Milanese Remodeling
by Mark Milanese
April 7th, 2015

Window Condensation: A True Case

Can windows make water?  I have a client who is convinced they can.

They believe one of the windows we installed on their home is defective because water shows up on the inside glass surface facing the room in the winter.  None of the other windows we replaced in their home make water, so they’re convinced the window in this room must be broken in some way and not as good as the rest – it wasn’t even as good as the “crappy” window the builder originally installed on their home… the original windows may have been drafty, but they didn’t “make water” on the inside of the house during the heating season.  Not only that, this bedroom was always colder than all the other rooms in the house.  They were disappointed and they wanted me to fix it. Now.

I immediately put on my “Sherlock Holmes Hat” to solve “The Mystery of the Window Making Water”…

Four years ago, we replaced all their windows with very high-performance models. They selected DH windows with an R-5 insulation value, a Design Pressure Rating of 55 and a Condensation Resistance Factor of 51.

U-Factor – 0.22

R-Value – Nominal 5

CRF – 51

DP – 55

These are very good overall ratings for insulation, resistance to air infiltration, water penetration and forced entry. We do offer a window with higher Condensation Resistance Factor, but lower R-Value & DP Rating. Since “warmer windows” and “cut down on drafts” were the two biggest reasons they initially listed for replacing their windows, I think they made the best choice for them…

I observed the conditions of the house and the room where the window was located, looking for anything that would affect humidity, glass surface temperatures and room air temperatures.  I followed them up the stairs to the north end of the house.  They opened a bedroom door next to the hall bathroom and we all entered a 12-foot by 12-foot room with one window. Here is what I found:


  • The door to the room was closed in the middle of the afternoon, which didn’t allow warm air from the rest of the house to circulate into the room and excess humidity in the room to dissipate.
  • As the door opened, I saw a wooden rack for air-drying clothes loaded with damp towels sitting in the room.
  • The bed was made with a thick quilt. The floor was carpeted and the window was protected by two layers – a heavy drape and a pleated blind.
  • This room was located at the end of the forced hot-air duct line and got the least heated air blown into the room from the furnace.
  • I found only one heat vent bringing heat to the room.  It was a floor vent located on the north side of the room, but not below the window.
  • The pleated blind closed over the glass fit so tightly to the window frame that no air from inside the room could touch and warm the interior glass surface.

These cumulative factors would have the overall result of making the air in the room more humid and colder than other rooms in the house and also making the glass surface colder…

Despite the factors that would raise the humidity, lower the temperature and keep heated air from warming the interior glass surface, there was no condensation on the glass on this freezing cold day, but there were indications of condensated water proving the window did sweat, at times.

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