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Supplimental Dehumidification for High Performance Houses in Hot & Humid Climates (Austin, TX)

  This is majorly geeky, but I’ve been thinking about this humidification issue for a few months now.  I want to talk about the next wave of building science thought about HVAC systems for our climate. 
  For most houses in American their Air Conditioner unit does an OK job of controlling the humidity of their house.  But, what about our Austin, TX climate where we’re considered a Hot & Humid climate zone?  Then, what happens to our houses when we make them incredibly air tight and efficient?  How well do we keep the humidity of these homes in control?  Have you ever been comfortable with a temperature but felt hot because of the high humidity.  And the reverse: Have you ever felt cold in your house but it felt clammy (think meat locker)?  Those were all issues where your AC unit was cooling but not necessarily bringing down the humidity level (latent heat load). 
  Well, I’ve come to the realization that the super high efficiency houses I’m building here in Austin really need some supplemental dehumidification.  Dr. Joe Lstiburek a well regarded building science expert says this about Austin’s climate: “All homes in this climate call for supplemental dehumidification; the reduced sensible load of high performance homes reduces the dehumidification the AC unit provides, extends shoulder seasons, and raises the impact of occupant-generated moisture.”  Our HVAC systems need to remove Latent heat and Sensible heat.  I’m simplifying this, but the latent load means humidity, sensible load means temperature on the thermometer.  Our high performing houses are able to hold their temperature better but sometimes need some help removing the moisture (humidity).  Thus the need for a separate dehumidifier with it’s own humidity stat.
  Here’s what I’m now recommending to all my clients:

This is a stand-alone dehumidifier made by ThermaStor.  The unit in the photo above is an Ultra Aire 90H that can very efficiently remove 90 pints of moisture per day.  This is a fantastic unit because it can be mounted in the attic near the HVAC equipment and give excellent supplemental dehumidification to a high performance house.  It includes a MERV 11 filter (very high quality), has a port to bring in ducted fresh air that can be dehumidified and then the dry/filtered/fresh air is ducted to the supply side of the furnace to distribute throughout your house.  Eric Rauser in my office drew up this sketch of how to mount this unit in a conditioned attic of one of our homes.

In my opinion this is a vital component of a high performance home built in Austin, TX.  -Matt Risinger
  For more reading on this topic see the DOE website for their take on this issue in our Hot/Humid Climate.
  Also, read this great Building America sponsored study of homes with different Dehumidifier methods with Building Science Corporation.

PS> If you have an existing house and want to help with your humidity control you might consider buying a small plug in model. I have a similar one to this Frigidaire model on Amazon that earned a Consumer Report’s Best Buy.  Buying with that link will help support my blog as I’m an Amazon associate.  You’ll get the same price and I’ll get a small percent of your sale.  Thanks!