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Old 07-10-2012, 12:06 AM   #1
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Radiant Barrier


I live in the deep south where summers are hot and humid. Design temp is 95 F. I have R 30-38 in the attic, R 13 in the walls, and R 4.2 silver wrap flex duct. No shade trees. On a sunny day, the attic gets to 120 F and above, with the roofing 140 F. If I start the AC in the morning and run it continuously on days when it gets up to (say) 98 outside, I can maintain 73 F around 52-53% RH at best. My split measured across the coil is 20 F, but at the return and registers, it's 12-13 F under these conditions. The AC is set to the proper speed and working right, there is no significant duct or return leakage, I've sealed around ceiling penetrations, etc etc etc; there's just no more low-hanging fruit to pick.

I've thought about wrapping the flex duct with more insulation, and that will certainly help, but it won't do anything about the heat transfer through the ceiling. So I'm looking at killing both birds by installing a radiant barrier in the rafters. Does anyone have experience with this? Does it really work? Has anyone used Enerflex sheets, which are held in place by friction? Or would the stapled rolls be the way to go? Does the whole attic have to be treated for it to be effective? My garage is attached and I'd just as soon not treat it.

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Old 07-10-2012, 06:35 AM   #2
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Radiant Barrier


I have the same issues on hot days in VA. When it gets hot during the day air coming out of my vents is maybe 70 degrees but at the air handler its a cool 50 something like it is supposed to be.

Your flex duct is only R4.2? Mine is R-6 i think. They sell R-8 now at home depot. Maybe you can try that? I replaced one duct with it and am going to replace the others as well.

Also for a radiant barrier I saw a really easy one that was radiant barrier chips. They were more expensive but it was a bunch of silver metal chips that you blew out of a machine kinda like blown in insulation. Looked like it would be way less of a pain than trying to lay down a barrier.

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Old 07-10-2012, 02:02 PM   #3
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Radiant Barrier


I would staple up radiant barrier foil to the bottom of the rafters. It will make a huge difference on the heat coming into the attic - especially in hot climates. it's pretty cheap and easy to do as a DIY project. It will bring you the last mile if you have already sealed the ducts, and provided good attic insulation. Search google for: radiant barrier and look a the different suppliers.
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Old 07-10-2012, 03:20 PM   #4
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Radiant Barrier


Doesnt that cause your shingles to overheat?
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Old 07-10-2012, 03:29 PM   #5
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Not really. Shingle temperature only goes up a few degrees (less than 10) over a radiant barrier. Considering that they are already going to be 150 or hotter, this is not significant enough to have a big impact on the roof.
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Old 07-10-2012, 05:06 PM   #6
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Radiant Barrier


I do remember seeing controversy about radiant barriers. It seems like they were in vogue for while and they worked for a while, but they get dirty and then they don't work anymore. My memory might be faulty, but the info should be pretty easy to find.
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Old 07-10-2012, 09:01 PM   #7
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Originally Posted by ryanxo View Post
I have the same issues on hot days in VA. When it gets hot during the day air coming out of my vents is maybe 70 degrees but at the air handler its a cool 50 something like it is supposed to be.

Your flex duct is only R4.2? Mine is R-6 i think. They sell R-8 now at home depot. Maybe you can try that? I replaced one duct with it and am going to replace the others as well.
Code was upped to R8 the year after my house was built but the builder lobby got it down to R6 the next year, and that's all you can find locally now. Anyway, I replaced an R4.2 duct several years ago with R6. It improved the split by as much as a couple of degrees over the 22' length. It would be far easier, more effective, and less destructive to the blown-in insulation to leave my ducts in place and wrap them with R6 duct wrap to give me an effective R10, which is what I've been considering doing once the cool weather returns.

I think moving from R6 to R8 would be even more minimal of an improvement than I observed going from R4.2 to R6. ISTR linking to a report demonstrating the diminishing returns several years ago in this forum. Anyway, I wouldn't recommend replacing more ducts without choosing one as a sample, measuring its air flow, measuring its temperature with a digital probe thermometer stuck up its grille, and also measuring the return temp, indoor RH, and recording the outdoor conditions, time of day, etc, all done after the AC has been running an hour or so. Then I'd replace that duct and wait until comparable conditions come around again and do the same measurements for it. I'd want to be really confident that replacing R6 ducts with R8 is going to make a worthwhile difference, because it will be an expensive PITA to do.

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Also for a radiant barrier I saw a really easy one that was radiant barrier chips. They were more expensive but it was a bunch of silver metal chips that you blew out of a machine kinda like blown in insulation. Looked like it would be way less of a pain than trying to lay down a barrier.
The Enerflex panels I mentioned are like 2' x 4' semi-rigid panels and insert between the rafters, bowing out in the middle, and held in place along their edges by friction. Very easy to install, easier than stapling in an awkward attic. The company says it'll decrease your attic temps by 30F! I'd like to hear some real world reports before I bite. They're not terribly expensive, but still...
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Old 07-10-2012, 09:19 PM   #8
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Radiant Barrier


I replaced 1 duct that was R6 with R8 and didnt see a difference but my AC has other issues. It's undersized and ridiculous so were not gonna get into that.

I feel you on not want to replace them. It really is a PITA bit if you wrap flex duct in more flex duct won't that make 2 vapor barriers and make condensation? I think thats what i read somewhere thats why I was hesitant to wrap my plenum with more duct wrap.

Those panels sound a lot easier haha.
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Old 07-10-2012, 11:38 PM   #9
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I feel you on not want to replace them. It really is a PITA bit if you wrap flex duct in more flex duct won't that make 2 vapor barriers and make condensation? I think thats what i read somewhere thats why I was hesitant to wrap my plenum with more duct wrap.
I've read the same thing, but it can be avoided by taping after you staple and taping the ends. I'm not sure how much of a real issue it is, though. For the last several weeks, one of my R4.2 ducts has been wrapped over about 85% of its length with raw fiberglass I kept from the R4.2 duct I replaced and some leftovers from another experiment. I haven't observed any condensation, and we've had a lot of 100 degree days and very humid weather, with the AC running up to 8 hours straight. The additional insulation did get me up to a 2 F split improvement on that duct. It's not going to be possible to do the entire length on every duct due to access issues, which is why I wanted to test the wrap over a segment.

I would just love it if a radiant barrier would make this a pointless upgrade at the same time it helps make my blown insulation more effective by reducing the heat load in the attic.
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Old 07-11-2012, 09:13 AM   #10
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If you want to re-wrap your ducts with insulation that has a vapor barrier, just slash the vapor barrier in the original insulation if that is possible.

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I would just love it if a radiant barrier would make this a pointless upgrade at the same time it helps make my blown insulation more effective by reducing the heat load in the attic.
It might be a semantic issue, but a radiant barrier will make you insulation less effective. It may, indeed, cool your attic, but the temp gradient across the insulation will then be shallower. Your investment return in your insulation will be diminished.
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Old 07-11-2012, 02:12 PM   #11
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Radiant Barrier


no one has mentioned attic ventilation..make sure the existing insulation isn't blocking your soffit vents and make sure you have sufficient roof vents ..this will relive the heat build up and reduce the cooling losses..might get you over the hump ..if you just have gable vents at each end of the house th that won't provide enough ventilation ...look up more info on the www re sizing etc..
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Old 07-11-2012, 03:54 PM   #12
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Radiant Barrier


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Originally Posted by ionized View Post
If you want to re-wrap your ducts with insulation that has a vapor barrier, just slash the vapor barrier in the original insulation if that is possible.



It might be a semantic issue, but a radiant barrier will make you insulation less effective. It may, indeed, cool your attic, but the temp gradient across the insulation will then be shallower. Your investment return in your insulation will be diminished.
Not really. The insulation will still be reducing the heat transfer to the occupied space. And the insulation itself won't be heated by radiant heat.
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Old 07-11-2012, 06:57 PM   #13
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Radiant Barrier


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Originally Posted by techpappy View Post
no one has mentioned attic ventilation..make sure the existing insulation isn't blocking your soffit vents and make sure you have sufficient roof vents ..this will relive the heat build up and reduce the cooling losses..might get you over the hump ..if you just have gable vents at each end of the house th that won't provide enough ventilation ...look up more info on the www re sizing etc..
There is extensive ridge venting and not enough soffit vents, but they are protected by those styrofoam baffles. I guess it works well enough as the attic air cools down rapidly once the sun goes down.
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Old 07-11-2012, 07:16 PM   #14
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Radiant Barrier


Attic venting does not really do much to cool the house if there is sufficient insulation. Attic ventilation serves to remove moisture from attics, not so much heat.
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Old 07-11-2012, 07:45 PM   #15
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Radiant Barrier


It would be great to have more insulation, but it's not going to help the heat gain in the suspended ducts. Anyway, my I/R thermometer indicates the ceiling gets no more than 5 F or so hotter over most of its surface than the indoor temp, which I understand to be acceptable. Same thing for the exterior walls. There are spots where the ceiling insulation is insufficient, but they're small, like in the corner of a room, and I would estimate they add up to a couple of percent at most. However, after I moved in, I found the area over the dining table was completely missed, and it was over 60 sq ft! Naturally, I had that taken care of.

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