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  1. #1
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    Nov 2011
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    Exclamation 2x6 Rafter Bays - Cathedral Ceilings

    I have a 470 sqft finished attic space. Everything is in good condition, drywall looks good. Whoever finished this attic nailed the drywall directly to 2x6 rafters leaving a 5.5'' (I think) space for insulation. It is currently insulated with old R-11 fiberglass.

    What are my options? Tearing down the drywall, or roof isn't an option.

    Should I just leave it alone?

    This is a non vented roof, on an old Tudor home (no signs of moisture or rot).

    PLEASE HELP!

    Last edited by FuZe; 12-18-2011 at 04:36 PM.

  2. #2
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    May 2011
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    I'd just leave it for now since it's a small space.. uless it's really cold up there in winter. IF there' no air movement form leaks, the old fiberglass probably performs as well as R20 in a vented attic where there's air movement. Make sure all ceiling penetrations for electrical if any are sealed really good.

    The only imrprovement you could make would be to pull down the drywall and use spray foam.


    Funny how 80+ y.o houses with sealed attic have roofs that are just fine (not rot or moisture issues. And moderns homes with vented attics and tons of insulation seem to have all the problems.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  3. #3
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    Quote Originally Posted by motoguy128 View Post
    I'd just leave it for now since it's a small space.. uless it's really cold up there in winter. IF there' no air movement form leaks, the old fiberglass probably performs as well as R20 in a vented attic where there's air movement. Make sure all ceiling penetrations for electrical if any are sealed really good.

    The only imrprovement you could make would be to pull down the drywall and use spray foam.


    Funny how 80+ y.o houses with sealed attic have roofs that are just fine (not rot or moisture issues. And moderns homes with vented attics and tons of insulation seem to have all the problems.
    Thanks, Glad to hear this!

  4. #4
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    May 2011
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    Look at it this way. The ceiling is probably about 500sqft (since it's sloped). IF it's -5F outside and 70F inside, Then with R11, the heat loss is 3400 BTU/hr. With R20 it would be 1900 BTU/hr. IF you have a house that has a total design heat loss of 70k BTU, that's a 2% difference in heat loss. So if you total heating bills are $1000 for the winter. You'd save a whopping $20 by tearing down the ceiling and re-insulating it.

    I know $20/yr defnitely isn't worth a major tear down.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  5. #5
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    Quote Originally Posted by motoguy128 View Post
    Look at it this way. The ceiling is probably about 500sqft (since it's sloped). IF it's -5F outside and 70F inside, Then with R11, the heat loss is 3400 BTU/hr. With R20 it would be 1900 BTU/hr. IF you have a house that has a total design heat loss of 70k BTU, that's a 2% difference in heat loss. So if you total heating bills are $1000 for the winter. You'd save a whopping $20 by tearing down the ceiling and re-insulating it.

    I know $20/yr defnitely isn't worth a major tear down.
    great work putting that into numbers.
    My advice and opinions come from hands on knowledge...and This Old House Hidden Content

  6. #6
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    Quote Originally Posted by motoguy128 View Post
    Look at it this way. The ceiling is probably about 500sqft (since it's sloped). IF it's -5F outside and 70F inside, Then with R11, the heat loss is 3400 BTU/hr. With R20 it would be 1900 BTU/hr. IF you have a house that has a total design heat loss of 70k BTU, that's a 2% difference in heat loss. So if you total heating bills are $1000 for the winter. You'd save a whopping $20 by tearing down the ceiling and re-insulating it.

    I know $20/yr defnitely isn't worth a major tear down.
    You are a genius...

  7. #7
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    May 2011
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    If people did some basic calculations...we're talking like 5th grade math here... you just need the formulas involved.... they might realize that a LOT of energy efficiency projects aren't worth the time and expense it.

    They make sense for new construction and during major demolition when you have walls open anyway. But gutting a house and even drilling holes every 16" just for the purpose of adding some insulation that might cause other building envelope problems like trapping moisture. Not good economics.

    Don't get me started on how replacements windows have in reality a 50-250 year payback over just installing storm windows.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  8. #8
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    Default Re: 2x6 Rafter Bays - Cathedral Ceilings

    Here's some basic math based on some numbers provided throughout this thread.

    Calculate Wall Loss Rate In BTU per hour

    R11 --- 470 sq ft X 75 / 11 = 3204.54 btu/hr

    R21 --- 470 sq ft X 75 / 21 = 1678.57 btu/hr ( about 48 % reduction in heat loss )


    Calculate Loss Per Day

    R11 --- 3204.54 X 24 = 76,908.96 btu / day

    R21 --- 1678.57 X 24 = 40,285.68 btu/day


    Heat Loss Cost for 90 days ( based on cost of $12 per 1 million btu of natural gas )

    R11 = $83

    R21 = $43

    This is only for this space alone and does not reflect the other heat loss costs for the rest of the home.
    Also , this only reflects the heat loss cost and does not reflect the heat gain costs for air conditioning.

    Other factors --- if the square footage is based on floor space and not the actual sq ft of wall and ceiling surface area --- the square footage in the calculations would need to be adjusted higher. Which would show larger heat loss numbers. If the existing insulation is *old * as stated then it's not likely at an R11 value but at a lower R value .
    "" an ounce of perception -- a pound of obscure "
    - Rush

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