This article was originally published in the November/December 1995 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 1995
Clearing Up Shady Issues
Our report, Measured Impacts of Air Conditioning Condenser Shading (FSEC-CR-827-95), does not show that shading A/C condensers cannot save energy (see Air Conditioner Shading Shows Minimal Savings, Sept/Oct '95, p. 7). It does indicate that the potential savings are small. The report shows that if done properly, as at Site 1, an improvement in A/C efficiency of about 1% can be realized. On the other hand, the data show that when shading is done improperly, A/C efficiency can be lowered. Proper and improper shading strategies appear to be delineated by the proximity of the landscape shading to the condensing unit and its air flow pattern. A proper attempt at A/C shading would use landscape that is sited so that the unit is shaded in the late afternoon hours, but air flow is not impeded--particularly the hot exhaust air plume. This usually means using larger specimens (or waiting for smaller ones to grow), so that the shade trees are at least 6 ft away from the unit. We do indicate that such shading is not cost-effective when done solely for the purpose of shading an air conditioner. However, often landscaping is placed around homes for other purposes (beautification), and allocating one or two medium shade trees to this duty might be considered a no-cost option.
Remedial Action for Mobiles?
Wood decay in mobile home walls is not a newly discovered phenomenon (see Moisture and Mobile Home Weatherization, July/Aug '95, p. 29). I'll venture to say that just about anybody who has done weatherization work on a mobile knows about many different types of moisture problems in these homes.
For remedial action, how about opening the bottom of the vapor retarder (which is acting as a vapor condenser) and shoving a vinyl-faced batt into the wall cavities? This gives you a compressed batt with a vapor retarder on the inside and stops the moisture from entering the wall. We've been doing it for years. Call Cal Steiner in North Dakota to find out how it works in a really cold climate. The windows become the condensing surface, but interior storms will keep the sills from rotting more. Vent the dryer to the outside and make sure the mechanical ventilation works and the client uses it. If the mobile's in really bad shape, recycle as much of it as possible.
Side-by-side testing? This is not economically feasible for weatherization programs that already know what works. Tyvek wrap? The permeability of Tyvek is not capable of allowing moisture vapor (in the quantities produced in a mobile home) to escape. Wall vents? We dumped that one a long time ago. I've got a really good research idea. Go out and ask people who work on mobiles how to fix the problems.
Author George Tsongas Responds:
Mr. Shaber points out that anyone who does weatherization work on mobile homes knows about many types of moisture problems, including wood decay. That is certainly true, but one of the main points of my article was the serious extent of decay in walls with an exterior vapor barrier (EVR). That has not been previously recognized. I've talked about the problem I described to dozens of people who regularly work on mobile homes, including the well-known trainers Cal Steiner and John Krigger, and not one of them was aware of it. Furthermore, I've taken numerous mobile-home weatherization training workshops from practitioners to learn as much as possible about the real world of mobile homes.
Many people who do weatherization on mobiles never touch the walls for a variety of reasons, and they often do not know, or check to find out, whether the wall has an EVR in place. Yet doing any weatherization that air tightens an older mobile home with an EVR can lead to significant wall wood decay.
Mr. Shaber suggests filling the wall cavities with vinyl-faced batts as a remedial action. However, the article concerned older mobiles that already have wall insulation and an EVR in place. Mobiles without wall insulation typically do not have an exterior vapor retarder, so they were not the subject of my article. Inserting vinyl-faced batts into walls where unfaced ones already exist would not be cost-effective. More importantly, the vinyl facing would not stop moisture from entering the wall. Unfortunately, adding a vapor retarder on the inside of the wall (the vinyl facing) would only reduce water vapor diffusion into the wall cavity. However, moisture migration into the wall cavity from the indoors occurs primarily by air leakage, so adding a vapor diffusion retarder has only limited value. It would probably not help to reduce decay. Adding an interior retarder might even make matters worse, because it might lead to slightly increased indoor relative humidities. Adding indoor storm windows would also increase indoor humidities and lead to increased wall decay.
While mechanical ventilation may help to reduce indoor relative humidities during cold winter periods in northern climates, it does little to reduce humidity levels in mild fall and spring periods, when the outdoor air has about the same absolute moisture content as the indoor air. Those are the crucial periods when it comes to wood decay.
In the article I suggested side-by-side testing of a number of alternative remedial actions, including installing a Tyvek wrap in place of the poly exterior vapor retarder. Tyvek has the highest permeability of any component in a wall assembly, so it readily allows water vapor transfer through it.
Side-by-side testing is the only sure way of determining what will actually work in the field to solve this problem. Guesswork by myself or others certainly is not prudent when we are dealing with people's homes. Moreover, what works in the dry Denver climate, where moisture problems occur relatively less frequently, may not work at all in the moist climate of the Pacific Northwest or in the hot and humid South. Only field testing in a variety of climates will provide indisputable answers. That will take external funding from an agency like U.S. DOE and input from both researchers and those who work in the field. Neither can do it alone. I pride myself on talking to as many people as possible (both researchers and people who work in the field) in trying to solve this real-world moisture problem.
Web Visualization Technique
I have had some interesting experiences with pressure testing houses (mainly mine). Before starting my pressure testing folly, I spent a few hours cleaning and vacuuming the areas where I planned to work. I then set up the blower door and proceeded to use a micromanometer, hot-wire anemometer, and smoke pencil to look for outside air sources and cavity air movement. The results of these tests are not always obvious to the novice experimentalist, and sometimes seem to contradict themselves.
Being a parent and homeowner, it was weeks (um, months actually) before I could revisit my project. My clean work area was overrun by filth. It was at this point that my wind tunnel experience from long ago reared its head. I fired up the door, ran downstairs, and there it was as plain as day. Flow visualization via cobwebs. It was beautiful to see the webs streaming up the cavity. Even the natural convection from the top of the water heater was obvious through a spider's eyes. Since cobwebs move under the smallest driving forces, weatherization crews can use this visual technique during home walk-throughs to identify pressure flows under normal conditions.
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A couple of errors appeared in the article Assessing the Integrity of Electrical Wiring, Sept/Oct '95. In the first full paragraph on page 6, the second sentence should have read This expression times 3.412 is the heat produced (in Btu) by the wiring over one hour of operation at a given current draw. In the third paragraph under the heading Field Use, also on page 6, the second sentence should have read They use a 10% voltage drop threshold for the test, and Jeff Allegretti reports that approximately 25% of the jobs flunk at the initial audit.
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