Help! My ceiling insulation dilemma

Ok, before I drone on and on about it — and I will — let me just get my problem out there and sum it all up. I need some advice.

Porch roof/ceiling

Our upstairs sleeping porch was once just a porch, so it has no attic space. After tearing down the old dropped ceiling and pulled out the old insulation, all that’s left between our heads and the elements are the joists, beadboard roof sheathing, 2″ foam insulation, and the roof membrane.

I just can’t figure out the best way to insulate this space before putting in the new ceiling. That’s my dilemma. So to my partners in renovation and other skilled experts: how would you fill this with insulation and a ceiling?

Consider the following:

  • The joists are about 5.5″ deep, so in my mind, I can’t put in insulation that’s thicker than 3.5″. (Next thickest is usually over 6″) Because I need to keep space for air to circulate above the insulation — between the insulation and the beadboard sheathing….right?
  • But the soffits (to the left in this pic) outside the house may not connect to this space very well, and definitely don’t connect to the actual attic space. At all. So any ventilation coming in those soffits has nowhere to go up out of the roof, because of how the porch slopes up and stops at the brick wall of the proper back of the house.
  • I can’t drop the joists or the ceiling down far enough to create proper attic space because it’ll cover the window and door frames. See lower left of picture above. So that’s not an option.
  • I need a vapor barrier inside the drywall ceiling. But can I have foamboard insulation between my drywall and the vapor barrier? Or on the other side of the fiberglass with vapor barrier? And what kind of foam? They make aluminum-faced and non-faced Pink stuff.

So here are the options I’m considering:

1. Putting nothing up against the sheathing, putting in 3.5″ faced insulation as close to the drywall as possible so there’s space above for air to circulate. And then drywall.

Obviously, this plan sucks for R values. This means I’d have R-20 or less between the 2″ roof foam and the fiberglass. That’s terrible, especially for a room that gets direct sun and connects to our bedroom with a door we’d like to keep open.

2. Putting foamboard up against the beadboard sheathing — either compressed Owens Pink non-faced foam or the Dow Tuff-R aluminum faced insulation — and then putting in the 3.5″ faced insulation as far down close to the drywall ceiling as possible. This would allow space between fiberglass and the foam board for air to circulate.

But does it matter if I have room for air to circulate, considering there’s nowhere for the air to go? I also read in one of my top notch reno books that aluminum faced foamboard (like the Tuff-R) shouldn’t be installed on  interiors because it can trap moisture. But I’ll have a vapor barrier below it, and Dow info says it can be used on “interior retrofit of existing walls under a new interior finish of 1/2” (minimum).” If there’s a vapor barrier below it, does it matter if the foam has another one facing the unfaced side of fiberglass?

3. Putting nothing up against the sheathing, putting in 3.5″ faced insulation as close to the drywall, and then putting 1-2″ Pink or Tuff-R foamboard across all the joists and then putting in drywall on top of the foamboard. (Could also add 1″ of foamboard against the sheathing too with this option.)

This option is best for R-values, especially if I can use 2″ foamboard. From the Dow info, it sounds like putting it right next to drywall is a proper use — and it serves as a vapor barrier. So should I still put faced insulation above it between the joists? Does it matter? Does the aluminum faced foamboard have a sufficient vapor barrier by itself?

Ok, see what I mean? I can talk about this all day, and the possibilities are driving me crazy. I’ve been googling and looking up data sheets on insulation materials and asking everyone I know this question. The Dow and Owens Corning sites are geared toward construction of new stick and sheetrock mansions with sloped roofs and conventional construction. Not 90 year old rowhouses with porch roofs that are now house roofs. Same goes for many of the DIY forums and sites I read. The problem is just so specific that I can’t quite find a convincing answer out there.

So if you know the answer to this, help a fella out in the comments.

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