Letters: March/April 2009
How Tight Is a Passive House?
I love your magazine and Web site. You inform me about cutting edge technology for advising, building, and renovating—backed up by tests and studies—it isn’t guessing. That is why I was troubled by your recent sidebar on a Passive House (“First U.S. Retrofit to Passive House Standards,” Nov/Dec ’08, p. 25). Your standards for an airtight house are minimal at 0.6 ACH. For EarthCraft and Energy Star standards we are looking for 0.6 ACH (natural) in a renovation. We are looking for 0.35 ACH in a new house. An ACH of 0.6 in a house is not tight by Energy Star standards or by me for new housing. I do have an engineering degree, but I am recognized for my common sense in building applications to obtain cost-effective energy efficiency. I say this because I don’t want you to think I am just finding fault with numbers. Keep up the good work.
Hampton Roads, Virginia
Author and architect Nabih Tahan replies:
I think that you are confusing natural airtightness—ACH (nat)—with airtightness under pressure. The Passive House requirement is 0.6 air changes per hour at 50 Pa (ACH50) of pressure difference. It is like trying to find a hole in a bicycle tube when the tube is sitting there naturally versus pumping it up and putting it in water and seeing where the air is leaking. During a blower door test, the house is pumped up and put under pressure so that it is easier to find the leaks and plug them.
I found two archived Home Energy articles that discuss this issue. The first one is “Introduction to Blower Doors” by David Keefe (HE Jan/Feb ’94, p. 17). You can find it on the Web at www.homeenergy.org/archive/hem.dis.anl.goveehem/94/940110.html. If you look at the end, under blower door terms, it defines ACH, ACH50, natural ventilation, and so on. The second one is “Infiltration: Just ACH50 Divided by 20?” by Alan Meier (HE Jan/Feb ’94, p. 35). You can find it on the Web at www.homeenergy.org/archive/hem.dis.anl.gov/eehem/94/940111.html.
This article explains how to convert from ACH50 to ACH (nat). The rough rule would be ACH (nat) = ACH50 divided by 20. The Passive House requirement of 0.6 ACH50 divided by 20 would be 0.03 ACH (nat). That is very tight, and it’s the only way to be able to trap the air and its heat and recycle it through a heat recovery ventilator. All the best.
Passive House Handles Grease
While reading the recent article “First U.S. Retrofit to Passive House Standards” (Nov/Dec ’08, p. 25), I noticed in one of the photos and even more clearly on the cover photo that a recirculating-type range hood was used.
I realize that this setup causes no depressurization to the building envelope and that that is a good idea. However, the output for the hood exhausts only inches below the ceiling. Over time, and depending on the type of cooking that is done, wouldn’t a grease spot develop on the ceiling (or a moisture problem)? I can’t believe that the filter(s) in this system remove all of the grease and oils (or water) in cooking vapors.
The reason I ask is that I am thinking about upgrading my own kitchen. It currently has a very old Jenn-Air system, and there is no convenient way to route venting from an updraft-type hood to replace it. Also, the cabinetry was designed around the Jenn-Air system, so there is not enough room under the cabinets over the stove top area to mount a conventional hood of any type.
Any comments on the possible ceiling grease/moisture issues would be welcome. (Any other suggestions are also welcome!) I realize that HE is not “Ask This Old House,” but because of the possible ceiling grease issue I felt that an e-mail would be appropriate. Thank you.
Syracuse, New York
Nabih Tahan replies:
We have been using our recirculating hood for 11 months, and have not yet noticed a grease spot on our ceiling. We have even cooked Wiener schitzel many times. It’s my kids’ favorite Austrian meal—breaded turkey fried in oil.
I wash the filter with warm water every three months. Our energy recovery ventilator has a red light that comes on every three months to remind me to clean the filters, so I clean the hood filters at the same time, and it seems to be enough. In our case, where we are trying to trap and recycle as much heat as possible, it would not have made sense to let that heat escape and then turn on the heater. It was worth the risk of having to paint the ceiling more often, which so far does not seem to be an issue.
I hope that helps you make a decision. It does not seem like you have too many options either way.
I just read your article “Fixing California’s Existing Homes” (Nov/Dec ’08, p. 16). Good job.
We are a HERS provider in North Carolina. We are in need of some software like the CHEERS version for existing homes. Since the CHEERS software is based on the California Title 24 building standards, what limitations do you think we may have for its use in North Carolina? Can you recommend any other similar software? We would like to use it for existing-home upgrades, energy efficient mortgages, and so on.
Ideally it would be good to find some software with simple inspection data input (good data library) that also produces a good customer report.
Energy Wise Solutions
Augusta, Georgia; Charleston, South Carolina; and Raleigh, Charlotte, and Boone, North Carolina
Author and software guru, Steve Mann replies:
Thanks very much. I’m glad you liked the article.
Unfortunately, the CHEERS software won’t work for you because it doesn’t calculate energy savings quite the way the national RESNET program wants, and its reporting is geared to the California Energy Commission’s requirements. However, if you read my previous article about REM:Rate (“REM:Rate 12.5—A Versatile Energy Application,” Sept/Oct ’08, p. 10), it sounds like it could fit your needs quite nicely. Another alternative is EnergyGauge (www.energygauge.com). It’s similar to REM:Rate but less expensive. I’ll be taking a look at it in the coming months.
Best Heat Strategy
My elderly mother will be moving in with me soon. She will have a living room and her bedroom. I live in an old house in
southeastern Ohio, and I have to keep the thermostat at 68°F or so when I am home, cooler when I’m at work. She is always cold when she visits, so we will have to have a heater in her rooms (the rooms are side by side; you walk through one to get to the other) to keep her warm enough. I am a nervous wreck about finding a heater that will be safe and not send my electric bill through the roof (I heat with natural gas). I have read that some power cords get hot after an hour or so of use. She may let her room heater run from November to March! Can you recommend a safe heater for ongoing use like this?
I’ve read Consumer Reports, but someone who said to be careful with it because the power cord kept getting hot used one of the ones they recommended. And there wasn’t any mention in the review of constant use. I have spent many hours researching on the Internet and still have not come up with anything I am really comfortable with. The Eco-heater sounds safe, but I haven’t found reviews on it yet. I had heard Eden Pure and those like that are safe, but the reviews are problematic and they are expensive! I would hate to put $350–$450 in a heater that seems to have so many problems and does not rate well. Anyway, I would really appreciate some professional advice.
Technical Editor Steve Greenberg replies:
It may well be less expensive to heat one or two rooms with an electric space heater than the entire home with a central furnace. That said, there’s no such thing as an inexpensive way to heat with an electric space heater, but you can minimize the energy use by making the spaces more energy efficient with the usual weatherization techniques: air sealing, insulation, and window treatments. In addition to reducing heat loss from the room, these help improve thermal comfort at a lower air temperature, since they minimize drafts and increase indoor surface temperatures. Warm clothing is another way to minimize the temperature needed to stay comfortable at lower temperatures.
Given that two rooms are involved, using two heaters and keeping only the occupied room at full temperature will also save energy.
There are more-efficient ways to spot heat rooms, including heat pumps (air conditioners in reverse that use electricity
to pump heat rather than create it); they come in configurations that install in windows, through the wall, and in ductless split configurations.These typically provide 2 to 3 times as much heat per unit of electricity as space heaters. There are also small natural gas-fired heaters that install through the wall (that is, they get their combustion air and vent their combustion gases through an outside wall). Both of these options have a significantly higher initial cost (including purchase and installation), but cost substantially less to run than space heaters.
Regarding space heaters, for safety we suggest the following features: tip-over switch, overheating protection, UL listing, and thermostatic control. It’s OK for cords to get warm, but not hot (that is, too hot to touch). Space heaters should be plugged directly into the wall outlet if possible, but if an extension cord is needed, be sure it’s rated at or above the current draw of the heater (typically this means #14 or #12 wire is needed in the cord), and keep the cord as short as possible. Hope this helps!
I have a question that nobody has an answer for. I hope you can help.
I’ve read extensive research on vented versus unvented attics. In short, it says that unvented attics in hot-humid
climates are generally good things, as long as foam insulation is used. The attics take a performance hit by actually letting 34% more thermal transfer occur between the attic space and ceiling surface. The decrease in attic temperature is partly due to less thermal load from the roof decking, cool-air infiltration from the conditioned space, and thermal transfer to the conditioned space. This is made up by duct efficiency.
The question I have—and I can’t find the answer anywhere—is, what about a dual-insulation setup? This will be very common as houses are retrofitted with foamed roof decks. The home will have the bulk of the insulation on the roof deck but also the existing insulation on the attic floor. Since the thermal load from the attic space will no longer be affecting the ceiling temperature as much (or hardly at all, depending on insulation depth), how does the energy efficiency of a dual-insulation setup look? It seems to me that if one could not only decrease and seal the attic with closed-cell foam but have the added benefit of the existing insulation, thermal transfer via convection (duct leakage), radiation (low insulation temperature surface on the roof deck), and conduction would be minimized.
I can’t seem to find a study where they looked at a dual-insulation situation. I can find foamed roof decks and vented attic floor insulation studies but none that have both. Again, this will happen in every retrofit application of closed-cell foam. What do you think?
Executive Editor Iain Walker weighs in:
If the ceiling is well insulated, then the only advantage of additional insulation on the roof deck is to mitigate attic temperatures for attic duct systems. Therefore, there are some diminishing returns involved. The effect of the additional roof deck insulation on building load is reduced if the ceiling is already well insulated. Also—if we keep the ceiling insulation, then the attic space is at a temperature between inside and outside, and the duct losses will be greater than if the ducts were inside the thermal envelope. In other words, there are some drawbacks to the dual-insulation attic that reduce the energy savings potential, depending on the duct losses in particular.
In the article, “New Homes are So Last Year,” (Jan/Feb ’09, p. 28), we regret confusing the GreenPoint Rated New Home and GreenPoint Rated Existing Home programs. The second sentence in the first paragraph should read as follows:
The existing homes program is modeled after the highly successful GreenPoint Rated New Home rating system, which launched at West Coast Green in 2006, and currently has 6,000 homes enrolled in the program, with another 7,000 in the pipeline.
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