[Cob] thermal bridging around windows (Robert Alcock)

[Robert Alcock]

Hi Shannon

thanks for the detailed reply. It sounds like condensation is more of a 
problem in cob buildings than I was aware! By the way, the cob in our 
main wall has been tested as having a thermal conductivity between 0.24 
and 0.3 W/mK. That's equivalent to about a US R-value of 0.5 per inch -- 
compared to the typical value of 0.3 per inch that you give at 
http://www.deatech.com/pipermail/coblist/2011/013007.html. I imagine 
this is because we added quite a bit of extra fibre (in the form of wood 
shavings) to our wall. We do have double-glazed windows with wooden 
frames, so what you say about the moisture condensing on the cob is 
probably true.

Robert
www.abrazohouse.org

Shannon Dealy wrote:
>
> 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
>