I have a home built in the mid- fifties that has a 12/12 roof, it has insulation with a paper backing stapled between the roof rafters.
Will I have any problems if I add more insulation in the joists and leave the rafter insulation in place?
It's not an ideal situation to leave it in place but you can. The paper was the old version of a vapour barrier. As you can tell it is not a complete barrier.
If you want to do it to todays standards, remove all the insulation and lay down plastic vapour barrier over the joists and ceiling and light boxes. Tape all the jionts and seams with red tuc tape. If the old insulation is in good shape you can reuse it and then put the new on top of that.
Although insulation is cheap, so you might want to go with all new stuff.
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