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Community and Q&A

Skylight – increasing its R value

Joe Pakus | Posted in Energy Efficiency and Durability on

I have a 4′ x 4′ skylight double pane filled with argon gas. It faces south and during the the winter in Massachusetts when the sun shines through, it provides plenty of light and passive solar gain.

But night time is another story. The heat loss is quite evident. The chill in the room and the snow cover melting off the glass reminds you of the heat loss.

I was thinking of installing a third pane of plexi or clear plastic to the interior jamb, it is nearly 12” deep. This removable pane would be for the winter only. My concern is, would there be an excessive build up of heat in the new dead air space on sunny days that may compromise the existing double pane lite?

Any thoughts, ideas or experiences.
Joe Pakus

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think heat build-up will cause any problems, but you may increase the chance of damaging condensation. An interior storm (or "energy panel") will lower the temperature of the innermost glazing an the skylight, and you may get condensation on the edges of the glass or the frame of the skylight in cold weather. Keep an eye on it to be sure you don't have a condensation problem.

  2. Joe Pakus | | #2

    Thanks for the advise. I take it that a good seal around the new stops for the energy panel is a must and try to be as dry as possible prior to the install. I get some condensation a few times during very cold spells, I was hoping this might reduce it too, but I will keep a close watch of it.

  3. 5C8rvfuWev | | #3


    many years ago I had a MA skylight with the same heat-loss (and gain in summer) at issue in a loft area. I was then able to source an "insulated" (i.e., padded) solar shade which of course was far from airtight ... but it did cut off the cold breeze on winter nights and -- because it was backed with mylar -- a big chunk of the summer heat gain. Perhpas you can still find such a product.

    Joe, too

  4. Eric Tollefson | | #4

    Along the same lines as #3, have you looked into cellular shades? I believe the manufacturers have some models which are designed to be installed on skylights and such. The double- and especially triple-cell versions claim to add some R-value. I think you can also get them with a reflective liner on the inside which adds some radiant barrier benefits.

  5. Dan Kolbert | | #5

    Won't help you with existing, but I wanted to put in a plug for a local company - Wasco, making reasonably-priced triple glazed skylights.

  6. Amanda Evans | | #6

    Also a plug for a local company here in Santa Fe that is making a nanogel aerogel skylight or an insert for retrofits that is supposed to be R-9. The company is called Aerolenz:

  7. Joe Pakus | | #7

    I'd like to thank you all for your input and will investigate the insulated shade approach. Around 12 years ago this was one of the better fixed skylights around and if it ever comes to replacing it I'll look into Wasco or the Santa Fe company.

    As a quick fix/experiment I hung a 2.7 mil clear vinyl sheet taped to the casing yesterday. No easy feat. After I secured the last side I stood back and looked, to my amazement there was an 'upward' deflection in the vinyl sheet. We had high winds yesterday and I attributed the upward deflection to wind entering the house and seeking a place to exit. This morning the winds are still here but the vinyl sheet is flat. And yes Martin I have a round circle of condensation in the middle of glass the size of a soccer ball. If it continues to grow and or drip...well off comes the vinyl sheet.

    Now that the wood stove is cranked up and the house is warming up for the day the 'upward' deflection has started again. I did not think rising heat would generate that much pressure.
    Thanks again people for your input and advise. Trying to stay warm...

  8. GBA Editor
    Martin Holladay | | #8

    What you call "rising heat" is air leakage at the edges of your skylight. If I were you, I'd get a ladder, a good headlamp, and a tube of caulk, and look carefully for cracks at the perimeter of the skylight.

  9. John Brooks | | #9

    Is it not also possible for the membrane to deflect even without an air leak?
    Doesn't stack effect pressure exist even without an air leak?

  10. GBA Editor
    Martin Holladay | | #10

    Joe wrote, "I hung a 2.7 mil clear vinyl sheet taped to the casing yesterday. ... After I secured the last side I stood back and looked, to my amazement there was an 'upward' deflection in the vinyl sheet."

    Clearly, there was a pressure difference between the air below the poly and the air above the poly.
    The air below the poly was at positive pressure with respect the air above the poly. That means that there had to be an air leak above the poly -- and the poly was closer to the outdoor air pressure than the air below the poly.

  11. Joe Pakus | | #11

    Martin & John,

    A few more thoughts to ponder... The sheet deflection may be a combination of both stack effect and air leakage in the new 'dead air' space. Yesterday was the first 24 hour cycle of observation. The sheet went from flat in the morning to upward deflection as the house warmed up and around high noon a downward deflection and back to flat as the evening came.

    Recalling when I recently shoveled off the 2 feet of snow on the roof, the snow around all four sides had melted back quite a bit. This was observed after the surface blanket of snow (top 6-8 inches) was removed. This leads me to believe that there is air leakage around the head, step & seat flashing. Mind you I never had a water leak from this skylight. Condensation drip yes. And since I've been reading GBA recently the fact that I just nailed the 1x8 T&G Vgroove knotty pine to the roof rafters is proof that the entire ceiling is leaking air. Yes now I know I should of place some type of air tight substrate first. Hey I did remember the insulation and the soffit and ridge vent..

    When it warms up I'll remove the casing and see what I can seal up between the jamb and roof rafters and check the interior plastic trim piece that butts up to the glass. Hopefully between sealing what ever air leaks that can be fixed without ripping down the ceiling and a insulated shade I can stay a few degrees warmer. I really appreciated the discussion and information.


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