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Inusllating unvented cathedreal ceiling w/exposed rafters in S. California

JMorales's picture

My client has two bedrooms with vaulted ceilings, 4/12 pitch, exposed 4x4 rafters 3-1/4" thick, under 1" tongue and groove boards. The ceilings are unvented and the home is located in Southern California.

My plan was to fill the cavities with Hunter H Shield panels 2 1/2". Thereby leaving a 3/4" gap of dead air. Then running full sheets of 3/4" Rmax R-Matte Plus-3 unde the rafters. Then taping the seams of the Rmax with Tyvek housewrap tape, to try and air seal as much as possible.

Also, there are 4 fiberglass jboxes in roof that will be getting LED surface lighting installed. I was planning on using fireblocking foam to surruond the jboxes.

2.5" Hunter polyiso=   R15.3

3/4" dead air          =    R2.77

3/4" Rmax              =     R5

Total Rvalue          =    23.07


What do you guys think?

  Uh, yeah... (post #206796, reply #1 of 1)


Uh, yeah... don't.

Look, the is alreay a nice loking finish on the underside of that deck there with the 4x and the tongue & groove.  Leave it, it looks nice.

Instead, focus the efforts on the area ABOVE that deck.

  1. Strip the old roof down to the deck.  
  2. Cover the old deck with an air barrier, like TyPAR.  This ia your primary air barrier, so seal it up well.  There are only going to be a couple penetrations so all should be well.
  3. Cover this with a layer of 1.5" polyIso foam board.  Between the gaps in the sheets on the horizontal, place 2x4's on the flat.
  4. Over this, place 2x4 furring rafters on edge 24"OC, secured down to the 2x4s you placed earlier.  
  5. Between these, you will rip shees of 2" polyiso down to 22.5" an place them betwen the furring rafters.
  6. Seal the seams with foam and aluminumn tape.
  7. Deck over this with new plywood or OSB.
  8. Install continuous soffit and ridge vent.
  9. Apply to favorite roofing product.

Now you have a well sealed, well insulated roof (R22) and didn't have to impact the living areas at all.  If you need more, you can add another 2" layer under the horizontal furring, giving you R36... but that's a little much here in mild SoCal!

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