Should I add more blown-in or start insulating the attic rafters instead?

Since you're in the Philly area, you're looking at a Zone 4 or 5 climate, which means you really want to be closer to R-49 or even R-60. Right now, that 12 inches of fiberglass is only getting you around R-30, and those thin patches are basicly thermal holes where heat is just pouring out. You definitely need more height, but just blowing more over the top isn't the whole fix.

The most important part before you rent that blower is **air sealing**. Those rough winter bills are usally more about air leaks than the insulation itself. You have to move the existing fluff aside and look for where wires, pipes, and wall tops meet the attic floor. Use some canned spray foam or caulk to seal those gaps. If you don't, the heat just drifts through the fiberglass like a filter. Also, check your ductwork for leaks while it's still accessible. If the ducts are leaky, you're literally heating your attic.

Stick to the attic floor rather than the rafters. Insulating the roofline is a different beast and usually requires spray foam to be done right. If you just put batts in the rafters while leaving the floor insulated, you can end up with a **moisture trap** that rots your roof deck. Plus, it's way more expensive and labor-intensive. When you do the blow-in, make sure you install **soffit baffles** at the eaves first. This keeps the new insulation from blocking your airflow, which you need to prevent ice dams in the winter and mold in the summer. Once those are in, you can level out the low spots and blow another 8 to 10 inches across the whole thing. Burying the ducts deeper in the insulation is acutally a good thing—it helps keep the air inside them at the right temperature as it travels to your rooms.

That air sealing part sounds like a massive headache. If I’m moving that old fiberglass around, am I going to be kickin' up a ton of dust and stuff? I’m worried about breathing that in or it getting into the rest of the house.

Also, what kind of foam do you use for the bigger gaps around the chimney? I saw some stuff at the big box store that says it's fire-rated, but I wasn't sure if that was overkill or actualy required for code.

For the chimney, skip the canned foam. Even the "fire-rated" orange stuff isn't meant for direct contact with a hot flue. You need to use sheet metal flashing to bridge that gap, then seal the edges with high-temp red silicone or some packed rockwool.

To handle the dust, grab a P100 respirator and a disposable Tyvek suit. It’s a lifesaver. If you're worried about the mess spreading, tape some plastic sheeting around the attic hatch area. Pro tip: use a garden rake to gently level out those thick patches before you start blowing the new stuff. It’ll give you a much more even base to work with.