Insulating a bonus room cathedral ceiling

Discussion in 'Insulation and Radiant Barriers' started by wargle, May 5, 2013.

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  1. May 5, 2013 #1

    wargle

    wargle

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    My son's house is in central Alabama. It has a pyramid style roof with a gable roof over the attached garage. The unfinished bonus room is over the garage and has a cathedral style ceiling. the rafters are 2x6. It's 68" from the top of the knee wall on each side to the collar ties. The collar ties are 8' above the floor. the floor is on 2x12's and is insulated over the garage. He wants to finish the bonus room and we don't know what to do about insulating the ceiling from the knee wall to the collar ties. There is no ridge vent on the roof over the bonus room and the rest of the roof is vented with two of the old fashioned turbines and there are sofitt vents all around the house. I can furr down with a 2x on the rafters to make them 7" deep. I am considering doing this and using some R30 batts slightly compressed with a 1/2" foam panel at each end of the batts to stop any air flow through the batts. The knee walls will also get batts and 1/2" drywall over all. Will this work since there is no ridge vent over the bonus room? The small area above the collar ties will be open to the pyramid portion of the roof as will the area behind the knee walls. I don't want to have to deal with trying to put up some sort of additional vent on the 12-12 pitch pyramid roof and spray foam insulation is too $$$.
     
  2. May 5, 2013 #2

    robertwc

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    I understand that you may think that spf is too much money. But, consider this. If you use spf then you would not have to vent the attic portion or incerase the size of your ac. What you suggested is not a bad approach, but if you check out some other homes that have spf in them you would see that your heat gain would be minimalized with it. In this day and age most people are looking for their best bang for the buck. With spf your electricity cost for adding the bonus room would be minimal compared to any other form of insulation. Spf will also allow the room to be more comfortable in the heat of the day even in the middle of the summer.
     
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  3. May 6, 2013 #3

    nealtw

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    If you don't take the advice givin by robertwc, your plan is the way to go but if the attic is connected to the higher roof it does not want vents. The question is, are the vents on the higher roof big enough to handle both.
     
  4. May 30, 2013 #4

    Perry525

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    Roof vents are not a good idea.
    When the wind blows over a roof, the roof acts like an aircraft's wing trying to lift off, this effect creates a low pressure area to the lee of the roof, that pulls air into the roof and out of the roof.
    This means on hot days hot air is pulled into the home.
    On cold days cold air is pulled into the home.
    It is better to have a sealed air tight roof, with plenty of polystyrene board below it, to provide insulation, from the sun's heat coming inwards and your expensive warm air going out during the winter.
     
  5. May 31, 2013 #5

    WindowsonWashington

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    If you don't vent it, SPF is best and easiest. If you do vent it, consider furring down the framing as you mentioned to get additional rafter total depths so that you can properly insulate while still leaving a 2" ventable space.

    Rigid foam works well in these cases.
     
  6. Jun 1, 2013 #6

    Perry525

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    Alabama is warm with high humidity.
    I imagine you want to keep the heat out?
    Keep in mind that the heat of the sun is mainly transferred by conduction.
    You will want the rafters and the roof isolated from the air inside.

    Leaving a two inch gap between the roof and the insulation, in most cases will provide a gap for air movement, to dry off any rain that may seep through. In this case you will encourage hot air into the room.

    You want to totally enclose the roof and rafters to stop the transfer of heat, (as much as you can) through the roof. Keep in mind that 13 inches of polystyrene will stop most conduction, but you have to be practical and probably settle for less. Filling the gaps between the rafters, then covering the rafters with three inches of polystyrene, then adding more rafters and more polystyrene will make a real difference.

    Then you need to think about the walls facing the sun.
     
  7. Jun 1, 2013 #7

    WindowsonWashington

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    Providing for venting is not for the purpose of drying out any rain that might enter the roof structure. That should be handled by the shingles.

    A picture of the exterior would help as well.

    How is the ceiling height on the interior? That will determine whether or not furring down the interior is worthwhile.
     
  8. Jun 1, 2013 #8

    Perry525

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    Many people over the years, in many places, have found that shingles do not keep out the rising snow melt on their roofs.
    While this is certainly not a problem in Alabama?
    Kindly advise why you thing a two inch gap is desirable?
     
  9. Jun 1, 2013 #9

    WindowsonWashington

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    The two inch gap between the underside of the sheathing and the insulation would be recommended if the roof system were vented via soffit and ridge vent combination.

    If the assembly is not vented, insulate the entire cavity and preferably with spray foam.

    Venting of the roof assembly has nothing to do with snow and everything to do with humidity.
     
  10. Jun 2, 2013 #10

    GBR

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    2" is best, but not in a hot/humid climate;
    "In vented cathedral ceiling assemblies a minimum 2-inch clear airspace is recommended between the underside of the roof deck and the top of the cavity insulation. This is not a code requirement but ought to be (only 1-inch is typically specified in the model codes). It is the author’s experience that typical installation practices and construction tolerances do not result in an airspace of at least 1 inch and rarely is it “clear.” Even when 2” clear space is provided, the rate of ventilation flow will be significantly less than in an open ventilated attic."

    "In hot climates, the primary purpose of attic or roof ventilation is to expel solar heated hot air from the attic to lessen the building cooling load. The amount of cooling provided by a well ventilated roof exposed to the sun is very small. Field monitoring of numerous attics has confirmed that the temperature of the roof sheathing of a unvented roof will rise by a few to no more than 10 F more than a well ventilated attic." From; http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation?full_view=1

    Test results show increasing the ventilation rate from 1/300 to 1/150; pp.7---- almost no difference in shingle temps; pp.8----4*F difference in plywood sheathing temp., pp.10--- less than 1% annual net effect for heating/cooling; http://www.buildingscience.com/documents/reports/rr-9801-vented-and-sealed-attics-in-hot-climates

    From pp7; " With 1:300 vent area, adding the radiant barrier system
    reduced the ceiling flux by 26%. Increasing the vent area to 1:150 improved the reduction to 36% (an additional 10 percentage point reduction) while increasing the relative humidity in the attic by
    6-10%. The average humidity without the radiant barrier system and with 1:300 was 53%, when the radiant barrier system was added the average was 54%, and when the vent area was increased
    to 1:150 the average relative humidity increased to 62%." From; http://www.google.com/url?sa=t&rct=...sg=AFQjCNHp4qU3vWO1vNceVxn2LqAJmH81PA&cad=rja

    Gary
     
  11. Jun 3, 2013 #11

    nealtw

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    If you're in a colder climate the venting has everything to do with snow and ice dams as well as humidity.
     
  12. Jun 3, 2013 #12

    nealtw

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    GBR: This house has closed cavities where the jack rafters meet the hip, so unless the ceiling is completely rebuilt there would be no way to ventulate the roof properly.
     
  13. Jun 3, 2013 #13

    WindowsonWashington

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    True, but it does still get cold in FL (certain parts).

    While the necessity drops considerably, it does have its place in even hot climate applications.
     
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  14. Jun 3, 2013 #14

    nealtw

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    Venting of the roof assembly has nothing to do with snow and everything to do with humidity.

    You want to be carefull when making statements like this. People living in different climates read this and may get the wrong answer to their question.
     
  15. Jun 3, 2013 #15

    WindowsonWashington

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    Agreed.

    I still stand by the statement, as does building science, that venting is for humidity.

    While venting is often cited for the prevention of ice damns, I would argue that ice damns are the result of a poorly sealed envelope and/or inadequate insulation layer.

    If ice damn prevention was solely accomplished by venting, those persons that have an insulated roof deck would be in serious trouble while they are usually just the opposite. Insulated roof decks normally perform better as a result of the materials used (i.e. SPF vs. loose fill insulation).
     
  16. Jun 3, 2013 #16

    nealtw

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    Venting of the roof assembly has nothing to do with snow and everything to do with humidity

    If this statement included closed cell foam, I might have agreed, but the if is always can you insulate enough over the outside wall to prevent heat from that wall getting to the roof sheeting when the snow is on it. The answer is most of the time, it often requires a close look at the size of the rafter and the birdsmouth cut in it.
     
  17. Jun 4, 2013 #17

    GBR

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    "Figure 2: Classic Control Approach—Keep heat from the interior from getting to the roof deck and then remove any heat that gets there. First, construct an airtight “lid” or ceiling plane. Second, insulate the top of the lid with lots of insulation. And then third, flush away any heat that gets to the roof deck by ventilating the underside of the roof deck with exterior air. Note the air seals at the top of the exterior wall. Exterior walls are like chimneys—you don’t want to vent the tops of them into your roof. Also, it is a good idea that the minimum thermal resistance directly over the top of the top plate be greater than the thermal resistance of the wall. So, if the wall is R-30, you want at least R-30 directly over the top of the top plate. Also, note that a 2 in. (51 mm) minimum airspace is recommended under the sheathing. Most codes call for only 1 in. (25 mm). The codes are wrong. In ice dam regions, where ground snow loads are greater than 30 lb/ft2 (146 kg/m2) you need 2 in. (51 mm). This is because ventilation only works to remove small amounts of heat. Therefore, you need a lot of it." From; http://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam

    I don't see the part where jack rafters meet a hip..... in this thread, perhaps another? I read "gable roof" and 28" knee walls with 8' to finish flat ceiling on a 12/12 pitch, with 2x6 rafters he will furr.... and "I don't want to have to deal with trying to put up some sort of additional vent on the 12-12 pitch pyramid roof and spray foam insulation is too $$$." so I showed him that minimal venting is fine as you introduce hot exterior air with moisture. Add foamboard on knee wall and on sloped ceiling on room side. Cover for ignition barrier if required per local AHJ. ADA the drywall to keep warm exterior air from air conditioned interior.http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach

    Wargle, any HVAC ducts in knee wall or top attic? Is there any radiant barrier?

    Gary
     
  18. Jun 4, 2013 #18

    nealtw

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    Sorry Gary, looks like I got this thread mixed up with another.
     
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  19. Jun 4, 2013 #19

    GBR

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    Been there, done that...lol. I wasn't trying to be funny by thanking you,(just acknowledging it), couldn't remove it afterward, no harm-no foul. Gary
     
  20. Jun 5, 2013 #20

    wargle

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    No radiant barrier and we plan to install ducts in the knee wall.
     

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