Blogger Mark Milanese continues his series on window condensation. To get caught up, read Part 1, Part 2, Part 3 and Part 4.

To answer my client who thought I’d installed defective windows because she saw condensation, I began to collect the data to use the American Architectural Manufacturers Association (AAMA) condensation resistance factor tool:

  • I used my sling psychrometer to measure relative humidity and found it was at 48 percent;
  • I measured the indoor temperature and found it was 70 degrees, and;
  • I measured the outdoor temperature and found it was 28

I entered the data I collected into the AAMA condensation resistance factor tool from the Internet connection on my smartphone and found the dew point temperature and the condensation resistance factor:

Dew point temperature:  49.48

Condensation resistance factor: 50.94

Remember, they had selected a window with a CRF – 51. If the glass got any colder or the humidity any higher, condensation could form under these conditions, according to the AAMA calculator, which recommended a CRF of 50.94 or higher.

Next, I measured the glass surface temperature and found it was 63 degrees, almost 15 degrees above the dew point of 49.48.  No water had condensated on the glass at the present because the interior glass surface temperature was still above the dew point.

The air temperature in the room was 70 degrees, so the glass temperature was impressively high considering the outside temperature and the tightly closed blinds…

My effort is always to delight my clients… I decided to offer to replace the sash with ones that have a higher condensation resistance factor (CRF-63) but a lower insulation value (R-4) for free and explain ways they could lower humidity and raise the temperature of the glass in the room so the glass remained above the dew point.

I began by explaining the dew point.  Then I showed her the ways to avoid condensation in the future:

  • Reduce the humidity in the room;
  • Increase the air temperature in the room, and;
  • Increase the surface temperature of the glass

Reduce the Humidity in the Room

  • Don’t air-dry laundry in the room, and;
  • Open up the windows on low-humidity days in autumn to allow water in the carpets, drapes, bedding and quilts to dissipate outside before closing up the house for winter.

If they still chose to air-dry laundry in the room:

  • Open the door to allow the moisture from drying laundry to circulate through the rest of the house, and;
  • Run a dehumidifier to take water out of the air.

Increase the Air Temperature in the Room

Because the room is at the end of the air duct line and there’s only one feed vent in the room:

  • Change the air filter on the heating system air handler annually, at least;
  • Make sure the air filter is the density recommended for their air handler, and;
  • Leave the bedroom door open as much as possible to allow the warm air in the rest of the house to warm this room.

Increase the Surface Temperature of the Glass

  • Do NOT block the air from the room from touching the interior glass surface!

The heated air in the room MUST be allowed to warm the glass in the window.  If the room’s air is not allowed to contact the glass and warm it, the insulation value of the window glass will lower toward outdoor temperatures – even with R-5 insulation.

Now, for the rest of the story…  My client’s final say on this matter:

“You can please stop telling me the science about what is going on in the room. As a matter of fact, don’t reply. Same room, same heat/cooling input, same blind closed all the time, same door closed, same drying rack….the old, supposedly crappier windows never condensed like this and the room was never this cold.  End of discussion. Don’t bother.”

Of course, draftier windows can impact humidity levels, too, but I am sorry to say, I gave up…

Sometimes our best efforts will fail.  I know this.  But I am still disappointed.  If I could have done something to make her home and life better, I would have.  I would have given her new glass with a higher CRF, if they were willing to decrease the R-5 insulation value to an R-4… My reputation is more valuable than the cost of two window sash to me…

The condensation on this window, in this room, in this home was NOT the fault of the window.  I could not solve their problem.  Only the homeowner was able to stop water from condensing on their window — with the help of good advice from an expert who was trying to help…


  1. Thanks for this great blog, Mark. For those also interested in using AAMA’s CRF Tool, please visit

  2. Mark,
    This is a great blog and one that I will print and keep as a superior explanation of the factors that control condensation and how the room variables can be adjusted. This particular homeowner obviously didn’t appreciate your very thorough analysis but I know many on the other hand that would embrace it. And in the end, you did the right thing. You swallowed your pride to please the customer. You are totally correct that the previous windows did not have condensation because their air infiltration was probably so high.

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