This article was originally published in the November/December 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online November/December 1997


Breaking the Habit at Habitat Linda Wigington's article Creating an Energy-Efficient Habitat (July/Aug '97, p. 23) did an excellent job of showcasing Habitat for Humanity's approach to building affordable housing and describing the important role energy and resource efficiency plays in housing affordability. Wigington states, though, that institutionalizing energy and resource efficiency is still a way off. The U.S. Department of Energy (DOE) is currently working with Habitat for Humanity International (HFHI) and Habitat affiliates to make sure this is not true.

DOE is supporting the Oak Ridge National Laboratory, Southface Energy Institute, Florida Solar Energy Center, National Association of Home Builders Research Center, and others to bring energy and resource efficiency to the forefront of affiliates' attention. We have provided numerous training sessions at Habitat regional workshops and to individual Habitat affiliates, with more planned for the future. We developed an energy efficiency checklist and HVAC contractor guidelines for the recently completed 1997 Jimmy Carter Work Project (JCWP). HFHI Green Team members used these materials to ensure that energy efficiency was adequately addressed during construction of each of the 52 JCWP houses and to educate affiliate leaders, construction volunteers, and homeowners on energy efficiency principles. We have also provided direct design assistance to Habitat affiliates that has included reviewing construction plans and sizing equipment. We are currently preparing technical bulletins and energy-efficient construction plans and guidelines to further institutionalize energy concepts.

The partnership we have created with HFHI is intended to assist them in moving quickly forward on their idea of sustainable construction. On a larger scale, the energy-efficiency materials, approaches, and concepts that we develop with Habitat will be useful for many more providers of affordable housing.

Mark Ternes
Buildings Technology Center
Oak Ridge National Laboratory
Oak Ridge, TN

More Clarifications I would like to note several clarifications on the recent article written by Stevens and Lubliner on ventilation in manufactured homes (July/Aug '97, p. 7). That article compares annual costs of furnace-based, whole-house ventilation systems with a continuously operating exhaust fan.

The intent of the article was to show that a continuously operating exhaust fan costs much less than an intermittently operating furnace-based system while delivering equivalent mechanical ventilation. Several details should be mentioned which are obscure or left out of the article.

  • The 1994 revisions to the HUD Manufactured Home Construction and Safety Standards did not eliminate passive inlet vents. Operable window vents or inlet ducts into the furnace cabinet are still both acceptable air inlets. However, our tracer gas research indicates these intentional inlet vents usually have only limited impact on the overall effective ventilation rate of the home, on the order of 0.01 to 0.02 ACH. In the case of window vents, the vents are often closed and do not often add to the effective leakage area of the house. The slot vents can admit as much as 5 CFM under certain circumstances, but the long-term average is much less than this. Similarly, the air inlet vents to the furnace may or may not have the intended effect, depending on whether they contain dampers, and whether the dampers are intentionally open or closed (or jammed shut). In summary, passive vents by themselves do not cause the house to be ventilated to levels suggested by air quality standards such as ASHRAE 62.
  • My second point concerns the pressure relief valve required by HUD to prevent pressurization of homes in temperate climates. As with window and furnace vents, the pressure relief valves now deemed to satisfy HUD's requirements actually add very limited net free leakage area to the shell of the house (on the order of 6 in2, in a house with an effective leakage area of at least ten times that amount). The pressure relief valve responds to a federal requirement, but not to house physics.
  • Most furnace-based systems rely on the air handler fan to circulate new air entering through the furnace air inlet; this fan accounts for 75% of the annual cost of running this ventilation system. If the ducts are very leaky, much of the new air will never make it into the house, but the 300W-500W furnace fan will run anyway, often on a duty cycle of eight hours or more. Especially in cold climates, this creates a comfort problem in addition to the added cost of using a large fan to circulate a small amount of fresh air. In some parts of the Northwest (such as Idaho) the furnace fan has been taken out of the loop to alleviate potential comfort problems.
What the reader should come away with from this article is that furnace-based ventilation systems, because they operate only intermittently and use a large fan to circulate air in most homes, result in very little long-term pollutant dilution or removal in a manufactured home. Continuously operating moderate-volume exhaust fans designed to run for long periods with quiet operation do a much better job of pollution dilution and removal.

Bob Davis
Ecotope, Incorporated
Seattle, WA


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