[Greenbuilding] Near-infinite Scandinavian breathing wall R-values
nick at early.com
Wed Feb 11 14:33:29 CST 2009
John Straube wrote:
>> The authors of a 1996 NESEA conference Scandinavian breathing wall
>> paper said they had been in use for 20 years. They estimated the 1/4"
>> felt would have a 300 year lifetime.
> I was at that conference.
> And I met the folks from Gaia. The proof is in the pudding: the energy
> numbers for their buildings built this way (and they tend to draw air
> in from the ceiling) are not as good as people who build insulated
> airtight builidngs the normal way.
After looking into this more, that would also be my conclusion. Fig 1 on
page 3 of one of Professor Imbabi's latest papers:
shows a 3.7" US R15 wall becoming R57 when 0.001 m/s of air flows
through it. Keeping 1 ft^2 of unaspirated wall 70 F indoors and 30 F
outdoors requires (70-30)/15 = 2.7 Btu/h. The airflow lowers this to
(70-30)/57 = 0.7, for a net savings of 2 Btu/h. Warming 60x0.001x35.3 =
2.1 cfm of air from 30 to 70 F requires about 85 Btu/h, a lot more than
the energy saved.
It looks like the breathing wall airflow has to be very small and
fragile to make this pay for itself, energy-wise, with no outgoing
air-air heat exchanger, 10X the lower limit of Imbabi's operating range:
10 TI=(70-32)/1.8'indoor temp (C)
20 TA=(0-32)/1.8'outdoor temp (C)
30 DATA 0.0001,0.0002,0.0005,0.001
40 FOR VEL=1 TO 4
50 READ V'air velocity (m/s)
60 RHOA=1.2'air density (kg/m^3)
70 CA=1000'air specific heat (J/kg-K)
80 RS=1/.365'static R value (m^2-K/W)
90 UD=V*RHOA*CA/(EXP(V*RHOA*CA*RS)-1)'dynamic U-value (W/m^2-K)
100 RDU=5.68/UD'US R-value (ft^2-F-h/Btu)
110 WALLLOSS=(70-30)/RDU'heat lost through wall (Btu/h-ft^2)
115 CFM=4000*60*V*35.3/10.76'cfm with 4000 ft^2 of wall area
120 AIRHEAT=(TI-TA)*CA*V/RHOA'air heating (J/s-m^2)
130 AIRHEAT=3.412*AIRHEAT/10.76'air heating (Btu/h-ft^2)
140 PRINT V,RDU,WALLLOSS,AIRHEAT,CFM
150 NEXT VEL
.0001 18.4247 2.170998 1.02764 78.73606
.0002 22.01065 1.817302 2.05528 157.4721
.0005 39.52315 1.012065 5.138201 393.6803
.001 122.0273 .3277955 10.2764 787.3606
But it seems this can be useful for ventilating a house, with 2 walls
and a bidirectional partition fan.
And a transpired mesh in a single glazed solar air heater (something
like a glazed Conserval wall) can act as a breathing wall, reducing the
heat loss to the outdoors.
> Of course, really well insulated houses dont need any more heat when
> it is sunny than is provided by normal view windows...
Perhaps they should have less insulation and fewer windows and more
solar air heaters.
>>> ... The flow is however not uniform, it goes through joints, holes,
>>> and cracks, bypassing the diffusive flow needed to get infinite R
>>> value. Nevertheless, practical rvalues of 100 or so are possible...
>> Not bad :-)
> Outrageously good! Problem is, NO ONE HAS BUILT ONE OF THESE THAT WORK
> OVER ANY AREA OF ANY LENGTH OF TIME.
They seem to be trying hard, in Scotland.
>I can't believe I, as an engineering professor, am saying this, but
>equations do not capture everything, and although sometimes (often
>times) simplifications work well enough to help us design (thankfully)
>there are many examples where simple static equations don't even come
>close to predicting reality. This is very clearly one of these cases.
There are many ways to do something wrong :-)
> Based on many personal conversations with Prof Timusk and many
> "wasted" hours of my past research, there are major practical
> limitations that cant be solved by doing more calculations. Building
> many houses and measuring performance is the only way to push this
> field forward.
Actual work? :-)
>>> The show stopper was that people could not access the insulation to
>>> replace it every year: if you suck air through a porous insulation,
>>> it is a filter, and needs to be changed every so often as dirt, dust
>>> and pollen accumulate. If you cant change all the wall insulation,
>>> then there are serious, and so far insurmountable, indoor air
>>> quality issues to deal with.
I guess we could make changeable wall insulation. The paper above says:
>PM filtration performance and subsequent rise in pressure drop across
>the fibre-based DI media have been investigated by Imbabi and Peacock
>. A single-fibre model was developed that predicted that PM10
>filtration with a first pass efficiency of 97.5% is achievable. This
>filtration efficiency is expected to improve with time due to dendrite
>growth, where the internal structure changes over time as branch-like
>dendrites form through the agglomeration of particles within the filter
>media. Some of these dendritic fibres will act as filter fibres,
>increasing the packing density. Since the collection efficiency
>increases with dendrite formation, there is no risk of efficiency loss
>The corollary of this effect however is an increase in pressure drop
>across the DI media with time - a phenomenon that will determine the
>lifetime of the product, i.e. the pressure drop will eventually exceed
>the capacity of the installed air handling system. Results from
>theoretical predictions and laboratory tests were used to inform the
>EnergyfloT cell design process to ensure that the lifetime of the
>product as determined by this limiting rise in pressure drop was well
>in excess of expected building lifetimes even in the most polluted
>urban and industrial environments.
I can imagine asking a Certainteed engineer for some numbers.
>> That didn't seem to be a problem in 1996...
> Not true. Not every house is a problem. It is a probabilistic problem.
> We see houses built by the hundreds with much smaller flaws, and 10%
> failure rates are considered major failures of these. One hand crafted
> house or swimming pool does not prove general truths.
Maybe that means we need to learn more to make this work well every
>> Bill Shurcliff proposed attaching a "lung" (picture a giant bellows)
>> to the outside of a house, with a fan that periodically inflates and
>> deflates it with house air, thus turning all the cracks and crevices
>> in the house envelope into bidirectional heat exchangers with latent
>> heat recovery, as in a camel's nose. An "infinite virtual lung"
>> might divide a house into two partitions with a fan between them that
>> periodically reverses. This could be very efficient, if done slowly,
>> with lots of heat exchange area.
>> Max Sherman at LBNL tested a finite lung in the 80s, but the lung
>> volume was small compared to the stud cavity volume, so there was
>> little fresh air exchange. They weren't able to test it further [but]
>> Someone else might, with a humidistat that runs Lasko's $60 2155A 16"
>> reversible fan with Grainger's 2A179 $88.15 programmable cycle timer
>> and its $4.37 5X852 octal socket whenever the house RH rises to 50%.
> Max knows airflow cold, but does not know too much about real houses.
> I doubt he has been involved in a single IAQ investigation.
>> What is the standard deviation of house envelope cracks? A few big
>> holes or cracks could make this work poorly.
> Whoa. Tough question. A whole field of research. In short, the
> standard deviation is very large, most houses leak worse than you
> might expect.
Breathing walls seem more predictable.
>... my good friend Terry Brennan has looked at thousands of dwelling
>units with IAQ problems. I could continue to list the people I know
>well and personally who are among the top indoor air quality experts in
>North America. Breathing air through your (potentially moldy, off
>gassing, particulate clogged) is a really really bad idea.
Would Terry agree? IIRC, he think earth tubes can be unmoldy.
>Yes, you can get away with it, much like you can pull the trigger in
>Russian Roulette several times before you notice a problem.
>Understanding the mechanism of how a revolver works, and knowing that
>there is a round in one of the chambers is enough for an expert to say
>"dont play the game if you value your life".
This would be different, if you could see where the round is.
> If someone wants to experiment carefully on their family (as Jan
> Timusk did, and pulled the plug on the Dynamic Wall house after less
> than 5 years) then I wish them luck and beg them to take extensive
> measurements. On the other hand, no professional can ethically deploy
> this system given what we already know about the health risks and
Lawsuits. This needs more work, and more vic^h^h^h beta test sites. And
maybe more instrumentation, eg a $75 2-watt PC that measures the temp
and RH in the air filter/insulation.
More information about the Greenbuilding