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Old 07-11-2012, 07:51 PM   #16
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Radiant Barrier


I sort of lost track of the fact that you are trying to address a duct-heating problem. It is late to suggest it since you've done a lot of work already, but I don't suppose that you considered sealing the attic and insulating the roof deck?

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Old 07-11-2012, 11:03 PM   #17
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ok... Empirical evidence only.
Wrapping ducts with more insulation works. I have two ducts that total about 25 feet of 16 inch steel duct in my attic. Before I added additional to the existing r6 insulation, the delta t at the far end was around 10 degrees (return to far supply). My delta t at the unit was 20 degrees.... I wrapped the ducts with craft faced r13 and stapled the edges together. I then did a second wrap of r13 craft faced and stapled those edges together. I then wrapped the double insulated ducts in radiant barrier. bottom line is that the delta t is now 20 degrees at the far end supply, as it should be... again, these are just my measurements and my experience.

Now to attic ventilation.

I have little to no eve ventilation. I only have small end vents. The high today was 111 in PHX....The high temp in the attic today was 160 degrees. Yes 160. My brother has replaced all of his eve blocks with screens, added 8 roof vents, along with his existing end vents. I talked to him today and his attic was todayhigh was 130. about 20 degrees over ambient. both our temps were measured right at the surface of the insulation, away from air leaks and ducts and vents etc. so venting works. 30 degree difference in heat load coming from the attic. What is really interesting is that some days when there is a little wind... you can feel the hot air coming out of the eve vents at my brothers house!!!! the roof vents are acting as air scoops... LOL...
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Old 07-19-2012, 03:06 PM   #18
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Radiant Barrier


Wow Bob, way to show how duct insulation can prevent heat gain in such a hot attic! I hear that mountains of insulation on the attic floor also make the attic temps irrelevant for the most part.

Tim - If you are wondering what you should do if you can only do one thing, I would ask myself this: how much insulation is on the attic floor? If there is plenty, I would focus on the ducts, making sure they are sealed and insulated to the max. If you are also in need of more ceiling insulation, you might opt instead for the radiant barrier which will benefit the ducts and ceiling at the same time.
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Old 07-19-2012, 03:18 PM   #19
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Radiant Barrier


oops, missed that you had R-30 to R-38. I'd still do the rb for now and possibly a duct upgrade later if needed. - JD

Last edited by JDale; 07-19-2012 at 03:21 PM.
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Old 07-19-2012, 03:23 PM   #20
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Radiant Barrier


I had a contractor come out to my house yesterday. BIGGG Company Best Value Remodelers. They wanted to put their "NASA tested" radiant barrier in my attic. They had this black box with 2 glass windows on the front. Temp gauge on bottom, fiberglass insulation, a slot then heat lamp. The sides were seperated. On one side they put their radiant barrier, on the other nothing. Obviously one side gets super hot 170+ and the other stays the same 87ish.

I had some Reflectix radiant barrier laying around from where I did my duct work. I asked if i could try that in his box and compare. The "shiny bubble wrap" from home depot kept the temp LOWER than his "NASA tested" stuff.

He had quoted me $5000 to put in this radiant barrier of theirs. I did the math using their square footage and I could buy the amount of stuff from home depot for $272.00. Folks I believe I am in the wronggggggg business. $4700 to staple 1800sq ft of some radiant barrier in an attic!? I would do 1 attic a month and call it a job!

Bottom line... don't get JIPPED! DIY! And... that shiny bubble wrap from home depot keeps your house alot cooler lol
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Old 07-19-2012, 04:05 PM   #21
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By the way..73 deg F and 53% RH at outdoor temp of 98 is actually fantastic..I recall reading in a trade magazine that your indoor temp should only be 10 deg F less than outdoor temp to avoid thermal shock to persons entering or leaving..assume it would be more of a problem with older folk like me S0, anyway if it's 105 outside you are likely going to achieve 80 which is more than acceptable...I think you are whipping a dead horse.Paying more attention to.ingress and egress by persons, window coverings, number and type of windows and any drafts would likely be more effective unless you reroute your cooling ducts to inside walls or maybe bury them completely with more insulation. Keep Cool!!
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Old 07-19-2012, 06:04 PM   #22
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80 degrees in a house!? They can make cars that drive themselves and phones that hold more games than a computer did 2 years ago and they can't make an AC that keeps a house less than 10 degrees below outdoor temps? I dont think so lol/

That would mean my house should have been 90 today!

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