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re: condensation on interior of thermal air column?
18 dec 2001
james johnson  wrote:

>i live in central texas and am contemplating building a new home with
>passive cooling paramount in mind.

san angelo looks central. nrel says it's 82.7 in july, with w = 0.0130,
ie vapor pressure pa = 29.921/(1+0.62198/w) = 0.613 "hg and dew point
td = 9621/(17.863-lnpa)-460 = 64.2 f. the average daily low and high are
69.1 and 96.2. what are you planning to use for thermal mass?

the coldest month is january, when it's 43.7 outdoors with a daily high
of 56.8. a south wall gets 1400 btu/ft^2-day on average, a level surface
gets 1010, and w = 0.0039, which corresponds to a dew point of 32.3 f.
>my idea is this:  the exterior of the house will be sheathed with standard
>metal building sheeting, with the 'highs' on 1' centers.  these highs are 1
>1/4" high.  a 'reverse run' sheet (that is, the good paint is on the back of
>the sheet) will be screwed to the first layer of sheeting, high to high,
>with the back of the sheet facing out.  this will create a 2 1/2" open
>channel between the highs of the sheets.  if this channel is left open top
>and bottom, as the sun shines on the exterior sheet warm air will rise and
>flow out the back.

sounds interesting. you might calculate the airflow and temperature rise 
with a chimney formula.

>actually, the roof will be done the same way and the air will flow up
>the wall, then up the roof and out a continous ridge vent.

that might work better if the bottom air exits at the eave, and more
fresh air for the roof enters there. 

>my question is:  how likely is condensation to occur inside this column.
>and most important, will condensation occur on the interior of the interior
>sheet?  this sheet will not have insulation up against it, instead will have
>an air space of 3/4" and a radiant barrier on the inside of the air space.

sounds like your new house won't have much insulation. about r2 for the
cladding plus r8 for the wall vapor barrier and r14 or so for the roof,
with downward heatflow.

>obviously, if condensation occurs on the inside of the inside sheet, which
>is actually the interior of my wall, i must re-think my plans.

if the room is colder than the outdoors, condensation may occur on
the outer vapor impermeable surfaces, but it looks like that won't
happen on an average day, with a 69 f min and a 64 f dew point.

if the room is warmer than the outdoors, condensation may occur on
the inner surface. it looks like that won't happen unless you run a
humidifier in wintertime. a radiant barrier that is also a vapor
barrier can help avoid condensation inside the wall. innovative
insulation in arlington, tx makes foil with and without tiny holes... 


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