GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Skylights and the laws of physics

Peter L | Posted in Green Building Techniques on

During wintertime in a Zone 5+ climate, even with good skylights like Velux Skylights (R3/R4). When during wintertime when it’s 20F outside and 68F inside and it’s snowing with a lot of moisture in the air. Every skylight will experience condensation of some sort, correct?

The laws of physics cannot be defied if you have a R-40 roof/attic and then a R3/R4 skylight. When it’s 20F or colder outside and inside the temps are 68F and humidity in the 40% range. Moisture will form on the skylight due to warm moist air rising and condensing on the colder skylight glass, correct?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. User avatar GBA Editor
    Martin Holladay | | #1

    Peter L,
    I haven't double-checked your numbers on the psychrometric chart, but you are absolutely correct on the principle. To reduce the risk of condensation, you have two choices: (1) increase the temperature of the innermost pane of glass (for instance, by replacing the double-glazed skylight with a triple-glazed skylight), or (2) lower the indoor relative humidity (for instance, by increasing the run time on your mechanical ventilation system).

    For more information, see "Rating Windows for Condensation Resistance."

    For more information on triple-glazed skylights, see these two articles:

    "A Passivhaus-Certified Skylight Hits the U.S."

    "New Green Building Products — February 2012"

  2. Zephyr7 | | #2

    I have the type of skylights you mention and yes, when conditions are right, mosisture does form on the inside of the glass. It’s not as easy as “when it’s 68 inside and 20 outside” though when determining when the condensation will form. As Martin mentioned, there is a complex relationship shown on the psychrometric chart that you can use to determine from temperature and humidity measurements when condensation will form.

    Basically, if the inside surface of the glass is below the dew point for the inside air in the house, you’ll get condensation. In my expierience, even when it’s very cold out, you usually get only a little condensation. There is usually not enough condensation to be a problem. What I found to be much more important is to air seal the skylight very well so that no indoor air leaked up into the skylight flashing and curb. We had that happen a little bit before the ceiling was finished and that made some drips that looked like small leaks. Once all the drywall was up and detailed the problem went away.


  3. User avatar
    Walter Ahlgrim | | #3

    Most skylights are tilted so any condensate runs to the low side of the glass where it drips into gutter a gutter and drains to the outdoors thru a weep holes. See the attached image. This drain generally works well sometimes you will get the right combination of weather to freeze the weep holes and the gutter overflows and get some drips. I recall 5 days of dripping over 12 years in St Louis MO.

    I will stay away from the laws of physic. But you may enjoy Goggling “latent heat of condensation” it seems all the energy absorbed boiling is returned when it water condenses.

    If you must have a natural light without windows, I much prefer Velux insulated sun tunnels. They deliver more light than a skylight 4x there size.


  4. User avatar
    Jon R | | #4

    Depends on airflows, etc, but should be more like < -20F outside to cause condensation under those conditions.

  5. User avatar GBA Editor
    Peter Yost | | #5

    With skylights and condensation, I find that about half the time condensation is related to glass temperature and the other half it's the lack of air seal/continuous insulation between the rough opening and the skylight unit. Most skylights have great flashing packages for those tall sidewalls but they are great fin radiators as well. So, yes, purchase the highest performing skylight you can, but make sure your installation is just as high performance.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |