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Need advice on powered attic ventilation
Last Post 03 Apr 2011 01:14 PM by cmkavala. 39 Replies. |
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wildblue
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Posts:37
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| 16 Jun 2009 11:55 PM |
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Interesting stuff but it’s all theoretical. Knowing this stuff is good but ultimately the specific situation should rule.
As far as the radiant barrier, solar gain in winter is negligible around here. In any event I’ll gladly trade what there is for more comfort in summer. Even if my cost savings for winter heating is a little less, we would much rather have the windows opened in summer than closed with the AC on. Additionally we have a very high efficiency dual fuel heating system. It’s not too expensive so I’ll be hanging it from the rafters.
I will install fiber glass insulation because cellulose burns and it absorbs water. It rains a bit here and there is much electrical wiring in my attic. Even if cellulose would perform a little better the few bucks a year are not worth the risk to me.
The bottom line is when I am done with all this I WILL be saving big on my current energy costs. |
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Dana1
Senior Member
Posts:6991
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| 17 Jun 2009 10:43 AM |
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With a foot of fiberglass in the attic, the radiant barrier will be a total waste of money, and the effect on heat-gain will be next to nothing, which is what the ORNL table points out (they DO measure that stuff, and their assumptions are conservative.) RB only makes a noticeable difference if the insulation layer is thin to VERY thin (even in cooling dominated climates.) If it's worth it in AC power savings at R19, but not R30 insulation levels in Phoenix, with 10x the cooling season, it's surely not worth it by any measure in Seattle at R30+. See: http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table4
The number in the table is the max you should pay per square foot to get a 25 year payback. Figure if you get a really good deal, and get it at $0.10/square foot, it's still not going to ever pay off, and the effect at R30+ will be academic (measurable with instrumentation, but not a noticable difference in comfort level.)
Canadian (and other) studies have shown that cellulose in the attic improves it's fire-resistance (it'll only light up with extended exposure to heat/flame), and fiberglass REDUCES fire resistance (since it passes some IR and allows superheated gas convection), but if you're not comfortable with it I'm not gonna push you on it. (Roof leaks can be somewhat messier to deal with using cellulose.)
But I WILL push you on this:
Rather than installing the radiant barrier, take that money and put it into another inch or three of fiber insulation (take it to 13-15" instead of sticking to the 12").
The net reduction in heat flux through the ceiling during the summer will be similar (or better), and it'll improve the wintertime performance as well. It's simply money better spent to achieve a comparable-to-better summertime result, with a more favorable annual result. Whatever the "...not very expensive..." cost of RB you come up with, just add it to the fiberglass installation. Unlike RB, the payback will be there.
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Dana1
Senior Member
Posts:6991
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| 17 Jun 2009 11:05 AM |
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FYI: With 3-4" of FG you're at ~R11, in which case the (very conservative) ORNL cost-effectiveness insulation value estimator recommends bumping it up another R30 to final of R41:
http://www.ornl.gov/~roofs/Zip/tmp/results21497.html
Try it yourself: http://www.ornl.gov/~roofs/Zip/ZipHome.html
(insert your actual fuel & fuel costs & zip. I used 980__ as the first 3 digits of the zip for climate correlation.) |
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wildblue
New Member
Posts:37
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| 18 Jun 2009 04:56 AM |
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Thanks. The money is not a problem. I'm interested in comfort not pay back. It's not like I'm going to remove all the widows in the house. The difference with or without RB will be minimal. There is already 3-4 inches of fiber glass so the added 12 will make 15 to 16 inches.
There is no way to tell if RB will be effective or not in this particular situation. Studies are averages and bell shaped curves. I believe this situation is an outlier. However once I blow it the insulation I CAN'T go back in and add the RB.
Additionally, cellulose is nothing but treated recycled news paper. Implying fiberglass is not more fire safe makes me go hmmmm.
Nuff said on this thread... |
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Dana1
Senior Member
Posts:6991
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| 18 Jun 2009 09:18 AM |
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Posted By wildblue on 06/18/2009 4:56 AM
Thanks. The money is not a problem. I'm interested in comfort not pay back. It's not like I'm going to remove all the widows in the house. The difference with or without RB will be minimal. There is already 3-4 inches of fiber glass so the added 12 will make 15 to 16 inches.
There is no way to tell if RB will be effective or not in this particular situation. Studies are averages and bell shaped curves. I believe this situation is an outlier. However once I blow it the insulation I CAN'T go back in and add the RB.
Additionally, cellulose is nothing but treated recycled news paper. Implying fiberglass is not more fire safe makes me go hmmmm.
Nuff said on this thread... Actually there, is a way, and it's a done-deal: The Nat'l Labs folks who actually test & measure stuff (in the field as well as labs) and have pretty-good models say definitively: " NOT!!" The effectiveness of RB diminishes with increasing R-value- right now you've got next to nuthin', and if all you were to do was add RB the effect would be substantial. But at ~R40 the biggest effect you would get would be be like going from R40 to R45, best case. Sure, the effective R45 would be a cut in heat gain from R40, but only a 9% difference (on an already75-80%% reduced heat gain.) Going from R10 (what you have now) to and effective R15 is more like a 33% difference. There's a nearly encylopedic synopsis of the radiant barrier testing that was chased quite intensively in the '80s here:
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-211-88.pdf
DO read the section on the effect of attic insulation level (starting
on P21 of the PDF pagination). (Note as an asided, that in their
analysis they presume that without RB the surface of the FG is ~15F
above the attic ambient, based on reasearch data not cited. They don't address the differences in emissivity between fiberglass & cellulose, since most of the research was done on the then-more-common R19 fiberglass batting.) Their conclusion (at the bottom of p25) is, "All other things being equal, the percentage reduction in ceiling heat transfer for attic radiant barrier systems as compared to standard attics should remain constant regardless of ceiling insulation level."So let's say your RB cuts the heat gain by 50% (way more effective than most installations- that would be an upper-bound). At your current ~R10 level that's a significant amount of heat rejection. But at R45 (about where you'll end up with 15" of FG) you've already cut the gain by 78%, so adding the RB is going to cut the remaining 22% of gain in half, only 11% of the heat gain you were getting before the increase in insulation level.
Putting it in perspective (on page 26): "...50% of a large number may be substantial, whereas 50% of nothing is still nothing."As the fiberglass gets ever deeper, the absolute amount of heat flux reduction is reduced, because it it only rejects a percentage function of what the insulation passes. That IS the way it works. Broadly tested in the field by the ORNL, it correlates well with the FSEC synopsis. The ORNL tables could be denominated in BTUs/hr as well as dollars- it'll scale linearly: http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table4 The net difference in heat gain of adding RB to an R38 attic in Seattle is ~1/5 of what you'd get out of adding RB to an R11 attic. (And you'll be around R45.) At R45 in the attic you'll be seeing far bigger gains from your glazing than heat flux through the attic. Going to low-e storm windows (or adding low-e window-films) will reject more heat than adding RB to the attic.
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jonr
Senior Member
Posts:5341
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| 18 Jun 2009 08:59 PM |
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All insulation suffers from diminishing returns on a $ saved per inch basis. And savings are even less than one would think (double the insulation is not 1/2 the heat loss) when one considers the entire building and other sources of heat loss.
Evidently radiant barriers being effective without an air gap is a myth that only people selling RBs promote.
Far better than blocking radiant heat in the attic is blocking it on the external surface. Aluminum shingles for example.
IMO, the best system for hot climates is Dupont AtticWrap (or similar systems with a sealed attic and ventilation under the roofing).
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Dana1
Senior Member
Posts:6991
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| 19 Jun 2009 10:18 AM |
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Posted By jonr on 06/18/2009 8:59 PM
All insulation suffers from diminishing returns on a $ saved per inch basis. And savings are even less than one would think (double the insulation is not 1/2 the heat loss) when one considers the entire building and other sources of heat loss.
Evidently radiant barriers being effective without an air gap is a myth that only people selling RBs promote.
Far better than blocking radiant heat in the attic is blocking it on the external surface. Aluminum shingles for example.
IMO, the best system for hot climates is Dupont AtticWrap (or similar systems with a sealed attic and ventilation under the roofing).
What makes the diminishing returns curve of RB different from other insulation is that it's a fixed ratio of the total heat flux, independent of the total R value. (Whereas additional R-value's reduction is a ratio of the R-values.) If you plot it out it's a steeper asymptote to zero with additional R, but it has a fixed cost independent of R. That makes it a great deal when the R-values are low since the same incremental cost of more insulation to achieve the same result is higher. But it's lousier deal than additional insulation when R-values are high, buying you only a fraction of the heat rejection that even more R-value would give, per dollar. The crossover point depends on your climate, since it's usefulness is far higher for cooling. The financial (not heat-rejection) crossover occurs at much lower R-values in heating dominated climates since the heating-season benefits of addtional R-value far exceed the heating-season benefits of RB. According to the ORNL calc, the crossover occurs below R40 in any climate (even Phoenix.) It's true that rejection of the radiated heat at the exterior of the structure is indeed more effective (by shade or reflectivity) than between the roof & attic floor, also much-studied by FSEC & Texas academics. But even there it's relative value can be trumped by higher R-value. In most heating dominated climates the R-value is already well beyond where paying extra for a reflective roof is worthwhile, even in the cooling season. But this is another situation where the annual energy costs are
net-negative in heating dominated climates, since it also reduces attic
temps in winter, not just summer. (Even if it had ZERO addtional cost up front , it's not the right solution in Seattle.) Even where reflective roofs make sense, SOME emissivity of the exterior finish is important for optimum performance. Very low-E surfaces like unfinished aluminum (with emissivites under 0.15) run at higher temps, since they can't re-radiate much heat, and their cooling relies on conduction to the roof decking (increasing the attic temp), and convecting heat to the exterior air, which isn't very efficient in low-turbulence low-wind conditions. See: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1411-03.pdf http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1220-00-es/roofing.pdf With a surface emissivity of 0.38, Galvalume is good stuff! I'd be curious to see how it's emissivity & performance changes over 10 years though- grit, algae, anything on the surface will likely raise it's emissivity considerably over time (just as dust on RB changes it's characteristics.) Whether 0.38 represents the optimal emissivity/reflectivity balance point for roofing in hot climates seems an open question.
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jonr
Senior Member
Posts:5341
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| 19 Jun 2009 03:22 PM |
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Yes, aluminum should be anodized for use as a reflector.
I believe white paint will both preserve galvalume and increase weathered performance.
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pablordstribe
New Member
Posts:1
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| 31 Mar 2011 07:50 AM |
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Powered Attic Ventilators this is a simple item to install, 4 in. diameter steel blade specially designed for maximum air movement., Operates automatically. Adjustable thermostat built-in.
http://www.thehardwarecity.com/products/Broan-358-Roof-Mounted-1200-CFM-Attic-Ventilator/0002862 |
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cmkavala
Veteran Member
Posts:4327
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| 02 Apr 2011 08:34 AM |
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its actually a 14" blade but, powered vents are a waste of time money and effort, while they do reduce the temperature in an attic, they also create negative pressure in the attic and suck out conditioned air from the duct work / living space
Invest in a reflective barrier instead |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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jonr
Senior Member
Posts:5341
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| 02 Apr 2011 10:18 AM |
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Not that I'm recommending any attic fan, but one fan blowing inward and another outward doesn't create the negative pressure problem.
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cmkavala
Veteran Member
Posts:4327
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| 02 Apr 2011 10:54 AM |
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Posted By jonr on 02 Apr 2011 10:18 AM
Not that I'm recommending any attic fan, but one fan blowing inward and another outward doesn't create the negative pressure problem.
it most certainly does, even the wind blowing past your house causes negative pressure |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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jonr
Senior Member
Posts:5341
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| 02 Apr 2011 11:04 AM |
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Technically all air movement involves pressures - but in this case, compared to a fan pulling only outward, it is effectively zero (or positive if you want to adjust it that way). Even comparing wind to the incredibly low through the attic velocities of these attic ventilators is silly (say by 100 to 1). With push/pull, 99.9% of the problem is gone and the rest you have even without the fans. |
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cmkavala
Veteran Member
Posts:4327
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| 02 Apr 2011 04:26 PM |
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Posted By jonr on 02 Apr 2011 11:04 AM
Technically all air movement involves pressures - but in this case, compared to a fan pulling only outward, it is effectively zero (or positive if you want to adjust it that way). Even comparing wind to the incredibly low through the attic velocities of these attic ventilators is silly (say by 100 to 1). With push/pull, 99.9% of the problem is gone and the rest you have even without the fans. Your statement is to broad......... even with a balanced fan situation/push -pull you have essentially made the attic a plenum the air moving thru the attic creates negative pressure and will draw air from the ceiling lid penetrations and duct leaks. The balanced fans help but don't eliminate the negative pressure. The same can be said with HVAC ductwork in a hot attic, it creates negative pressure within the duct when the air handler runs drawing in hot air from the attic space. |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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acwizard
Basic Member
Posts:265
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| 02 Apr 2011 05:13 PM |
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This negative pressure is being extremely blown out of proportion. If there is plenty of louver or soffit which is required by code , then adding an attic exhaust fan will improve the temperature of the attic space. The heat transmission to the hvac ductwork is far more of a concern than negative pressure.I would recommend a heat sensor to differentiate from natural heat in the attic space versus a fire. You do not want air movement in the attic during a fire. Yes winds do create natural pressure differences in the attic or for that matter in the interior space as well. This a very big problem in high rise construction along with the chimmey effect. |
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jonr
Senior Member
Posts:5341
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| 02 Apr 2011 05:27 PM |
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Agreed. If there is a concern about something on the order of -.05 pascals in a push/pull attic setup, one can block the exit fan a tiny bit and get a positive pressure in the attic. |
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cmkavala
Veteran Member
Posts:4327
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| 03 Apr 2011 12:30 PM |
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Posted By acwizard on 02 Apr 2011 05:13 PM
If there is plenty of louver or soffit which is required by code , then adding an attic exhaust fan will improve the temperature of the attic space. Roof ventilation is not something that is well regulated even in the strictest of code enforcement areas and not knowing the age the home it may or may not have adequate ventilation. Radiant barriers will also reduce the temperature of attic space with out the expense ongoing electricity use and will not cause negative pressure. Just adding a fan is a bad idea and can actually be counter productive in saving energy. |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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cmkavala
Veteran Member
Posts:4327
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| 03 Apr 2011 12:52 PM |
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<!-- #BeginEditable "body" --><!-- End Header -->
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| Back to Contents Page | Home Energy Index | About Home Energy |
| Home Energy Home Page | Back Issues of Home Energy |
Home Energy Magazine Online November/December 1995
trends
in energy
Drawbacks Of Powered Attic Ventilators
Powered attic ventilators, already suspected of using more energy than they save, can also create excess moisture, structural problems, discomfort, and combustion safety problems for home occupants, according to a recent study. John Tooley of Natural Florida Retrofit, and Bruce Davis of Alternative Energy Corporation's Applied Building Science Center in North Carolina, presented "The Unplanned Impacts on Houses by Powered Attic Ventilators" at the 1995 meeting of the Energy Efficient Building Association.
The paper describes research conducted on eight homes over a period of three months. As a result of this research, Davis said that he wouldn't recommend the use of powered attic ventilators. He emphasized, "If someone chooses to use a powered attic ventilation strategy, they need to do additional performance tests and take responsibility to be sure that it won't cause other problems." The potential for hazardous conditions is particularly high in homes with combustion gas appliances, because the ventilators can create negative pressures that cause backdrafting.
Tooley and Davis took measurements from July through September 1993 at eight homes in Research Triangle Park, North Carolina. Each attic contained passive ventilation in addition to a powered ventilator. All the houses had some depressurization when the ventilator operated, with pressures ranging from -0.5 to -2.5 Pascals (Pa). The tests showed that the powered attic ventilators, on average, drew 231 cubic feet per minute (CFM) of conditioned air out of each house and caused, by themselves, 0.72 air changes per hour (ACH). This flow from the house to the attic ranged from a low of 104 CFM to a high of 646 CFM, and ACH ranged from 0.38 to 1.2.
The houses had air volumes ranging from 13,000 to 32,000 ft3. The high figures above were for the largest house in the study, which had two ventilators and noticeable air leakage paths between the house and the attic. Tooley and Davis concluded that all eight of the sample homes wasted energy due to the high leakage of conditioned air into the attic.
Two houses had combustion safety problems resulting from the ventilators operating in conjunction with other mechanical equipment. At one, the water heater, located inside the conditioned space, stayed in a complete backdraft mode when the ventilators were operated with other equipment. During the seven minutes of the testing cycle, the area containing the water heater reached 40 ppm (parts per million) of carbon monoxide, and at five minutes the water heater flue contained 700 ppm of carbon monoxide (see "Combustion Safety Checks," HE Mar/Apr '95, p. 19). The study also reports anecdotal cases of combustion safety problems, including a Colorado family of three who died from carbon monoxide poisoning when an attic ventilator caused the furnace to backdraft.
Another two houses had moisture problems resulting from ventilator-induced negative pressures. At one house, a chain of events started with the repair of a leaky return duct for the air conditioner. Condensation occurred on the tighter (and colder) duct, which was located in a framing cavity, and moisture began to appear on the ceiling Sheetrock of the first floor. The suggested remedy was to increase the powered attic ventilation and turn on bath fans to remove moisture from the structure. This actually exacerbated the problem by increasing the pressure difference, moving more humid outside air through the building cavity, which created more condensation on the sheet metal duct. The moisture saturated the Sheetrock, which eventually fell to the floor. Discomfort problems were found in two study houses, one of which also had safety problems, while the other had moisture problems.
Tooley and Davis concluded that if a powered attic ventilator is to be used, the installer should provide a good air barrier between the ceiling and the attic, and ensure that adequate net free vent area is provided in the attic. The amount of air the fan moves must also be considered so that it can be properly sized (a bigger fan is definitely not better in this case), and pressure measurements should always be used to confirm safe operation.
Powered attic ventilators are commonly used in some regions of the United States, such as the Southeast and the Midwest, and they have a positive reputation. They are actively promoted by builders, roofers, HVAC contractors, ventilator manufacturers, do-it-yourself retailers, and even some utilities, although few utilities are sponsoring installation programs as they once did. Davis said that ventilators are sometimes used as a quick fix to meet attic vent codes in homes with an unusual roof structure or design.
Most powered ventilators are thermostatically controlled, with variable settings. Davis said that some manufacturers are also promoting these or similar products as crawlspace ventilators to help remove moisture, but this use can also cause other problems if it is not properly evaluated.
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| Figure 1.Effects of powered attic ventilators on eight North Carolina homes. Excessive air flow and house depressurization caused problems in each of the study houses. All of the houses were wasting energy (and money), while some also had moisture and discomfort problems, or even a serious backdrafting hazard. |
Ted Rieger is a freelance writer based in Sacramento, California, who specializes in energy issues.
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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jonr
Senior Member
Posts:5341
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| 03 Apr 2011 01:10 PM |
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All that studying and not once did they look at using a push/pull configuration to eliminate the negative pressure - or even explain that they were only studying exhaust only setups.
That being said, it is good to be aware of the effects of building pressure, positive or negative. Water heaters, bathroom fans, dryers, unbalanced suppy/return ducts can all cause negative pressures. If outside air needs to come in, allow for it in a controlled place, not through the walls.
They had a good point about negative pressure undoing the effects of a weak underslab radon fan. |
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cmkavala
Veteran Member
Posts:4327
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| 03 Apr 2011 01:14 PM |
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Posted By jonr on 03 Apr 2011 01:10 PM
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That being said, it is good to be aware of the effects of building pressure, positive or negative. Water heaters, bathroom fans, dryers, unbalanced suppy/return ducts can all cause negative pressures. If outside air needs to come in, allow for it in a controlled place, not through the walls. absolutely! |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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