[Greenbuilding] indoor humidity levels and how (NOT !) to lower them

RT 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.

=== * ===
Rob Tom
Kanata, Ontario, Canada

< A r c h i L o g i c  at  Y a h o o  dot  c a  >
(manually winnow the chaff from my edress if you hit "reply")

More information about the Greenbuilding mailing list