[Greenbuilding] Exhaust Only Passive Inlet Ventilation

[Bob]

Sacie,
It's more than the temperature differential.  Assuming an temperature of 
65F and an RH of 40%, from your approximations, you will get 
condensation on any surface that reaches about 42F.  If it is in the 
high teens outside, say 18F, condensation on windows is not unexpected.  
I suspect that there are other factors at work as well, curtains that 
are close to the window can restrict airflow over the glass, "trapping" 
a layer of relatively stable air against the glass where the smaller 
volume will chill more rapidly than if it were being replenished by air 
at the full general temperature, for example.  Another possibility is 
that there are leaks at the trim joints around the window.  I see this 
more often than leakage associated with the window itself; the shim 
space is simply not well sealed in many cases, so the joint "seal" of 
the trim is the critical barrier.  This could increase the localized 
chilling of the living space air at the window.

On the second question,  "small exhaust fan" is not much to work on.  I 
agree with Keith.  You should have blower door testing done.  The 
presence of leakage through the shim space can be checked at that time.  
A well equipped tester should also be able to measure the flow through 
the exhaust fan, as well as assess if or how much depressurization it is 
creating.  You may be able to find someone local by checking the 
www.natresnet.org site for a list of raters serving your area, or 
www.energyconservatory.com for a list of blower door equipped people in 
your area.  If there is a community action agency, or the equivalent, in 
your area, it may be equipped and willing to take on side jobs.  Granted 
it's still a small circle of friends, but growing all the time.

I hope this helps.
Bob Klahn

Sacie Lambertson wrote:
> Nick, John et al,
>
> This morning is typical.  High teens outside, RH in the 40s inside, 
> with temps in the mid to high 60s and a nice bead of water across the 
> base of every window in the house, including those that are 
> inoperable.   Simple difference between indoor and outdoor 
> temps?   Comment please?
>
> Second question: if there are no vents below or above (an open, 
> therefore multi-story house with no ability to close either up or 
> down) and a small exhaust (don't know how many cfm pulled) in the 
> main center floor, what is the effect?   House tight, but many many windows.
>
> thanks,  Sacie
>
>
>
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