# An improved solar attic

When a local couple considered a "solar addition," it turned out the first and second floor levels of the south wall were mostly shaded by evergreens, including trees on the neighbors's property, but the roof had lots of sun. It's about 20'x40' with a 4' high ridge running east and west. I suggested removing the south roof and extending the north roof up to the south beyond the ridge at the same angle to make a shed attic with an 8'x40' transparent south wall. Soldier's Grove solar attics used blowers to move hot air down to the building, but water is simpler and uses less power and makes heat storage easier in a slow draindown system with an unpressurized heat storage tank on the ground. The house has a gas furnace.
NREL says 1000 Btu/ft^2 falls on a south wall on an average 30.4 F January day with a 37.9 max in Phila, so a \$320 8'x40' Dynaglas wall would collect 900x8x40 = 288K Btu on an average day, or more, with a reflective deck to the south and/ or a reflective overhang (which could also prevent overheating in summertime.)
With 120 F water in an 800 Btu/h-F auto radiator and fan near the peak and insulation under the roof, it might look like this, viewed in a fixed font:
--- 1/320 |---|-->|-------www--- 34 F --- | 288K/6h x = 48K Btu/h | w w 1/800 w | | |
120 F
Opening the circuit at x makes the solar (Thevenin) equivalent temperature 34+48K/320 = 184 F, like this:
1/320 184 F ---www--- | x | w w 1/800 w | | |
120 F
... which might collect (184-120)/(1/320+1/800) = 14.6K Btu/h or 87.8K Btu/day, enough to make lots of hot water for showers (with a \$60 300'x1" PE pressurized pipe coil in the heat storage tank) and provide some space heat. The attic temp might be 120+14.6K/800 = 138 F. We might also circulate some attic air through a filter and the 2nd floor of the house.
This seems efficient, even if the upstairs needs little heat. A dark mesh near the glazing (eg black aluminum window screen) with 70 F house air flowing through it could greatly lower the heat loss by convection through the glazing, as in a Scandinavian "breathing wall." With 70 F air near the glazing, we might capture up to 288K-6h(70-34)320ft^2/R1 = 218.9K Btu on an average January day. Polycarbonate blocks longwave IR, but there would still be radiation loss from the mesh to the glazing.
http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic metric U-value of a breathing wall, as corrected:
Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where
V is the air velocity in meters per second, Rhoa is air density, 1.2 kg/m^3, Ca is the air's specific heat, 1000 J/(kg-K), and Rs is the wall's static thermal resistance in m^2-K/W.
Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall), Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)
If 1 meter per hour (0.0055 fpm) flows through 320 ft^2 of mesh, the total is 17.5 cfm... 100 lfm up into a 1' wide x 2x4 cavity 8' tall with a 0.29 ft^2 cross section (29 cfm per linear foot of wall) would not increase the glazing loss much over still air. In that case, the total airflow would be 320x29 = 9300 cfm. So it looks like a wide range of airflow is possible, depending on how much heat the house needs.
With no mesh and 2 glazing layers, eg sliding glass doors, we might have this:
2/320 Tsun ---www--- Tsun = 34+259K/6/(2/320) = 304 F. | x | w w 1/800 w | | |
120 F
.... which might collect (304-120)/(2/320+1/800) = 24.5K Btu/h or 147.2K Btu/day with a 120+24.5K/800 = 151 F attic temp. (We datalogged 157 F in December of 1995 in the sunspace of our phone-booth-size experiment at Ursinus college.)
We might start by drilling holes from the inside of the attic with a long bit near the ridge next to the present rafters, then Sawzall a rafter-size piece of roof from the outside and slip a 12' 2x6 into each hole parallel to the rafter and bolt the bottom foot to the rafter, then build the new roof, then remove the old one (or not) and build the wall...
I bought a used 1984 Dodge Omni automobile radiator for \$35. We might attach a \$50 Lasko 2470 cfm 90 watt window fan to it... 800/5 = 160' of fin-tube pipe with no fan would cost about \$320. I like that better, since it is simpler and uses less electrical energy and the fan may not last very long over 100 F. (My 640 ft^2 Dynaglas solar attic has a \$433 2764 cfm 275 W Swedish Multifan with special ball bearings and lubricant rated for 311 F.)
Nick
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Good thinking! Tom
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When a local couple considered a "solar addition," it turned out the first and second floor levels of the south wall were mostly shaded by evergreens, including trees on the neighbors's property, but the roof had lots of sun.
It's about 20'x40', with a 4' high ridge running east and west. I suggested removing the south roof and extending the north roof up to the south beyond the ridge at the same angle to make a shed attic with an 8'x40' transparent south wall. Soldier's Grove solar attics used blowers to move hot air down to the building, but water is simpler and pumps use less power and make heat storage easier in a slow draindown system with an unpressurized heat storage tank on the ground. The house has a gas furnace.
NREL says 1000 Btu/ft^2 falls on a south wall on an average 30.4 F January day with a 37.9 max in Phila, so a \$320 (\$0.02 per peak watt :-) 8'x40' Dynaglas corrugated polycarbonate wall would collect 900x8x40 = 288K Btu on an average day, or more, with a reflective deck to the south and/or a reflective overhang (which could also prevent overheating in summertime.)
With 120 F water in an 800 Btu/h-F auto radiator and fan near the peak and insulation under the roof, it might look like this, viewed in a fixed font:
--- 1/320 |---|-->|-------www--- 34 F --- | 288K/6h x = 48K Btu/h | w w 1/800 w | | |
120 F
Opening the circuit at x makes the solar (Thevenin) equivalent temperature 34+48K/320 = 184 F, like this:
1/320 184 F ---www--- | x | w w 1/800 w | | |
120 F
... which might collect (184-120)/(1/320+1/800) = 14.6K Btu/h or 87.8K Btu/day, enough to make lots of hot water for showers (with a \$60 300'x1" PE pressurized pipe coil in the heat storage tank) and provide some space heat. The attic temp might be 120+14.6K/800 = 138 F. We might also circulate some attic air through a filter and the 2nd floor. The present furnace might spread the heat around the house.
This seems efficient, even if the upstairs needs little heat. A dark mesh near the glazing (eg black aluminum window screen) with 70 F house air flowing through it could greatly lower the heat loss by convection through the glazing, as in a Scandinavian "breathing wall." With 70 F air near the glazing, we might capture up to 288K-6h(70-34)320ft^2/R1 = 218.9K Btu on an average January day. In full sun, with 225x340 = 76500 Btu/h entering and (70-34)320 = 11520 leaving the attic, one attic temp upper limit would be 120+(76500-11520)/800 = 201 F. Polycarbonate blocks longwave IR, but there would still be radiation loss from the mesh to the glazing.
http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic metric U-value of a breathing wall, as corrected:
Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where
V is the air velocity in meters per second, Rhoa is air density, 1.2 kg/m^3, Ca is the air's specific heat, 1000 J/(kg-K), and Rs is the wall's static thermal resistance in m^2-K/W.
Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall), Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)
If 1 meter per hour (0.0055 fpm) flows through 320 ft^2 of mesh, the total is 17.5 cfm... 100 lfm up into a 1' of 2x4 cavity 8' tall with a 0.29 ft^2 cross section (29 cfm per linear foot of wall) would not increase the glazing loss much over still air. In that case, the total airflow would be 40x29 = 1160 cfm. With no water flow, we'd have something like this:
Ta = 70+I/1160 = 95 F. | 1/320 | 1/1160 184 F ---www-------www--- 70 F I -->
I = (184-70)/(1/320+1/1160) = 28.6K Btu/h. It looks like a wide range of airflow is possible, depending on how much heat the house needs.
With no mesh and 2 glazing layers, eg sliding glass doors, we might have this:
2/320 Tsun ---www--- Tsun = 34+259K/6h/(2/320) = 304 F. | x | w w 1/800 w | | |
120 F
.... which might collect (304-120)/(2/320+1/800) = 24.5K Btu/h or 147.2K Btu/day with a 120+24.5K/800 = 151 F attic temp. (We datalogged 157 F in December of 1995 in the sunspace of our phone-booth-sized experiment at Ursinus college.)
We might start by drilling holes from the inside of the attic with a long bit near the ridge next to the present rafters, then Sawzall a rafter-size piece of roof from the outside and slip a 12' 2x6 into each hole parallel to the north rafter and bolt the bottom foot to the rafter, then build the new roof, then remove the old one (or not) and build the wall...
I bought a used 1984 Dodge Omni automobile radiator for \$35. We might attach a \$50 Lasko 2470 cfm 90 watt window fan to it... 800/5Btu/h-F-ft = 160' of fin-tube pipe with no fan would cost about \$320. That's simpler, it uses less electrical energy, and the fan may not last very long at 151 F. (My 640 ft^2 Dynaglas solar attic has a \$433 2764 cfm 275 W Swedish Multifan with special ball bearings and lubricant rated for 311 F.)
Nick
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No it's not.
See the problem?
--
Cheers,
Bev
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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The solar attic is not improved?

No.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

You have no idea what I'm talking about, do you?

Then you weren't paying attention. All you need to do is scroll up a few messages and it will all be clear.
Or not.
--
Cheers,
Bev
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The Real Bev wrote:

I'm reading this from alt.solar.thermal, and I have no idea what you are talking about.
What I do know is that not including *any* of the thread (see your original post in this thread) is abysmal usenet practice. Why Nick even responded is beyond me.
Cheers, Jeff

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Jeff Thies wrote:

Congratulations, Jeff, at least ONE person got my point.
For the rest -- Losers who quote NOTHING of the post to which they're replying have nothing at all to say worth reading because they aren't smart enough to let the readers know what they're talking about. Alternate: They're so enamored of seeing their deathless prose in print they don't care that nobody knows what they're talking about.
--
Cheers, Bev
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Thanks for explaining. Abby and m Ransley sometimes do this. It can be an interesting puzzle with a solution. Other times, a complete mystery. Amusing cluelessness.

I imagined you meant the original attic wasn't solar, so the new design was not an "improvement." I meant that it seemed to be an improvement compared to the 20 or so solar attics built in Soldier's Grove, WI about 25 years ago when that sensible little town turned down \$6 million from the Army Corps of Engineers to build dikes and moved its commercial district uphill (after too many Kickapoo River floods) and then passed a law saying all new commercial buildings must be at least 50% solar-heated (see the Solar Today story at http://www.ece.villanova.edu/~nick , if you like.) Most of their large empty solar attics have sloped glazing with a large blower moving hot air down to the lower part of the building and a motorized damper to keep building air from rising back up into the cold attic at night, but that uses big ducts and lots of electrical power, and it's hard to store heat from warm air. Some buildings store no solar heat. Others store heat in rock beds, hollow floors, and shelves of canned goods in the IGA supermarket.
As to "improvements," this newer design might have a higher solar collection efficiency, with cool air next to the glazing losing less heat to the outdoors. It might also collect and store heat in water, which requires less electrical power and makes for easier heat storage and water heating for showers, and the glazing is vertical, which allows an overhang for summer shading to avoid overheating and extend the glazing lifetime and make more of the attic space useful for people. Corrugated greenhouse polycarbonate glazing (eg "Dynaglas") has largely replaced the fiberglass glazing used in Soldier's Grove, since it costs less and installs easier and lasts longer, with no maintenance. My roofer neighbor envied my solar roof as he watched it go up in a day or so in 4'x12' sheets, with no sheathing, felt or shingles. It costs less than a normal roof.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Nick
Are you sure you are not Rod 'the Rocket' Speed's alter ego?
You like to suddenly cross post solar and frugal stuff all the time half way through a thread plus you have a similar pattern of deny, deny, deny when you do not have a clue, comparable to wet paper bag, blotto and ignorant pig.
I was surprised that as the rocket you mentioned swamp coolers tho.
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I apologize, for suggesting Nick could be the Rocket
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Yes. Rod is often correct, but I reluctantly killfiled him because he is sooo obnoxious.
Nick
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Jeff Thies wrote:

That shouldn't make a difference. As far as I can tell, every article in this thread, including the article that started it, and everything that Bev said and that she was responding to, has been posted in alt.solar.thermal.

If you think that's not a good thing, then it might be interesting to take a look at the message Bev responded to. If you do, you will notice that that message didn't quote any context either. I wonder if that might even be related to Bev's point.
- Logan
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Logan Shaw wrote:

Gee, ya think?
First it was "<sigh> September again." Then it was "Oh Lord, another AOL newbie." Now it's "Oh Lord, another Google goober." Wonder what will be next and how much worse it will be...
--
Cheers,
Bev
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<snip>

Actually no...

What we have here is one of those rare quirks in usenet. Each message has an identifier that is randomly generated. This is usually unique because of the large number of possibilities, but as luck would have it, in this case it was not.
Reading here in alt.solar.thermal, Nicks post was the start of a new thread, not a follow up.
Since '95, I've seen this only once before... Caused a stir then also!
Cheers, Jeff

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Jeff Thies wrote:

I'm not following you, but it's no big whoop either way. From where I stand, it seems like Bev responded to someone named "tom" who wrote a message whose body was just this one line:
Good thinking! Tom
And, unless Usenet has mangled the posting badly, he was not starting a new thread, because his posting has a References: line. And Bev was replying to him.
But, I don't see much point in pressing the issue, since Usenet is fairly chaotic sometimes, and not everybody always sees the same set of articles in the same order. Sometimes articles never even make it to certain sites, etc., etc.

misc.consumers.frugal-living.
- Logan
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Logan Shaw wrote:

Me too. Not many of us left. I think I'm going to bail...
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Cheers, Bev
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We might discuss the economic and technical aspects, vs "You must be crazy to think that will work," "Your computer can't even make Pi," "Your attic can't work, because you haven't installed enough heat pumps," and so on.
Dynaglas polycarbonate costs \$1/ft^2. Installing a 4'x12' panel takes 10 minutes. It lasts 20 years but might go longer with a sloped reflective overhang for less summer sun and more winter sun. Shingles cost about \$0.30/ft^2, plus \$0.10 for felt plus \$0.50 for OSB? So materials are a wash, but there's a large labor savings. How much? The solar heat is worth about \$1/ft^2 per year.
How much heat will a dark absorber lose to cold glazing by radiation? How can we make a dark mesh that will work with a 1-10 meter per hour face velocity? Black aluminum window screen costs about 25 cents/ft^2 in 4' rolls, but its pneumatic resistance is likely too low. Typar is cheap, about 10 cent/ft^2 IIRC, but its pneumatic resistance is high. How about poking some holes in Typar? How many? How big? Would a geotextile material work better?
http://www.reemay.com Typar comes in wide rolls, eg 9'x111.1'. It's porous, 0.0032L/s-m^2 at 75 Pa, ie 0.00063 cfm/ft^2 at 0.3" H20, eg 0.2 cfm for an 8'x40' wall, and charcoal on one side. It lasted 5 years on a south wall of my house, exposed to the weather, but it has a lifetime guarantee :-)
Nick
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... \$145 for a 9'x100' roll.

If 1 cfm = 16.6Asqrt(HdT) = 16.6Asqrt(dP) in a 1' chimney with a 1 F temp diff and dP = 0.075x(1/460-1/461) = 3.54x10^-7 psi and a 0.01"H20 fan pressure makes dPf = 3.61x10^-4 psi, it might push 0.01 cfm through an A ft^2 hole in a 1 ft^2 surface if 0.01 = 16.6Asqrt(dPf/dP) = 531A, so A = 1.88x10^-5 ft^2, ie 0.0027 in^2, ie a 0.059 inch diameter hole, about 1/16".
So we might put up an 8'x40' 32 cfm Typar wall with 2x4 studs on 4' centers and poke big nailholes on a 1' grid from the south. With luck, the edges will open more at higher flows, keeping the pneumatic resistance low. A layer of black window screen might make it last longer and reduce the heat loss by radiation.
Nick
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Oops. That's a 3.2 cfm wall. For 32 cfm, we might poke 0.59" holes, or 0.29" pencil-sized holes with 0.04 "Hg. For uniform air distribution, we might make the pneumatic resistance of the mesh large compared to the pneumatic resistance of the stud cavity.
Nick
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