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re: basement humidity (do i need dehumidifier and, if so, would this setup work well)
12 jul 2005
blueman   wrote:

> writes:

>> jay  wrote:
>>>...i can keep the basement's humdiity level below 60% as long as i leave 
>>>the blower motor on the furnace/ac system turned on all the time.  this 
>>>causes some of the basement air to circulate throughout the rest of the 
>>>house (with the rest of the house still having humidity below 50%). would
>>>i be better off with a dehumidifier rather than leaving the blower motor
>>>on all summer?
>>a dehumidifier would add significant heat to the house in summertime...

> i right in interpreting that you are basically saying that
>a dehumidifier is going to be relatively helpless against a cold slab
>with warm, humid outside air?

it would work, if the air leakage is not too large. for instance, if c cfm
of outdoor air with wo = 0.0134 pounds of water per pound of dry air leaks
into a 55f 60% rh wb = 0.0051 baltimore basement on an average july day,
a 40 pint per day dehum could keep up as long as 24hx60c0.075(wo-wb) < 40,
ie c < 45 cfm, or more, if the dehumidifier and the incoming air warm the
basement, but that uses lots of electricity and makes lots of heat, about
1.6x40x1000 = 64k btu/day. you might run a 5k btu/h window ac 13 hours per
day to remove all that heat :-) 

it can be a lot more energy-efficient to use a fan to circulate air between
the basement and the house when the basement is humid, warming the basement
and cooling the house.

>(plus we have the added comlication of dampness (not puddles) seeping
>in during rainstorms?

maybe we can adjust the gutters and downspouts and
slope the soil away from the house to improve that.

>so if a large cold slab stores so much moisture...

the long term moisture content of concrete might be 5% of the rh
of the air surrounding it. concrete weighs about 150 lb/ft^3. 

>...what is the best thing to do?
>        - heat the slab?

maybe a little, in wintertime, by moving air from the basement floor up
into the house. a baltimore basement with 10 cfm of outdoor air leakage
might be 53 f in january, with pa = 29.921/(0.62198/0.0025+1) = 0.120 "hg
vs ps = e^(17.863-9621/(460+53) = 0.440 "hg at 100%, and 100pa/ps = 29%.
circulating some air between the house and the basement could lower that
and add desirable humidity to the house, with a little more heating fuel.
(unless the home is 100% solar-heated :-) outdoor air warmed to 70 f would
have 100pa/0.748 = 16% rh. a 4"x1000ft^2 50k pound slab can slowly store
(0.03-0.01)50k = 1000 pints of water as the rh of the basement air rises
from 20 to 60%.

>        - circulate *in* hot air from outside (even if it is humid) to
>          attempt to heat up the slab?

i don't think so, if it's more humid outside, in the absolute sense.


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