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Perforated Radiant Barrier Insulation

  1. Jul 10, 2010 #1
    Will installing perforated radiant barrier insulation in the attic reduce the power bill significantly.
     
    Last edited by a moderator: Jul 10, 2010
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  3. Jul 10, 2010 #2

    Evo

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    It needs to be used in conjuction with other insulation, but it sounds like it could improve things a bit if you have central air conditioning running through your attic in a hot, sunny climate.
    http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11680 [Broken]

    The perforated type would help to prevent moisture condensation.
     
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  4. Jul 10, 2010 #3

    Chi Meson

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    "Significantly," I strongly doubt it, unless there is little or no insulation there to begin with.

    What kind of attic, where in the attic are you thinking of placing it, where do you live, what insulation is already there, are you wanting to stay warmer in winter or cooler in summer...
     
  5. Jul 11, 2010 #4
    My house is hip construction, wood roof with shingles, installed over R32 blown insulation. I live in Pensacola, Florida. I want to stay cooler in summer at a lower cost.
     
  6. Jul 11, 2010 #5

    Chi Meson

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    For this purpose, I would recommend http://www.bobvila.com/wwwboard/messages/painting_asphalt_shingles_202587.html"

    Radiant barrier will reflect the infra red radiation coming from the hot underside of your roof. But it will just reflect it back to the same place. You will want to be sure that the attic space is well ventilated, perhaps with a fan.

    A reflective roof will prevent that radient energy from being absorbed by your roof in the first place, and the attic space will be significantly cooler with much less IR coming from the roof underside.

    Downside: Our culture doesn't tolerate change very well, and white roofs are a big change.
     
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  7. Jul 11, 2010 #6

    russ_watters

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    I'm not so sure its that simple: unless you are in a very southern climate, you lose more heat through your roof in the winter than you gain in the summer.
     
  8. Jul 11, 2010 #7

    Chi Meson

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    Painting your roof white would not make you lose more heat in the winter. Here in New England, the solar gain from radiant heat through the roof would not even make it to the top of a good R-30 fiberglass insulation. Radiant barriers are to be used in conjunction with good ol' fiberglass which will slow the loss through conduction.

    I do agree that a radiant barrier, and painting your roof white, are most advantageous in warmer climate where you want to prevent radiant solar gain. This is why I'm not even considering painting my roof. I did, however, deliberately choose the lightest shade of asphalt shingle that was possible at the time. I also use a "whole-house" fan which sucks air in from all windows and blows it out through the attic, all but eliminating heat gain through the roof, plus keeping a breeze going in every room.

    This is sufficient for Connecticut (and I don't need AC), but if I lived in Florida (and I could not possible live there) I would have a white roof.

    But to get back to the OP, the radiant barrier probably would reduce cooling costs, but don't expect to see a very visible dip in the electricity bill. I'd expect (and this is a very rough guess) at least a two year payback on the investment (depends on how hot it is and how cool you want it). But as this option will actually raise the temperature of your attic space, make sure that attic ventilation is good.
     
    Last edited: Jul 11, 2010
  9. Jul 11, 2010 #8

    russ_watters

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    I didn't make that clear enough. I was talking about the net loss. Let me try again:

    In summer (during the day), you gain a lot of heat due to the sun hitting your roof. You also gain some heat through your roof due to convection with the hot outside air. The black roof is a negative in the summer.

    In winter (during the day), you gain some heat due to the sun's rays hitting your roof. You lose a lot of heat due to convection with the outside air. The net result is a loss of heat through your roof - and the net loss is greater if your roof is white than if it is black. So a black roof would be a benefit during the winter.

    There are some cases where the benefit of a black roof in the winter would outweigh the cost of a black roof in the summer. Particularly for those in the far north who have little or no air conditioning.

    One area where white roofs are cerainly better is flat, white roofs of commercial buildings. The reasons are:
    -Since the roof is flat, the solar gain in summer is a lot higher than the solar gain in winter. So the penalty in the summer is a lot bigger than the benefit in winter.
    -The benefit in winter is snakk because most commercial buildings generate a lot of heat internally and require cooling even in winter.

    As such, white roofs are pretty much the standard in commercial buildings these days.

    BTW, a radiant barrier helps in both winter and summer. How much, I'm not sure.
     
    Last edited: Jul 11, 2010
  10. Jul 11, 2010 #9

    Chi Meson

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    Sounds as though we are mostly in agreement, although I am confident that the blocked solar gain in summer is much greater than the blocked solar gain in winter. I am certain that in Florida a white roof would save more in summer cooling cost than it would require in winter heating costs.

    A well insulated and ventilated attic is first priority anyway .

    Whether a radiant barrier helps in winter strongly depends on the type of heat. Obviously, homes with radiators, or wood stoves would save some of that escaping radiation. If there is only forced air heat, there would be significantly less high-energy radiation to save.
     
    Last edited: Jul 11, 2010
  11. Jul 11, 2010 #10
    Sorry, but I put it hear because there is much opinion for and against the use of radiant barrier insulation. Feel free to move it to home improvement. But distributors of the insulation claim a 30% savings, but the opinion here is much less. Isn't that debunking the distributors claims.
     
  12. Jul 11, 2010 #11

    Chi Meson

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    That sounds like the famous "up to 30%" line. "Up to 30%" happens to include 3%, 2% and even 1%. When comparing the use of the radiant barrier to "nothing at all," then easily you will have 30% reduction. But if you already have R24 in the attic, it's going to be much less of an improvement. Not to say there is no reason to install it. I am again confident that you would benefit from it and that it would ultimately pay for itself in a few years.

    Did I mention that your attic should be well ventilated? In Fla, with radiant barrier above your insulation, the attic temperature could easily surpass 180 Fahrenheit every day if air flow is insufficient (I think that's actually a low-ball estimate; I'm a New Englander and still can't comprehend Southern Summers). That will degrade the structure, degrade the roof, degrade the insulation...everything!

    I'd paint the roof white in your situation. Start the trend.
    "Hey, it's cool! Literally!"
     
  13. Jul 11, 2010 #12

    Ivan Seeking

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    Your choice of forums was perfectly acceptable and entirely appropriate. :smile:

    [It seems there was a bit of confusion about the scope of the forum. We address all advertised product claims]
     
    Last edited: Jul 11, 2010
  14. Sep 3, 2010 #13
    Actually, it helps a lot more than people might think, and is most effective in hot, sunny climates.

    There's a device which thermally measures the temp of one's walls, ceiling - wherever it's aimed. The readout is in deg F.

    When I lived in Vegas, it was in a brand-new house. I was aghast at my air conditioning bill, but no amount of air conditioning seemed to take the edge off what felt like being gently roasted in the afternoon. A neighbor who did home improvements had the thermal device and simply recommended I buy half a dozen rolls of aluminum foil and lay sheets, shiny side up, on top of the insulation between the rafters in the attic.

    I took spot measurements of the ceiling throughout my house at the same time in the afternoon throughout the week, along with other measurements (outside air temp, inside air temp, relative humidity and sky conditions (sunny), then did as he said. Afterwards, I took measurements for another week.

    That single measure cost less than $30 (and a bucketload of sweat) but reduced the mean temperature of my ceiling by more than 4 degrees, and the comfort level inside improved - we were able to set the thermostat higher as we had noticeably less radiant influx entering the house from the attic.

    Additional measures also helped cut the cost of cooling significantly, though wound up costing far more:

    Rear patio cover: $2,300
    Solar window screens: $680
    Insulating and thermally reflective blinds: $800
    Attic fans and wire: $120

    The patio cover was the most expensive, but as it faced the southwest, it dramatically helped keep the entire side of my family room, living room, and kitchen - the living spaces - very much cooler throughout the afternoon. I'm sure I didn't recoup its expense in terms of reduced cooling bills in the 3-1/2 years I lived there, however!

    As for the aluminum foil, I'm fairly certain I recouped its expense in less than 30 days.
     
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