[Greenbuilding] Night Flushing Humidity Issues

[nick pine]

"elitalking" <elitalking at rockbridge.net> writes:

> I feel fortunate to live in a location where during the summer, the night time cools about 30F from the highs.  I have made a commitment to live without air conditioning here in the mountains of Virginia.  I have incrementally been upgrading the tightness of my house, having installed new continuous thermal barriers of 3" iso foam over most but not all of the exteriors walls and below cathedral ceiling.  Piece by piece, I am covering all the holes that remain.  

Have you done a blower door test?

> We have been in the practice of opening windows at night and aggressively changing the air in the house with window fans to cool the mass inside the house.  Most days, we can get the house to below 70F.

NREL says July is the warmest and most humid month in Sterling, VA... 1910 Btu/ft^2 of sun falls on the ground and 1040 falls on a south wall on an average 75.6 F day with a 64.1 low and a 0.0136 humidity ratio... 580 falls on the ground and 950 falls on a south wall on an average 35.4 F December day with a 45.0 high. You might dehumidify with an equilateral A-frame on the lawn full of solar-regenerated clumping kitty litter in summertime and solar-heated water in wintertime. The deep ground temp is 53.8. Earth tubes could cool on humid nights. 

>  We close the house off during the daytime to block out air delivered heat.

Do you ventilate every night by the clock, or only when outdoor air is cooler than indoor air, using a differential thermostat or passive plastic film dampers?

>I have installed an HRV.  We run the HRV during the day to deliver fresh air when windows are closed to block heat out. 

Do you need to do that, or is the house natural air leakage sufficient for fresh air? You might only run the HRV when the house CO2 level is above 1000 ppm. 

> The peak temperature outdoor air has much lower relative humidity than indoor air from night flushing.

How about the absolute humidity, ie the humidity ratio in pounds of water per pound of dry air?

>  I can observe the condensate drain.  Under normal fair weather days, there is negligible if any condensation.  My hunch is that the interior and exterior absolute humidity is about the same, so cooling the outdoor air closer to the indoor night flushed temperature never reaches dew point.  

If you can cool the outdoor air all the way to the indoor air temp with no condensation, the outdoor air may have lower absolute humidity. You might add a sensor to the condensate drain and only enable night ventilation if there is no condensation. Or use a smart ventilation controller that only night-flushes when outdoor air is more comfortable in an ASHRAE-55 comfort calculation or when it has less absolute humidity. IIRC, the ASHRAE comfort zone has an upper right corner at 80 F and w = 0.0120, at 66% RH.

> However, night flushing does result in high humidity in the house.  I have now been observing humidity for the past year more closely.  The result of night flushing is to seasonally build up humidity in the house.  This time of year, the house has some residual dryness from the heating season.  I think the material of the house is a humidity flywheel as well as a thermal flywheel. 

In an airtight house, hygroscopic materials can beneficially store dryness, with the house closed up on humid nights. Like thermal mass for solar heat storage. You might fill up your house with books and papers and clothing... http://www.ornl.gov/sci/buildings/2010/Session%20PDFs/129_New.pdf Tim Padfield has lots of humidity insights... http://www.conservationphysics.org/ http://www.conservationphysics.org/isothrm/isothrm.php

> However, this material is gradually wetted with increasingly high indoor humidity.  Late summer, wood work swells and there is some mold.  This goes away when it dries out again as heating begins in the fall.  

Sounds like your house can store dryness well. You might add humidity to your night ventilation decision criteria.

>However, late summer the house can be in 70% range.  This slows down the rate of drying for anything in the house.  How much is our health at risk? I presume there is some dust that becomes air born from mold.  I learned from this list that floating pathogens do not readily dry up allowing more contagions.

You might enjoy a 2-watt 7KV DC electrostatic media filter, eg http://www.nsaw.com/enviro2.htm "After only twenty minutes, the equivalent of two air exchanges, "EnviroSept" removed over 45% of particles in the 0.09 to 0.5 micron range and over 77% of the particles in the 0.5 to 1.0 micron range." 

http://www.engineeringtoolbox.com/particle-sizes-d_934.html gives:
 
Dust Mites                    100 - 300
Mold Spores                   10 - 30
Mold                          3 - 12
Spider web                    2 - 3
Spores                        3 - 40
Talcum Dust                   0.7 - 90
Clay                          0.1 - 50
Copier Toner                  0.5 - 15
Liquid Droplets               0.5 - 5
Anthrax                       1 - 5
Yeast Cells                   1 - 50
Atmospheric Dust              0.001 - 40
Bacteria                      0.3 - 60
Radioactive Fallout           0.1 - 10
Smoke from Natural Materials  0.01 - 0.1
Burning Wood                  0.2 - 3
Tobacco Smoke                 0.01 - 4
Viruses                       0.005 - 0.3
Typical Atmospheric Dust      0.001 to 30

Nick
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