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To get R-49, can faced R-30 be layered on top with R-19?

lookloan | Posted in General Questions on

I had asked a question yesterday on spray foaming under the deck for a long single family house. It seems no mater how you twist the options, the math shows just for the roof insulation is over $21K so almost 30K to insulated this 2000 sf new construction house. And 30K is after  I removed some of the insulation steps to do myself. 

I started looking at insulation that is available (both faced and unfaced) I started looking at fiberglass and Rockwool that  I could work with since the ceiling between the 1st floor and attic is 10 inches deep – 24 OC trusses.  
Under the roof deck is also 24 OC and 10 inches deep.   

So looking at the pieces of the puzzle, my question is if I put face R-30 in the ceiling, can I then overlay R-19 unfaced over the installed R-30 below to come up with R-49.  Doing it this way is only 10% of the closed cell option – yes less than $3000.  If figure if I do it this way, I cut down dramatically on the insulation costs and I noticed I am not hearing from the installation installer after suggesting this by email.  I would like to add I dont want to spray the fluffy stuff over the R-30 as I think it’s messy but I am open to other ideas. Thanks in advance.

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, you can layer batts the way you describe. You want to make a cross-cross pattern with the upper layer laid perpendicular to the lower layer to minimize convection currents between batts.


  2. lookloan | | #2

    Thank-you Zephyr7

    1. Expert Member
      MALCOLM TAYLOR | | #3


      I answered your last post but completely misunderstood what type of roof you had. It's probably my fault but I'm completely lost. What is your roof assembly from top to bottom? How do these two 10" cavities relate?

      1. Expert Member
        BILL WICHERS | | #4

        I’m assuming the OP has a typical open attic floor formed by the upper level ceiling drywall and the lower chords of the roof trusses.

        From the question the OP posted in this thread, it doesn’t sound like a hot roof arrangement.

        For the OP: you should probably clarify exactly what you have. If Malcolm misunderstood what you had in a previous thread, I might have too in this one. It’s important to get this right so that you don’t have problems.


  3. lookloan | | #5

    Thanks for asking for details. The house is a one floor 26x84 rectangle with 12 foot ceilings. The architect used what is called attic trusses that are designed with 2x10 rafters along the top with a 12 pitch, and 2x10 on the bottom, that makes up the ceiling for the first floor. Basically it has a barn/ modern ranch look. I was going to have spray foam R-49 on the gable ends and the under-deck of the roof in the 2x10 rafters but the quote of 21-22K is a lot of money so I started retinking the insulation. Because these area attic trusses, the center of the attic is a full walk up and it looks like a 8 to 10 foot wide bowling alley with 2x4 studs 24 oc on each side of the bowling alley. There is one Anderson Series A window on each end gable side. There are about 44 or 46 trusses that are 24 oc. The sheathing is on the roof and it is shingled. The town inspector wanted a ridge vent on the top so I am glad he asked for it as I can now let the underside of the roof breath with open soffits rather than having them foamed in. BTW I am in Zone 5.

    Studying this today, I noticed faced R30 FG batts that are 9.5 inches thick would rest perfectly in the 1st floor ceiling, which is the 2x10 bottom rafters of the roof trusses which is also the attic floor. Then my question was after I fill in the R-30 in the ceiling which is also under the attic floor, can I then in the attic, lay R-19 across the top of the 2x10 rafter floor to get R-49. It accomplishes the same R49, the roof breaths, I don't have to do the above gables with foam and is less than 3K vs 22K. I would also insulate at some point the center attic alley which is a large square meaning their are 2x4 sides and a 2x4 cross stud on the top that will make for a flat roof for this alley area. I would also insulate just section of the gable ends that have windows that fall in this section.

  4. Expert Member
    NICK KEENAN | | #6

    Do you want the attic to be inside or outside of your conditioned space?

    1. lookloan | | #7

      Basically it's 50/50 - The center of the attic will be the only usable space, which I described as a 10 foot wide x 8 foot tall bowling alley that is 84 feet long with a Anderson Series A window on each end. The stairs from the first floor to this walk up attic space is about 1/3 of the 84 feet on once side and 2/3s on the other. I may use the 1/3 side for an office and the HVAC person was tinking of one Mitsubishi air handler in the 1/3 side and one one the 2/3 side. This is 84 foot long center section has a subfloor at the moment and the two sides opposite the alley walls is the floor rafters from the trusses.

      The two opposite sides of this center alley has a sloping roof from 8 to zero and will have no use. Basically I was originally going to spray foam the entire roof and gables which looking back would gain no additional space beyond the center alley. These two sides of the center are sectioned off by the 24 oc wall on each side of the alley. If it gets too complicated in the attic, the HVAC can be installed in the full walkout basement under the 1st floor I'm thinking.

  5. Expert Member
    NICK KEENAN | | #8

    So the building envelope goes across the top of the ceiling in the edges, up the walls between the edge and the center, and in the center it goes across the first floor ceiling for 8', up an 8' wall, across the attic ceiling for 10', down an 8' wall and then across another 8' ceiling?

    Where it's running over ceilings it's perfectly fine to roll out two batts of insulation perpendicular to each other, that's done all the time. You should also price blown-in, that's the other really common way of doing vented attics. I would make sure the attic outside the insulation is vented, and that the airflow isn't obstructed. Since this is a somewhat tricky shape you want to make sure there is good air sealing for the entire length of the envelope, and that where the ceiling joins the first floor walls there is continuous insulation and air sealing. The other thing to look at is the vertical walls in the attic, you may want more insulation than on the exterior walls because the vented part of the attic will be pretty hot in summer. They should be pretty straightforward to put foam sheathing on the exterior and either batt insulation or blow-in in the cavity.

  6. Expert Member
    Peter Engle | | #9

    In this arrangement, air sealing the connection between the vertical walls of the attic "alley" and the floor joists is super important. You don't want to allow unconditioned attic air from the triangular spaces to blow into the floor cavities under the center of the attic.

    You should read this article:

  7. Expert Member
    BILL WICHERS | | #10

    I would put rigid foam on the outside of those studs on the sides of the “bowling alley”, and be sure to air seal everything all the way to the ceiling drywall under that space. I would use blown cellulose for all of the attic floor outside of that “bowling alley” area. I’d put batts between the studs on the sides of that “bowling alley”.

    If you are going to put a ceiling of some kind in the “bowling alley”, then I’d install blown cellulose above it. You need to make sure you’re leaving air gaps between the roof sheathing and the top of the insulation around the “bowling alley” area.

    Note that you’re basically building one of the “story and a half” cape cod style homes here, so you’ll have all the same insulating and air sealing concerns those houses have to deal with.


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