[Greenbuilding] Drying House in Humid Season

[conservation architect]

It has been noted on this list the problems associated with night flushing in humid climates such as here in Virginia in the summer.  I have engaged in this practice for many years, because it brings down the temperature of the house to low 70s most nights and keeps the house below 80 most days when outside air temp is in 90s.  My house has most of a high standard retrofit continuous foam thermal envelope.  I have an HRV.  I choose not to get ERV because I did not plan to install active dehumidification.   To do so,  I tolerate sometimes 80% relative humidity in mid-summer.  I am not willing to accept that one must have refrigerant driven dehumidification to achieve healthy conditions.  However, for my clients that expect a full comfort standard, this is what I must specify.  The people who choose to live without air conditioners acclimate to higher amounts of ambient humidity.  I went to primary school in Central Florida with no air conditioners.  None of us complained because ac was not yet expected in 1960s.  



The last few years I have been observing more carefully relative humidity, where previous years I only observed temperature.  I have observed that a house as a whole goes through a cycle of steadily becoming more humid through the summer and peak in early fall and then go into a drying mode when heating air.  The drying peaks in early spring (end of heating season).  It is my observation that the house as a whole acts somewhat like a desiccant in that it adsorbs humidity in the early part of summer as the conditions deliver humid air to the space.  The house's initial dryness in early spring dampens the air relative humidity in early part of humid season.  However, steadily, the house's capacity to adsorb humidity is maxed out and air relative humidity rises farther.  This creates the slow drying conditions of contents that cause the problems associated with humidity.  This past year, I have added absolute humidity to my observations with the use of a psychometric chart using temperature and relative humidity.  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.  



This was a particularly humid summer. Early summer was rainy and cooler than normal.  This resulted in elevated humidity.  One day I observed 71F-88%RH in the house.  This was alarming, so I decided to heat the house above comfort in order to dry it out.  Like a clothes dryer, I was elevating the temperature to lower the relative humidity and increase evaporation.  That day was in mid 80's, so was not an extremely hot day.  Since I was not going to occupy the house I turned the HRV off to increase my temperature to avoid delivering new humid air. I lit the direct air delivered wood stove to a mid-winter full size fire.  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.  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.  



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.   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.  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.  With those risk managed, humidification is much lower energy consumption and environmental risk than the refrigerant driven dehumidification. 



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.  



Very interesting subject.  I hope to hear many insightful comments from the experienced and knowledgeable participants from this list. 



Eli  




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