Letters: July/August 2007

July/August 2007
A version of this article appears in the July/August 2007 issue of Home Energy Magazine.
Click here to read more letters.

Fiberglass or “Filterglass”?

I liked the article about your energy-efficient home in a recent Home Energy (“Design, Construction, and Performance in Ohio,” Jan/Feb ’07, p. 36).  Sounds like a well-thought-out home.  The only comment I have is regarding your use of fiberglass in the walls.  I was an energy auditor for a Vermont weatherization program for six years and have done hundreds of infrared scans on fiberglass-insulated homes.  Fiberglass, or “filterglass,” as I like to call it, just does not live up to its billing unless it is installed in an airtight cavity.  Now your walls sound pretty airtight, but I think you would have been much better off using spray-applied cellulose in the walls (the 2-inch exterior foam board is excellent, by the way).  I think we have to send a message to the fiberglass manufacturers and to the builders still using it that their fiberglass batts are an inferior product. 

I built my home with fiberglass insulation before I learned these things and have since tightened my 2,000 ft2 home to 600 CFM50.  However, on a cold day, all of my outlets on exterior walls are pitch black and a source of air leakage.  I was very conscientious about installing the fiberglass.  Your type of construction, with blue board and airtight Sheetrock, is probably the only way you can get away with using fiberglass. 

By the way, have you had any problems with CO in your home due to your attached garage?  

Geoff Wilcox
Weatherization Technician
Vermont Office of Economic Opportunity

Author Allen Zimmerman replies:
I appreciate your kind words about the house. I had a number of other readers of Home Energy respond with positive feedback about the article.  Regarding your comments about the insulation: In all my courses, students are required to be able to list and thoroughly discuss the advantages and disadvantages (pros and cons) of alternative materials, procedures, and techniques for specific applications. After careful consideration, individuals can then make an informed choice. This is the procedure that I used in selecting insulation for my house, and you will note that the result was three different types of insulation, depending on the location and application. 

As you noted, the airtight wall construction techniques negated the problem of air intrusion and the concurrent lowering of the R-value of fiberglass.  Therefore, the result of my analysis was that the increased cost of the spray-applied cellulose was not justified in this design.

We have not had any problems with CO in the attached garage, but of course always practice safe techniques in this regard. Thanks again for contacting me and sharing your thoughts.          


In an article in the Solar and Efficiency Special Issue, “HERS Inspections of New Homes” (p. 7), the opening photo caption wrongly identified the duct-testing device as a Duct Blaster. The device in the
photo is a Retrotec product,
the Duc-Tester.

In the article “Saving Water Indoors,” (Water/Energy Special Issue, p. 30) the total cost of toilets in Table 5 should be $726.

Surprising Turnabout

To quote the first sentence in the article “Energy Star Changes Approach to Programmable Thermostats, (Mar/Apr ’07, p. 10),  “Energy Star has promoted programmable thermostats since 1995, estimating that consumers will save 10%–30% on their heating and cooling bills.” My own experience has shown this estimated energy savings to be on the high side. Savings will vary based on the setpoint/setback temperature, hours of setback, and efficiency of the building, among other things.

For my own house, with a 95% efficient gas furnace and weekly nighttime setbacks from 70ºF to 62ºF (60 hours total) and work week setbacks from 70ºF to 62ºF (40 hours total), total weekly setbacks 100 hours, midwinter savings are about 5%. This house in its original configuration consumed about 4 Btu/ft2/HDD (heating degree-day), and with an ongoing energy retrofit is now using about 2 Btu/ft2/HDD. I would think a less-efficient home would enjoy a larger percentage of energy savings by setting back the thermostat, as the setback temperature would be reached sooner. I love the Honeywell programmable thermostat I am using but will admit that my brother Dave, a programmer/analyst, did the programming.

On a related note, to take full advantage of a deep thermostat setback, proper furnace sizing is critical. The statement “A properly sized furnace will run nearly full time on the coldest day of the year” does not take into account an aggressive setback. The recovery (pickup load) is substantial in a cold climate, and getting the house back to the setpoint will take forever with an undersized furnace. A two-stage furnace does add some flexibility to the equation, but exactly the right-sized furnace for a given home will maximize energy efficiency and allow for deep thermostat setbacks.

For the record, my house has a heat loss of 390 Btu/hour/ºF or 31,590 Btu/hour at the design temperature for Minneapolis. The furnace is a 66,000 Btu input high fire and 45,000 Btu input low fire, and once the house is up to temperature, the furnace will maintain 70ºF on low fire on all but the very coldest days. It is my understanding that the two-stage furnace saves energy with the electrically commutated motor (ECM) running at low speeds. I track both gas and electric use and can see lower kWh used with the new furnace.

Jim Wickham
via e-mail

An Exhausting Problem

I saw your article “Oversized Kitchen Fans—An Exhausting Problem” (Jan/Feb ’99, p. 37) online about kitchen fan size and makeup  air. We are planning a new kitchen and are looking at getting a 36-inch five-burner gas cooktop along with an external vented fan, as we cook a lot.

Can you give me advice regarding fan sizing?  On one side I understand I may only need a 120 CFM fan and on the other I need a 900 CFM fan—which is correct?  Splitting the difference, I found an external-mount fan with 435 CFM—would this be appropriate?

Secondly, regarding makeup air, is the appropriate rule that I need a makeup air inlet the same diameter as the exhaust vent?  Does this need to have a fan also (for our purpose—I understand that commercial units have fans) or will sufficient air just get sucked in?  If we need a fan, can you recommend a quiet inline fan, and should the fan CFM match the exhaust CFM or is some differential OK?  We live in New York City (gets hot and cold) so would ideally want to be able to close the vent altogether when the kitchen fan isn’t on, to prevent drafts.

Max Horn
via e-mail

Executive Editor Ian Walker replies:
For commercial kitchens with continuous use and a desire to limit odor spread, large air flows are necessary. However, for a residential kitchen this is not the case.  I recommend using a relatively small fan (you mention 120 CFM and that is fine).  Then you will not need the makeup air inlet or fans that you would need with a bigger fan, and the smaller fan will likely be quieter.  This smaller fan would meet the requirements of ASHRAE Standard 62.2 that governs ventilation for acceptable indoor air quality in homes. 

For a quiet fan, look for a low sone rating in the HVI product directory (www.hvi.org/associations/4692/files/CPDFeb07FullDirectoryRev.pdf).  I recommend a fan in a hood over the cooking surface, as this is much more effective than the downdraft fans that have to move huge quantities of air to achieve similar effectiveness.

Getting Even with Your Heater

Thanks for an excellent source of weatherization and building science information.

I recently spoke with a colleague who is a HERS rater. He is consulting on a project in Maine, and the homeowner/ builder doesn’t know if his small 40,000 Btu gas stove, without any distribution system, in the central living room will effectively reach the far corners of this 1,400 ft2 tri-level condo in Portland, Maine. It has an excellent building shell and will be very tight. The problem is, though we know that the 40,000 Btu capacity stove will be enough to keep this home at 70ºF even on the coldest days, how uneven will the heat be?

Will the living room have to be brought up to 75ºF just so the third-level bedrooms get up to 65ºF? This house won’t go through much fuel, but how uncomfortable will it be due to the uneven temperatures? How do you reasonably project the level of even temperatures and comfort in this house that has no mechanical distribution? Can you? Are there any houses that work well without central ducting, fans, and so on?

Kevin Hanlon
Concord, New Hampshire

Technical Editor Steve Greenberg replies:

This is a question that a good simulation model should be able to answer in some detail. But one observation I have from experience in spaces with natural-convection circulation is that since warm air rises, there will be significant stratification such that the hottest rooms will be on the highest level, and the coldest on the lowest level, even though that’s where the heat’s coming from. If you are near the hot metal of the stove, radiant heat will contribute to your thermal comfort and may balance the fact that the air is at a lower temperature. Any closed doors blocking access to the heat source will effectively block heat distribution.

Other considerations for a tight building:

  • Where’s the combustion air for the stove coming from? Ideally, the air is ducted from the outside.
  • How are cooking and bathing vapors being exhausted?
  • How is ventilation air being provided?

Inverted Question

Could you please answer a question for me? Living off the grid, am I better off using fluorescent 12V DC lights or 120V AC fluorescent lights through my inverter? Ohm’s law seems to say I will use less power through the inverter, with a 26W 120V light using 0.2166 amps with a 0.5-amp inverter draw for a total of 0.7166 amps, and a 26W 12V  DC light drawing 2.166 amps. Somehow logic tells me this has to be wrong. What am I missing?

Bruce Feaver
via e-mail

Steve Greenberg replies:
Your logic is good. You’re actually applying Watt’s Law (power = volts x amps), but then looking at current instead of power, when power is what matters. If you’re using a 26W lamp, and it actually draws 26 watts from either the 12V  or the 120V  system, to first order it doesn’t matter: the system is delivering 26 watt-hours of energy for every hour you use the lamp. But there are likely to be significant second-order effects.

The first question is, what is the real input power for the two? Is it really 26 watts in both cases? There may well be differences in ballast efficiency and in how hard the ballast drives the lamp. In the case of the 12V version, there is likely a slightly bigger power loss in the wiring from the source to the lamp (depending on wire sizes and lengths). In the case of the 120V version, there’s the extra inverter loss to account for. It wasn’t clear where you got the 0.5-amp inverter draw; typically inverters run in the 90%–95% efficiency range, unless they are very lightly loaded. Thus if you draw 26 watts at 120 volts through the inverter, and the inverter is 90% efficient, the DC power delivered to the inverter is 26/0.9 or about 29 watts. If the lamps really both run at 26 input watts to the ballast, you’re likely to be better off with the DC version.

Heating Myth?

I have hot water heat in my home. My contractor says it would be less expensive to keep the thermostat at one temperature versus lowering the temperature during times when I’m not at home. Uses less energy.  I say that is incorrect; that the real issue is the temperature outside and other factors. If it is 30ºF outside and 60ºF inside, it takes a certain amount of energy to maintain the 70ºF inside temperature. If it’s 30ºF outside and 70ºF inside, it takes that much more energy to maintain the additional 10ºF inside.

The contractor maintains that the energy spent getting from 60ºF to 70ºF would offset and pass that saved using the programmable thermostat and only raising the temperature when I’m at home. Who is right?

Kevin White
Elkhart Lake, Wisconsin

Steve Greenberg settles the question and busts a myth:
You are correct. It’s true that it takes more energy during the recovery period from setback than it would have during that period to maintain the temperature (since you are not just maintaining a steady-state heat loss but also increasing the temperature of the air and thermal mass of the building). But during the cool-down period when the setback was initiated, that energy was released into the space to help meet the (lower) load, so the extra energy put in during warm-up is recovered during cool-down. And of course it takes less energy during steady-state conditions to keep the building at a lower temperature inside. A properly used setback temperature control does indeed save energy.

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