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13234Re: passive solar windows

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  • LarenCorie
    Oct 27, 2013
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      Posted by: "Martha Olson" mbo1955@...

      > I am in E TN.

      Hi Martha;

      Nice country. my whole family is from that region.
      Tennessee does not have a mandatory state building
      code for one and two family dwelling. Perhaps you
      are in a city jurisdiction, or financing requires building
      to IRC.

      > The windows in question are 3' x 5' - two in the LR,
      > one in the BR. They will be shaded by a "hedgerow"
      > of fast growing locust trees, about 15' away, already
      > 15' tall,

      So, they will shade the windows during the Winter.
      Trees at that distance are not good for Solar.

      It sounds like they are also double hung windows,
      which are not good for Solar, for a couple of reasons.
      Besides having a relatively smaller glass area, their glass
      is also set deeper into the window, so that do more self-
      -shading than casement, awning and fixed glass windows.
      The worst thing about them (for Solar heating) is that their
      screen are on the outside, blocking large portion of sun-
      -light from even getting to the glass. That sunlight blocking
      is not included in the SHGC. Some people say they will
      take the screen off in Winter, but there are many reasons
      why that practice stops being done.

      > plus a large shade tree that already blocks the early
      > morning sun.

      "Early morning sun" can be coming from a fairly wide
      range of angles. Though its azimuth (basically compass
      direction) will be the same, for the time of day, in all
      seasons, the altitude (above the horizon, will change
      significantly. In general, the best way to shade East
      and West walls is with hedges, close to the wall.
      Since 6AM (Solar time) is always very near due
      East, South facing windows (or just East of South
      which is ideal orientation for a house) which are
      easy to shade with modest overhangs, make real
      good morning sun windows. East facing windows
      don't give very good Winter light, and blast you
      with annoying sunlight as early as 4AM in Summer.
      South facing windows are a lot nicer to wake up
      to, so it is nice that you put a South facing window
      in the bedroom.

      > the BR is 180 sf with 8' ceilings, the LR is 200
      > sf with high ceiling that opens to a 100 sf kitchen.

      Why 8ft ceiling? Cathedral ceilings make small
      spaces feel much more spacious, and are good for
      using high vents for natural ventilation.

      > It will be suitable for the solar storage scheme that
      > you promote because of the "mezzanie" area over
      > the kitchen and bath.

      Sounds like you have cathedral ceiling in part of
      the house, like living/dining room. Many people
      (myself included) like to spend more time in the
      bedroom, than living room, so for a spec home
      you may be able to draw a few more potential
      buyers by giving a little more spacious feeling to
      the master bedroom. This is not just for people
      who want to get away from their kids. With flat
      screen TVs and laptops, more and more people
      do their living in the bedroom. Personally, I do
      not even need a living room. I live in the bedroom,
      and can entertain in a combo kitchen/dining space.
      But, that is a different subject. Back to windows ;O)

      There is no need for Solar storage, unless there
      is enough Solar gain, to have Winter overheating on
      a regular basis. However, there are several other
      advantages to insulating the roof, rather than ceiling,
      and for using insulated attic space as utility space.
      For instance, it is an excellent location for a Heat
      Pump Water Heater which takes excess heat from
      the house, and transfers it to the domestic hot water.
      They do a very good job of dehumidifying interior air,
      which is a significant issue in modern energy efficient
      homes, year round. Since warmer air rises, it is an
      optimal location for heat storage (like water heater)
      but it is also the best location for thermal mass for
      natural cooling, because is can absorb rising heat,
      while naturally trickling down cool air during the
      heat of the day. At night, it is easy to vent out
      heat through the roof, and open roof vents do
      not vent out cooler interior air, or vent in hotter
      daytime outdoor air.

      > If I sell it to someone who is interested in solar
      > aspects I will send them your way for consultation.

      My plan is to not do much (maybe no) more custom
      design work, and to write books and do stock plans.

      > I have sized the front porch so it can be used for solar
      > collection with the addition of sliding doors. But it is a
      > "spec house" so too soon to say if the buyer will want one.

      I am thinking that I recall you being in Chattanooga (?)

      > I'm not wanting to buy specialty windows.
      > Right now the choices are low-e coating
      > or no coating (if allowable), essentially
      > the same price. They are argon filled.

      There are other options, besides the IRC specification
      of U = 0.35 (primarily Low-E) windows. IRC allows
      you to show, either by simulation (RESCheck, or other)
      or by certain testing (blower door, etc) that the house
      will be as energy efficient with your variations, as it is
      expected to be, if built to code minimums. So, if you
      have done another part of the house (such as wall
      and/or roof insulation) to exceed code, you can
      simply show (using basic heat loss calculations)
      that the house will out-perform a code minimum
      house, even though your windows have a higher
      U factor..

      So, if you are smart and building with 2x6 24"oc
      using advanced framing techniques to reduce your
      building cost (and increase profit) and especially
      if you are also using FPSF to reduce foundation
      cost and increase energy efficiency, you should
      be able to show that you far exceed code energy
      standards. Basically, in your climate, a house
      can be designed/built, that costs less to build,
      than conventional, and will require almost no
      purchased heating and cooling..

      Lets look at another, more straight forward way
      to show that an unshaded South facing window will
      outperform the code windows. Code for your Zone
      4A location asks for a Ufactor of 0.35 (R2.86) You
      could use windows with 1" clear glass, that is R 2.38
      and lets in 75% of sunlight, but let's use your number
      of "62%" to include window framing, too. (this will
      be simplified by focusing on just two months). You
      also mentioned a SHGC of 0.28, for the Low-E. So,
      let's look at an average Dec-Jan day. Temperature is
      40°F, and the sunlight on a South wall is 990BTU/ft²
      (that is standard weather data for Chattanooga)

      So the Low-E window will gain 0.28 x 990 = 277
      and will lose 0.35 (70-40°)24hr = 252
      For a net gain of 277-252 = 25BTU/ft²-day

      The clear glass window will gain 0.62 x 990 = 614
      and will lose (70-40°)24hr/R2.38 = 303
      For a net gain of 614-303 = 311BTU/ft²-day

      If your building inspector understands that,
      it should show them that clear glass would be
      over six time more energy efficient than your
      Low-E option for unshaded South facing
      windows. But then, your South facing
      windows are shaded, so your calculation
      would need to include a realistic factor
      for that, which might be hard to determine,
      and to get the inspector to believe, unless
      you actually monitored it, during a heating
      season. So, I think your best option is
      good design, with better-than-code insulation
      value in other parts of the house, such as using
      low/no cost options, like 2x6 24"oc, advanced
      framing, foam sheathing, and dense-packed
      cellulose insulation, along with FPSF foundation,
      and I-joist rafters with open ceilings and exposed
      beams spanning between the tops of the walls.
      Using those highly economical strategies, in your
      climate, clear window glass would be a non-issue,
      and you will have a house that is relatively cheap
      to build, that would have very little heating beyond
      normal interior generated heat from daily living and
      electric usage. It should also be easy to cool, with
      night venting and a small mini-split heat pump, that
      would also do the heating

      -Laren Corie-
      Natural Solar Building Design and
      Solar Heating/Natural Cooling/Energy
      Efficiency Consultation Since 1975
      www.ThermalAttic.com (many new
      photos and pages, coming soon)

      Read my Solar house design articles in:
      -Energy Self-Sufficiency Newsletter-

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