[Greenbuilding] Condensation and shutters

[nick pine]

Corwyn <corwyn at midcoast.com> writes:

>Doug Kalmer wrote:
>> ... there is a considerable amount of condensation on cold nights.

> That condensation is releasing 970 BTUs of heat per pound of water to 
> the outside as it condenses...

The dew point of 70 F air at 50% RH is (70+460)/(1-(70+460)ln(0.5)/9621-460
= 50.5 F. If a shutter with an Rs R-value inside an R2 window leaks indoor air
on a 30 F night and 50.53 = 30 + IR2 = 30 + (70-30)2/(Rs+2), condensation
will happen if Rs is greater than 1.896. 

                 50.5 F
         Rs      |         R2
70 F ---www---------------www--- 30 F
              ------->
                 I

Increasing the shutter's R-value beyond that won't change the dew point
temperature at the indoor glass surface. It can INCREASE the heat loss
through the window by making a new condensation heatflow path with a low
thermal resistance in parallel with the R0.68 slow-moving airfilm
resistance at the glass surface, ie by making the dry R2 window
into a moldy rotting R1.32 window.

Nick
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