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  1. #1
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    Nov 2007
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    Radiant barriers

    I've been toying with the idea of installing radiant barrier material on my rafters. Does this actually help reduce heat in the home (by its reflective qualities) or would simply adding more insulation be the better route? Combination? I'm guessing this helps the home envelope and would decrease both heating and cooling bills. I figured you fine professional type folk would know, Twilli in particular.

  2. #2
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    Aug 2003
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    Fort Worth, TX
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    Quote Originally Posted by speedymonk View Post
    I've been toying with the idea of installing radiant barrier material on my rafters. Does this actually help reduce heat in the home (by its reflective qualities) or would simply adding more insulation be the better route? Combination? I'm guessing this helps the home envelope and would decrease both heating and cooling bills. I figured you fine professional type folk would know, Twilli in particular.
    I have spray-on radiant barrier on the underside of my roof deck. This summer will be the proving ground for that material, as we had it applied this past October, just as the hot weather ended. On the few warm days we've had since then, the house stayed cool.

    For heating, I think the best bang for the buck is good insulation level in attic, adequate in walls, energy efficient windows, and tightening of the building envelope via sealing gaps in construction. You can have a blizzard of insulation up in the attic, but if the structure leaks air like a sieve, your furnace will run a lot anyway. All that thermostat senses is the temperature around it. You could have R-50 in the attic...leave a window open next to the thermostat and that heater will run and run. Infiltration is a reduced equivalent to leaving a window open.

    Radiant barrier will reduce the amount of heat a house can get from the attic in winter due to less solar heating of the attic. In summer, this is exactly what you want; in winter it works a bit against you. That is where going after other aspects of the building envelope can work in your favor (windows, caulking, etc.).

  3. #3
    Join Date
    Jul 2000
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    Northern Wisconsin
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    I keep seeing the threads on radiant barrier in attics and have yet to research it. That being admitted to I wonder if this is just another product that was brought onto the market to do a specific job and has been expanded to be a cure for everything.

    Unless what I learned many years ago about weatherization and building envelopes I don't see where putting something on the underside of the rafters in a conventional attic would be cost effective compared to what I understand to be accepted and in most cases code requirements for attic construction.

    Let me elaborate:
    If you live in an area where high solar gains along with high ambient temperatures are the norm..... why wouldn't you be putting a light colored roofing material on in the first place to reduce the energy transfered into the attic? In these conditions a dark colored roofing material will deteriorate faster due to extremes in temperature both from it's surface and also from baking from underneath. With this in mind...... why would you put a product under your roof that will reflect, block or slow down the movement of the energy coming through from the roofing material? In my mind that will just speed up the deterioration of the roofing material and possibly the decking it's attached to.
    Ventilation in attics is the most important aspect of proper attic construction. With proper ventilation you don't get the heat build up. With the temperature of the attic staying close to what the outdoor ambient is you don't get the transfer into the home assuming your attic insulation is at or above recommended levels.

    Maybe this whole thing is about ductwork located in the attic. I'm happy to say that we don't have much of that in this part of the country. When we do have to located ductwork in the attic we seal it and insulate it to the same level that the attic is insulated to. R-40 is the norm around here with a lot of attics insulated to R-50.

    I guess before I'd apply anything to the underside of my roof deck I'd make sure my attic was ventilated to 150% of what was minimally recommended and insulated at or above code. With those two things done and if I still had issues with downward heat coming through my attic in the summer I'd either replace my roofing material with something lighter in color or install a power ventilation system for those times when the natural venting couldn't handle the removal of excess heat.

    Just my opinions.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  4. #4
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    If your air handling equipment and ductwork are not in the attic, you are better off just adding more insulation.

    There are conflicting reports, but the DoE, which has no product to sell, and a couple of utilities in cooling dominated states, that are very motivated to not have to build new power plants, recommend the radiant barrier be installed on the under side of the roof deck, or rafters, if the ductwork or air handling equipment are in the attic in cooling dominated climates.

    I have seen a couple of "independent" studies quoted that show the RB is always better on the floor of the attic, but almost everything I have seen in support of it came from companies with a product to sell.

    My personal experience working in attics in North Texas is that the RB blocks more heat from the attic when it is installed on the under side of the roof deck, or rafters. The attic are temperature not always greatly reduced, RBs don't do anything for conductive heat transfer to the air that contacts the roof, but there is a night and day type difference in the radiant heat transfer to objects and surfaces in the attic. I have been in a lot of customers attics before and after the RB installation, and it made me a true believer in having the RB at the roof line in my climate.

    In my last house, just the Sherwin-Williams E-Barrier paint, which is not as good as the best spray on products, which are all >20% less effective than real radiant barriers, dropped my peak attic temperatures by 25.

    Nothing beats a roof that doesn't let the roof decking get hot in the first place.

    This summer I want to do some data logging with a black ball thermometer in the attic, and temp/RH loggers in my ductwork, to get some before and after data on how much the E-Barrier paint actually reduces the radiant heat energy entering my attic, and specifically its affect on heat gain in my duct system.
    I think a lot of people observe that the air temperature isn't necessarily significantly lower with a RB, but air temperature isn't a very good indication of how much radiant heat energy is entering the attic.
    If more government is the answer, then it's a really stupid question.

  5. #5
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    Quote Originally Posted by mark beiser View Post
    This summer I want to do some data logging with a black ball thermometer in the attic, and temp/RH loggers in my ductwork, to get some before and after data on how much the E-Barrier paint actually reduces the radiant heat energy entering my attic, and specifically its affect on heat gain in my duct system.
    I think a lot of people observe that the air temperature isn't necessarily significantly lower with a RB, but air temperature isn't a very good indication of how much radiant heat energy is entering the attic.
    I'd also like to do similar data gathering, although I can't have a baseline of prior to radiant barrier installation like you'll have, since the product is already installed. Nevertheless, where might one pick up a black ball thermometer (I know what they're for...mean radiant temperature - MRT - gathering)?

    I also agree that attic air temperature is one thing, radiant heat transfer is another, since radiant heat does not warm the air, only objects the rays of radiant heat strike. A ventilated attic reduces air temperature, and may to an extent reduce the surface temperature of the roof deck underside and rafters. For that to happen, a significant amount of air must flow through the attic, which has its own caveats when forced air ventilation is considered.

    A radiant barrier is a passive product. It requires no ongoing energy consumption to do its job. Not everyone will want light colored roofing materials; some homeowner associations will not allow it, or metal/standing seam roofs, which in light colors are also reflective of solar heat gain. I have to agree...if HVAC equipment is in an attic in a cooling climate, radiant barrier is a great addition. I will have more empirical data regarding this matter as the year progresses, I hope.

  6. #6
    Join Date
    Jul 2004
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    322
    If you don't believe radiant barriers work, try this simple experiment. Take two identical water glasses, and cover the outside of one with ordinary aluminum foil. Fill each glass with the same number of ice cubes. The ice in the foil-covered glass will take significantly longer to melt. Works with hot water too.

  7. #7
    Join Date
    Feb 2008
    Location
    Lucas, TX
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    103
    Light color roofing material doesn't have that large of an effect over dark colored
    material. The reflectance difference is not large. They are starting to make
    roofing material with a much high reflectance percentage - 30 - 60%, this would
    be a good way also to go.


    Adam Krolnik

  8. #8
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    Aug 2004
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    North Richland Hills, Texas
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    Quote Originally Posted by adamk View Post
    Light color roofing material doesn't have that large of an effect over dark colored
    material. The reflectance difference is not large. They are starting to make
    roofing material with a much high reflectance percentage - 30 - 60%, this would
    be a good way also to go.
    My ideal roof would be a white standing seam metal roof. I don't have the name of the company at hand, but there are colored coatings for metal roofs that will reflect upwards of 98% of the solar energy that hits them, depending on the color chosen.
    Obviously it would have to be kept clean, I own a rather nice pressure washer.

    Unfortunately a metal roof is a little out of my budget range at this time, and for my house, the ROI isn't very good.
    Maybe I'll get "lucky" this spring and have some grapefruit sized hail that damages my roof decking. Then I could use the insurance money to pay for most of the cost of a metal roof, and get a 10-15% discount on my insurance to boot.
    If more government is the answer, then it's a really stupid question.

  9. #9
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    Aug 2004
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    Quote Originally Posted by shophound View Post
    Nevertheless, where might one pick up a black ball thermometer (I know what they're for...mean radiant temperature - MRT - gathering)?
    Good question, I was thinking of making one, but found this:
    http://www.pirometro.com.br/Kimo/FT%...la%20preta.pdf

    Just need to get my hands on one. I'm thinking of using the one with the PT100 thermistor in it so I can mount it in the attic and run wires down to a logger that will read a PT100 probe.
    Sticking to type K thermocouple probes may be more in my budget range though.

    The black ball is ideal for this kind of testing, and should give a much greater idea of how effective a RB is, at reducing attic heat gain, than just looking at the air temperature.

    I had also hopped to have a thermal imager by this time, but the boss decided a new service van, 14' enclosed trailer, and graphics wraps for both vans and the trailer were more important.
    I have seen more than enough comparative thermal imagery of roof decks with and without radiant barrier products to convince me though.

    Even just using a laser thermometer, even though it is very inaccurate for the task, there is an obvious difference.
    Last edited by mark beiser; 03-14-2008 at 05:18 PM.
    If more government is the answer, then it's a really stupid question.

  10. #10
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    Feb 2008
    Location
    Lucas, TX
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    > Hoped to have a thermal imager...

    You missed out.... Saw one many years ago at a college fair. He pointed it at the steam pipes and showed where there was missing insulation and you could see the hot pipe...

  11. #11
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    Aug 2003
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    Fort Worth, TX
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    You could make a cutout the diameter of the black ball out of non reflective insulation board, and stick the ball in so only the top half receives radiant heat. The bottom half would sit in insulation. Hmm...could work?

    Off hand I'd think you'd need some parameters for your data gathering. Sample taken same time of day, sun angle when measurements taken, outdoor ambient temperature and humidity, clear or partly cloudy skies, etc. = attic data for those conditions. But I'm sure you're already all over that. What would be further interesting is to extend that data to run times for your a/c system, along with your planned measurement of output temperature and humidity. The payoff will be when you have similar conditions of observation from one season to the next, with the difference being the radiant barrier addition.

    had also hopped to have a thermal imager by this time, but the boss decided a new service van, 14' enclosed trailer, and graphics wraps for both vans and the trailer were more important.
    What a letdown... *sigh*


  12. #12
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    Aug 2004
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    Quote Originally Posted by shophound View Post
    You could make a cutout the diameter of the black ball out of non reflective insulation board, and stick the ball in so only the top half receives radiant heat. The bottom half would sit in insulation. Hmm...could work?
    That is an idea I have toyed with, but the more I think about it, the more I think it is just fine, maybe best, to have it free hanging in the attic to monitor the true mean radiant temp, rather than trying to limit it to only being exposed to the source.
    Maybe I'll have to get one of each if they are not to expensive. I have a feeling they are going to be pricey though...

    Off hand I'd think you'd need some parameters for your data gathering. Sample taken same time of day, sun angle when measurements taken, outdoor ambient temperature and humidity, clear or partly cloudy skies, etc. = attic data for those conditions. But I'm sure you're already all over that. What would be further interesting is to extend that data to run times for your a/c system, along with your planned measurement of output temperature and humidity. The payoff will be when you have similar conditions of observation from one season to the next, with the difference being the radiant barrier addition.
    I'll have an outdoor temp/RH logger too, and cloud cover can be pulled off the weather data.
    I figured the temp RH logger(s) in the ducts would make it obvious when the system is running.

    I want the results to be somewhat more than just anecdotal evidence, but don't have anything close to the budget, or time, to make it truly scientific.
    I also want the testing to be completely objective, so I want to make sure the sensor choices and placement do not favor an outcome. I know what I believe to be true, but I'm after real data, not support for my opinion.

    What I would really like to do is build a number of sets of different styles of test homes, enough of each style to have a control and several with a variety of RB, ventilation types, and duct/air handler placements, with hundreds, even thousands of sensors per house, but I don't have a government entity or public utilities budget, or the credentials to get the financial backing of one. If I was independently wealthy enough to be able to do it with my own money, I most likely would not care about the subject in the first place!

    What a letdown... *sigh*

    Yeah, but I do like the new van.
    Last edited by mark beiser; 03-14-2008 at 06:35 PM.
    If more government is the answer, then it's a really stupid question.

  13. #13
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    Aug 2003
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    Quote Originally Posted by mark beiser View Post
    Maybe I'll have to get one of each if they are not to expensive. I have a feeling they are going to be pricey though...
    I still have two more float balls in my cooling towers that I need to change out. I'm wondering if I took one, dried it out, spray painted it flat black, drilled out the top where the float rod threads in, and then made a fitting to accept a regular mercury bulb thermometer...how would that work? If it did it could be done on the cheap...sure seems a waste to toss two copper float balls away.

    I'll have an outdoor temp/RH logger too, and cloud cover can be pulled off the weather data.
    I figured the temp RH logger(s) in the ducts would make it obvious when the system is running.
    At the museum I am constantly logging and trending data.Temperature, humidity, dew point, wet bulb, etc. One of the instruments we've used for logging purposes is a system called "Hobo"...it consists of small data loggers than you attach to a laptop to set their parameters, then set in the environment you wish to monitor. The loggers can later be uploaded to the computer, where software is then used to create trend data, etc. Pretty cool stuff. About once a month I meet with the conservator to review the data, and it's helpful to see how well we're doing.

    I
    want the results to be somewhat more than just anecdotal evidence, but don't have anything close to the budget, or time, to make it truly scientific.
    I also want the testing to be completely objective, so I want to make sure the sensor choices and placement do not favor an outcome. I know what I believe to be true, but I'm after real data, not support for my opinion.
    I know what you mean. I've had little pet theories blown apart more than once at the museum by actually letting the data tell me what's going on vs. what I had in my brain. A lesson learned via indisputable data is a lesson not soon forgotten.

    What I would really like to do is build a number of sets of different styles of test homes, enough of each style to have a control and several with a variety of RB, ventilation types, and duct/air handler placements, with hundreds, even thousands of sensors per house, but I don't have a government entity or public utilities budget, or the credentials to get the financial backing of one. If I was independently wealthy enough to be able to do it with my own money, I most likely would not care about the subject in the first place!
    I've had similar thoughts...from a sheer radiant barrier test method of building two mock-up "homes" in my backyard, say a four by four footprint of an attic, with a gabled pitched roof, and a space below sheetrocked and insulated like conventional construction. Put them side by side and install sensors to collect data for varying solar exposure and ambient conditions. Would be interesting. Wouldn't look all that great in the landscape, however.

    It would be great if someone would do what you're proposing, so some good comparison info could be created and analyzed.

    If I were to somehow become independently wealthy, you couldn't drag me out of my woodshop long enough to be bothered with radiant barrier studies, I suppose.

    Yeah, but I do like the new van.
    Saw it the other day at the conference. It does look sweet.

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