Insulation challenge - exposed wood beams with 6" tongue/groove slats on top form ceiling with attic above

Question

1950s Los Angeles ranch style house with 32F lows in winter and 95-115F highs in summer. Ceiling above family room has exposed 6x6 wood beams with 1x6 pine tongue and groove slats resting on top. Above this ceiling is the bare wood slats with an inch or two of blown in insulation. No paper, no housewrap, no drywall, no other insulation, nothing but dust. Roof above barely-standing-room attic has turbines and ends of home have gable vents, no soffit vents where roof line meets walls.

Access to attic is through 30x30" entry.

How to insulate above slat ceiling?

After removing existing blown in insulation, I'm thinking: (1) lay housewrap and use some kind of caulk/sealing around edges imperfect air seal (2) drywall* screwed to slats with mortar between edges, expanded form all the way around to complete the air seal (3) blown in insulation on top

Of course, I'll spend time looking for air gaps and use spray foam to seal those cracks.

Answer 1

Your goal here is a significant improvement in energy efficiency without causing any unwanted side effects, such as moisture buildup in the wrong places. With that in mind, I'd keep it simple.

• Lay 4 mil poly sheeting right over the existing insulation. Unless the stuff is filtering through your ceiling and making a mess, just leave it alone. Lap joints 2 feet or so. This will provide an effective seal. It's not critical to create an actual balloon out of your house. You just want to eliminate the majority of airflow between the conditioned space and outdoors.
• Blow the appropriate amount of cellulose over the poly.

That's pretty much it. You'll probably need to build a simple barrier around your access port to contain the insulation, and be sure any recessed light cans are rated for insulation contact. Otherwise, don't overthink it.

Answer 2

You live in a relatively warm, humid environment so I definitely would not recommend poly sheeting, especially on the interior surface. That's a great way to get condensation and wood rot. You need to have a way for the vapor to escape and the structure to dry out, which airtight plastic doesn't provide.

You don't mention what type of blown insulation you have now: fiberglass, cellulose, or mineral wool. The simplest and most cost effective way to deal with it is to just blow more insulation on top of what you have (Personally, I greatly prefer blown cellulose or rock wool batts over any fiberglass.) You can easily rent a blower from any big-box hardware store and blow some insulation in a weekend.

Like @Isherwood mentioned, make sure any recessed lights are IC rated or put barriers around them to keep the insulation at a safe distance. Even better, replace them with airtight led fixtures.

The next step up is to add soffit and ridge or deck vents, closing off the gable vents, so cool air comes in as low as possible and exits as high as possible. This will help drop your attic temp during the summer cooling season.

The next level of improvement, as you considered, is removing the existing insulation, air sealing your ceiling, and re-insulating. I'd suggest either the house wrap or drywall, not both. Using both would be redundant. Overall, I think the drywall would be easiest to install and seal.

If you want to go all-in, you can add roof vents, then put solid foam sheet insulation on the bottom of your roof deck to make your attic a conditioned space for storage or an extra room.

FWIW, I just removed all the insulation (R-3 to R-6) in my attic, air sealed, and am re-insulating with rock wool batts. (I would do blown cellulose, but my lady didn't like the dust factor when blowing.) Just the air sealing alone lowered my heating bill this past winter, with only half the house insulated to R-15. I expect even better when I finish insulating the ceiling to R-45. If you want to know more about the insulation removal, just post a separate question.