Wednesday, December 26, 2007

House Ventilation

Now that the insulation is in, the house has few ways for air to get inside or out. This is good because air will not infiltrate through holes in walls, floors, or duct seams in walls and crawl spaces. Air tightness makes heating and cooling more efficient since outside breezes won't blow air in.

The next steps address increasing tightness for heating and cooling energy efficiency, and whole house ventilation so that air exchange keeps air fresh and healthy.


Aside from sealing ducts and installing vapor barriers and weatherstripping, this house is tighter because of insulation characteristics and the characteristics of the heating/cooling system.

The insulation is predominantly blown-in insulation. We have used icynene insulation. Blown-in insulation does a better job of sealing stud cavities and ceiling joists than batt insulation, particularly when there is wiring in a wall or ceiling. Insulation, vapor barriers and weather stripping are not the only things impacting tightness - the heating and cooling system impacts house tightness as well.

Why does the Heating/Cooling System impact Airtightness?

Traditional heating and cooling is done with forced air (using hot air or frigid air). Large temperature extremes in heated or cooled air cause it to expand or contract - and since a house cannot contain this pressure, it breathes air in and out. It doesn't matter if the house is well sealed or not - air will be expelled or drawn in anyway.

For example, if a furnace heats air from 70F to 150F, this expands its volume by about 13%. If 1/10 the volume of the house is affected, this will displace 1.3% of the volume of the air in the house: it will force it out when heating, or suck it in during cooling, whether or not the house is tightly sealed. A 2500 square foot house would breath about 260 cubic feet of air each heating cycle!

This house's heating and cooling system uses low-temperature difference hydronic - so heating or cooling operates over longer periods of time Heating is done with lukewarm water (95F) and cooling is done with cool (not chilled) water. The result is that the air inside the building expands or contracts less, and the house doesn't breath as much as a forced-air heated or cooled house. Using smaller temperature variations reduces the amount of air breathed in and out.

By comparison, we might expect that if heating goes to only 85F (radiant floor), then the effect would be roughly 1/5 as much as above, or 50 cubic feet of air. So, by using lower termperature vairations in heating and cooling, the house breathes less, and the house truly is tighter.


One of the downsides of a tight house is that it doesn't leak air, you have to design in ventilation. This house's ventilation has summer and winter modes of operation. In either case, we strive to be energy effiicient - either using ventilation to cool the house in summer, or use heat recovery for winter ventilation.

Summer ventilation is accomplished by sucking (warm) air from the ceilings of the second story of the house and venting it outside. This is easy because the second story air return ducts are in the ceilings (see heating and cooling system design).
Cool fresh air is introduced into the first floor by means of an EarthTube. Since the air is both blown out and drawn in with low-power fans, air pressure in the house is near neutral. Whole-house ventilation is naturally achieved since summer heat causes air to rise, where it is ultimately vented out. Summer ventilation doesn't increase house cooling requirements because the EarthTube cools the incoming air.

During winter air inside the house is warmer and more humid than outside air. Straight ventilation would introduce uncomfortable cold drafts, and extra demands on the heating system. A heat recovery ventilator from FanTech is used to heat and humidify incoming air using the outgoing air stream. This air exchange is balanced, so that air pressure in the house is roughly the same as air pressure outside the house. Warm humid air is less likely to seek a path out through walls, windows and doors.

Finally, exhaust vents such as the kitchen hood, bathroom fans and fireplace should not have to fight a tightly sealed house. In our house the EarthTube (summer) and/or HRV inlet (winter) can be activated when the exhaust fans turn on, guaranteeing that air drawn into the house is coming in through desirable intakes.

Ventilation is an Art

Compared with Heating or Air Conditioning, Ventilation is unfamiliar territory in residential construction. As pointed out in a posting on the GreenerHouse Google Groups, it is so new that there are seminars to educate the public (not to mention contractors). The speaker, Judy Robertson, presented a talk "Putting the V back in HVAC" and pointed out that residential contractors are experienced with HAC - not HVAC! Some interesting bits from the seminar:
  • Green builders and homeowners are desparate to find residential HVACcontractors. Today's residential heating/air-conditioning contractors are not familiar with ventilation. Our HVAC contractor is a crossover contractor - they do part commercial and part residential. Several people asked me for our HVAC contractor's contact information.
  • The larger Fantech ventilation fans are quite noisy but can also move a lot of air, while the smaller Panasonic fans are whisper quiet by comparison. I intend to do a test run with the house ventilation fans to see how they fare.
  • Piping inlet air directly to desired rooms is important, because air flow rates for ventilation are a lot lower than those typical of forced air systems. Very important for bedrooms, since so much time is spent there. Check this in your house!
  • Heat recovery ventilators are less efficient with lower delta-T. It is important to duct exhaust air though conditioned space as much as possible prior to connection to the Heat Recovery Ventilator.
  • The amount of ventilation (air exchange) required for a building is a function of many things. ASHRAE has a minimum value based on square footage. The baseline air exchange rate we had chosen barely met this number.
There are air quality / energy tradeoffs between insufficient and excessive ventilation. If is was possible to automatically measure objective and subjective air quality, this would be easier.

1 comment:

  1. I feel more folks should be required to read this.Finally exhaust vents such as the kitchen hood bathroom fans and fireplace should not have to fight a tightly sealed house.Really great info. It gives us free valuable information..and when I see posts like this.

    PVC Ventilation Ducting


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