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Ventilation Strategies for Texas Homes – Good/Better/Best

PART 1 OF THIS POST IS FOUND HERE TO READ FIRST.
  We’ve all heard the acronym HVAC – Heating Ventilation and Air Conditioning right?  In general the Ventilation for Texas homes has come from one source, infiltration.  What is infiltration?  This is the air that leaks in around windows & doors.  Most Texas houses have ducts in their unconditioned attic and when the furnace fan kicks on those ducts leak 30-60-90 cfm of air into the attic.  That puts the house into a negative pressure situation and that pressure gets relieved by air leaking in around recessed cans in the ceiling or around vents or outlets on exterior walls.  Texas houses are generally VERY leaky structures.  Infiltration is a very poor way to Ventilate  a home.  This method brings with it all the pollutants from the air source.  Have you ever noticed dust marks on the drywall around ceiling registers?  That’s probably dust and fiberglass deposited by attic air leaking in.
  BUILD Tight – Ventilate Right should be our motto.  We want to build the tightest shell possible then introduce fresh air on our terms.  If you’re in the Hot/Humid Southern US, it should fresh air that’s been filtered and dehumidified. So, let’s get down to the strategies to achieve this for our climate zone.  Here’s my Good Better Best:
Good: Central Fan Integrated Ventilation with Continuous Exhaust
This is my Risinger Homes standard and this system works very well. 

 

From Building Science Corporation Article Linked Above
Aprilaire controller set for 10 min per hour of fresh air for this house.

Better:  Good System Plus Ventilating Dehumidifer
This is the same system as above but adds an Ultra-Aire Ventilating Dehumidifer to the HVAC system (instead of the portable plug-in model in the good system).  The fresh air pipe goes to the Dehum first then to the return side of the furnace for distribution.  This is really the premier system in terms of comfort for a house in our hot/humid climate zone.  This system is a bit more expensive but is vastly more comfortable for my clients than any house they’ve lived in before.  See this video for an overview of a house with an Ultra-Aire XT150 installed.

Ventilation piped to the Dehum first.  I’m with Architect Eric MacInerney in a house we built with Heimsath Architects. 

Best:  Better System Plus the Addition of a ERV & HEPA filtration
In this system we un-couple the Dehumidifier and the ERV so they are separate and fully independent ducted systems not connected to the furnace.   This system has several advantages.  The ERV is able to move some of the moisture (energy) between the incoming/outgoing air streams so the fresh air is dehumidified before entering the house.  The ERV has a small efficient fan to bring in outside air and exhaust stale air.  The ERV has separate controls and can have a carbon/particle filter to remove pollen & ozone before entering the house.  HEPA filtration is also easy to add to the ERV if necessitated by clients with Asthma or Compromised Immunity.

Courtesy of Building Science Corporation Article “Read This Before you Ventilate”

You will notice that in all three ventilation strategies there is a supplimental dehumidification component.  I found this interesting exerp from a Building America Ventilation Guide in relation to our Humid/Hot climate and the ASHRAE 62.2 Ventilation Standard:
If the target ventilation rate is to be maintained in a humid climate, active dehumidification should be considered. Consider a 2,500 sq.ft., four-bedroom home. The required ventilation rate would be 63 CFM. In a hot-humid climate like Sarasota, Florida, according to the ACCA Manual J Table 1, the outdoor 1% summer design condition is 92°F with a coincidental wet bulb of 79°F. The moisture content of the air is 129 grains per pound of air. A pound of air occupies about 14.3 cubic feet. So our target ventilation will move about 4.4 (63/14.3) pounds of air into the building per minute. Also, with it comes 5,676 (4.4×129) grains of moisture. In an hour, we have transported 340,560 grains of moisture. There is 7,006 grains of moisture in one pound of water (roughly one pint). Thus, the target ventilation will bring in 48 pints or 3 gallons of water during one-hour of operating time; that is equivalent to moving 72 gallons of water through the home in a 24-hour period.

With no active dehumidification, the home accumulates water until the grains per pound of air indoors is equal to the grains per pound of air outdoors. In the Sarasota, Florida, area where the moisture levels are 129 grains per pound of air outdoors, it will be 129 grains per pound indoors. If the home had 8-foot ceilings, it would contain 20,000 cubic feet or about 1,428 pounds of air. At 129 grains per pound, there would be about 26 pints of water in vapor form in the air at all times. If the AC system was oversized and it could easily maintain 75°F with very little run time, the indoor relative humidity would be 98% and the dew point would be 74°F.
The discussion above sounds extreme but if care is not taken to implement a good dehumidification strategy in humid climates, durability issues can result. There is no doubt that ventilation is required; standard construction practices now result in very tight buildings. Without ventilation, there would be no way to remove indoor pollutants. However, we must understand that when we move outdoor air through the building to ventilate to remove pollutants, in humid climates we are also moving water through the building at the same time. A dehumidification strategy should go hand in hand with the selected ventilation strategy.

Thanks for reading this pretty geeky post!  I also want to thank Joe Lstiburek and the good folks at Building Science Corporation who have some amazing building science research all posted free for the world to learn.  I wish you the best in your building endeavors. 
Remember BUILT Tight – Ventilate Right!
Best,
Matt Risinger
Risinger Homes is a custom builder and whole house remodeling contractor that specializes in Architect driven and fine craftsmanship work. We utilize an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy and durable homes.
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