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Use less insulation in South-facing wall?

mapnerd | Posted in Green Building Techniques on

I’ve been discussing the frame wall assembly of our South-facing wall with my architect. The home uses some passive solar design, so the South wall is tall (just under 21′) with a lot of windows (19). I am using 2″-3″ of exterior rigid foam on the other walls, but he suggests going with less foam and using OSB sheathing on the South wall. His reasons are 1) there is very little insulated wall on the South elevation, so the rigid foam may not make as much of an impact, and 2) this would enable us to use standard 6-9/16″ extended window jambs. Does it make sense to reduce insulation to allow for more standard-sized window jambs? I’m concerned about my sheathing falling below the dew point temp.

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Replies

  1. Riversong | | #1

    Some passive solar design? With a 21' tall south facade with 19 windows? That sounds like excessive glazing and a lack of passive solar design principles, such as proper glass to floor area and glass to mass ratios as well as appropriate shading.

    I would distrust anyone who advises the use of OSB sheathing, since it's highly vulnerable to moisture. And I would distrust even more the advice to eliminate the thermal break on the wall that has the most concentrated framing with little room for cavity insulation.

    I would suggest getting a different architect who understands both passive solar design principles and the concept of thermal bridging.

    Jamb extensions are easy.

  2. J Chesnut | | #2

    Michael I don't recall your location.

    Has your architect successfully designed a previous passive solar home? Focusing on passive solar over super-insulation (or Passivhaus) in my opinion requires a much greater level of experience and learning through trial and error.

    If you are using windows with poor performance and they are poorly installed allowing for much air leakage then maybe the added insulation will have little extra impact, but why accept compromise from the get-go?

    I don't understand the comment about the OSB. What was the alternative? Maybe your architect is less worried about the jamb extensions and more concerned with the window attachment and exterior flashing details. These construction details can become more difficult as deeper insulation packages are introduced.

    Martin recently blogged about determining minimum thicknesses when using exterior rigid board:
    https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

  3. user-659915 | | #3

    21' solar wall? Sounds like your gains will be awesome, and not in a good way.

  4. user-757117 | | #4

    Out of curiosity, what climate zone are you in?

  5. homedesign | | #5

    This seems to be a common misunderstanding.
    The belief that South Facing Glass galore is the "ticket".

  6. mapnerd | | #6

    I am in Omaha NE (climate zone 5). Here's a link to the plans - http://dl.dropbox.com/u/2226850/Plans.pdf. Feel free to lay into me with suggestions/criticisms/ideas. Thanks.

  7. user-757117 | | #7

    I see a number of references to "sunshade/collectors". Specifically, what type of collectors does this refer to?

  8. Riversong | | #8

    That south wall is the most important wall for interruption of the thermal bridging through all the concentrated framing elements. But, more importantly, is meeting IRC shear bracing requirements (shear panels ≤ 25' oc) and lateral wind load resistance for that 2-storey wall.

  9. mapnerd | | #9

    I added another page to the link above with the sunshade details. I also forgot to mention that we do have 4x6 support beams at 6'oc running vertically along the South wall.

  10. J Chesnut | | #10

    Michael,
    I'm a designer who has been working with the German PassivHaus approach for the last couple of years. I have 6 years experience working with residential architecture firms. I've come to respect the PassivHaus approach because the principles favor simple solutions and the designer is provided with a quantitative energy modeling tool that informs the design.

    I would want to be convinced by some means that your 'passive solar' design will work as intended. The drawings don't offer enough information to do that. Has the designer quantified anything with energy modeling? Or can the designer point to work within their built portfolio that has functioned as intended? From the vantage point of my work experience I think many architects "shoot from the hip" when asked to design passive solar and energy efficient systems.

    Passive solar design requires a VERY good designer WITH a track record of succesful built projects in order to avoid the unintended consequences that can come with poor passive solar design - overheating, heat loss that causes too much discomfort at night/non-sunny days, "supplemental" mechanical systems that end up consuming more energy than intended. I'm not experienced enough to achieve this.

    At this stage of design the heating load can be determined with energy modeling based on some assumptions of a targeted air change per hour (ACH) goal. Your building footprint is large. The more square feet you start with the bigger the energy demand you start with. How was the quantity of window surface in each cardinal direction determined? How were the shading elements sized? What are the ACH goals? What is the targeted R-Value for each of the assemblies? What are the window specs? What are the mechanical systems? Does the passive solar gains need to be distributed to certain rooms? These are all questions your architect should have a good handle on.

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