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Insulated Jam Extensions for High Performance Windows

T_Barker | Posted in Energy Efficiency and Durability on

Even high performance windows have relatively very poor insulating qualities through their frames. For example, fiberglass, reasonably large 4’x6′, tilt and turn windows, with an R=8 center of glass, will have an overall window R=6 or less. Since the frame is such a small portion of the overall area of the window assembly, obviously the R value of the frame itself is very low. Probably R=1 or 2.

But if you are willing to go with “innie” or even “mid-way” window installation in thick walls (e.g. 4″ of exterior insulation), why not add INSULATED exterior jam extensions? Even if they have to overlap the frame and protrude say 1/2″ towards center of glass (so there is less thermal bridge).

Are there any companies selling insulated exterior jam extensions?

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  1. charlie_sullivan | | #1

    Thermal buck,, might be what you are looking for.

    I would like to see a window company sell the whole setup.

    Another thing I've thought about is a double-pane low-e argon innie plus a second double-pane low-e argon outie in the same opening. That means you can buy mass-market windows and save money. But it does entail some aesthetic and functional challenges.

    1. T_Barker | | #3

      Thanks Charlie. I'll take a look at Thermal Buck.
      I agree, the window manufacturers should be doing this as a package.

      1. T_Barker | | #8

        ThermalBuck looks like a great product. However, it seems that you have to to do an "outie" installation, and you have to use flanged windows. I'm pretty sure I'm going to use flangeless windows, installed "mid-way" in the wall assembly.

        1. T_Barker | | #11

          On second thought, I don't see why you couldn't use flangeless and/or put the window where you want with ThermalBuck. I'm going to take another look.

          1. qofmiwok | | #17

            Did you come up with a detail using flangeless windows?

    2. creativedestruction | | #13


      Installing a pair of double glazed units in sequence would result in poor visible light transmittance. Multiply the VLT for the two units (eg. 0.49 x 0.49 = 0.24). The wide air gap in between the units would allow convective loops, negating insulating value of the gap even if both units are airtight enough to minimize interstitial condensation. They could together achieve maybe R6 or R7 whole window performance. Forget about SHGC. And cost-effective energy savings.

      1. johngfc | | #14

        Jason -
        I'm interested you're so negative about this.

        Looking at Cardinal glass, a local manufacturer, their LoE-180 glass has visual transmittance of 79%, U=.31 (air) or U=.26 (argon). So a "double double" window would have VT something less than 0.64 - not bad. Our recent quote for triple pane windows (whole window U ~ 0.18) averaged ~ $75/sq ft. For specific, well protected locations, I've wondered whether it wouldn't be a considerable cost saving, AND increase in U value, to construct such a double double-pane window. With, say, a 1/2 to 3/4 in space between the IGUs, I'd think such a window would be close to R 9.5 for air fill, and R 10.7 for gas - U .105 and .093, respectively. In my book, those are darn good windows. Conservation Technology has a variety of gaskets for window construction so suitable materials are readily available. Using e.g. ZipR for some of the framing, one might even be able to design a frame as or more efficient than commercial (this is of course only for a non-opening window). One might need to accommodate condensation between the IGUs (dry out in the summer?) or other details, but I suspect even if you're paying full labor, this could be an attractive option for windows well protected by a porch roof or similar.

        What do you think about this?

        1. creativedestruction | | #16

          You're taking about a direct-glaze or site-built frame sort of setup, which is different than the innie & outie mass market windows idea suggested. That has its own challenges but if you've got time and patience to secure and seal something like that up correctly then go for it. Fixed windows are underrated and much cheaper than operable.

  2. Expert Member
    Michael Maines | | #2

    In Passive House training you learn to "over-insulate" the window frames on the exterior. Several window models are available where you can bring the exterior extension jambs right up to the edge of the clear opening, leaving space for insulation over most of the window frame. I don't know of any products designed for this application. I have specified Zip-R to wrap the exterior window buck before applying extension jambs; maybe that would work for you.

    1. T_Barker | | #4

      Michael, any chance you have a detail drawing as an example?
      I'm concerned that simply wrapping the exterior window buck will still leave a substantial energy bridge through the window frame. Similar to the thermal bridge at a slab to foundation joint if you don't carry the insulation up the slab edge. A surprising amount of heat loss.

      1. Expert Member
        Michael Maines | | #7

        Sure, here you go. It's not notated but the top is exterior and the cladding is shown with two lines as it was cedar shingles, returning back into the window. The Zip-R sheathing and relatively thick cladding hurts the insulating value of the exterior insulation.

        I just found another one, not as clear but with a more conventional arrangement.

        1. T_Barker | | #9

          Excellent, thanks!

          In the 1st image is that a double stud wall? I'm assuming the insulation portion of the ZIP-R is the "over-insulating of the window frame".

          In the 2nd image what is the 1x3 (I'm guessing) backing at the exterior end of the plywood buck attached to, if anything?

          This whole window installation issue needs to be "kicked up a notch". It's like everyone installing a window is either not worried about the air seal and heat loss around/through the frames (probably 98%), or those who care, try their best to hack together a solution every time they install a window.

          Upon further investigation, it seems like most of the European high performance windows do have better insulated frames, and in one North American case so far, the thermally broken aluminum frames are well insulated.

          1. Expert Member
            Michael Maines | | #12

            Yes, the first one is a double stud wall with 2x6 exterior wall, no sheathing, just Mento on the exterior (with Simpson strap bracing) and 2x3 vertical battens for the rain screen.

            Yes, "over-insulation" is the Passive House term, which I think is a weird choice but that's what they call it. The insulation portion of the Zip-R is the over-insulation. In this example it's not covering much of the frame, but it cuts heat flow from the wall framing and installation gap.

            The second example is a 2x4 wall, 1/2" sheathing and exterior I-joists for insulation. Again with Mento at the outside, and 1x3 vertical strapping. When dense-packed the Mento bulges out and the 1x3 is not enough to ensure air flow. I think you're asking about the 2x4 that wraps the window buck. It's only attached to the buck and provides stiffness and nailing.

            For Passive House designs I use the PHPP energy modeling program which is exhaustive in the input details. We not only model the glass and frame, but also the installation detail and even the spacers between glass panes, orientation, how far it's inset and the level of shading from all sides. There is certainly energy lost through the frames but it is not enough to worry about on must projects, only the ones going for the highest levels of energy performance.

            There are a lot of different ways to make windows, and a lot of different ways of installing them. Most high-performance designers and builders obsess over their details. But it's not a one-size-fits-all situation.

  3. ERIC WHETZEL | | #5

    You may find this video helpful:

    We over-insulated our frames, giving the windows a 'frameless' look. You can see photos towards the end of this post:

    I think it's debatable whether this is worth the effort. I've heard opposing opinions from PHIUS certified professionals. Even in our situation, a thermal bridge remains through all of our plywood window and door bucks.

    It's plausible, especially in a retrofit situation, that you may need to address this area to make up for other areas of heat loss in the building that are more difficult/impossible to address in order to meet certification requirements.

    In general, the windows and doors are already a fairly significant thermal bridge (e.g. even R-11 window vs. R-40 wall), so plywood window and door bucks aren't adding a significant amount of overall heat loss to your wall assembly.

    On Passive House drawing details for windows sometimes you see rigid foam spec'd for the exterior and interior window sills, sometimes one or the other, or even none at all.

    Will you be going for official certification through PHI or PHIUS? Will you be using PHPP or WUFI to do an in-depth energy model for your home?

    1. T_Barker | | #10

      "In general, the windows and doors are already a fairly significant thermal bridge (e.g. even R-11 window vs. R-40 wall), so plywood window and door bucks aren't adding a significant amount of overall heat loss to your wall assembly."

      Good point. It just seems a shame there isn't a better way to do this. I'm going to take another look at ThermalBuck as well.

      No, unfortunately no official certification. I'm with Joe Lstiburek when it comes to certifications.

      I haven't looked at the energy modelling close enough yet. I was considering a combination of Wrightsoft, BEOPT, and some of Borst Engineering's software.

      1. Expert Member
  4. ajc_electric | | #6

    I agree and recommend Thermalbuck

  5. Deleted | | #18


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