Insulation Question

I did work in an older home where the ceiling was the underside of the roof. We used the Styrofoam boards in full width areas (between 2X4's) and paper backed fiberglass in narrower areas.

We installed the insulation and then installed drywall over that.

The install looked good and the second floor was warmer/winter and cooler/summer.

As far as un-faced fiberglass, I don't know the pros or cons of the product as I have never used it.

WOW - Built around 1918, what a piece of history that is!

Hope this helps - Good Luck!

4" thick fiberglass paper-faced on one side is going to be around R-13, which is not much for a roof. Two 2" foam boards is less than R-13 - at least the foam boards I see at Home Depot are less. Foil-backed is ever so slightly higher R value, not much. I have found both foil and paper backed let a lot of fiber into the air while working with them, only the completely plastic sealed batting contains most of the airborn filaments while wresting with the stuff.

Would your funds allow you to install a light "drop ceiling" so you could fit 6" thick fiberglass and not have to be concerned about adding weight to rafters?

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The issue is an oddly shaped roof. Only 2 rooms with an area seperating them for stairwell and 'landing'. So major peakage to work with. I would post a pic but have not figured out how to yet. So no chance of drop ceiling. My major concerns of course are insulative values and preventing moisture problems. I'm leaning towards un-faced, no kraft paper, fiberglass with that foil, bubble, bubble, foil material over it for reflective value in keeping heat in during winter months and heat out during summer time.

Then I would use the un-faced no paper batting product, and depending on humidity in your area I might use that plastic bubble backed foil product, too high of humidity then not. Consider the condensate when working a ceiling job . Good luck, best part will be when it's done!

There's this thing about ice dams forming on roofs that are insulated wrong, destroying the roof eventually with rot. Look into the proper way to treat your house. Apparently, you are supposed to insulate the room ceiling, but ventilate the attic. You don't have a ceiling, it seems, so this is a strange problem. Maybe a sheet insulation over the rafters leaving an air space above them. May still need a ridge vent, and soffit vents.

Otherwise, I would recommend looking into spray foam. It really stops drafts.