Moisture Check - Theory

[Toby Anderson <anderstc@nv.doe.gov>]

Way Cool Habib!

Thanks for all that NEAT info. 

Aesthetically speaking, how does Carl's inside/unplastered walls look?

>>>Yesterday was moisture recording day for my unheated load bearing
utility building. As usual, the moisture content at the the three sites is
stable around 14.6 %. This building is finished and the probes are built
into the walls.

I'm glad that all the readings were below the "safe" level which if I
remember was 18%MC.

What probes (type/manufacturer) ya got in thar? 

>>>It seemed like a good time to visit Carl Schlichting ... Interestingly, only
the outside of  the building is covered, with a single coat of cement
stucco. ...We did a very casual moisture survey: ...The moisture content
varied between 14.4 and 14.9% throughout the house.

Alright.

>>>At three sites the deep reading was higher than the mid bale site, the
variation was between +.3 to +.5% MC. During the cold spells of -17
degrees C, Carl observed moisture on the outside walls, these
disappeared with warmer temperatures.

>>>What does it all mean? ...Comments, observations, please.

That, my SB friends, was quite an important scientific observation.

Why?

It means, IMO, that the infamous dew point was, for these extreme
conditions, NOT within the wall, but on the outside surface of the wall.

SO what?

1. This gives evidence that there is NO ICE DAM inside the walls, and will
make code officials very happy. 

2. It explains why, with standard SB construction techniques, as
opposed to standard stick-built construction techniques, stucco can be
used in cold winter climates such as in Canada and Minnesata..

3. And maybe better yet, with more data, it might keep the WOBby,
vapor-barrier-heads quiet :-)  (grin)

I have a theory, which I give below, that I ran by Balehead several
moons ago that matches this phenonomen exactly.  Balehead thot it had
some merit. I mention John cuz he's a bit more respected than I, and I've
been known to have some WAG theories (wild ass guesses). 

Just to make sure we are not talking about apples and oranges, I have a
few questions:

By "moisture", do  U mean liquid water (as opposed to ICE)? 

Did he see steam or vapor moisture?

By "warmer", do U mean above freezing? 

If U could find out roughly what this temperature was, it would be very
helpful.

Did Carl observe the moisture during the day or night?

The outside stucco surface was wet not because the outside air
condensed on the stucco, but because the vapor in the air coming from
the inside of the house condensed. Why? For 2 reasons:

1. Because if it's freezing outside, there ain't much vapor in dat air. That
much moisture, must be coming from the inside.
2. In the winter, air flows from the inside to the outside because there is
more pressure inside due to warmer temps and higher humidity (i.e
pressure is directly proportional to RH*T). So, at the outside surface of
the stucco, air is exiting and thus outside air is pushed away from the
surface.

Again I repeat:
This means, that the infamous dew point was, for these extreme
conditions, NOT within the wall, but on the outside surface of the wall.

IMHO, when the temperatures got warmer, the moisture disappeared,
because it actually evaporated into the surrounding air.

Here's my theory, exactly as I put it to Balehead, with the background
that I was talking about a SB house in a cold winter climate:

*********************************************************************************
As the heat moves to the outside surface, it does so because for 2
reasons:
 1. Conduction: Heat transfer caused  solely by a difference in
 temperature - heat flows to the lower temperature. Heat is caused by
 molecules bouncing off each other.
 2. Convection: Heat transfer by convection is when the heated
 substance (the vapor) moves from one place to another.

This heat warms up the cold air penetrating in from the outside.  At some 
point wtihin the bale or on the stucco, there isn't enough heat to warm
 the air enough to prevent condensation. When condensation occurs,
 heat is given off, further warming the air. My point? IT may be possible
 that this condensation point occurs close enough to or directly in the
 stucco.  IN addition, it may be that during the day, the third type of heat
 transfer, i.e. radiation from the sun, is warm enuf to warm up the
stucco,  so that any vapor will come out thru the stucco, and any liquid
water  near the stucco will vaporize and then continue thru the stucco..

Balehead's responce:
>>>Yo!  Another theory.  I think it has merit.  Under some conditions, the
dew point would be at or near the stucco, and then could easily dry out
in the sun.  
*********************************************************************************

The moisture (liquid water) on the stucco didn't freeze, because the
stucco was warm enough to prevent freezing for probably 3 reasons:

The stucco was warmed by:

1.  Radiation from the Sun 
     If we knew the temp and stucco thickness, I think we could make
some rough calculations.

2. Convection: heat from the heat stored in the vapor is transfer from the
inside of the house to the outside of the house.
     If we knew the outside temp we could make some rough calculations.
The inside RH would help also, but we could make some assumptions.
    
3. Heat from "the heat of condensation" is produced when the vapor
traveling from inside to outside condensates.

On retrospect, it still may be possible, that when it gets colder and at
night, that the dew point may have moved into the middle of the stucco
(this is probably bad),  but this is unlikely as described in the next
paragraph.

If no damage to the stucco occurs over time, then the dew point did not
occur within the stucco. Condensation, on dense surfaces (like stucco),
usually occur on the surface because the temperature throughout the
stucco is probably very uniform.. i.e. stucco is a thermal bridge - it
conducts temperature very well.

So if condensation occurs on the outside surface, we have a good
situation as described above. If it occurs on the inside surface of the
stucco (the stucco/bale interface, we have a very hopeful situation as
described below:

My thots (and I am pretty sure Balehead agrees, as he was the instigator
of the wicking theory) are that if the dew point moves to the bale surface
or into the bale, that the bales, act like a sponge and absorb the water,
without freezing (no ice dam), and wait for warmer weather to
evaporate the liquid & breath the vapor out again....

>>>There is a second floor reading room at the head of the stairs. The
bales under the picture window are unparged on both sides and the
temperature in these bales was 8 degrees F. warmer than the bales with
the single coat of exterior stucco. Carl commented the bales were
happily exhaling a lot of heat.

Probably more air is moving thru these bales since it is more permeable
than the bales with stucco. More warm air means the bales are warmer
due to convection of the vapor. The air higher up in the building may be
warmer also because warm air rises.

I wud be very interested in hearing any other comments i.e. any personal
email to Habib, could U copy them to me also?

Thanks,

DTT