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Community and Q&A

Exterior insulation and large windows: Worth it?

JacobInTN | Posted in Energy Efficiency and Durability on

I’m planning on building a small house this year and am wanting a very efficient structure but one with a lot of glass (because the lot has an amazing view).

My plan is to use 2×6 advanced framing (when possible), Rockwool between studs, air sealed exterior (sheathing, meeting foundation/roof, penetrations, etc), 10-12″ of dense pack cellulose in cathedral ceiling with Rockwool above roof decking and now have to decide on exterior insulation.

As shown in my attached plans, a large portion of the back of the house will be glass (close to floor to ceiling). My question is, given the lack of efficiency of that many sqft of glass, would it really be worth it to use exterior insulation on the rest of the house where there isn’t?

The rear with all the glass faces south and will have overhangs to control exposure throughout the year. My thought is given the structure will be relatively tight, even without exterior wall insulation I will end up with a much more efficient structure than any house I can buy in this area.

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  1. Expert Member


    Yes the windows may well dictate how much effort you want to put into the other assemblies, but the only way to know is to run heat loss calculations. To make even a ball-park estimate, more information is needed - particularly the location of your build and the size of what look like substantial windows on the north side. if you can provide that you will get much better answers to your question.

  2. Peter Yost | | #2

    Great project for BEopt.

    This free, online energy modeling tool allows you to easily compare all the options and optimize along the way.


  3. onslow | | #3

    The short version of my answer would be - yes it still helps to have a good envelope, but the best window performance is critical as well. For the size of the house relative to the window areas, I would question the value in advance framing for the two walls most likely to benefit. By the time you frame the windows , doors, and headers in I think the amount of savings will be trivial. An engineer will also have to address the racking forces for local wind loads and transfer of roof loads to foundation. It may all be moot for those reasons alone. Also, overhangs are over-rated because the shoulder seasons are where most fall down. You haven't mentioned your climate and whether air conditioning is required.

    When finalizing window decisions in my own case, I had benefit of extra insight into how large a window was really needed for the great views our lot afforded. A friend had recently completed a home near by with similar views and had installed soaring windows like those frequently seen in magazine homes in the mountains. Great visual effect that, in my opinion, really doesn't do much for the view ultimately. Additionally, it required some very extreme overhangs and beam design to make it all work. Watching the large glass areas flex in the strong winds that come with the view adds an element of fear that may not be desirable.

    I did take my friends advice and dragged a ladder along with some old light stands that I had to hold pieces of furring to represent the bottom and top limits of imagined window openings. The ladder was necessary to mount the furring strips, as well as create a perch that matched our final projected floor plus chair height. This was post site clearing of the many trees that needed to go for the house, driveway and septic. If you are still early in the process you need to be sure the views you are after can really be obtained. It would be very stressful to commit to building the house on the only available space for septic fields.

    After many visits and some spirited discussions with my wife, the views were settled upon and the window sizes selected. The tallest is 5' 8" starting 18" above floor and I probably could have shaved 3-4 off that with no loss of view. That would have been helpful when working out the roof pitches and the headers required for the span. The big "view" window is composed of three mulled units that span almost 11'. The reason behind not doing one extremely large panel was both monetary and structural. Guaranteeing that a header beam will not sag over time is almost impossible. The risk of creating a creeping load point in the middle of a huge sheet of glass seemed to great. Also remember, one is generally sitting while looking out the window. All that extra glass will just be sky or the underside of the overhang.

    Code restrictions in your area may also dictate glass selection for the windows. In some high wind areas of my state, the code can call for glass units capable of near hurricane force winds. Another fun feature of code involves windows near doors. If a window is set too close to an exterior door, tempered glass may be required. Same for windows that are set too close to the floor line. You are mentioning the desire for nearly floor to ceiling window dimensions, so be sure to look into the code requirements if you must go this big. Some manufacturers may not offer the option or charge substantially for the special upgrade. Coating options may also be restricted.

    When re-"viewing" your site and the landscape, it may help to take along phone or camera and take pictures to print out - without cropping from original image. It may prove constructive while sorting out what exactly you are "seeing" versus feeling when on site. The most difficult parts of dealing with what constitutes a view are the unstated feelings each person has. My wife actually loves the "into the forest" view on one side of the house over the miles of mountains view on the other side. Photos of both views do not look anything like we see them.

    A jumble of random trees is her preferred "view" and a set of distant diminutive "mountains" is my preferred view. At least that is what the photos reveal. Eye and brain combine to elicit very different perceptions of views and merely having a large window may prove unsatisfactory on many levels. Final floor height relative to the lot surface will profoundly affect your perceptions as well as proximity to any grade changes (maybe less so in a prairie).

    I think you will find that the really giant windows work best with views when you approach them from a distance. A large window situated 40' opposite an entry door can create a dramatic effect that loses impact when standing two feet from the window. Unless the window overlooks a dramatic grade change like a bluff or canyon. The best choices are up to your site and your perceptions.

    Our lot was heavily wooded and difficult to read visually. A great bonus we uncovered when clearing for building was a better perception of the underlying contours and a nice counterpoint to the more distant views. Having some anchor points in the mid field of the view helps to give weight to the distant points. Photographers frame landscapes with a least one foreground item such as a tree or rock to place you in the scene more firmly, frame your views similarly by constraining window size and being aware of the foreground elements you include.

    Hope that was worth the long walk around the park. Best of siting. Oh, and one last will have to wash them. Do you have a place to stand safely.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      As he usually does Roger has offered a thought provoking a thorough take on the issues. I'd add to his analysis that window size and location should also be thought of from the perspective of how it affects the relationship of the interior of the house to its immediate surroundings. Do the large windows help make the two feel like one? Windows have a number of roles apart from framing views.

  4. onslow | | #5


    Thank you for the kind words. I had decided to shut myself up before going on even longer.

    If you have patience, I would like to add a comment on an extreme window house plan. Back in college, I had opportunity to tag along with some architecture students that had volunteered to clean the mud out of the Mies van der Rohe Farnsworth house along the river in Plano, Illinois. (Flooding up to four or five feet over floor level was a more frequent occurrence than is generally reported. )

    It was thrilling to have a chance to visit what was a well known icon in the world of sophisticated architecture, but must say that living in the house would be an acquired taste. The glass box certainly did give one a grand view. Perhaps living through the seasons might make up for some of the plans practical deficiencies regarding furniture and life as actually lived. While isolated enough when built, an inescapable feeling of vulnerability is what I finally concluded made me less enthusiastic about the structure and its endless glass as a home. Indeed, story was that the patron for whom it was created forced a recalcitrant Mies to add a bank of cabinets to provide some privacy on the bathroom end of the core. He would not call it a wall.

    Living in/on a glass walled patio might be for some, but not me. Visits to apartments in the glass wall skyscrapers so popular in Chicago similarly left me wanting a bit less of the outside world being so very present. Maybe being a caveman at heart and afraid of heights to boot is my limitation not the norm. I would agree that windows have many hidden attributes and hope that Jacob still has time to explore his site with some in mind. It may well be that the site demands having the barrier between inside and out melt away. Just beware of the structural and thermal ramifications.

    As the Farnsworth house is entirely structural steel and glazed, I do believe, with single pane glass like used in skyscrapers (it was 1950 or so) I can only imagine how cold the walls would be in winter and how relentless the summer sun would be. The vast floating patios would likely be usable only a few weeks a year depending on how one felt about bugs and humidity. 35 years of life near Chicago has made me treasure all the more where I now reside.

    However, everyone is different, so viva la differance' or words to that effect.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      I'm very sympathetic to that view. Architecture's most basic aim is place-making. And what distinguished places from undifferentiated space is enclosure. Very large windows detract from a place feeling enclosed. I have never sat in an apartment near a curtain-wall where I felt comfortable - not physically, but psychologically.

      That said, I think there is a right amount of glazing for each wall - and sometimes that is much larger than energy concerns would dictate. This is from a house I built a couple of years ago overlooking the Strait of Juan de Fuca. What part of that window would you not want to be there?

  5. walta100 | | #6

    Worth It? That depends on your goals.

    If you want to build a code compliant house with 2x6 wall in most places yes you will need exterior insulation.

    If the question is, will the insulation ever save enough fuel to pay for its self? The only way to know is to build a model of the house in BEopt.

    If the question is, is this window to big? We all know windows make bad walls, and pointless glass is wasteful.


  6. JacobInTN | | #8

    Wow, this was my first post here (after a couple years of reading) and I'm amazed by the thoroughness and thoughtfulness of the replies. Thank you all so much for the information - I definitely have more to consider.

    To follow up on my original glaring omission, I'm in zone 4 (NE TN) so we definitely have A/C and I will be adding a dehumidifier (hopefully ERV too) because the shoulder seasons tend to result in my current residence being >80% relative humidity.

    I'll definitely start to run some calculations and see what makes good economic and comfort sense. Attached is the lot I'm purchasing and the view I'll be attempting to frame with windows.

    1. Expert Member
      MALCOLM TAYLOR | | #9

      Beautiful view. You want to see that as often as you can! Good luck with your build.

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