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  1. #1
    Join Date
    Mar 2006
    Posts
    11
    I know this is off topic and if there is a better forum to do this in, please let me know. I was wondering if reflective insulation works or is this some sort of gimmic. I live in Maryland as for climate. I am getting an addition put on and i was going to put this against the stud work and then insulate with fiberglass batts too. I have read the philosophy behind it and it makes sense, but that doesn't always correlate to reality.

  2. #2
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    Have you been to Building Science? I've yet to see bad information there.

  3. #3
    Join Date
    Jun 2004
    Posts
    459

    does it work

    Heat is conveyed by three methods, conduction, convection,and radiation. Since all matter containing heat trasmits radiant heat energy, the energy will pass through the air without affecting it; radiant energy must be absorbed first by an object. Radiant energy cannot pass through reflective material, like a mirror, the reflective surface reflects it back. Low-E glass is a good example of using a reflective material to put radiant energy in check. If I'm not mistaken, I believe glass makers use a very thin layer of metal oxide added to the glass when manufactured. This is what gives low-e glass its 'mirrored' image when looking at it.

    Yes it does work, however I do not know how cost effective it is in residential construction. You could achieve the same results by stapling aluminum foil (shiny side toward living area) before finishing the wall. lol

    [Edited by re2ell on 03-16-2006 at 10:20 AM]

  4. #4
    Join Date
    Jul 2004
    Posts
    322
    It absolutely does work if done right. You need an air space next to the foil. see http://www.ornl.gov/sci/roofs+walls/radiant/index.html

    It works better in the roof than walls. Also works better at blocking heat in hot climates than retaining heat in cold climates.

    A simple experiment will prove this. Fill two large identical glasses with exactly the same number of ice cubes. Wrap aluminum foil around one the glasses. The ice will last substantially longer in the foil-wrapped glass. Also works with hot water.

  5. #5
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Radiant barrier

    I am a homeowner in a hot-humid climate and recently all the new homes in my neighborhood are being built with radiant barrier (RB) in the roof decking. I believe it would be excessively cheap construction to omit it here in S.Texas -- not so sure about your locale. With new construction, there is no labor cost and modest material cost, just specify roof decking manufactured with the reflective foil on one side. This foil will reflect approx. 99% of the radiant energy back outward. According to a study by FSEC (Florida Solar Energy Center) in their state they measure between 8% and 12% electricity savings because of using RB. You will be able to feel a tangible reduction in heat when you go into your attic in summer.

    I had always thought of this as a great cooling-season method, I understand studies have shown lesser benefit in heating conditions. Whether it is still worth doing, I am not prepared to say for your case.

    With old construction, the labor saving method is a special water based paint applied to the attic. The materials are somewhat costly but it can be sprayed on in a day. With spray the coverage is good, however it requires a licensed contractor -- no DIY possible. The paint is at best 70% reflective, but the labor savings have pretty much obsoleted the old method below.

    The oldest RB method is a heavy foil which comes in 4-foot width rolls, 100 or 250 feet in one roll. It is 99% reflective (as is your kitchen foil) but very labor intensive to staple it up along the roof line. Some rooflines are simpler than others, and require less labor. The only reason you cannot use kitchen foil is that it tears easily, the commercial product is fiber reinforced and won't tear. Otherwise kitchen foil has the same reflectivity. Some people advocate laying it flat on top of attic insulation, I think this is a poor idea because the side facing the airspace *will* surely get dusty and it will lose much effectiveness.

    You can buy foam boards with reflective foil on one side. I believe the only drawback to this, is much higher material cost than the foil in rolls. In this application I think the foam part is useless as insulation, but it is handy structurally.

    Surprisingly RB works well with the airspace above or below the foil. But where it touches solid material on both sides it will not work.


    Hope this helps -- Pstu


  6. #6
    Join Date
    Mar 2006
    Posts
    11

    Moisture Problems?

    Are there any moisture problems that will come about with having such a solid insulation? Are there ways around it? I have newer house (built in 89) and stapling it to the rafters from the inside would not be that difficult. There are so many different types, which one should I use for this application?

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Moisture issues

    Here is where I would like to have a building science expert second my opinion. Radiant barrier (RB) does not need sealed gaps to work effectively, in fact it does not care. Moisture cares about sealing, what you want to avoid is a vapor barrier in the wrong place.

    My foil type RB has thousands of tiny holes to prevent it from being a vapor barrier, no matter how I install it. If you were to install the foam board type, and leave a certain gap between pieces, I think you could guarantee no moisture problems from your project. I would think in terms of a half inch gap or so, just enough to allow air to circulate if it wants to (and with temperature or humidity differences, it *will* want to migrate).

    Our usual practice is to employ a ridge vent at the roofline, or alternatively a series of discrete roof vents very near the top. And our regional code asks for an equal vent area at the eaves, there has been much debate about whether this code does any good in hot-humid climates (and the code is violated quite often too, with few ill effects). The ideal is that air gets warmed between the RB and roof decking, then migrates up and out the roof ridge. In your climate you probably are needing to ventilate already, I just wanted to mention it.

    I would also do some net research about what works in your climate. Building Sciences Corporation ("BSC"/Joe Lstiburek) has published a book titled "Building Guide for Hot-Humid Climates" and also there is one for three other climate types. Maryland ought to be in "Mixed-Humid Climate" I think. There ought to be some well educated gurus who can judge the climate issue, published out there. I'll let you know if I find anything, but give me plenty of time.

    Best wishes -- Pstu


  8. #8
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,448
    - under roof radiant barrier
    - living space walls and ceiling radiant barrier and insulation combined will give you the best result.

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