[Greenbuilding] indoor humidity levels and how (NOT !) to lower them
archilogic at yahoo.ca
Fri Dec 30 16:20:19 PST 2011
On Fri, 30 Dec 2011 16:28:00 -0500, nick pine <nick at early.com> wrote:
> Reuben Deumling <9watts at gmail.com> writes:
>> ... Are there other tricks for lowering humidity indoors?
> turn on a reversing fan in a house partition wall to draw air in through
> half of the house envelope and exhaust from the other, then vice-versa,
> turning all the cracks and crevices into extremely efficient
> bidirectional air-air heat exchangers
Assuming for a brief moment that the above scenario could actually take
place in the Real World ...
Exhausting warmed, humid indoor air through the cracks and crevices in the
envelope would result in condensation of that moisture in the envelope
materials (assuming that there is a reasonable amount of insulation in the
That moisture would support microbial activity and quite likely subsequent
rot of framing materials.
Reversing the air flow would then bring the microbes from that fetid goo
broo into the interior air environment where it would then be breathed by
the house occupants eventually wreaking havoc (or worse) upon their health.
Not good. And certainly no "air-to-air heat exchange" taking place either,
for what should be obvious reasons.
In the Olde Days (ie early 1970s), some people did attempt to make DIY
air-to-air heat exchangers using polyethylene sheets tacked to 1x2 wood
frames to serve as the heat exchange surfaces separating incoming and
exhaust air streams but you can pretty much imagine what became of them.
However, in the Real World, instead of the above fictional scenario
happening, in winter, infiltration will occur at points below the Neutral
Pressure Plane (NPP) and exfiltration will take place above the NPP, with
increasing intensity as distance from the NPP increases. This process is
already happening in Reuben's house but obviously, the natural air change
rate is insufficient to deal with the current humidity levels.
Having a fan in an interior partition moving air horizontally from one
side of the house to the other is unlikely to do much of anything in terms
of altering humidity unless the fan is moving air past a sufficiently
large surface area of a desiccant medium or a condensing surfaces.
=== * ===
Kanata, Ontario, Canada
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