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wildblue
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| 05/17/2009 5:20 PM |
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My attic gets as hot as a sauna when the sun is shining. Although the attic is well ventilated with soffit, gable and roof vents there is absolutely no air movement. Between the giant fir trees behind the house and the homes on either side I seem to be blocked from any breeze. However the house is exposed to full afternoon sun from the street side. I intend to add radiant barrier to the rafters and more insulation but I don’t think it will be enough because of the lack of air movement. Even if I can keep the heat out of the living space, I know that an attic that is too hot is not good for the roof materials.
I plan to install a powered gable mount attic ventilator. However with all the other vents I’m pretty sure an exhaust fan will short cycle and suck air from the near vents and still leave most of the attic hot and stagnant. I don’t want to block the vents or install multiple fans. There are also other potential problems with powered exhaust because I have a gas hot water heater.
My idea is to reverse the fan and pull cool air from outside into the attic rather than try to exhaust the hot air. I live in the Seattle, WA area. Today it is beautiful sunny day and 70 degrees outside with a slight breeze. It rarely gets above 90 here but even that would be 50 degrees cooler than the air in the attic. Even though it’s 70 outside I could bake a cake by just putting it in the attic.
I believe that blowing outside cool air into the attic will cause air to flow up and out the roof vents, possible even setting up a draft that will cause cool air to be pulled in from the low soffit vents. I did a trial with a cheap low power box fan hung in front to a gable vent and it did help. I think a high power fan will do the trick.
Does anyone see a problem with this idea? BTW, the ceiling is well sealed to prevent air leaks into or from the attic. |
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sastexan
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| 05/22/2009 12:58 PM |
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We have a powered attic fan - did roof mount instead of gable to keep the gables open for natural convection.
Short cycling isn't an issue with a fan, like with an A/C compressor, which is more efficient after it has been running for a period of time - the power use is flat as it runs.
Most fans come with a thermostat and only turn on at the preset temperature, say 120 degrees F.
The fan will pull cooler air from the soffits and gables from the outside as it blows out the hotter air sitting in the top of your attic, as long as you have adequate ventilation. Use this formula to determine if you have proper ventilation.
http://www.askthebuilder.com/B335_Calculating_Ventilation_Area_of_Attic.shtml
Good luck. |
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Dana1
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| 05/22/2009 1:07 PM |
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In Seattle summer humidity levels are dry enough that you can get away with positive-pressure ventilation. In hot-humid areas pressurizing the attic would force humid air into cooler conditioned space with condensation potential. I'm not sure what problem you're trying to solve here- why not let the attic run hot? Do you have high heat gain into conditioned space through the ceiling?
It's been heavily studied, but overall cooling & energy consumption savings of active ventilation is very modest in when it works, and often ADDS to the cooling load on air-conditioned spaces. If it's comfort & efficiency you're after, air conditioning will usually provide equal or better bang/buck. Only if you're ventilating with self-powered drive like solar attic fans, or wind-powered ventilators can you get a grid-efficiency boost out of it. One reference, of many: http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/ (If you're only saving 6% with a self-powered ventilation in FL, you can imagine the relative LACK of value for powered attic ventilation in Seattle!) The amount of cooling you can get is pretty minimal unless you move a LOT of air, and the higher the pressure differential, the more likely it is to be self-defeating. Spend the power in the AC. But managing the solar gain is also huge...
The radiant barrier will lower the attic temp, but that will be lower year round- the shoulder & heating seasons it will be lower too, presenting a larger delta-T across the attic insulation, resulting in a higher heat load. Radiant barrier on rafters usually results in a annualized net higher energy use in heating dominated climates. But radiant barrier at the conditioned-space ceiling (or attic floor) helps year round- particularly if the attic insulation is fiberglass, and R38 or less.
If it's R19-R38 fiberglass insulation batting between joists at the attic floor and you can add more insulation, blowing 6" of cellulose over it will improve both summer & winter performance more than just the additional R20 might imply. The cellulose will block the radiated downward heat better than FG, and it'll block convection currents within the insulation better, largely stopping the R-value loss that fiberglass usually sees in high delta-T cold-side up configurations. If you blow over the tops of joists, it'll also form a thermal break on the short-circuit formed by low-R value wood, increasing the net whole-surface R-value, and it'll fill in any gaps, etc.
If you already have Fiberglass batts rolled over the joist tops rolling it back and installing perforated radiant barrier at the joist tops and reinstalling the top batting can also provide real benefit. Simply laying radiant barrier on top of the FG works too, but is more prone to degradation from dirt than if sandwiched. Be sure to use a breathable type RB to avoid vapor trap & mold issues (definitely NOT a bubble-pack RB application.)
Exterior shading for the windows on the S & W sides can also make a significant dent in the summertime solar-gain. On the S side it won't take much awning-type shade to get there at 47 degrees N. latitude, but the W side it takes a real shade (like a tree or a shutter.) Mirrored translucent window film can also make a big dent if shading isn't a realistic option. (But it'll cut your gain somewhat more than it'll cut nighttime losses during the heating season, but probably not enough to make it worth peeling off seasonally every year.) |
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glenfotre
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| 05/22/2009 2:38 PM |
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While I realize that it may be considered a 'redneck solution', I just hang a 20" box fan on the inside of the gable vent and run it 24/7 all summer. The attic access is right below that vent into the garage, so it helps to cool the garage as well as I leave the access open when the fan is on. |
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jonr
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| 05/22/2009 5:56 PM |
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If you use push/pull fans, ie, one on each gable end, one blowing in, one blowing out, then you minimize any pressure change or leakage.
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wildblue
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| 05/23/2009 6:52 AM |
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Posted By Dana1 on 05/22/2009 1:07 PM
In Seattle summer humidity levels are dry enough that you can get away with positive-pressure ventilation. In hot-humid areas pressurizing the attic would force humid air into cooler conditioned space with condensation potential. I'm not sure what problem you're trying to solve here- why not let the attic run hot? Do you have high heat gain into conditioned space through the ceiling?
It's been heavily studied, but overall cooling & energy consumption savings of active ventilation is very modest in when it works, and often ADDS to the cooling load on air-conditioned spaces. If it's comfort & efficiency you're after, air conditioning will usually provide equal or better bang/buck. Only if you're ventilating with self-powered drive like solar attic fans, or wind-powered ventilators can you get a grid-efficiency boost out of it. One reference, of many: http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/ (If you're only saving 6% with a self-powered ventilation in FL, you can imagine the relative LACK of value for powered attic ventilation in Seattle!) The amount of cooling you can get is pretty minimal unless you move a LOT of air, and the higher the pressure differential, the more likely it is to be self-defeating. Spend the power in the AC. But managing the solar gain is also huge...
The radiant barrier will lower the attic temp, but that will be lower year round- the shoulder & heating seasons it will be lower too, presenting a larger delta-T across the attic insulation, resulting in a higher heat load. Radiant barrier on rafters usually results in a annualized net higher energy use in heating dominated climates. But radiant barrier at the conditioned-space ceiling (or attic floor) helps year round- particularly if the attic insulation is fiberglass, and R38 or less.
If it's R19-R38 fiberglass insulation batting between joists at the attic floor and you can add more insulation, blowing 6" of cellulose over it will improve both summer & winter performance more than just the additional R20 might imply. The cellulose will block the radiated downward heat better than FG, and it'll block convection currents within the insulation better, largely stopping the R-value loss that fiberglass usually sees in high delta-T cold-side up configurations. If you blow over the tops of joists, it'll also form a thermal break on the short-circuit formed by low-R value wood, increasing the net whole-surface R-value, and it'll fill in any gaps, etc.
If you already have Fiberglass batts rolled over the joist tops rolling it back and installing perforated radiant barrier at the joist tops and reinstalling the top batting can also provide real benefit. Simply laying radiant barrier on top of the FG works too, but is more prone to degradation from dirt than if sandwiched. Be sure to use a breathable type RB to avoid vapor trap & mold issues (definitely NOT a bubble-pack RB application.)
Exterior shading for the windows on the S & W sides can also make a significant dent in the summertime solar-gain. On the S side it won't take much awning-type shade to get there at 47 degrees N. latitude, but the W side it takes a real shade (like a tree or a shutter.) Mirrored translucent window film can also make a big dent if shading isn't a realistic option. (But it'll cut your gain somewhat more than it'll cut nighttime losses during the heating season, but probably not enough to make it worth peeling off seasonally every year.)
Yes, The problem is high heat gain through the ceiling. I'm doing the prep work to add blown in insulation and I am trying to wire in AC smoke detectors but is gets so hot in there even when it's only 70 outside I can't stay up there.
It seems to me if I could cool it down by blowing in cooler air from outside my AC would hadly ever kick in. I think push-pull fans on either end may be the ticket.
Right now there is only about 3-4" of blown in fiberglass. I could lay perforated RB on it and then blow in more insulation but doesn't RB need an air gap to be effective?
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jonr
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| 05/23/2009 7:44 AM |
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Absolutely. a RB needs an air gap and it can't collect dust.
Ideally a radiant barrier is foil bonded to the underside of the roof sheathing.
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wildblue
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| 05/23/2009 10:38 AM |
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Posted By jonr on 05/23/2009 7:44 AM
Absolutely. a RB needs an air gap and it can't collect dust.
Ideally a radiant barrier is foil bonded to the underside of the roof sheathing.
That's what I thought. Like the health studies about coffee where half say it will kill you and half say it is good for you, the jury is still out on the dust thing.
In any event I'll staple a good quality breathable RB to my rafters then blow in 12"" of FG. That is if I ever finish the smoke detectors and sealing all the ceiling air leaks I an find.
I'm putting in push-pull fans at both ends first so I can work up there. I can alyways turn them off.
But this weekend, no home improvement projects. I going down to the Columbia River at Umatilla to fish for bass and walleye.
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Zorro Olmer
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| 06/14/2009 1:47 PM |
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| I have a two story house just north of Seattle. For the few days a year that I would like air conditioning it is just is not worth the investment. I have installed a powered roof fan, plus I built an extra attic door (for the summers) with a a gable end fan mounted to it, to pull air up through the house. These are wired to one twelve hour timer switch. Any time I want the house to be as cool as it is outside I just open a window and turn on the fans. Most days I just wait till the sun goes down and turn in it on so that it runs till a couple hours after bed time. It takes about twenty minutes to get the entire house down to the out side temp. When I run the fans they move air over the wall surface in the house and remove the heat that they would otherwise radiate back into the rooms. When It gets really hot I open the windows downstairs, on the shady side of the house and run that part of my sprinkler system for a few minutes, to speed things up. This whole setup may not be very green, But I sleep just fine, anyway you look at it. |
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jonr
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| 06/14/2009 6:09 PM |
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I do the same. A whole house fan often creates enough breeze and cooling that AC isn't needed.
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Dana1
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| 06/15/2009 4:56 PM |
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Posted By wildblue on 05/23/2009 10:38 AM Posted By jonr on 05/23/2009 7:44 AM
Absolutely. a RB needs an air gap and it can't collect dust.
Ideally a radiant barrier is foil bonded to the underside of the roof sheathing.
That's what I thought. Like the health studies about coffee where half say it will kill you and half say it is good for you, the jury is still out on the dust thing. In any event I'll staple a good quality breathable RB to my rafters then blow in 12"" of FG. That is if I ever finish the smoke detectors and sealing all the ceiling air leaks I an find. I'm putting in push-pull fans at both ends first so I can work up there. I can alyways turn them off. But this weekend, no home improvement projects. I going down to the Columbia River at Umatilla to fish for bass and walleye.
RB on the rafters is not recommended in heating dominated climates, since it rejects solar gain during the heating & shoulder seasons, with a subsequent net increase in annual energy use. RB placed on the conditioned space side of the fiberglass still rejects a significant portion of the solar gain, but adds at least some insulating value during the heating season. (The "air gap" in this installed configuration is essentially the inter-fiber gap of the insulation.) But even that is probably a net performance loss in Seattle. And as the R-value of the "real" insulation rises, any benefits/detriments of the radiant barrie fall dramatically.
If you have 12" insulation in the plan, forget about the RB- it would only show measurable benefit with fiberglass, (and then, not so much, especially in the PNW.)
Open-blown fiberglass in attic floors loses significant R-value at higher delta-T due to the low density allowing convection currents. This can be mitigated by as little as 3" of cellulose over-blow atop the FG, or going with cellulose altogether. Unlike FG, cellulose is opaque to infra-red, and doesn't NEED radiant barrier to maintain it's R-value in a hot attic. And unlike FG, it GAINS a small amount of R-value at higher winter-conditions temperature differences. But it weighs more, and in the event of a roof leak can sometimes get quite heavy before the leak shows though to the interior.
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mark custis
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| 06/15/2009 9:10 PM |
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I am clueless about Seattle. To get air to go out you have to let air in.
My guess is your soffit vents are not. |
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wildblue
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| 06/15/2009 11:16 PM |
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Exactly. I have lots of open soffit vents, some roof vents and gable vents but the air just does not move up there.
Anyway I have my push-pull fans up and working now and they work great. Instead of 120 it's only 80 up there and my AC doesn't run when it's only in the 70s outside.
I'll actually be able to work up there and finish the wiring for my smoke detectors. |
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jonr
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| 06/16/2009 6:22 AM |
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Fiber insulation is opaque to infrared. Very easy to show with an infrared thermometer.
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Dana1
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| 06/16/2009 10:49 AM |
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Posted By jonr on 06/16/2009 6:22 AM
Fiber insulation is opaque to infrared. Very easy to show with an infrared thermometer.
Depends of the fiber.
There's a huge body of evidence (from people who measure stuff carefully, including FG manufacturers, Oak Ridge Nat'l Labs, etc. ) that fiberglass passes significant amounts of IR, and that it's performance improved by radiant barrier. This has been a well known, well documented drawback to the product for at least 50 years now. (It's the very reason why foil-faced fiberglass insulation was invented.)
A classic study from 25 years ago that clearly demonstrates the IR differences between cellulose & fiberglass lives here:
http://txspace.tamu.edu/bitstream/handle/1969.1/6841/ESL-HH-85-09-33.pdf
A short writeup of where/when RB might be appropriate lives here:
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-DN-6-86.pdf
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jonr
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| 06/16/2009 1:40 PM |
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They have learned a lot since then. Later studies show this non-linear performance of fiberglass to be due to convection within the insulation. Yes, convection is a real problem with fiberglass.
There is an almost imperceptible IR effect within the first 1/2 inch of fiberglass. Not worth worrying about and also easily confirmed by anyone with a hot surface and some fiberglass insulation.
The old study also shows that cellulose is improved by a radiant barrier. |
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Dana1
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| 06/16/2009 2:02 PM |
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This may be major thread drift, (or not), but perhaps the infra-red (IR) & fiberglass (FG) vs. cellulose thing needs more verbiage.
With the fiberglass being semi-transparent to IR, some amount of IR is absorbed by the fiberglass, but it doesn't happen at the surface, but somewhere inside the insulating layer. This absorption raises the temperature of the fiberglass itself, but being insulator, that heat gets trapped (since it's insulated on both sides by more insulation), resulting in the non-linear temperature profiles in that Texas study (http://txspace.tamu.edu/bitstream/handle/1969.1/6841/ESL-HH-85-09-33.pdf). In higher-temp studies the highest temperature in the attic is sometimes end somewhere in the insulation(!), not the attic air, not the roof that's emitting the IR(!). How deeply into the insulation this IR-heated fiberglass zone lives depends on the density and depth of the insulation, as well as the intensity of the incident IR.
In the cited study, the top surface temperature of all fiberglass thicknesses tested was ~3-10F below the radiating roof temperature (increasing delta-T with inreasing roof temps), where as the cellulose surface stayed 10-27F below the roof temp. At a roof temp of 145F the FG surface was ~137F, where the cellulose surface temp was only 116F (at a roof temp of 143F). Add a coupla degrees to the cellulose temp to get rough parity with the FG test condition, you're still talking ~19F lower temperature that the body of the insulation need to insulate against. See figures 3 & 5 in the document.
What's going on here? When cellulose or other IR opaque material is used, the IR absorption all occurs at the top-most tiny fraction of an inch, where it's reasonably high emissivity re-radiates the heat back into the attic since it doesn't have insulation to slow or stop it. It's convection-cooled to the temp of the attic air (or very near that temperature), so you get the full R-value effect whether there's radiant barrier in place or not. The RB DOES improve the performance , but in this case it's adding effectively another ~R3, (See figure 8 in the study document) but that's in addition to the full R-value of the cellulose. (But adding another R3 of cellulose would be cheaper than adding RB.)
In the fiberglass at 145F the top layer is effectively running ~15-20F over attic ambient(!). That means instead of 12" of insulation (or R36 @ ~ R3/inch typical for open-blown FG) against ~120F, it's having to insulate against ~140F. But it's actually somewhat worse than that, since the interior layer of fiberglass is at an elevated level. When it's only 6" (R19-ish) or less the cut in performance from this effect is HUGE, and radiant barrier gives a similarly huge boost.. But as the fiberglass gets ever thicker it becomes something of a "who cares?". At a nominal ~R36 adding RB may be cost effective in cooling dominated climates, but probably not in heating dominated climates. At ~R50 the performance difference may be measurable with proper instrumentaion, but negligible in practical terms (and not cost-effective even as a DIY project.)
In other studies (I tried to google it, didn't find it right off- I think it was a Florida State University study from the late '80s), adding radiant barrier to the conditioned-space side of the fiberglass still improved the cooling season performance of low density R19 fiberglass batting, but only about half as much as putting it on the attic-space side. This effect is somewhat documented in figure 11 of the Texas study- other studies are clearer on this.
The relatively paltry effects of active ventilation, and comparatively better performance gains with RB are demonstrated in figures 12 & 13 in the study.
But if it's 12" of blown fiberglass, fuggeddabout active ventilation OR radiant barrier- they're not buying you enough additional coolth in Seattle at that insulation level to be worthwhile- 3" of cellulose overblow would buy you more (and improve the wintertime performance of the FG significantly.) |
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Dana1
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| 06/16/2009 3:30 PM |
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There's a pretty-good first-order estimate of the 25 year present-value savings per square foot of adding radiant barrier to rafters in a range of US cities here:
http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table4
Other tables on the same page give NPV for adding RB to the attic floor, or adding more insulation, etc.
According to table 4, in Seattle, with R30 of attic insulation installed adding RB to the rafters is only worth ~3 cents/square foot. If you have R38 installed adding RB is only worth 2 cents/square foot. And that's over a 25 year analysis! 12" of fiberglass is typically in that range, so if you can install it for $0.02/ft^2, go for it! If not... (What, the material alone costs 10x that? I'm shocked! :-) )
In Phoenix or Albuquerque, if you had only R19 of attic insulation there's some decent payback though.
But more insulation may still be cost-effective (in shorter time frames than 25 years.)
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jonr
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| 06/16/2009 3:37 PM |
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> In higher-temp studies the highest temperature in the attic is sometimes end somewhere in the insulation(!),
The study (using reasonable temperatures) shows otherwise - the temperature always decreases as you go into the insulation.
>In the fiberglass at 145F the top layer is effectively running ~15-20F over attic ambient(!
The study always shows the insulation surface temperature lower than the roof temp (the only temps they disclosed).
> adding radiant barrier to the conditioned-space side of the fiberglass still improved
Do you have any credible references that suggest radiant barriers under attic insulation (ie, with no air gap)? Of course clear plastic also helps fiberglass performance.
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Dana1
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| 06/16/2009 4:40 PM |
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A lower bound for the ambient can be inferred by the surface temp of the cellulose in that study. The FG surface was nearly 20F above that of the cellulose under similar roof temp conditions. (It's unfortunate that they didn't spare a thermocouple to track the "attic simulator" ambient in that lab-study. It's also unfortunate that they didn't do tests on thicker samples for both cellulose & fiberglass.) I'll just say it's unlikely that in that lab setup that there would be a 20F ambient difference between the two tests that explains the different result.
The studies where the temps interior to the insulation end up higher than the attic ambient are/were at reasonable temps, but were over time-of-day- not static laboratory conditions, but IIRC the deltas were significant. I'll try to dig them out (along with the low-density FG with RB on the conditioned space side.)
This one refers to some of the early FSEU work http://txspace.tamu.edu/bitstream/handle/1969.1/6870/ESL-HH-86-11-11.pdf?sequence=3 (and also seems to show lower heat flux through the ceiling with the RB on the floor rather than the rafters.) But doesn't show the (unusual, which is why I remember it) placement of RB below vs. above the insulation.
Can you point me to any study demonstrating that the temperature non-linearities in FG under hot-roofs are purely (or even predominantly) a convection effect? Convection in FG is much more pronounced when the warm side is below, since the denser cooler air sinks and stays at the bottom and the warmer air floats up to the top surface. Warm side up may have convection issues in the upper layers if/when the radiated heat is absorbed by the FG in middle layers to temps higher than attic-ambient though... I'd be very curious to know if/how radiated heat from above or even elevated attic air temperatures can induce currents counter to the buoyancy effects within the fiberglass. If there's a convection effect here, it's in the uppermost radiation-warmed zone of the fiberglass (which sorta demonstrates the translucency/partial-absorption thesis...) But if there's better evidence with an alternate theory of causation, I'd love to see it! |
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