[Greenbuilding] Drying House in Humid Season

[conservationarchitect at rockbridge.net]

Corwyn / Topher Belknap and Nick Pine responded to my message describing using heat in summer to dry out my house.



Beyond my experience, what strategies do others use other than refrigerant driven dehumidification for restraining humidity in house when ambient humidity is high. Assume reasonable control of humidity at source such as ventilation at baths and kitchens.  If all living in humid and mixed humid climates must have refrigerant driven dehumidification to maintain healthy conditions, we are doomed. Often old equipment stops working after refrigerant leaks into atmosphere.    







Corwyn / Topher Belknap wrote in response to my original message show in italics.  My fresh comment shown in brown.  



> I got the house to 104F.I found that the absolute humidity went way up 

> over what would be expected if no moisture was added to the air.This 

> was the moisture being evaporated from house contents inside thermal 

> envelope.

Right.

> Since the house was closed up, I would assume minimal infiltration.

Ha.  Good luck with that.  Did the pressure in the house go up with that 

temperature increase?  No?  Then the house has leaks.  Even the best 

sealed houses would exchange 1/2 their volume of air over an afternoon.



I am sure that I still have leaks in the house.  Though closing the house up and turning off HRV reduced the infiltration over open and on HRV. 



> I would propose that intermittently heating a house that has humidity 

> build up is a way to manage the humidity.The mold is dried and must 

> start all over. Yes, control humidity at the source where 

> possible.However, when the ambient conditions are humid that strategy 

> is incomplete.

> 



The problem with this is that most things which retain moisture, 

exchange it fairly slowly in response to changing conditions (as you 

observed).  I wouldn't expect a change made over the course of a day to 

last more than a day when conditions are returned to what they were.  It 

might be useful if you had a sudden influx of moisture, or if it was 

done at one of the transition points into a drying regime (to speed up 

change that was going to happen anyway).



How long the increased dryness would last would be proportional to how much moisture was removed. Hot air sure works in clothes dryers.  

> 

> This heating is a is appropriate where the house has low mass, 

> allowing the temperature to be easily raised and returned to normal.I 

> would also propose that we take steps to build up the desiccant 

> capacity of the house such that fewer cycles such as this would be needed.

> 



The higher the desiccant capacity, the less well this is going to work.



This summer I did experiment with using bentonite clay (desiccant) with the aspiration to dehumidify the house non mechanically.  My experience validates your statement.  The rate of adsorption of humidity was way too slow to cycle on a daily basis.  However, I did achieve the ability to dry out the clay by setting it in a solar cooker (car parked in the sun) in a short time. Hot air evaporates liquids.   



> Because we are taking away the humidity in the humid seasons, it may 

> result in the house being dryer in the early heating season.A strategy 

> of humidification might be considered.I am aware that there is risk if 

> air is flowing through the thermal envelope and cooling to dew point 

> as a result of higher indoor humidity.However, if a well designed and 

> built thermal envelope with a thick layer of foam that is high enough 

> proportion of the total R value such that humidified air that would 

> penetrate to the warm side will be above dew point in the expected 

> coldest conditions, this could work.

> 



Maybe.  On a day like you were discussing, 80's and high humidity 

outside, the dew point is going to be in the high 70s,  With a 

temperature inside of 104 and evaporating moisture that would otherwise 

have been accumulating, the dew point of any intra-wall surface is 

ALREADY going to be below the dewpoint, no cold conditions required.  

Nor is that surface going to have had time to adjust to the suddenly 

increased temperature.  In other words, you might potentially be driving 

moisture into the walls from both inside AND outside.  Not good.



This statement I do not understand.  My understanding is that humid air is drawn to dry air or rather air with low relative humidity.  How can low RH air push moisture in the other direction? Is that what creates vapor pressure?

> 

> With those risk managed, humidification is much lower energy 

> consumption and environmental risk than the refrigerant driven 

> dehumidification.

> 



Got numbers for that?  Do you not have any cheap sources of cold?



Do you mean refrigerant driven dehumidification?  What other controlable sources of cold are there?  Air conditioners and dehumidifiers rely on cooling air to dew point and then reheating to achieve lower RH.  With AC the ambient conditions reheat.  Straight dehumidifier uses another energy source to produce heat to lower Rh.  

> 

> Perhaps leaving the windows open during the day and not pulling cold 

> moist air into the house at night is another way to reduce humidity 

> risk.Though the absolute humidity would be about the same, the higher 

> relative humidity resulting from the higher temperature would be less 

> vulnerable to humidity problems.Although, I contend it is less 

> comfortable.

> 

If you assume a set temperature inside, you are better off letting in 

night air than day air (at least in my climate).  Check out a weather 

graph which shows temperature and dew point (I use weatherspark).  You 

may see that a warm day in a shoulder season starts with the temperature 

at the dewpoint, and as the day warms the temperature rises 

significantly, and the dewpoint follows it but at a much lesser 

magnitude.  So the time of lowest *absolute* humidity is at night.  

Pulling in colder, less absolute humidity air in at night is better than 

pulling in warmer, higher absolute humidity air in the day, particularly 

if that air is warmer than the inside.







My observation using RH and Temp to determine absolute humidity from Psychrometric chart showed that on fair weather days, the absolute humidity stays about the same inside, but RH fluctuates with temperature.  On rainy days moisture is delivered by weather.  On those high RH rainy days of mid 70'sF, ventilation alone is not effective at reducing humidity.  



I checked out weatherspark.  This looks like a great source of information.  I specified a graph that that tracks temp and RH on an hourly basis.  

July 15, Roanoke, VA illustrates my point. 

5am 72F-96%RH, DP 71F, Humidity Ratio .0164#humid air/#dry air

4pm 90F-44%RH, DP 65F, HR .0133#ha/#da

The absolute humidity is higher for minimum temp.  

In this case, I am comparing outdoor minimum temp with outdoor maximum temp.  However, I was observing indoor minimum temp which is higher than outdoor min and max RH inside, which though high is still lower than outdoors. Indoor absolute humidity compares to absolute humidity at peak outdoor temp. 



Looking at these fair weather days, the shape of temperature graph and RH graph are mirror images of each other.   





Nick Pine writes: IIRC, concrete absorbs about 1% of its weight in water as the RH of the surrounding air rises from 40 to 60%, and unpainted wood and paper and fabrics absorb more... http://www.conservationphysics.org/isothrm/isothrm.php I will be studying this diagram.  You can also estimate the absolute humidity with a Clausius-Clapeyron approximation: vapor pressure Pa = RH/100e^(17./863-9621/(T+460)) "Hg, with RH in % and T in degrees F, and humidity ratio w = 0.62198/(29.921/Pa-1) pounds of water per pound of dry air. For example, 70 F air at 50% RH has Ph = 0.374 "Hg and wh = 0.00788 pounds of water per pound of dry air... 50 F air at 60% RH has Po = 0.220 "Hg and wo = 0.00461 pounds of water per pound of dry air. Air weighs about 0.075 lb/ft^3, so replacing 2000 cfm 70 F house air at 50% RH with 50 F outdoor air at 60% removes 2000x60m/hx0.075(wh-wo) = 29 pounds per hour (vs 29 pints per day with a compressor) of water vapor. This is an option, only when outside air is 50F and 60%RH. When does that occur in mixed humid climate? W  >This also gives dew point.  Absolute humidity and dew point are directly >proportional.  I observed that on a hot fair weather day (no >precipitation), the absolute humidity was the same inside and out.  If >inside was 75F-80%RH and outside 95F-45%RH, the absolute humidity and dew >point were approximately the same.  Although the hotter air would be more >affective at drying and avoiding humidity problems. I believe that Smart Vents do this sort of calculation... http://www.smartvent.net/    >I found that the absolute humidity went way up over what would be expected >if no moisture was added to the air.  This was the moisture being >evaporated from house contents inside thermal envelope.  Since the house >was closed up, I would assume minimal infiltration.  After a few hours of >this, I opened up the house to vent out the heat and moisture.  Returning >to night flushing I was able to return house to low 70s and 10% lower RH by >morning.  This returned my house to an earlier time in the humid season. "Smart ventilation" might do this more often with less energy, in times of low outdoor absolute humidity. How do you respond to high RH ambient conditions?  This product is marketed for ventilated crawls spaces, I presume.  As unoccupied, comfort is not a consideration. I suppose if you could easily identify the dry ambient conditions that would dry the house, this would be an alternative to heating the house. However, this would mean ventilating the house when air is low RH. This is usually when the air is hot.  
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