[Greenbuilding] [BULK] Re: Near-infinite Scandinavian breathing wall R-values
nick at early.com
Fri Feb 13 06:00:30 CST 2009
Lawrence Lile writes:
>OK. Heat doesn't leave the space by convection. If you don't count
>the heat you added to the furnace making the breathing air, it looks
>great! But you did heat that air, and that energy does count, and that
>heat DOES leave the breathing air channel by convection and conduction.
>And the inside of the wall, just behind the moving air breathing
>thing, is still 72F, and the outside is still (say) zero. Heat still
>moves by conduction, just about the same rate as it did in the
>conventional wall, driven by the 300 year old formula.
Picture cold outdoor air entering the outside of a wall, as in Fig.
The paper says "As cool ventilation air is drawn into a warm building
through the breathing wall, air flows inwards in the opposite direction
to the heat being conducted outwards as shown in the figure below. The
contra-flow of mass versus heat fluxes results in the cool air picking
up heat that would normally be lost through conduction, effectively
yielding a reduction in the dynamic U-value of the wall and higher
overall insulation efficiency..."
(Equation (1) on page 4 for dynamic U-value is wrong in the paper above,
and corrected in
So we still have to supply heat to the air in the room, but the wall
R-value increases, so the room needs less total heat than a room with
non-breathing walls and the same amount of ventilation air entering
directly from the outdoors.
Picture an 8' USR13 cube with 15 cfm of ventilation air. With no
breathing walls, its conductance (ignoring the floor) is 5x8'x8'/R13 =
24.6 Btu/h-F. Keeping it 72 F inside on a 32 F day with no air-air heat
exchanger requires about (72-32)(24.6+15) = 1584 Btu/h. If 15 cfm flows
in throught the walls and ceiling, V = 15cfm/(5x8'x8') = 0.047 fpm, ie
0.00024 m/s, with a static metric R-value 13/5.68 = 2.29 m^2K/W, which
makes Rd = (e^(1200VR)-1)/(1200V) = 3.23 m^2K/W, ie USR18.35, so the
cube's thermal conductance is only 5x8'x8'/R18.35 = 17.4 Btu/h-F, and it
only needs about (72-32)(17.4+15) = 1297 vs 1584 Btu/h, ie 18% less
But it seems to me we can save more by dividing the cube in 2 parts with
a periodically-reversing fan in the partition wall that turns the
breathing walls into bidirectional heat exchangers. This would be like
breathing through a scarf on a cold day. Breathe out, and the scarf
material captures some of the heat and moisture. Breathe in, and it
gives it back. How can we model this? Residential low-density fiberglass
insulation has about 0.5 lb/ft^3. The ASHRAE HOF lists the specific heat
of "glass wool" as about 0.16 Btu/lb-F, so 1 ft^2 of 3.5" unfaced
fiberglass might have 0.5x0.16x3.5/12 = 0.023 Btu/F, with lots of
surface. Warming it from 32 to 72 takes about 1 Btu. If 0.047 fpm flows
into a wall and 0.047cfm(72-32) = 2 Btu/h, so this seems like the right
Dan Antonioli writes:
>One thing that would be helpful to the list is to remember that this
>isn't an engineering forum. It's a green building discussion list.
Same thing :-)
>If you want to work with engineering formulas and math, please explain.
>Just like your teachers asked you to do.
My old Latvian friend Lisa used to say "Dahlink, never apologize, never
explain." John Wayne said it in 1949 (in "She Wore a Yellow Ribbon.")
Oxford teacher Benjamin Jowett said it on October 7, 1893.
Don Eyermann writes:
This reminds me of another quote:
When we play tennis or walk downstairs we are actually solving whole
pages of differential equations, quickly, easily and without thinking
about it, using the analogue computer which we keep in our minds.
What we find difficult about mathematics is the formal, symbolic
presentation of the subject by pedagogues with a taste for dogma,
sadism and incomprehensible squiggles.
from Structures: Why Things Don't Fall Down, by J. E. Gordon
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