re: help is here!
27 jun 2000
terry reynoldson wrote:
>> >the outside surface may be cladded with dryvit outsulation which
>> >collects solar energy
>> ...the dri-vit i know doesn't collect solar energy...
>did you even bother visiting the page that i linked to in the message above.
>here it is again: ( http://www.build-green.com/product_dir/dryvit.htm ).
i didn't see any solar energy collection there, after reading the entire
website. some excerpts:
...when conventional methods are used, the r-equivalent of a solid,
10 inch thick wall is less than r-22. when the same wall is formed
with xps foam panels, its thermal performance jumps to r-50! in other
words, a framed wall would have to be insulated to r50 to produce
the same results as a concrete wall made with lite-form.
r50!? does your engineer arno dyck have any evidence or calculations
to support this? the drawing shows 4" of dri-vit foamboard over an 8"
block wall with 1/2" of plaster on the inside. i'd make that about r18.
how dryvit outsulation turned a lighting system into
a heating system!
design innovation took hold at the construction of the mission forest
offices in mission, kansas. the 13,440 sq. ft. structure would be
heated by electricity in the form of fluorescent lighting. that is,
3-4 watts per square foot equaling enough btus to heat the entire
for $13,440 per year. (btw, watts compare to btu/h, as power, vs energy.)
secondarily, some heat would be generated by office equipment such as
computers and typewriters along with that of the employees themselves.
that might make sense for an office, but electric resistance home heating
is expensive and environmentally unfriendly.
the mechanism chosen to control the lighting system is accustat.
here, tiny fuse-like bulbs contained inside the accustat regulate the
temperature of the offices, maintaining it at 75 f. after work hours,
if the temperature falls below this point, fluorescent lights are
turned on automatically...
why 75 f?
but heating by fluorescent light could not have been achieved
successfully without an effective method for storing the heat in the
walls of the building.
and to do that, just the right insulation system was needed...
dryvit outsulation offered the key element for energy efficiency
at mission forest...
insulation does not store heat. if the temperature is 75 f during
the day and 75 f at night, the concrete stores no heat at all, right?
the structure could achieve the same (low) energy efficiency without
just how efficiently has the outsulation--no furnace--building
operated over the years? total energy costs are less than one-third
that of a conventional building...
and though the structure employs only electricity for heating,
there are still substantial savings over electricity usage for
furnace-heated commercial buildings of similar size. more specifically,
a traditional facility uses $2.75-$3.25/sq. ft. at mission forest,
the cost averages $1.00/sq. ft.
more than $4,000 per year for a 4,000 ft^2 house, with its smaller
volume and larger surface to volume ratio. hardly economical. better to
burn the fuel used to produce that electricity, or use a heat pump or
a small cogen system with close to 100% vs 30% power plant efficiency.
>> i read about "a mass wall behind glass" on your website, --snip--
near the end of "learning screen 26."
how is passive solar heating used in energy-efficient houses?
the normal method of providing passive solar heat is to use windows
as solar collectors... the most common problem with passive solar
heating is too much glass. this leads to overheating, even in winter...
overheating is easy to fix in wintertime with an exhaust fan and a
thermostat. the problem with too many windows is excessive heat loss
at night and on cloudy days.
how can passive solar heat be stored?
one method of avoiding overheating and storing heat is to provide
thermal storage within the house. if the south window area is more than
6 - 7 per cent of the floor area, then additional mass has to be built
in. look for heavy building materials, such as gypsum wallboard, bricks,
tile, or concrete that can absorb the excess heat during the day and
release it at night.
a house can be 100% solar heated with less concrete, if it's used more
effectively. consider a 48'x48'x8' tall house with superior (tm) walls
and ceiling ($3/ft^2, installed, where i live, with 2" of concrete facing
the living space inside r5 styrofoam insulation and 7" concrete studs
on 2' centers with a 1x3 nailer on the outside) with r19 fiberglass fill
and 1" of styrofoam and some sort of siding (ie us r29 walls), with
foamboard under a hydronic slab and 4% of the floorspace as r4 windows.
its thermal conductance is 96ft^2/r4 = 24 btu/h-f for the windows plus
1440/29 = 50 for the walls plus 2304/29 = 79 for the ceiling. air leaks
of 0.2 house volumes per hour add 0.2x48^2x8/55 = 67, for a total of 220
btu/h-f. near phila, pa, with no electrical energy usage, this house
needs 24h(70f-30f)220btu/h-f = 211k btu/day of solar heat in january.
suppose we keep the living space air 70 f for 6 hours per day, and let
its temperature decrease over the rest of the 18 hour day. its thermal
capacitance is at least 48^2x4"/12"x25btu/f-ft^3 = 19200 btu/f for the
slab plus 1440ft^2x2"/12"x25 = 6k btu/f for the walls plus 9.6k for the
ceiling, a total of 34.8k btu/f, with a slowly-moving room air film
conductance of about (2304+1440+2304)1.5 = 9072 btu/h-f, ie a natural
charging time constant rc = 34.8kbtu/f/(9072btu/h-f) = 3.8 hours.
over 6 hours, the concrete increases to tmax = 70-(70-tmin)exp(-6/3.8)
= 55.6+0.206tmin. the discharg time constant rc = 38.4k/220 = 158 hours.
over the next 18 hours, the concrete and air temp decrease to tmin
= 30+(tmax-30)exp(-18/158) = 3.23+0.892tmax = 52.8+0.184tmin, so the
dawn room air is tmin = 64.7 f, and the concrete reaches tmax = 68.9 f
after 6 hours on an average january day. more concrete or more surface
(eg concrete interior walls or corrugated surfaces made with reversed
aluminum wall forms) would decrease this day-night temperature swing.
a 48'x8' tall sunspace with r1 glazing with 90% solar transmission
collects 0.9x1000x48x8 = 345.6k btu of sun on an average january day.
if 211k goes to living space heat, the remaining 134.6k btu can make
the average sunspace temperature 30+134.6k/(6hx48x8/r1) = 88 f. a
transpired mesh collector (eg dark greenhouse shadecloth) near the
south house wall would make the sunspace cooler and more efficient.
a solar closet in the sunspace or a concentrating solar trough attic
(in lieu of the sunspace) could keep the house warm on cloudy days.
two $400 7'dx5' tall plastic agricultural tanks with 3000 gallons of
150 f water cooling to 80 f could store 3000x8(150-80) = 1680k btu,
enough to keep the house 70 f for 8 cloudy days in a row.
another method is to utilize solar energy indirectly. in this case,
look for concrete walls behind south-facing glass (mass walls) that
become warm and in turn heat the house...
this so-called "new technology" sounds like an inefficient trombe wall,
as patented by edward morse of salem, mass in 1881.