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  1. #1
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    Spray foaming attic roofline vs using Batt insulation on roof line?

    In all the energy effciency threads on this board. I always see the recommendation of using spray foam to seal the attic envelope. Why isn't batt insulation ever recommended? It is of course less R value, but also a LOT cheaper!

  2. #2
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    The foam is so much tighter than the batt insulation. And depending on how much i really don't think the foam is all that much more expensive at least in my area. They are pretty much neck and neck on the costs. Myself im torn between the two options but am leaning toward batt for a remodel and foam if it's a new construction. But forgive me if im wrong but isn't the r value of foam less than batt insulation?

  3. #3
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    Bats do nothing ot stop air leaks and the mosture it can carry and further, for a roof deck, I'd go wth closed cell. The wood deck is like a wick for moisture diffusion into the attic. Closed cell with slow it but still trnasmitt a LOT of moisture. I know form experience.

    On top of it, in order to get even R20, you need a lot of batts which reduced headroom in a oftne already cramped attic space. Ideally you get to R30-40.

    I think if possible, I'd insulate above the roof deack and put i na cool roof design, then insulate under it to seal it up completely.

  4. #4
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    Quote Originally Posted by newstudent View Post
    In all the energy effciency threads on this board. I always see the recommendation of using spray foam to seal the attic envelope. Why isn't batt insulation ever recommended? It is of course less R value, but also a LOT cheaper!
    That should read:
    It is of course less R value, has practically no vapor barrier, won't work, but also a LOT cheaper!

    If I remember correctly, in your previous thread, we went over this.

    I think your really the pink panther in disguise.
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  5. #5
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    Quote Originally Posted by newstudent View Post
    In all the energy effciency threads on this board. I always see the recommendation of using spray foam to seal the attic envelope. Why isn't batt insulation ever recommended? It is of course less R value, but also a LOT cheaper!
    Where is your Life Cycle Cost analyis indicating that Long Term is more expensive?

    Short term thinking gives short term solutions.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
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    Obviously it is less expensibe up front do u have any studies proving your point?

  7. #7
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    Have you ever tried installing fiberglass batts over your head? Loads of fun.

    Already mentioned above by my colleagues why foam on a roof deck is superior to batts is air and vapor resistance. Batts seldom perform to their rated R value because of these things. Also, the hotter figerglass becomes, the more its R value degrades. Foam does not have this problem.

    Finally, permanence. Foam will outlast batts on a roof deck many times over.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
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    Ok, that makes sense, are there any studies out there that show how much better foam is than fiberglass bats? If it is 3x the cost but only 20% more efficient, the return on investment might be very long. So long as to not be economically beneficial.

  9. #9
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    Quote Originally Posted by newstudent View Post
    Obviously it is less expensibe up front do u have any studies proving your point?
    There is a whole website dedicated to building sciences. http://www.buildingscience.com/index_html That's a good start. Then there is simple logic. Air movement carrier sensible heat energy and moisture, latent heat energy. Fiberglass is a nice air filter, but does not stop air movement. Foam does. The R value/in of foam is higher, so for a given cavity thickness, you get more R value. Foam does not compress or degrade significantly over time, and therefore hte R value stays constant. IF Fiberglass gets damp, or compresses, it loses R value. Further, if air moves through fiberglass, its R value also drops dramatically. That's why you find that 1980's and 90's homes with fiberglass underperform long term compared to calculated values. These homes seem to struggle the most in extreme weather as stack effect and reverse stack effect play a major role in air leakage rates.


    Remember, many, if not most things doen commonly in the building trades are not better. It's simply easier and/or cheaper or requires no regular maintenance for a homeowner and is therefore disposeable. Remember, nothing lasts forever. Its' either serviceable or disposeable. While there are some benefits to their modern counterparts, I wouldn't nessesarily call them superior. Plaster (more durable, anti-microbial, no dust during installation), Tile & slate roofs and copper gutters & flashing (100 year lifespans, more tolerant to water exposure (doesn't dissolve in water like gypsum), real stucco (100+ year lifespan with minimal maintenance, low air leakage rate, some insulation value and thermal mass), Brick & stone walls (thermal mass, durability, indefinite lifespan with tuckpointing every 50-80 years, self healing with softer lime based mortar), wood claboard siding (can be refinished indefinitely if cared for).

  10. #10
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    Quote Originally Posted by newstudent View Post
    Ok, that makes sense, are there any studies out there that show how much better foam is than fiberglass bats? If it is 3x the cost but only 20% more efficient, the return on investment might be very long. So long as to not be economically beneficial.
    Florida Solar Energy Center (FSEC) and buildingscience dot com are full of studies pertaining to insulation. A lot of our replies are based on our own consideration over these studies, plus field experience. I don't know where you are getting your 20% number above. Several variables come into play when considering batt fiberglass performance for a given R value vs. the various foam options available.

    For your discussion, if you're mulling over roof deck insulation because the building in question is in a predominantly cooling climate, consider "cool roof" options vs. insulating the roof deck. One FSEC study shows a reflective metal roof outperforming a foamed roof deck with equivalently built homes with ducts in the attic. This performance metric included peak cooling demand and overall reduction in cooling costs. While the foamed roof deck delivered the coolest attic temperatures, that came at the expense of placing the entire attic, plus the added surface area of the roof deck itself, into conditioned space. Overall heat flux through the insulated roof deck was higher than the insulated ceiling plane with reflective roofing, at the same R values. Not even the reduced heat gain to the ducts in this attic were enough to offset the overall gain through the deck.

    Are you beginning to see, if you want to approach attic temperature control with quality of thought, that there is little room for easy, slap-it-up-there solutions?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
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    "Are you beginning to see, if you want to approach attic temperature control with quality of thought, that there is little room for easy, slap-it-up-there solutions? "

    none of my questions involve slap it up there solutions. All of them are expensive procedures well over $1000. ..... and the 20% was just a random number.

    I found this one article at building science
    http://www.buildingscience.com/docum...oof-assemblies

    but it does not address foam vs fiberglass batt effciency

    "Then there is simple logic. Air movement carrier sensible heat energy and moisture, latent heat energy. Fiberglass is a nice air filter, but does not stop air movement. Foam does."


    OK, now quantify that statement, how much more effcient is foam vs batt insulation and what is the ROI?

  12. #12
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    This link has some info... but don't have links to specific studies off hand. Specific hard numbers would depend on your air leakage and what each insulation type costs you. IF you have good compettive spray foam guys in your area, it will be a closer comparison. At some point it also matters what you want to accomplish in terms of performance.


    http://www.creativeconservation.com/...ted-attics.htm

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