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Community and Q&A

Argon vs. Krypton fill in IGUs

JonathanRupp | Posted in Energy Efficiency and Durability on

We’ve finally had it with all the interior shrink wrap plastic covering our remaining single pane (fixed) windows so i am now looking to re-glaze them with IGUs. (We live in the greater Boston area in a house surrounded by trees filled with walls made from single pane windows occasionally supported by a 2×4 wall, so utility costs and comfort is dominated by heating – ~$1000-1200 p.a vs. no more than $100 p.a. for cooling)

Our existing window frames have relatively limited depth, so optimal IGU thickness is 5/8″ although I can squeeze in 3/4″ thick IGU. The larger windows need thicker glass, limiting me to 3/8″ spacers without alot of work on the window frames

I know Krypton is optimal with the 3/8″ spacer, however, i have not gotten any glass companies to give me a quote on the additional cost of krypton over argon (most have said they have never even seen one outside a trade show, which seems problematic), or the exact U factor performance boost, so i cant calculate if it will be worth the additional cost. So far i’ve gotten:

IGU price of roughly $16-18 / sqft with Solarban 60 and argon fill. (with the Argon fill costing about $2-3 / sqft). -I have lots of tree shading, so very limited to no benefit to going to a higher solar gain coatings, and the solarban 60 will match the smaller movable window sashes i’ve already replaced

Center of window U factor of 0.24 (day) and 0.29 (night) using solarban 60, 1/2″ spacer, argon fill, @0F) – and because the windows are large, the center window U factor is the only one that really matters, and i am not replacing the window frames.

I know going to a 3/8″ spacer will increase the U factor, but as i’ve seen several graphs showing Ufactors by spacer thicknesses, with one showing only a minor loss in argon performance at 3/8″, and another showing substantial loss.

Question 1: How big of a U factor penalty will i face moving an argon fill window to 3/8″ spacers over a 1/2″ spacer at winter temperatures in Boston (which is warmer than 0F on average)?

Question 2: How much lower will the U factor go with a Krypton fill at a 3/8″ spacer at similar temperatures?

Question 3: What exactly is the price difference between argon and krypton? And would there ever be a payback scenario where it would make sense, assuming these windows also last 50 years?


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  1. GBA Editor
    Martin Holladay | | #1

    You're not a glass company like Cardinal. You're a homeowner. You're not going to design and build your own IGUs. You are going to purchase what's available from your suppliers.

    Are you building your own fixed windows, or are you buying new windows? In either case, ask the window supplier or the IGU supplier what the glazing specs are. You probably won't be able to get krypton, and you may not be able to specify the gap between the panes.

    What you want to do is compare U-factors. The lower the U-factor, the better. In your case, you want a high SHGC, not a low SHGC.

    Here are links to two relevant articles:

    "All About Glazing Options"

    "High-Solar-Gain Glazing"

  2. JonathanRupp | | #2


    Correct, i am not building an IGU, rather I am buying assembled IGUs to re-glaze my existing windows. And as i am buying the IGU, i have to give the specifications for either the IGU company or glazing contractor (who then relays the specifications to the IGU company) to make the IGU, including the glass size, thickness, & coatings; spacer thickness, material, & color, and gas fill. The IGU company then sources the coated glass and spacers from the glass companies (but the IGU company typically have everything in stock for common sizes).

    It is much easier and cheaper to re glaze existing windows than it is to replace a whole window, as long as the frames are in good shape. All you need to do is carefully remove the exterior stop bead, pop out the existing glass (which is easy when the glazing compound is 50 years old), remove the remaining glazing compound (which is kind of a pain when the compound is 50 years old), install flexible butyl tape on the interior frame edge, place small spacers on the bottom of the frame, put the new IGU ontop of the spacers & against the tape, glaze around the edges with silicone, and finally carefully reinstall the stop bead without hitting the new glass with the hammer. It took about 2 hours for my first small, 52" x 25" window.

  3. JonathanRupp | | #3

    In my first post the "glass company" should have been "glazing contractor" but almost all glazing contractors call themselves glass or window companies

  4. jackofalltrades777 | | #4

    Cardinal is the most common glass supplier for windows but there is also Guardian glass. Just more options to consider.

  5. Jdmillar | | #5

    The benefit from krypton will be dwarfed by the frame losses. Don't bother.

    Edit: more usefully: I don't know the difference in price except it is "large." The benefit is not. It isn't easy to justify in high performance windows in a high performance house. You haven't told us much about the rest of the house but I would bet there are "lower hanging fruit" to save a lot more btu/$.

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