An improved solar attic

[nicksanspam]

... $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

[Abby Normal]

Perhaps, then do not crosspost the spew if you cannot handle the critcism.

[nicksanspam]

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

[Jeff Thies]

What's wrong with felt or multiple layers of black screen (could be fiberglass). Seems to me that you don't want much distance between holes. Surely the thermal conductivity of Typar is not that great. I'm think of getting the heat off the collector...

Cheers, Jeff

[nicksanspam]

The tiny airflow (32 cfm for an 8'x40' wall) just provides insulation, eliminating convection heat loss from the mesh to the glazing, as in a Scandinavian breathing wall. The fin tubes collect the heat, which moves from the south to the north side of the Typar by conduction and up to the fin tubes by convection.

Nick

[nicksanspam]

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 :-)

But less air saves more energy, if outdoor air flows in through the wall with no heat exchanger: with 0 C outdoors and 20 indoors and a square meter of R5.7 wall losing 20Ud = 24000V/(e^(6840V)-1) watts and V m^3/s flowing in through the wall requiring 1200V = 20Ud watts of warming, V = ln(21)/6840 = 0.000445 m/s, ie 1.6 m/h is optimal, and the wall loses 0.534 W, with a 1m^2x20K/0.534W = R37 effective metric R-value, ie US R213 :-)

How does this compare with ASHRAE's 15 cfm per occupant fresh air standard?

Nick

[nicksanspam]

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 :-)

But counting air heating energy, the total is 1200VdT(1+1/e^(1200VRs)-1), which increases with airflow. If 30 cfm of 0 C outdoor air flows through

4000 ft^2 of metric R7 (US R40) exterior surface in a typical 40'x60'x8' US house with no heat exchanger and warms to 20 indoors, V = 3.8x10^-5 m/s, with 0.914 W/m^2 of air heating. The walls and ceiling (eg 8" fiberglass with 9" TGI joists and studs) would lose 0.914/(e^1200V7-1) = 2.4 W/m^2h, for a total of 3.31 W/m^2, ie metric R6 or US R34, including fresh air heating as needed, with (68-32)4000ft^2/R34 = 4235 Btu/h at 30 cfm. This would only work well in an extremely airtight house.

In which case, maybe it's better to take the R40 with simpler non-breathing walls losing (68-32)4000ft^2/R40 = 3600 Btu/h and add an 80% air-air heat exchanger (eg some bidirectional breathing walls) losing about 0.2x30(68-32) = 376, for a total of 3976 Btu/h.

Nick