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Old 07-07-2014, 11:18 AM   #16
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Quote:
Originally Posted by beenthere View Post
Op might be able to save 3X as much by sealing and adding insulation. Or only half as much, since we don't know how well sealed his house is or isn't.

RB still lowers the attic temp tremendously, which lowers the attic temp, helping to increase the effect of the insulation on the attic floor.

I go into a lot of very hot attics here in PA.
You are right beenthere but I have yet to be in a home that is well sealed if it wasn't done already.

I stand by my statement that if the home hasn't previously been sealed, it will save considerably more money (and be healthier) if the attic floor is air sealed and then insulated to a proper depth.

If there isn't HVAC or ductwork in the attic, the application of a radiant barrier is wasted money in PA. Above a certain R-Value depth, the impact of a radiant barrier is imperceptible and can actually keep some of the passive heating energy that may enter the attic at bay. This is a not an issue or concern as you get into the cooling degree day dominated climates.

If the attic is hot, the first thing that we always look at it ventilation. Convection (with enough intake and exhaust area) should handle removal of most of the heat.

Quote:
Originally Posted by digitalplumber View Post
My son had his roof redone last month. Tech shield added and we are in Texas. 2 weeks later he had all of the old insulation, a small amount of old blown in and 3 inch, removed. I had figured with the change in weather here he would be complaining, but he has not and his ac is handling it well.

Highly recommend the use of it!
In Texas, radiant barrier has application value if used properly. In PA, not so much. The question was originally about PA.

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Originally Posted by Perry525 View Post
Not many people know this....a Polar bears fur is only two inches thick.
Yet with a body temperature of 37C (much like us) it is happy to play in outside temperatures of minus 20C.
Why? Because not only does its fur act as insulation it also acts as a radiant barrier, reflecting the bears heat back into its body.

The motto, radiant systems can reflect heat back into the home, or reflect heat from the sun. Or even do both!
Please explain the "radiant" aspect of a polar bear's fur. I think you are combining terms. My dogs fur is less than 2" thick and if you look at her on an IR camera, she will have a much lower surface temperature than me even though her core temperature is 3-4 degrees higher.

It isn't because her fur incorporates some sort of radiant barrier, the fur insulates the body and slows the radiation of heat outward.

These are not radiant barriers in the traditionally though up home improvement application and are merely layered insulation.

Heat move via radiation, conduction, and convection. By insulating an item, you slow one or more of these.
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Old 07-07-2014, 11:26 AM   #17
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TVA research paper on Radiant barriers: http://repository.tamu.edu/bitstream...pdf?sequence=3


If you are considering a radiant barrier for your home, please read the study completed by the TVA in the attached pdf. As read through the study, you will see that they the only significant reductions in ceiling temperature flux come at what would be considered terribly inadequate levels of insulation (R-11 or R-19).

Please re-read the summer sections. You will see that they discount the effectiveness of any radiant barrier at insulation depths of R-30. Energy star recommends a minimum of R-50 now.


As stated in the report multiple times, the effectiveness of radiant barriers is not "statistically significant" (page 177 top left) at insulation levels of R-30. R-30 is still inadequate for this area. Proper insulation levels are 1.67X times that depth. The effectiveness of radiant barriers at that depth is effectively zero. Radiant barriers for winter applications are minimally effective when compared to adding insulation and in some cases (inadequate insulation) they will translate to a lessening of home performance.

If you are having summer heating issues, the proper installation of the radiant barrier is along the rafters, not across the insulation. If you place a radiant barrier across the rafters and have too little insulation, you will actually make the wintertime conditions worse. Radiant barriers, used in these applications, do not have the universal applicability that insulation does.

The section that you highlighted regarding dust has to do with winter performance of radiant barriers and not summer performance. A Radiant barrier’s summer effectiveness and emissivity is reduced by dusting. The lack of change in winter performance has as much to do with the air barrier aspect of a radiant barrier applied to the insulation than with the emissivity of the radiant barrier. The increased emissivity in the radiant barrier as a result of the dust will translate lessened summer performance.

All of these dusting issues are mitigated by the proper placement of radiant barriers for controlling summer heat. If someone is proposing that they lay the radiant barrier on your insulation, they are doing you a disservice. Your issues are with summer heat. Radiant barriers must be applied to the rafters. This is absolutely best practice for summer heating issues.


Radiant barriers do have a place in residential energy efficiency if there are precluding circumstances. If you don't have access to proper insulation depth (i.e. insulated roof deck), a radiant barrier might have some application value. In situations such as that, spray foam is more likely a better solution. Regardless, most homes suffer from other issues that are not corrected by Radiant barriers.

Improper insulation levels and unchecked air infiltration and exfiltration from the home are the two largest culprits when it comes to issues of energy efficiency.

Making sure that the home is not taking on unconditioned air, has properly sealed air ducts, and proper insulation (R-50) will make exponentially more impact to comfort and eliminating heat gain than an improperly placed radiant barrier.
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