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re: seek passive solar design faq/guide
22 may 1996
william r stewart   wrote...
>> >> >>one could shorten the warm-up time of the enclosure and increase
>> >> >>the amount of heat delivered to the rooms by making the enclosure
>> >> >>virtually massless--by greatly reducing its dynamic thermal capacity.
>> >> (sounds like a solar panel to me :-)
>> but they are also very expensive, and normally live on the roof, which makes
>> them fundamentally different from these inexpensive sunspaces. 
>sure, but they will last 10-30 times longer than a sheet of mylar stapled to a
>wooden frame.

i suppose they might, but why would you do that? i might attach my 30 x 20'
sheet of very clear mylar only around the perimeter of a sunspace, stretched
over some curved galvanized pipes. if you were to look in a catalog, like
stuppy's (800) 877-5025, or geiger's (800) 4 geiger,,, or visit your local commercial greenhouse supplier,
you'd see some nice aluminum extrusion clamps like uni-lock (geiger 33-sl16,
$0.80/linear foot on page 101 of their current catalog) which make recycling
5 cent/ft^2 poly film as easy as changing a big bedsheet (on a calm day :-)
every 3 years, or less often, if you hang some 80% greenhouse shadecloth
(eg stuppy's 23691 polypropylene product at $0.139/ft^2) over the outside
in the summer and inside in the winter. only 2% of the oil in the us is
used to make plastic, btw, and most greenhouse poly film is recycled.

unfortunately you won't find the very clear mylar in either catalog yet... 
we have to wait a few months until bayer or dow start making it again, and
armin and replex plastics start distributing it again. this polyester film
was once used extensively in 100% solar-heated japanese greenhouses, but
people stopped making it for a while, because oil was so cheap. it should
last longer than poly film eg 5-10 years, longer with a seasonal shadecloth
cover, and cost a bit more (10-20 cents/ft^2 ?) for now, it seems reasonable
for reasonable and serious alternate energy enthusiasts (if any) to start
with greenhouse poly film over curved galvanized pipes, and upgrade to 
mylar some time in the next 3 years.

>before someone makes a decision about which route they will take,
>they have to consider whether or not weather considerations prevent the long term
>use of mylar staple (or nailed or glued, etc) sheets.  in my area (washington, dc),
>we get just enough high wind bursts every year to make a flimsy sunspace impractical
>for serious consideration.

if you look on page 106 of the geiger catalog, you will see their small
01-4c442 blower and 01-2ck610 blower fan installation kit used to inflate two
layers of poly film for better insulation and protection from wind fatigue.
this particular technique was invented at rutgers years ago, and it works,
in winds short of hurricanes. most growers let their 50 watt blowers run
all winter, keeping their $2000, 3,000 ft^2 greenhouse walls inflated. but
nowadays, now that electricity is more expensive ($1/watt/year), it seems
better to have a wind switch, something made with an outdoor microswitch from
grainger, and just turn on the blower when it's windy outside. once inflated,
the poly pillows should stay inflated for a few minutes.

it seems even better and cheaper and more ephemeral, as bucky fuller used to
say, to use only one layer of plastic film for a lean-to-sunspace on the
south wall of a house, and put a transparent motorized damper in the bottom
half of one window and a window fan and a plastic film one-way damper in the
upper half of another window, blowing air into the house, and turn on the fan
and open the motorized damper during the day when the house is cooler than
68 f and the sunspace is warmer than 80 f, say, using two thermostats in
series with the fan, and turn on the fan with the damper closed whenever the
wind blows hard and the house does not need heat, to make a slight vacuum
in this reasonably airtight sunspace.

>even if the force of the wind didn't carry it off in the first 2 months, wind-swept
>items, such as sticks or other debris, would puncture and rip the surface, 
>requiring replacement or unsightly patching.


thousands of greenhouse growers gone wrong... 

>> brick sunspaces have lots of thermal mass behind glass with no night
>> insulation, as do many trombe walls and direct gain houses. this is a
>> very inefficient way to heat a house in a partly cloudy climate, because
>> the thermal mass stores lots of solar heat during the day, and most of that
>> solar heat disappears through the glazing at night. 
>this ignores the sunspaces with insulating materials, that are tracked and
>easy to move.  just look through you back issues of home energy or home power
>for examples of window coverings between r4 and r8.

i have no desire to interfere with your twice-daily rituals, but you haven't
yet answered my question about the effect of air leaks around the edges on 
the actual r-value of those tracked and easily-movable and inexpensive (?)
curtains, which would insulate several times less effectively than a house
wall, if they worked perfectly. consider if you will, will, or if you can,
the effect of a 1 cfm air leak on an 8' x 8' "r8" curtain in front of an
r2 window on a 30 f day. chacun a son systeme. my taste is for inexpensive,
high-performance, automatic systems. 
>too bad you could'nt have come down this weekend for our yearly tour
>of solar homes.

if i'da knowed i'd been there. i like those tours. helped run some, even.

>one house i visited had virtually the entire south wall covered in windows (r3)
>with two large track-guided r8 shades (4 sheets of reflective mylar with air 

lovely. i saw something like that in a barn conversion designed by architect
harrison frakur in washington's crossing, pa, on a solar house tour in 1980.
it was very dramatic, as many architectural entertainments are. it was only
two layers of partly-aluminized mylar film, a single piece that deployed as
a u shape with tracks on the sides and a dead-weight roller at the bottom,
and a motorized roller at the top. these were over some 8 x 8' sliding glass
doors mounted way up on the west wall of a 3 story barn with a cathedral
ceiling, completely open above, and when the sun went down, they would come
down too, automatically, "reducing the heat loss by 90%." it was said you
could still see through them dimly when they were closed. they were open
when i was there. i wonder if these are a commercial product now, and how
much they cost, and what their real r-value is, including air leaks...

you still haven't answered my question about which brand of movable window
insulation you plan to use, and how much it costs, including installation. 
how about an insulated frame wall with a fan or a motorized damper and a
couple of thermostats, for "movable window insulation"?

those barn people had a woodstove in the middle of their barn, with a bare
black exposed 30' chimney, going right up thru the ceiling, dramatically.
they could well have used a double or triple wall fluepipe with a fan at the
top to blow down outside air or air from the ceiling inside the outer pipe...
i wonder if you will have such an air-air heat exchanger in your house.
harder to do this with a fireplace, especially without a fan, since to
make it work naturally, the air outlet from the house needs to be above
the air inlet to the house. but a small fan like that would have a high cop,
and it would keep the flow from reversing on a summer day. what will your
fireplace efficiency be, vs a woodstove? do you care?

>the house had less than .2 ach, was heavily insulated, and the 
>occupants said their fuel bills were less than $30/month in winter.

good. unbelievable, even. were they counting their electrical fuel bill?

>> of course people have preferences and that's fine. but let's recognize
>> that that's a lifestyle choice involving a compromise with aesthetics and
>> money and fossil or wood fuel consumption. let's not drag in false physics.
>let's not focus on one or two aspects of physics, and forget about infiltration,
>re-radiation, and convection.  that would give one false results in any
>thermal energy equation trying to determine overall qin to qout.

i agree. perhaps we can more usefully talk about that when you understand
how conduction and thermal mass work. and rc time constants. this time
constant idea is useful. once you learn it, you can instantly eyeball the
solution to that simple differential equation, and find how the temperature
of a passive solar structure changes with time, when the sun is not shining.
find the r, and find the c and multiply them together. bang: 200 hours for
that 4' cube, 800 hours during a cloudy week, 53 hours for your new house.

here is your house starting a cloudy week in january in richmond, where the
average outdoor temperature is 36 f. suppose the thermal curtains are closed
24 hours a day, and you are neither burning wood nor electricity, and the
house starts off at 76 f, water walls and all.

t(t) = 36 + (76-36) exp(-t/53), where t is in hours, ie

       76 after 0 days,
       61 after 1 day,
       52 after 2 days,
       42 after 3 days.

>> a low-thermal-mass sunspace could be a commercial plastic film greenhouse
>> adjacent to a house, costing 50 cents/ft^2, put up by 1 person in one day,
>> more like a tent than a building. the glazing might be poly film costing
>> 5 cents/ft^2, with a 3 year guarantee, changed every 3 years in an hour,
>> and recycled. 
>if it withstands weather conditions for more than 3 months and your neighbors
>still talk to you.

i'd just say "faith without works is dead." :-)
or maybe "works without faith are dead."
or "10,000 growers can't be wrong."
or "it's my house, not yours."
and stick out my tongue.

>> or it might be flat very clear polycarbonate glazing, costing $1/ft^2,
>> with a 10+ year solar lifetime, that comes in rolls 49" wide, so it might
>> be simply attached to 2x6s on 4' centers in a simple lean-to sunspace that
>> forms the weather south wall of a house.
>again, you have the serious concerns of infiltration, neglible insulation (>ri)
>and weather to overcome, not to mention the sickly yellow appearance after 
>a year in the sun.  if that's not a problem, by all means, knock yourself out.

polycarbonate tends to turn grayish after 10 years in the sun, not yellow,
and that process takes longer if it's shaded in summertime, and i found in
our test structure that if it's attached with some spongy tape or silicone
caulk underneath and a 1/8" x 1/2" aluminum cap strip outside, with a screw
every 6" or so, it leaks no air. r1 insulation is fine in a low-thermal-mass
sunspace that's 80 f for 6 hours a day. do the arithmetic, will...

>> another way to make a low-thermal-mass sunspace is to make the steep south
>> roof of a house with the same single-layer corrugated polycarbonate plastic,
>> with an insulated attic floor. again lower first cost than normal
>> construction (no shingles, no tarpaper, no sheathing, and 4' x 12' panels
>> that attach with a few hex head screws) and it can collect solar energy,
>> with a low power fan that blows warm air from the peak of the attic down
>> a large cheap uninsulated duct (eg a poly film tube duct that costs 50
>> cents per linear foot), into the house when the sun is shining. house air
>> might return to the attic through a $200 2' x 2' motorized damper that lets
>> daylight into the house when it is open. the value of such daylight might
>> be $200/year of electrical power savings, vs the fluorescent equivalent,
>> if it is well distributed inside the house, as well as the aesthetic value
>> of daylighting.
>one point you seem to miss over and over is the lack of a thermal 'flywheel' by
>which to dampen the large temperature swings large amounts of solar gain can

i don't undertand you here. i'm counting on that, inside the house.

>if you lessen this by reducing the solar insolation received, then 
>you lessen the amount of solar heat your structure can utilize.

i really don't understand what you are talking about here. do you?

>the lessons learned by overheating in the day have been hard won;

yeah, remember the old ranger, death valley days? 20 mule team borax?

>don't ignore them by
>offering simplistic paper drills that have too many questionable assumptions.

i'm not doing that... sunspaces are no-brainers. solar closets are a
little more complex, but still trivial physics.
bubblewalls are interesting...

>> >> if you design your house to be superinsulated, it has been shown that,
>> >> even for houses orientated to minimise passive solar gain, addittional
>> >> energy costs for space heating can be made insignificant.
>> absolutely true. but at what price?
>perhaps this is the crux of your argument. 

i'm very concerned about price and performance. i think these things
have to be economical to be widely used, on their own feet, today,
not "ecomomical" with government subsidies...

>i will address this as it applies to me.


>1.  the house i am in the middle of finalizing...

somebody was telling me yesterday over lunch how amazing it is what we can
think ourselves into believing, once we have made up our minds. nobody is
immune to that... i struggle with that all the time, and sometimes lose.
i suppose you do too. but we have to make up our minds about some things,
and the further along a house plan gets, the harder it is to change...

even were we not stubborn.

>will have a 'normal' amount of windowspace.  the south side, however,
>will have approximately 50% of the windowspace.

for the view, or for light, or for solar heating?

>my wife prefers a certain level of natural light, so i must
>satisfy her desires as well.

absolutely :-) wives are hard to find... does she like skylights?
or clerestory windows on the south side with reflective and insulating
dampers, perhaps made of foamboard, with small motors, hinged at the top? 

>my first inclination was to have an earth-sheltered home, with full
>exposure to the south, but she felt that would be like living at the
>mouth of a cave.  

it's a bit like that, and more expensive than a normal house. but my friend
bill freeborn's house is very nice, inside and outside, and his annual
electric resistance heating bill is only $58 (a design miscalculation :-),
in this 5,500 dd climate. he often goes south for months in bitter winters,
and he turns off all the electricity in the house when he leaves, including
the backup heat, and leaves the manual exterior thermal shutters covering
his sunspace glazing wide open, 24 hours a day. one very cold year, he left
a max/min thermometer in his house when he left, and when he got home a few
months later, he checked it, and the minimum house temperature was 68 f.

bill is a retired doctor, a no-nonsense person, given to truth-telling.

his house has a long time constant, which is good for several reasons. for
one, it allows easy manual control, since the interior temperatures change
so slowly. bill and his wife get ready for winter by closing some open
windows around thanksgiving (underground seasons are delayed) to make the
average house temperature gradually rise from 70 f to 72 f (their preferred
winter temperature), over two weeks. another good thing is that his house
cools off very slowly when there is no sun. another is that we can design
houses like this with just a couple of monthly averages, vs more detailed
weather data, eg hourly weather data for the last 30 years from an nrel/noaa
cd-rom. but these houses cost more, and as you say, not everyone wants to
live in one. our challenge is to equal their performance above ground,
inexpensively. and yes, aesthetically and reliably and safely and so on.

>so there will be no added costs for windowspace or window type. 

i'm not sure why you say that.

>2.  i will be using the same amount of window insulation that i would
>if i didn't have solar gain, given that i am an energy-efficiency nut.
>therefore, no additional costs will be incurred for extra window insulation.

you mean that if you could keep your house at 70 f for 2 weeks without sun, 
with no wood heat or electrical consumption, with windows that had no thermal
curtains, you would still add the thermal curtains? i could see plain old
curtains for privacy, and to make the room lighter at night, but...

>3.  the only added cost will be for the installation of thermal storage
>containers next to the south facing windows.

you said these would be 4" away from the south facing windows?
can you see well through them?

>previous installations have totalled $2000.  given the thermal flywheel
>effect, they will pay for themselves in around than ten years.

i can believe that. but you might be better off financially if you put the
money in the bank, especially if the water walls take up $50/ft^2 interior
floorspace and block the view through the windows. systems that pay for
themselves in 1-3 years would help solar houses catch on. and when we make
them close to 100% solar, there's an economical bump as we eliminate the
backup system, or have an inexpensive double-duty backup system like an
electric water heater that seldom turns on. if solar house heating is to be
a hobby, why not make it a high-performance hobby, like sailing :-) why make
a house with a 53 hour time constant when you can easily make it longer?
long enough to passively bridge a lot of cloudy days in a row...

"solar houses" seem to be moving in the superinsulated direction, ie they
are becoming more expensive houses that are heated by their tenants and
their electricity use, with lots of insulation and very little glass. direct
gain houses have an optimal ratio of south glazing to heat load, and once
we are there, the solar fraction can't be improved much by adding more glass.
they could often use more thermal mass, especially for natural cooling.
your water walls are a good step in that direction. we are having a little
heat wave here, close to 90 f during the last few days, and i just closed
the doors of my stone/foam house which is 69 f inside. it's 67 f this morning.
i would like to make an "enthalpy controller" to do something like this
with dampers, automatically.

wash, dc is strangely absent from the 239 monitored cities in the nrel solar
radiation data manual (too much local hot air?), but baltimore is there,
and richmond, va looks a bit warmer, with an average july temp of 78 f and
an average daytime high of 88 and nightime low of 68. so let's see... if you
kept your house at 80 f max during the day, it might gain 6 hr (88-80) 150
= 7200 btu on an average july day, and the 7k btu of thermal mass would only
have to move 1 f to provide that heat :-) so if you ventilate your house at
night, and close it up during the day, and don't use much electricity, you
probably won't need mechanical air conditioning. a window ac for sometime
dehumidification, maybe.

which is less expensive, a house with normal levels of insulation and air
infiltration and a reasonable number of low-r windows, and more thermal mass
than an average house, eg one block wall, with exterior insulation (2" r10
dri-vit?), and some kind of low-thermal-mass sunspace (maybe we should invent
a new word for this, since "sunspace" is already taken) and some sort of high
temp thermal mass space to store heat for multiple days and make hot water,
or or a superinsulated airtight house with tiny expensive windows with ritual
thermal curtains and backup fuel and no integral hot water or natural cooling?
you pick :-) this may have more to do with religion than physics or economics. 

david boyer's $1000 r1 20 x 100' poly film greenhouse did ok this winter
with 200 free 55 gallon drums full of water inside. a house with r20 walls
might only need 10 drums. the rc time constant product is important, but
c may be cheaper than r... 

>4. i still intend to have supplemental heat, as well as cooling.

once you have made up your mind about something, the reasons don't matter.

>my wife insists on *some* kind of fireplace.

she should have one. i like fireplaces filled with silk flowers...
a small ceremonial fire might be nice on christmas eve.

>> ...and how do you reduce water heating costs? superinsulation doesn't
>> heat water. can we try to look at more of the whole picture here, and make
>> new houses that have these functions as beautiful integral parts, rather than
>> adding on kludgey afterthoughts, box after box after box, more expensively?
>solar water heating will be in my design from the start.  it will be active, and
>will be supplemented by the ground source heat pump on overcast days.

and you like renewable energy? 
and you say you like passive systems?
you haven't said you wanted to save money...  

>> >some of the low-e windows i have seen lately have been close to what
>> >nick had mentioned; one was sc=.59 and another 'heat mirror' sc=.41
>> they are also more expensive...
>i would be using them anyway, in order to reduce heat loss in winter and heat
>gain in summer.

once we have made up our minds about something,

no longer


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