, etc.) or isolated from the room with curtains or shades. Or the fog was actually on surface 2 (compromised seals).

    Extech pro meters are rated +/-3% or +/-5% (depending on model). However, RH sensors are highly prone to drift, so a year-old meter could be off several more percentage points than rated accuracy.

    The only way to get around this is to buy a meter that can be calibrated. I've recommended the Extech RH305 kit to several clients. It includes a hygro-thermometer, calibration media and a nice case. It's rated at +/-3% and sold at Home Depot for about $120 (also see http://bit.ly/1Q2gdSb). Anything costing less will eventually be off.
  • I have been giving Accurite Temp/RH meters (~$10) to all of my clients when I perform an energy assessment. I usually have 4-8 meters lined up on a shelf in the Living Room. I also use several Accurite remote reading Temp./RH sensors. I have found that the temp. is almost always within a degree and RH within 1-2%. Rarely, one is off by more. I have also learned that these meters, as with most inexpensive meters, will not read below about 20%. I have experienced one remote sensor that failed after years of being outside. It failed at always reading 20%. You do need to let them acclimate for at least 20 minutes or more for a stable reading. A meter costing over about $100 will measure down to around 5% RH.
  • Trying to put it in perspective:
    From the Psychrometric Chart, for 70 degF air at 50% RH, the dewpoint is 50 degF., which is a rather low temp for insulated glass at 38 degF outside.
    If inside were at 60 degF, the RH would have to be at 70% for a 50 degF dewpoint
    If inside were at 80 degF, the RH would have to be at about 34% for a 50 degF dewpoint.
    What is the inside temperature? Is the condensation on ALL windows or localized, where they may be a local source of moisture?
    If he wants a really accurate RH, he needs to look at a CALIBRATED meter.
  • Don't forget the obvious--thermostat set back, interior window covers that shield the glass from the average room temperature and radiant heat, and create a convective current down the face of the glass. Notice that condensation almost always begins to accumulate at the bottom of the glass. It is the microclimate that we must consider.
  • Often the root cause of the condensation may be poor airflow/circulation to the area. Operating the furnace with continuous fan may help.
  • The dew point is the main thing....Brad Cook says it best.

    The bottom of the window has a lower surface temperture than the top of the window because of convection.

    In cold country where the temperture is below freezing, humidity is not the issue, it is surface temperature.
  • It's easy to estimate the glass surface temperature. The difference between the indoor air and the glass surface temperature is the result of the interior "air film", which we assume to have an R-value of 0.68 for vertical surfaces. This assumes a convective velocity of 1 fps.

    As a point of reference, dead air space with no convection is about R-1. At the other extreme, the outside air film is assumed to have an R-value of 0.17 @ 15 mph (22 fps) and 0.25 @ 7.5 mph (11 fps).

    The temperature gradient across the glass is proportional to the R-values in the path. A low-e window with a u-factor of 0.3 might have a COG u-factor of 0.25 (R-4). Note that NFRC u-factors already include the air films, so the internal air film represents around 17% of the thermal gradient based on NFRC test procedures.

    The OP indicates condensation @ 38F ODT. We don't know IDT but let's say the air in the immediate vicinity of the glass is 65. In this example, we can expect the surface to be about 60F.
  • (continued)... Presumably the windows are decent since the home is only 4 years old and has geo. But even if the windows aren't that great, say high .30's whole-unit u-value (R-3 COG), the glass surface is only going to be 6F lower than adjacent air. I can't believe the ambient dew point is anywhere close to that, considering Jerry measured 34% with a pro meter (although we don't know at what temperature).

    You can play with the numbers, but as I noted in my initial comment, it seems obvious that the affected windows must either be well insulated from room air (e.g, curtains), thus greatly lowering adjacent air temperature, or they're near a moisture source. For example, what if the steam humidifier is set up to operate the blower off cycle, and some windows are in the path of room temperature supply air charged with steam?

    BTW, here's a handy guide from Cardinal with COG and generic whole-unit u-factors for its full line as well as competitive products (http://bit.ly/1nUhKge).
  • I agree with David Butler's assessment. Other than a deep recess or curtains making a microclimate at the windows, my experience is that the HVAC is likely to be discharging highly humid air directly at the windows. We managed this on one project by installing outdoor temperature controls which turned off the humidifiers below a set point. We played around and ended up at 24 degrees on that house with its architectural unbroken steel frame windows (!), if this is your issue and solution you would probably find a much lower temp will work with normal (good) windows.
  • As I recall a saturated salt / watersolution in a cup that is in a plastic bag gives you 75% RH. You can use that just to see if the meters are right as you will want something for instantaneous spot checking. Aside from that I agree with the previous comments and am a strong believer in data logging over a long time period (week to month) to develop a movie instead of a snapshot.
  • Doug wrote: "...you would probably find a much lower temp will work with normal (good) windows."

    Yeah, except in this case the windows condensed at 38F outdoors! If steam is delivered without heat (e.g., return air temp), it wouldn't be surprising to see condensation at 38F. The solution in that case would be to redirect the air.
  • Agreed