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Insulating (and ventilating?) dropped ceilings

pbout | Posted in Green Building Techniques on

Looking for advice on how to insulate (and if to ventilate) sections of house under what is nominally an unvented cathedral ceiling, but has a section of flat lid. Details attached. 

First section is above the new addition part of the living room, the roof has 2×6 framing and is unvented below the roof (Delta Trella underlay provides a bit of ventilation for the standing seam metal roof). There is no insulation on top of the roof deck here, so I plan to start with 2″ of closed cell spray foam below the deck. With the flat sheetrock lid (supported by 2×6 spanning ceiling), there is a triangle that I’m not quite sure what to do with (highlighted in blue on cross section). I was considering furring down the 2×6 to by 4″, and adding 7.5″ Rockwool batts. But where the roof tapers, this will be difficult. Could I use blown-in cellulose or fiberglass in this space, maybe held in place with netting before I install sheetrock? If I spray foam under the deck and in the rafter tail blocking, can I close it up with no ventilation? 

The second section in question is on second floor above the master bedroom. Roof here has 3″ polyiso above the deck, and still no venting. In the rest of the upstairs, the ceiling is vaulted, furred down by 2″, and 7.5 Rockwool batts. In this room, there is a dropped ceiling with 2×6 spanning the room at 8′ height. There will be closed cell spray foam in the rafter blocking areas. The area in question is highlighted in blue again (lower image in detail). Do I need to ventilate this space? How can I insulate it? How can I soundproof it from the loft? Could I use blown-in fiberglass/celulose over the flat ceiling, then something in the batts between studs on small vertical wall between blue area and loft? 

2400 sq ft 2 story house in CZ03 coastal, zip 95060. HRV ventilation. Mini (multi) split for heat/cool (18,000 Btu/h each upstairs & downstairs).
Thanks everyone!

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  1. BrianPontolilo | | #1


    Since you already have insulation above and under the roof deck, it seams like you will want to treat these areas as cathedral ceilings. And although you have an attic space that could be unconditioned, glancing at your plans, it seems like it may be easiest to bring it into the conditioned space. Keep in mind that with all of those tricky transitions that you have to deal with, air sealing is your first priority. I'm sure there are some members with more experience detailing these areas who will chime in.

    As far as sound proofing, I know that lots of people use insulation as sound proofing, and it works, to some degree, depending on the type and density of the insulation, but there are better methods, including gaskets and channels that are part of the drywall assembly that help stop sound transmission, that you can use, depending on how it is for the space to be really quiet.

    Here are a couple of links that should be helpful:

    1. pbout | | #3

      Thanks Brian. My air seal layer is at the sheathing level of the building envelope, with some internal spray foam in transition areas. If I’m treating them as unvented cathedral ceilings, then I just need to keep the minimum R ratio of foam to fluffy. 2” if closed cell in my CZ should cover it. For the fluffy, do I need to have it right up against the spray foam, maybe nets under the rafters? Or can I loose fill the space?

      For sound, it’s mostly between ithe loft (part of the hallway) and the bedroom.

  2. Expert Member
    BILL WICHERS | | #2

    I have a similar space above my kitchen. I have 5.5” or closed cell SPF on the underside of the sheathing, and nothing above. I left the space empty and it basically behaves as a soffit. I’ve had no problems.

    Regarding sound insulation, what are you trying to insulate against? Rain noise on the roof? Sound insulation works to limit the transmission of sound from one area or room to another. Putting insulation for sound above drywall won’t do anything to make a room quieter if the noise source is within the same room, you need sound “traps” for that which are almost never used in residential construction (they tend to be ugly).

    If the intent is to limit sound transmission, then use a dense product. The mineral wool sound insulation works well. Even better is to hang your drywall using resilient channel to decouple it from the studs/rafters. Best is a mix of those two things, plus a double layer of 5/8” type X drywall with green glue between. Think of “sound insulation” as three things: a barrier to stop the sound (which means heavy mass like 5/8” drywall), an absorbent layer to soak up what makes it through the barrier (mineral wool and green glue), and a decoupling element like resilient channel to limit sound transmission between structural memebers. Double stud walls also help limit sound transmission.

    Insulating for sound is different than insulating for heat. If you can let us know what you’re trying to achieve with the sound insulation you may get more detailed ideas.


    1. pbout | | #4

      Thanks Bill for those great details. As noted in my response to Brian, for sound, it’s mostly between ithe loft (part of the hallway) and the bedroom, so between two spaces. I can try one of those approaches you noted.

      1. Expert Member
        BILL WICHERS | | #5

        Depending on how much you want to spend, I’d do this:
        1- use 5/8” drywall on the walls you want to insulate for sound
        2- fill the wall cavities with the mineral wool insulation made for soundproofing
        3- hang the drywall using resilient channel
        4- use double layers of 5/8” drywall with green glue between

        I’d always do steps 1 and 2 as a minimum. The next big jump up in sound insulating effectiveness will be using resilient channel. Step 4 will probably be the least bang for the buck compared to the others, but it will help.

        Make sure to use the putty sheets to seal any electrical boxes in the walls you want soundproofed. Soundproof walls need to be detailed just like you would for airsealing — any gaps will “leak” sound.


  3. pbout | | #6

    I'm hoping to revisit this thread. The key question for me: on the detail attached to the first post in this thread, if the blue shaded spaces are insulated (enough to prevent condensation), can they be closed up for good? Or do I need to either feed them with an HRV supply/exhaust, or add vents to make them part of the living space? I'd rather not do either of these, unless it will cause a mold or other problem down the line.

    Note that my whole-house air seal is outboard of wall and roof sheathing. If these spaces need to be air sealed as well, I could do that. I guess if they aren't air sealed to the living space, then technically they are part of the living space...?

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