Proper way to install insulating batts in flat roof?

[TomYoung]

Hi all:

I'm going to be installing batts into the ceiling (from below) but I'm not sure if there's a "correct" way to secure the batts to the joists. The old insulation (insufficient, only 4" in a 9" cavity) was secured in place by stapling the flanges to the 9" face of the joists. From the standpoint of having a secure moisture barrier it seems like the flanges should be stapled to the 1-1/2" faces of the joists.

Is one method preferred over the other?

TIA.

Tom Young

[Joe]

Many contractors ignore the paper 'barrier' on the insulation and put

6 mil PE film over the studs, rafters, whatever. In your case, that could be an elegant solution and easy to do. Might be even more desirable in a ceiling situation.

Joe