[Cob] thermal bridging around windows (Robert Alcock)

[Shannon Dealy]

Hello Robert,

There are two different issues here:

   1 - dew point
   2 - circulation

Areas of the house that go through cycles of higher humidity: kitchen, 
bathroom, areas where people spend lots of time (we exhale roughly 3 
quarts of water per day).  The dew point occurs when the air temperature 
drops below the point at which it can continue to contain the quantity of 
moisture it currently holds.  Cold surfaces such as exterior walls and 
windows on cold days cause the air at/near these surfaces to give up their 
moisture, causing condensation.

In high moisture rooms of the house, for part of the day, the moisture 
content of the air is considerably higher than the house as a whole, so 
these are the areas where you will find the most condensation occuring.

Once the moisture spike is over, some or all of the condensation MAY
re-evaporate as the humidity levels in the air drop and equalize through 
the house.  This is a function of the overall average humidity in the 
building and how the air circulates, particularly past the area where 
condensation occurs.

With regard to cob walls generally, windows tend to be deeply set into the 
walls, so there is (depending on design) typically very poor circulation 
of air past the inside of the windows, even if the room itself has good 
circulation.  This seriously limits the rate at which the condensation 
will evaporate, both due to the poor air circulation, and the localized 
lower temperature zone near the window which results from the poor 
circulation.

Cob is a very poor insulator, and as I recall, you mentioned that the cob 
around the windows is only 4" thick.  If this is the case, it is quite 
possible that your cob around the windows is a poorer insulator than the 
window.  If so, then this cob itself is the coolest point and where 
moisture will condense first.  Even if the cob isn't the cold point, 
sometimes the window frames (aluminum is a serious offender here) are 
poorer insulators than the window, in which case, the frame and cob in the 
immediate vicinity will be where moisture condenses.

To fix the problem you could significantly increase the thickness of the 
cob around the windows (helping to stabilize it to the temperatures of the 
surrounding cob call), or you would experiment with some thick layers of a 
high straw plaster on the interior cob around the window to hopefully 
boost the R-value above that of the glass (at which point the glass may 
become a problem).

[snip]
> breathe out water vapour. But cob walls are breathable, and so in general, 
> the humidity inside a cob house should self-regulate in a way that doesn't 
> happen in non-breathable walls, hence there shouldn't be a generalised

Actually, most wall systems are (historically) breathable, and people over 
estimate the rate at which cob is able to transfer moisture.  It is a very 
slow process.  To get a better feel, I would suggest you get something to 
monitor relative humidity and compare the inside and outside of your house 
as you go through periods of high and low humidity outside.  Earthen 
structures that are historical buildings (tourist destinations) with lots 
of people walking through but no one ever using kitchen or bathroom 
facilities, have serious problems with moisture from the breathing of 
people walking through.

Even though I keep the windows cracked open year-round in my tiny house 
(to deal with all the moisture I breathe out), at wetter times of year
the interior humidity can spike above 95%.  This is considerably higher 
than in my cob shop building where I spend far less time (and hence 
generate less moisture).

FWIW.


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
Phone: (800) 467-5820 |          - Natural Building Instruction -
    or: (541) 929-4089 |                  www.deatech.com