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  1. #11
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: Insulating a large cathedral ceiling

    Cold Canadian, floors are different than walls and ceilings. In walls, there is a cold vertical wall and a warm vertical wall. The air against the cold wall cools and falls, the air next to the warm wall warms up and rises. The air that falls ends up next to the warm wall at the bottom and the warm air that rises to the top ends up next to the cold wall, The air is in constant circulation.

    In a ceiling, the bottom surface is warm and the top surface is cold so there is circulation there as well. In both the walls and the ceiling, the added R3 for a 3/4" space is the maximum available. Increasing the air space does not increase the insulation value.

    But in a floor, the top surface is warm and the bottom surface is cold. The air next to the top surface warms up and stays there, the air at the bottom cools down and stays there. For that reason, as the spacing between the boundaries increases, so does the insulation value.

    So yes, it sort of works the same, only better. You just have to take in the dynamics that occur around the rim joist. The rim joist needs to be insulated and sealed if possible.

  2. #12
    Join Date
    Dec 2012
    Location
    Southern Ontario Canada
    Posts
    38

    Default Re: Insulating a large cathedral ceiling

    Much thanks,

    Your 110% correct about that area of the rim joist, sealing where the rim joist meets the floor and the sill meets the foundation wall is area of high air infiltration on old construction.

    SC

  3. #13
    Join Date
    Dec 2007
    Location
    MN
    Posts
    455

    Default Re: Insulating a large cathedral ceiling

    Quote Originally Posted by keith3267 View Post
    You are right that additional insulation reaches the point of diminishing returns very quickly, but the codes are not made on return on investment calculations, they are for the most part political. You can make a case though that there is an unseen ROI. If no one increased their insulation, then there would be more demand for energy and the cost of energy would go way up. Because so many people increase their insulation and furnace efficiencies, the cost of energy is lower and that reduces the visible ROI.
    Lol ! I wouldn't say the required insulation value is political. Try living in a place with cold winters and see if you think the same thing.
    The minimum required R value is based on a return on investment; for comfort and reduction related to HVAC operating cost and size. Exceeding the minimum amount of insulation isn't necessarily a bad thing if you can bring the heat loss or gain down to the theoretical zero mark. In most cases the cost of insulation isn't all that expensive and as one regular here , Canuk , says it doesn't cost a dime to operate or maintain.
    The cost of energy isn't going down.

  4. #14
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,381

    Default Re: Insulating a large cathedral ceiling

    Quote Originally Posted by bsum1 View Post
    Lol ! I wouldn't say the required insulation value is political. Try living in a place with cold winters and see if you think the same thing.
    The minimum required R value is based on a return on investment; for comfort and reduction related to HVAC operating cost and size. Exceeding the minimum amount of insulation isn't necessarily a bad thing if you can bring the heat loss or gain down to the theoretical zero mark. In most cases the cost of insulation isn't all that expensive and as one regular here , Canuk , says it doesn't cost a dime to operate or maintain.
    The cost of energy isn't going down.
    Vermont and Upper Michigan cold enough for you. I also think you misunderstand me, although I can see why if you are only reading my posts to this thread. I am not totally against additional insulation, but I want people to understand that throwing on more insulation does not always give the expected returns. In fact it rarely does. But "green" groups pressure local authorities to raise the requirements for insulation and that is political.

    A wall insulated to R-11 is going to reduce heat loss by about 91% compared to R-1, A wall insulated to R-19 is going to reduce heat loss by about 95% compared to R-1. Not much difference is there. OK this is an unfair comparison as even an uninsulated wall has two surfaces and a cavity so it will actually have an R value of around R-4, not R-1 so going from uninsulated to R-11 saves about 63% on heat loss through that wall and going to R-19 saves about 80%.

    A ceiling is a different story. It normally faces an unheated and ventilated attic. There is only one surface and no cavity to trap air. So it starts off around R-1. An R-19 saves about 95% of the heat. Going to R-30 saves about 97% and R-38 saves about 97.5%. People expect a huge savings going from R-19 to R30 or R38 and as you can see, the difference just isn't there.

    As for the cost, there is a big difference between the cost of additional insulation in new construction vs. a restoration vs. adding insulation just for the sake of adding insulation to existing construction. Its much cheaper with new construction, a little higher for restoration, but to tear out walls and add insulation and support for it and then redoing all the finishing just for the insulation is usually not justified by economics.

    If you want to increase the energy efficiency of a home, I strongly recommend that you have someone use a thermal imaging camera to map out the areas of high heat loss. You might find that very small amounts of strategically placed insulation will save much more money that an additional thick blanket of insulation over everything, especially if you miss the significant heat loss sources, which is often done.

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