Thursday, January 5, 2017

Design - Indoor Air Quality

According to one of the contributors to Christina Fisanick's book, "Eco-Architecture (Opposing Views on the Merits of Green Building)", the EPA says that outdoor air is 2 - 5 times healthier than the average indoor air.  Any house like ours that will be sealed well enough to be energy neutral would have an indoor air problem without purposeful control of pollutants.

Indoor air quality can be manged by a combination of two strategies:  controlling the amount of internal pollution in the first place followed by continuous replacement of stale air with fresh air.

Control Measures
  • Combustion ventilation:  Vented range hoods; vented water heaters; heating stoves with piped in make-up air and sealed combustion chambers; no open fireplaces
  • Moisture control:  Range hoods; bathroom exhaust fans; dryer vents; dehumidifiers if necessary
  • Low or no volatile organic compounds (VOCs):  Formaldehyde-free building materials; low or no VOC paints, finishes, carpet and upholstery; outside storage for and use of VOC cleaning supplies and shop chemicals such as acetone and paint thinner
  • Radon mitigation during construction or as a retrofit
  • Air barriers between attached garage and living quarters; no duct work penetrations

  • Enough strategically placed operable windows for adequate ventilation when the     outside environment cooperates
  • Heat recovery ventilator (HRV) or energy recovery ventilator (ERV)either free-standing or tied into the central HVAC to replace 30 - 50% indoor air with outdoor air every hour 

How Does Our Design Stack Up?
We will have a range hood matched to the BTU output of the burners but I remain ambivalent as to its configuration.  Venting to the outside is best in a normal situation but the disadvantage of venting is that, in winter, a lot of heat is lost.  Since we will have an ERV, maybe it makes more sense to use a hood with high-end filtering to comb out the worst pollutants then duct the ERV close enough to the hood to remove the remainder.  I need to do more research and consultation on range hoods before we make choices.

We will have no other sources of open combustion such as stoves and fireplaces but will have a gas tankless water heater and gas clothes dryer properly vented to the outside.  Since we are building new, we expect to have good control over VOCs.  Radon gas will be intercepted and led to daylight by the gravel backfill in the French drains and the AGS conduits and we have used plastic sheeting under the slab.  Air barriers between the garage and living quarters with no un-caulked penetrations are not only required by code but make perfect sense and we will be in compliance.

We are deliberately limiting operable windows to the least number necessary for proper ventilation because fixed panes are cheaper and are less likely to leak air. Since there are a lot of days here in the St Louis area when it is too cold or too hot or too humid to open the windows, we will depend mostly on an ERV to exchange +/-30% of the indoor air each hour year-round.  (Heat Recovery Ventilators (HRV) are better suited for colder, drier climates; the ERV functions like a HRV but also helps to control incoming humidity.)

Energy Recover Ventilator
Ours will be a free-standing ERV since we will have no conventional heating or air conditioning with which to integrate it.  The installation will be so unsophisticated that it becomes a reasonably easy DIY operation with perhaps some professional help in balancing the system.  The ERV will ventilate the bathrooms, eliminating the need for separate bathroom fans and, as discussed above, will likely replace a vented range hood.

As a value-added feature, the ERV will be situated so that it pulls air through vents in the partition between the living quarters and the earth contact wall so as to bring more air into contact with the wall than would otherwise be possible, thereby enhancing the performance of the AGS system.  
ERV in summer mode.  Hot, humid incoming air is cooled
and dried by the outgoing air by passing near each other
in the heat exchanger without actually physically mixing