Before y'all draw out the pistols, can I make what I think is a clarification,
or at least a distinction?
Bower is talking about the idea that walls will pass fresh air, acting as a
filter and potentially as a heat recovery mechanism, by moving counterflow to
the conduction in the wall (or ceiling). Salmen has referred to what sounds to
be a different thing altogether, an internally located wall which has plants
whose purpose (or one of them?) is to remove toxins from the air. OK so far,
you guys?
I'm going to take Bower's fork here, now. Many folks seem to be confusing
building assemblies that are permeable to water vapor with ones that are
permeable to air. The first, if properly designed, is likely to be just fine -
the issue is merely a rate problem - can you design the assembly for no net
accumulation of moisture above what will cause molds, decay, etc. The second
is very tricky if you want to make an enery-efficient building. The assembly
must have a relatively high resistance to airflow, uniformly spread across the
assembly, or else no filtration and no heat recovery, 'cuz all the air comes in
through the few holes, bypassing the would-be filter and heat recovery
membrane.
I believe that Steve Loken and John Timusk, Canadian building science guru,
both carefully built dynamic wall homes, and neither saw the results that were
predicted. That of course may be due to faulty design, not good enough
workmanship, or poor material choice. The Gaia Group of architects in Norway
has been designing buildings with air permeable ceilings, which they presented
in a very well-attended session at NESEA in 1996. Dag Roalkvam told us that
the pressure drop across these ceilings is only 1 pascal. Since the stack
effect in most cold climate buildings is larger than this, there were those in
the audience who didn't believe that what he was describing, possible in
theory, was actually occurring. It was quite controversial. I realized that,
in order to make the building work the way he described (depressurizing the
house with a passive stack, bringing in fresh air through the ceiling assembly,
filtered and warmed) that the house would have to be *extraordinarily* tight.
If it wasn't, then ANY hole in the envelope, especially those low in the
building, would become the inlets instead. I asked Dag how much of the air
flow was bypassing the ceilings - his estimate was only 30%.
Afterward, I asked Jim White of CMHC, and Joe Lstiburek, how tight they thought
a home would need to be to begin to work as Dag described. They thought below
200 CFM @ 50 pascals. I've only tested one building that tight. So, what may
be (barely) possible to do in a couple of cases may be quite hard to replicate
in practice. No home I've tested with a chimney has been that tight, for
example, and no home with double hung windows, either.
So I still aim to make a building as tight as I can (the tightest ones I've
tested have been ones I've designed and supervised) and bring in fresh air via
mechanical ventilation. I'm very interested in more "natural" solutions, but I
remain somewhat skeptical. And all I could think of in the Gaia ceilings was
the mouse droppings accumulating in the cellulose insulation filter over time -
but hey, maybe the cellulose will disinfect them? (or should we put wooden
cutting boards in the attic on two foot centers?)
Marc
And what connection does John Salmen have with salmonella?
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