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radiant insulation

Discussion in 'Home Power and Microgeneration' started by John & / or Maryln, Feb 19, 2006.

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  1. SQLit

    SQLit Guest

    Most of the claims are just that. Call your utility and see what they
    recommend. I would want the name and number of any contractor that they
    recommend. Please post it here.
    The government pages I have been to do not list radiant foils as a method of

    Insulation works cause there is trapped air in it. The more volume the
    better the insulation.
    Fiberglass, and cellulous are the ones used by the home builders. Rock wool
    not so much now days. So what is a couple of inches of Mylar doing to trap?
    Ahh you say but what about the radiant energy? I say bunk...

    If this stuff really worked, then why are the new home builders using it?
    Could it be that there are alternatives for less money? Or will FHA even
    accept this product? Tyvek the house wrap was not instantly accepted in the
    beginning. These claims have been around for 10 years, obviously some one
    is keeping the industry alive.

    Why would you be worried about radiant heat anyway? If you have good
    insulation above the ceiling it is not a problem.

    I added R-30, about 8 inches, of cellulous to my attic last July, cost
    ~$245. I signed a check. August's a/c bill was 50% less than July. I live
    in the southwest desert.
  2. Having used reflective mylar "space blankets" for personal use, I can't see
    why a foil shouldn't work on a house. I use one on my unheated waterbed.
    It doesn't stop the bladder from cooling down, but it prevents the
    temperature of the bladder from cooling _me_.
    ime, the problem with rock wool has nothing to do with insulative ability
    and everything to do with availability (which, in turn, is limited because
    of shipping costs). I did my entire last house in Roxul, but it's
    unavailable here because you can get 2-3 times as much fiberglass
    insulation in a truck to send it out here (Rock wool basically doesn't
    I think that must have been "why aren't..."?
    Because all heat loss is radiant?
  3. daestrom

    daestrom Guest

    Radiant insulation *can* work, in very specific circumstances. But most of
    the claims are over-inflated and don't really reflect 'typical' performance.
    The DOE has done some studies with these and came to same conclusions.
    Trying to assess its performance by claiming some 'R' value is usually
    useless. Manufacturers set up the most promising conditions, measure the
    performance and publish the 'R' value that would correspond to that. But
    other operating conditions can cut the effective 'R' by a factor of five.

    The radiant surface must *not* be in direct contact with other material on
    one side. It is important to be able to fasten it *under* the rafters in an
    attic, *not* over the flooring (dust accumulation can make the foil about
    useless in just a couple of years if allowed to accumulate).

    Most homes lose a lot of their heat via convection and foil doesn't stop
    that. It can keep an attic cooler in summer to help A/C costs.

  4. What if you don't have tar-paper shingles? I've heard both clay
    and metal roofing products are popular.

  5. Guest

    Well, what's a roof for? Snow and steepness aren't factors in Florida.
    Wind and dehumidification are. New houses might be happier with flat
    EPDM roofs under a few inches of sand or stone, with some water. A few
    plants might provide more shade, but the water they evaporate does more
    to keep their leaves cool than to keep the house cool.

    An airtight Florida house might have a LiCl roofpond under a plastic film
    greenhouse to concentrate LiCl during the day and let it absorb water
    vapor from the house at night to keep the house cooler and drier.

  6. Admittedly, the idea of a new kind of insulation has it's charms for me. And
    when I read that mass insulation just soaks up the heat as much as it can
    hold but then it has to give it up whereas the foil bounces those invisible
    infrared rays right back at you.Well I'm thrilled, but then when I came to
    the bottom line of what percentage of savings I could expect, the numbers
    don't seem to correspond to the theory. So ... I think I may play with it as
    nitetime window covers, but I'm not ready to bury it behind drywall until I
    understand more.
    thanks for your


  7. Guest

    Walk on the dry stone, with water underneath.
    Let it absorb house water vapor through a ceiling hatch.

  8. Refrigerators heat up while they're cooling the contents, too. Surely this
    just requires an intelligent design to ensure that heat flows in the right
  9. Ron Purvis

    Ron Purvis Guest

    A radiant barrier is primarily helpful in the deep south where you have
    almost no heating and a lot of cooling. I live in Greenville, SC and it is a
    tremendous help here. The typical household here will use AC daily at least
    April to October, whereas the heat will only be used sparely for a couple of
    months. In my previous apartment that had pretty good insulation, we used
    the heat for less than 10 days in a whole year. On the other hand, we used
    the AC at least 180 days. If we had a decent radiant barrier on top of the
    normal insulation, we would probably have used the AC only a little more
    than the heat.
  10. Ron Purvis

    Ron Purvis Guest

    That type of analysis is worthless. You can't tell us exactly how much the
    problem is due to poor quality of materials, installation, or manufacturer's
    defect. Also you have no way to show the way that the wind hit the shingles
    differently due to site considerations. The only way to be able to draw a
    reasonable conclusion is to test under controlled conditions.

    Ron Purvis

  11. daestrom

    daestrom Guest

    John, this is *exactly* the kind of 'double-talk' you want to look out for.
    Stuff like, "Mass insulations (foam boards/extruded polystyrene) even though
    they may have high R-values(depending on thickness) will eventually
    stabilize at that constant ground temperature of 55 degrees or lower
    depending on climate. Therefore the mass insulation provides a constant
    drain on the radiant system and the slab. "

    Frankly, if they understood heat transfer, they would know that the slab
    will *always* reach an equilibrium temperature between the heat flowing into
    it from above, and the heat flowing out of it to the ground below. Even
    with their product. The best you can hope for is to raise that equilibrium
    temperature to a comfortable level by reducing the heat flow to the ground
    below. But they're trying to make it look like their product isn't subject
    to the laws of physics and it somehow 'breaks' the heat transfer path
    preventing heat loss from the slab to the ground underneath.

    Aluminum sandwiched between two layers of foam can have some benefit by
    minimizing the radiant transfer across foam cells on one side to those on
    the other. But this same reduction in heat transfer can be achieved with
    simply thicker foam board. But their product does look like it is easy to

    But what is the actual heat transfer coefficient for a layer of this product
    versus a 1/2" or 1" of foam board? I suspect the *real* difference isn't in
    the heat transfer performance, but in the ease of installation, toughness
    and price. It would seem if their product were remarkably better, they
    would say so with numbers from an independent laboratory, not some paragraph
    of mumbo-jumbo about 'thermal breaks' and 'seeking the cold ground below

    Foil insulation spanning under joists of a radiant floor system is the sort
    of niche that radiant insulation works very well. It reduces the downward
    radiant heat transfer to an unheated basement or crawl space. Because of
    the orientation, there is already very little convective heat losses in such
    a situation, and no conductive losses. So now that we're down to just
    radiant losses anyway, this becomes the 'best bang for the buck'.

  12. stu

    stu Guest

    daestrom, I'm in the process of putting a new floor in my house, while I
    have it up I was planing on putting in fibre-glass insulation (about R3 as
    the joists are 4 inches), with foil insulation under the joists. This is
    more for stopping summer heat gain. although another thought I had was just
    to block the sub-floor ventilation on the really hot days.
    any ideas or links you have would be great thanks


    p.s. melbourne australia, so no freezing temps
  13. Ron Purvis

    Ron Purvis Guest


    No is telling you that you have to do anything. You don't want to put a
    radiant barrier under the roof: then don't. However the assumption that you
    saved $10,000 by not doing so is is only a very wild guess. I believe that
    you are vastly over estimating the amount of heat gain in the shingles.
    According to studies that are reported by Oak Ridge National Laboratory, the
    gain is only 2 F to 10F.

    The Florida Solar Energy Center says 2F to 5F depending on color of the
    shingle.There are warranty statements from Elk, GAF, Celotex, Owens Corning,
    Certainteed, and Tamko that allow the use of TechShield as roof decking
    without affecting their shingle warranties as long as the shingles are
    installed according to their installation instructions. (See

    I think that you looked at something you didn't understand and so you made
    assumptions that are completely invalid. Kind of like the ancient Norse
    heard thunder so Thor was out there in the heavens making the noise. If you
    choose not to find out the actual facts, so be it. You are as free to
    believe what you wish. Just like the member of the Flat Earth Society.
  14. Yeah, you're probably right. Theory is great - in theory :)
  15. Ron Purvis

    Ron Purvis Guest


    Anyone who knows me, knows stretching the truth is not what I do.. I said
    what I believe is the truth. Dropping the amount of heat that entered the
    apartment from radiation by up to 50% would be very significant. For a fair
    part of that period we only used the AC for one to two hours daily. All of
    those days would not need the AC if a radiant barrier was in place. The vast
    majority of the days, the AC was in use for less than 3 hours. Again, I
    think that if a radiant barrier had been in place we could have made due
    with fans and not used the AC at all on those days. We would have been able
    to use fans to keep it bearable.

    Yes, I do think that we could have made do only using the AC a little more
    than the heat if we had a good radiant barrier on top of the normal
    insulation that is already in place. Other areas will have different
    results. The Greenville area gets hot in the afternoon but the mornings and
    evenings gets cooled off from the mountain air to the north of us. If you
    can keep the sun from radiating heat into the home, you can save an awful
    lot on cooling the place. Other areas of the country, even others of SC,
    will be different in the results they achieve. However, the ORNL report
    says "The tests to date have shown that in attics with R-19 insulation,
    radiant barriers can reduce summer ceiling heat gains by about 16 to 42
    percent compared to an attic with the same insulation level and no radiant
    barrier." That is nothing to sneeze at. Especially if the largest part of
    the heat gain is from radiation.

    As for your last comment, I can back up everything I have said. You can
    check everything I say by doing research and find that I am correct. In none
    of you posts that I have seen, has there been any rational basis for you
    posts. So keep your quotes to yourself.
  16. daestrom

    daestrom Guest

    Yes, I agree that your product can certainly be easier to install and
    probably suffer less damage in the process. I said as much in my last post.
    And as you point out, it combines two construction needs in one easily
    installed product (both a layer of insulation and a vapor barrier).

    But that isn't really so much a result of using a radiant barrier in the
    middle of it, as it is the particular foam material and polyethylene barrier
    and PE foam laminate, right?
    For the 1/2" thick product? Compared to a 1/2" layer of rigid foam at R
    values between 2 and 3.3. While your stuff looks like a good and useful
    product, the layer of "97% pure aluminum" doesn't seem to really add much in
    the way of insulation value. Perhaps it adds to the overall toughness and
    ease of installation (or vapor blocking), but as a 'radiant barrier' it
    doesn't seem to do much for it.
    Thanks, but I can't seem to get that link to work. Lowe fishing boats?

  17. daestrom

    daestrom Guest

    One of the problems with radiant barriers is they lose their effectiveness
    if they get dust/dirt on the radiating surface. (dust/dirt radiate IR about
    as good as ordinary building materials) So to maintain it's effectiveness,
    most manufacturers recommend installing on the upper side of the space to be

    This means it works good under floor joists to minimize radiant losses
    downward from a floor into a crawl space. Or mounted on the rafters in an
    attic so hot roofing doesn't radiate energy down onto the insulation on the
    attic floor.

    But rolling it over the top of attic insulation, it will get dirty and lose
    it's effectiveness pretty soon.

  18. Ron Purvis

    Ron Purvis Guest


    Sorry for the way I worded my post. I did not mean laying the radiant
    barrier on top of the attic insulation. What I meant by saying "on top of",
    was "in addition to". It is just the local manner of speech. I do understand
    that you just can't lay the radiant barrier on top of the insulation that is
    lying on the attic floor. It must be properly installed or it will lose its
    effectiveness in a fairly short period of time. I would also like to point
    out that most people tend to ignore placing the radiant barrier in the
    walls. In an area like mine, this could be a big help.
  19. daestrom

    daestrom Guest

    Ah.... 'low-e'. In the other link, somehow the '-' got lost and ''
    is for something entirely different. Now that I have the right URL, I'll go
    read the pdf.

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