DIYbanter - View Single Post - basement humidity (do I need dehumidifier and, if so, would this setup work well)

writes:
jay wrote:

So far 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...

Here's a calc for the minimum basement slab temp needed to keep a basement
less than 60% RH for a typical year in Baltimore, using long-term monthly
weather data, assuming a fairly airtight house. It looks like the basement
has no problem staying below 60% in wintertime (months 1-4 and 11-12 below),
when outdoor air is dry and the 55.3 F ground supplies enough heat to make
the natural slab temp more than the min slab temp required for 60% RH, given
the moisture in outdoor air.

Psat = e^(17.8653-9621/(460+53.2)) = 0.413 "Hg at 53.2 F and 100% RH in
January, and the vapor pressure of outdoor air with a w = 0.0025 humidity
ratio is Pa = 29.921/(0.62198/w+1) = 0.120 "Hg, so RH = 100Pa/Psat = 29%,
approximately, for a slab that doesn't store moisture.

20 ASLAB=1000'slab area (ft^2)
30 RGRND=10'R-value of deep ground (h-F-ft^2/Btu)
40 TGRND=55.3'deep ground temp (F)
50 CLEAK=10'air leakage rate (cfm)
60 RHMAX=60'max basement relative humidity (%)
70 TAC=80'AC temp (F)
80 RAC=50'AC RH (%)
90 DATA 31.8,.0025,34.8,.0027,44.1,.0037,53.4,.0052,63.4,. 0083,72.5,.0115
100 DATA 77.0,.0134,75.6,.0131,68.5,.0106,56.6,.0070,46.8,. 0048,36.7,.0032
110 DIM T(12),W(12)
120 FOR M=1 TO 12'month
130 READ T(M),W(M)'average outdoor temp and humidity ratio
140 PA=29.921/(.62198/W(M)+1)'vapor pressure of outdoor air ("Hg)
150 TSMIN=9621/(17.863-LOG(100*PA/RHMAX))-460'min winter slab temp (F)
155 TSNAT=TGRND+(T(M)-TGRND)/(1/CLEAK+RGRND/ASLAB)*RGRND/ASLAB'nat slab temp
160 PRINT M,T(M),TSMIN,TSNAT
170 NEXT

month outdoor min slab natural slab
air (F) temp (F) temp (F)

1 31.8 34.03717 53.16364
2 34.8 35.98914 53.43637
3 44.1 44.13639 54.28182
4 53.4 53.22431 55.12728

The average daily max in May (month 5 below) is 74.2 F. A 66.2 F slab might
lose 24h(66.2-55.3)1000ft^2/R10 = 26K Btu/day to the ground, which might
come from a minimal 230 kWh/mo of indoor electrical use or a 90 W 2470 cfm
intake window fan running 26K/(2470(74.2-70)) = 2.5 hours per day, or less,
on a dry that's warmer and drier than an average May day.

5 63.4 66.21149 56.03636

It looks like AC will help for the next 3 months, with basement-house air
circulation, which would reduce the AC load. At 80 F and 50% RH, a 400 cfm
basement return would make the slab about 55.3+(80-55.3)(1/400+0.01)0.01
= 75.1. With Pa = 0.5e^(17.863-9621/(460+80)) = 0.524 "Hg indoors and Psat
= e^(17.863-9621/(460+75.1)) = 0.890 "Hg near the slab, RH = 100Pa/Psat = 59%
in the basement, approximately. The slab might give (75.1-55.3)1000ft^2/R10
= 1980 Btu/h of sensible cooling.

6 72.5 75.61597 56.86364
7 77 80.12378 57.27273
8 75.6 79.4524 57.14546

The average daily max in September (month 9) is 78.5. This might be another
ventilation month, or an air conditioning month, if it's warmer than average,
or a dehumidification month, if it's cooler.

9 68.5 73.23895 56.5

The average daily max in October is 67.3, so it's a dehumidification month,
with possible help from ventilation.

10 56.6 61.41242 55.41818

And we might do nothing again in months 11 and 12.

11 46.8 51.05957 54.52727
12 36.7 40.35092 53.60909

A TMY2 hourly simulation would show less energy use, since some days are drier
and/or warmer than average in winter and drier and/or cooler in summer. And
concrete can store lots of water, about RH/2K % by weight, in RH% air, and
it takes little additional house heat to reduce the basement RH and dry out
the concrete and desirably increase the house RH in wintertime. A 4"x1000ft^2
50K pound slab can slowly store and evaporate (0.03-0.01)50K = 1000 pints of
water as the RH of the basement air rises from 30 to 60% and falls back again.

Hour-by-hour smart ventilation controls that manage heat and moisture storage
in a basement might reduce the AC and heating load and eliminate the need for
a dehumidifier.

Nick

Not sure I understand this, but am 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?
(Plus we have the added comlication of dampness (not puddles) seeping
in during rainstorms?

So if a large cold slab stores so much moisture, what is the best
thing to do?
- Heat the slab?
- Circulate *in* hot air from outside (even if it is humid) to
attempt to heat up the slab?
- ????