[Greenbuilding] Drying House in Humid Season

Topher topher at greenfret.com
Mon Nov 11 11:29:56 CST 2013


On 11/11/2013 11:13 AM, conservation architect wrote:
> Absolute humidity and dew point are directly proportional.
I would phrase this as they are measuring the same quantity with 
different units.

> This was alarming, so I decided to heat the house above comfort in 
> order to dry it out.
I find this a generally confusing way to talk about this process.   
People often talk about 'drying out the air' with heat, and this is just 
wrong.  'Drying out the air' requires *cold*. I try to say things like, 
increase the temperature to evaporate more water (which can then be 
flushed from the house).
> 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.

> 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.
>

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).
>
> 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.

> 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.
>
> 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?
>
> 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.
>
> Very interesting subject.
>

Yup.  I explained dew point to my 6 year old niece, and I told her once 
she got it internalized, she would be halfway to being able to replace me.

Thank You Kindly,

Corwyn / Topher Belknap

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.GreenFret.com/
topher at greenfret.com

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