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BS* + Beer

Choosing and Using High-Performance Windows

Windows are a major source of air leakage, so be armed with the right information when selecting a window package. Begin by knowing the features that impact energy and comfort most.

This is one of Emu’s pilot projects with middle-of-the-wall window placement and exterior insulation over the window frame. Photo courtesy of Emu

This episode of the BS* + Beer Show features Passive House expert Enrico Bonilauri talking about high-performance windows. It has been said that even the best windows make terrible walls. Windows are often the largest source of heat loss in a home. So which window features impact energy and comfort most? Operation style, frame material, glazing specs, size and location are some of the considerations when choosing and optimizing a window package. Enrico discusses these topics and others in the context of his study, “Emu Report on Building Standards.”

Enjoy the show!


Enrico Bonilauri is co-founder and “chief geek” at Emu, a company focused on training, consulting, and analysis for high quality building envelopes and healthy indoor environments, with extensive knowledge and experience in the International Passive House standard. Enrico is an Italian native and registered architect there. He holds degrees from Germany, Italy and Australia, and is the lead trainer for Emu’s Certified Passive House Training program.

The BS* + Beer Show schedule

The next show is on June 6, 2024, from 6-7 p.m. ET.

Use this link to register for The BS* + Beer Show


Kiley Jacques is senior editor at Green Building Advisor.


  1. [email protected] | | #1

    Very informative and interesting BS + B webinar. I was surprised by the relative lack of overall contribution of air leakage to overall window energy loss compared with the performance of the other factors listed.

    Prior to reading Enrico's recent article and watching the webinar, I would have listed air leakage as a top two in window energy performance loss. Now I would be curious to see a breakdown of the data points used regarding window style, material, and manufacturer and how they compared, especially air leakage as a percentage of overall performance loss.

    1. Expert Member
      DCcontrarian | | #3

      But do the infiltration numbers reported for windows reflect reality? If you look at old houses, very little air is coming through the pane/sash assembly, but they cut a hole in the side of the house to put the windows in and only sealed it in a cursory manner and didn't insulate at all.

      And infiltration numbers for new windows are reported for when the window is new. What happens in a decade or more when half the weatherstripping is gone and the sashes have warped and don't fit tightly any more?

      1. Expert Member
        Michael Maines | | #5

        Enrico's data was from a survey of 50 homes he had access to; as I recall, they all met or exceeded current code minimum standards, so the same may not be true for older windows. That would require a much larger study.

  2. gstan | | #2

    I have maintained for years (and still do) that the most cost effective window
    solution available (with todays technology) is relatively inexpensive windows
    with interior insulated shutters. Unfortunately, it's so little used that it's
    an essentially unknown concept.

    1. charles3 | | #4

      I was under the impression that interior insulated shutters are risky because thermal shock can crack the glass. Is that incorrect?

      1. Expert Member
        Michael Maines | | #6

        The problem with interior shutters or insulated blinds, from what I understand, is that unless they are airtight, they do little to actually change window performance. They do help with radiant heat gain or loss, so they feel more comfortable. I have not heard of glass breaking due to interior shutters or curtains.

        1. charles3 | | #7

          Thanks for your reply. You wrote, "I have not heard of g(l)ass breaking due to interior shutters or curtains." Even airtight R-14 shutters? I assume you respect Joe Lstiburek. It was he who discouraged me. Perhaps climate zone is relevant?

          1. Expert Member
            Michael Maines | | #8

            Yes, I know Joe and trust his experience. I have seen many of his presentations and read many of his articles but don't recall that topic coming up. I can imagine that heat gain with R-14 airtight shutters could make the glass spacers/seals fail, it's just not something that I have heard about. "Thermal shock" usually refers to a fast change in temperature, such as pouring hot liquid into a cold glass container.

  3. charles3 | | #9

    Thanks again for your reply. Why do you think R-14 airtight shutters would cause heat gain? What if the shutters were painted white? You wrote, "'Thermal shock' usually refers to a fast change in temperature, such as pouring hot liquid into a cold glass container." How fast is fast? Suppose we have a window that has effectively been held at outdoor nighttime winter temperature by the shutters. Further, suppose the indoor temperature is nice and comfortable in the 70'sF. Now open the shutters and expose the glass to the indoor air. Would the glass be OK regardless of climate zone?

    1. Expert Member
      DCcontrarian | | #11

      "Why do you think R-14 airtight shutters would cause heat gain? "

      Light from the sun passes through the glass and hits the shutter, and warms the shutter. Some of the energy the shutter absorbs is then radiated back through the glass, but not very much. There's two reasons for this. First, the amount of energy radiated is determined by the temperature difference between two objects. The surface of the sun is roughly 10,000F warmer than the shutter, the shutter might be 100F warmer than the outside. So there's going to be a lot less radiation. Second, the frequency at which a body radiates is determined by its temperature, the sun radiates white light since it is so hot, which passes well through the glass. The shutter is going to radiate in the low infrared range because it's not very hot and modern windows are designed to block infrared radiation. So even though the shutter is insulated, it's going to be much easier for it to shed heat into the room than back through the window, so it's not doing much to block solar gain. One thing to keep in mind is the better the window, the less effective the shutter is, or conversely, the worse the window the better the shutter works.

      "What if the shutters were painted white?" The more the shutter can reflect light back at the same frequency it comes in, the more heat it can keep out of the room. So maybe not white but instead highly reflective like aluminized mylar. However, keep in mind that most modern windows have a solar heat gain coefficient of about 50%, which means they block 50% of the light that goes through them. So even with a reflecting shutter half of the heat gets trapped in the room.

      Again, the better the window, the less the shutter helps. Which is what I think Joe L. is getting at.

  4. charles3 | | #10

    This is a subject that I think might merit not only detailed discussion but laboratory experimentation and analysis to ascertain safe parameters. Let me share all of the info I have from Dr. Lstiburek on the subject. His original warning is at

    Then my follow-up email exchange with him went as follows:

    Joe: The cracking was due to thermal shock when the really cold interior pane of the double glazed window assembly suddenly saw interior warm air. The risk is much less with triple glazed windows that are gas filled. With R-5 and R-6 windows available today it is hard to justify interior window insulation. I had the problem in 1980. Things have changed in 40 years.

    Me: You wrote, "With R-5 and R-6 windows available today it is hard to justify interior window insulation." Well, you've undoubtedly heard it said that "The best window makes a lousy wall." Even an R-10 window seems like a lousy wall to me. If we can make windows thermally act like walls, on winter nights, I think we should. The question is how to do so without damaging the window. Is there a climate where interior window insulation works, and how much R-value can the insulation have without risking damage to the window?

    Joe: It is a pretty safe bet that an R-10 interior window insulation for an R-5 window will work in cold and very cold climates.

    Me: When you were experimenting with window insulation in 1980, how much interior/exterior temperature differential were you dealing with when the cracking occurred? Do you suppose low-e coatings have any effect in this situation?

    Joe: Toronto. Today’s windows are 250 percent better than what I was dealing with. Yes, low e matters.

    Me: How do you get a feel for what window R-value, what insulation R-value, and what temperature differential constitutes "a pretty safe bet"?

    Joe: I guessed. You are on your own now.

    Me: In 1980, did your glass crack just as much regardless of whether your insulation was interior or exterior?

    Joe: Only when it was on the interior.

    Me: I once saw my brother pour hot water on an icy windshield to de-ice it. I was sure it was going to crack, but it didn't. So I'm wondering what margin of safety tempering and laminating provides. Any thoughts?

    Joe: Tempering the glass is a big deal.

    Me: A big enough deal that R-25 interior insulation would carry no risk of cracking in your climate?

    Joe: Maybe. Try it and see.

  5. Mtn_hombre | | #12

    Fantastic presentation by Enrico, very informative and very thorough. Would love to see that window cost/performance chart with detailed labels! (I know, not to be) I believe that Enrico stated (as Michael mentioned in a comment) that this data was from their analysis of 50 homes that are up to recent code standards, I'm sure that has a large impact on what is the greatest efficiency hit of a home. I have Alpen windows in my own build, love them.
    Also, Enrico mentioned three window manufacturers that he likes in the bang for the buck category; Alpen, Innotech, Wife? I couldn't make out/not familiar with the third one?

    1. Expert Member
      1. Mtn_hombre | | #14


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