Can I replace 4" fiberglass batts with thicker ones in my attic?

Question

We live in an early 1970s ranch style house. Single story. Attic formed by 2x4 trusses on 24 inch centers. The insulation is R13 batt laid on top of the ceiling drywall but this has been removed in some areas such as over the kitchen by a prior owner.

I am planning to replace the batts room by room with R30 or thicker batts. Is there any problem with doing so? The insulation will be higher than the 2 x 4 and there would be a small gap between batts due to the trusses.

Could I also lay additional unfaced batts perpendicular to the batts in the truss bays over the R30 batts? There would be some compression.

I did some searching in the forum and did not see a set of similar questions although What's the best way to insulate an attic with 2x4 joists? is close.

Loose fill would work over the bedroom and kitchen areas, but the living room has a cathedral ceiling so batts would seem to be the best choice there.

Answer 1

I'm not an expert in this area but this is what I would do. If the ceiling joists are only 2x4s I would not use batts that are higher than the joists themselves and then run the additional batts perpendicular. This would leave a significant gap allowing hot/cold area to infiltrate from the sides. I would just put in the R13 in the empty bays and then run the R30 perpendicular to this. This way you have no air gaps.

If you blew in you would eliminate all this and actually get better coverage and fill then you would with batt insulation. If you don't mind paying for someone to blow in, they would be done in a few hours and the blow in type I've had done isn't itchy so if you go up in the attic and need to push it around you aren't feeling like all prickly later. I have a two story so I had to go up in the attic with a small leaf rake to clear some of the eave vents. Since you are single story you may be able to take a leaf blower and blow any insulation from the vents from the outside.

Answer 2

Depends on the R-value you want to achieve. If R30 is all you need, put batts between the rafters and call it good. If I were you (provided the R13 was unfaced) I would leave it and put the R30 over the top. It's best if it is perpendicular to prevent compression, but I have laid them parallel without any major compression issues.