13234Re: passive solar windows
- Oct 27, 2013Posted 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
> The windows in question are 3' x 5' - two in the LR,So, they will shade the windows during the Winter.
> one in the BR. They will be shaded by a "hedgerow"
> of fast growing locust trees, about 15' away, already
> 15' tall,
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"Early morning sun" can be coming from a fairly wide
> morning sun.
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 200Why 8ft ceiling? Cathedral ceilings make small
> sf with high ceiling that opens to a 100 sf kitchen.
spaces feel much more spacious, and are good for
using high vents for natural ventilation.
> It will be suitable for the solar storage scheme thatSounds like you have cathedral ceiling in part of
> you promote because of the "mezzanie" area over
> the kitchen and bath.
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 solarMy plan is to not do much (maybe no) more custom
> aspects I will send them your way for consultation.
design work, and to write books and do stock plans.
> I have sized the front porch so it can be used for solarI am thinking that I recall you being in Chattanooga (?)
> 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'm not wanting to buy specialty windows.There are other options, besides the IRC specification
> Right now the choices are low-e coating
> or no coating (if allowable), essentially
> the same price. They are argon filled.
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
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
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