--- On Fri, 12/12/08, Jeff Gephart <[log in to unmask]> wrote:

Houses don't breath.  They should be built tight and ventilated with equipment appropriate to maintain a consistent and appropriate air exchange (when occupied) to control moisture levels, provide adequate fresh air, and dilute indoor air contaminants of all sorts.

 
I'll have to disagree with Jeff on this, though the disagreement might be based on the ambiguous meaning of "breathe".
 
A tightly-built house will not "breathe" air sufficiently without mechanical ventilation, but it can (and I would say should) "breathe" moisture, just like our skin and our clothing.
 
The typical family of four will put approximately 2.5 gallons of moisture per day into the indoor environment through the normal requirements of living (cooking, bathing, washing, breathing, plants).
 
A 2000 SF 2-storey house, with a thermal envelope (walls and ceiling) that are relatively permeable to moisture diffusion (5 perms avg) will diffuse approximately 1.25 gallons of water per day at 70F/40%RH indoor and 20F/75%RH outdoor. This will eliminate half of the daily moisture production, and a modest air exchange rate will easily eliminate the remainder.
 
In addition, a building with high moisture-storage capacity materials (generally natural materials) will buffer the daily and seasonal variations in RH (just as thermal mass will buffer temperature swings). A wood-frame house with wood sheathing can safely store 50 gallons of water. Fill those walls and ceiling with cellulose and it can store as much as 200 gallons of moisture reversibly, but only if the thermal envelope is not vapor sealed to the inside.
 
Proponents of natural building (e.g. straw-bale, cob, plaster) understand the value of moisture breathability, storage and buffering. The rest of us would do well to rediscover it.
 
- Robert