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  1. #1
    Join Date
    Jul 2002
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    Foam vs Fiberglass

    We are building an addition ourselves (literally) and have researched foam vs fiberglass insulation. I would love to go with Foam insulation, but the price of it might put it out of our range. I know that foam insulates much better than fiberglass because it restricts air flow.

    I was thinking, if I took the time to caulk every seam where the OSB panels meet, seal the top and bottom plates, pretty much caulk every gap – wouldn’t that eliminate all the air flow. I also have the exterior of the house tyveked.

    Would this be a way we could use fiberglass and get close results to that of foam. In effect, If I do a perfect job sealing, there shouldn’t be any air flow – correct?

    Any opinions?

  2. #2
    Join Date
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    You may be able to get it tight, but you'll never be able to achieve the same tightness as spray foam.

    I had a guy spray foam my shop. It's the tightest, most energy efficient building I've ever been in. I keep the temperature real low, and it uses about 1/4 tank of oil a season.

  3. #3
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    You may get close to what foam would do.

    Any penatrations ,like wiring and plumbing in the top or bottom plates should be sealed ,as well as around windows and doors.
    Last edited by dash; 02-25-2008 at 12:41 PM.

  4. #4
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    The other thing to consider is humidity. Make sure you have a good vapor barrier, or you might degrade the R-value of fiberglass.

  5. #5
    Join Date
    Nov 2007
    Location
    Tucson AZ
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    Check out blown cellulose

    There are many reasons for going with fiberglass in bats, but as you note, sealing the volume of the insulation is needed, and this is not easy.

    Foam takes care of the "easy" part because it will fill all the potential leak points automatically.

    Foam gets some of its efficiency because it is effectively solid and so does not allow any circulation of air currents or the transfer of moisture. Fiberglass is by nature a loose collection of fibers and will allow air circulation (think of the permeability of a cheap furnace filter).

    An alternative is blown in cellulose. It is inexpensive, does not allow easy air circulation within it. Only drawback is that it is not a natural moisture barrier, so it is no better than fiberglass in that respect.

  6. #6
    Join Date
    Dec 2006
    Location
    Dallas
    Posts
    189
    Where are you located? You should also factor in windows as a cost benefit analysis parameter. You can create a 3D plot of fibergalss vs foam, vs window investment. Somewhere in there is a sweet spot for best ROI.

    If you ever go to sell the home, you are unlikely to recoup the foam investment for an addition.

  7. #7
    Join Date
    Jul 2002
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    79
    I live in Pittsburgh, PA.

    We have replaced all of the windows in teh existing house, and of course we will be putting in all new windows in the addition.

    so Fiberglass may be OK, if all is sealed very well?

  8. #8
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    Fiberglass will be ok, just not as good as foam.

  9. #9
    Join Date
    Feb 2003
    Location
    Huntsville,AL
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    are all of your studs straight?
    are all exactly the same distance apart?

    if not, then the bats may leave gaps!

    why not put a layer of 2" tk T&G Styrofoam on the outside & keep the ambient out ?
    & put in cellulose --

    I hope you did not put in vinyl windows --
    cheap ones sag, & gauld.

    faom does add some structural strength.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  10. #10
    Join Date
    Jul 2002
    Posts
    79
    Studs look good. Too late for the exterior foam insulation as I put the siding up already.

    Could I do a layer of 1" foam insulation on the inside, then do batts??

  11. #11
    Join Date
    Jul 2002
    Posts
    79
    Studs are pretty good as I framed everying myself. The lumber that came from Lowes was excellent. Very, very straight lumber. Was kinda unbelieveable

  12. #12
    Join Date
    Feb 2003
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    Huntsville,AL
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    no-
    the secret to ALL insulation is the entrapment of air -- then having small pockets of air to prevent transfer by convection

    if you crush the fiberglass bats, you change the R value!
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  13. #13
    Join Date
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    Quote Originally Posted by cem-bsee View Post
    no-
    the secret to ALL insulation is the entrapment of air -- then having small pockets of air to prevent transfer by convection

    if you crush the fiberglass bats, you change the R value!
    Does that include spray foam?

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