What is this reddish wood-fiber insulation in my 1955 attic?

That sounds exactly like **Silva Wool** or a similar redwood bark insulation product that was popular in the Pacific Northwest during the 40s and 50s. It’s essentially shredded wood fibers, often salvaged from the lumber industry, and it was marketed as a high-end natural alternative back then. Weyerhaeuser was a big producer of this stuff and you see it all over Oregon and Washington in mid-century builds. The reason it burns for a second then goes out is because it was treated with fire retardants, usally borates or ammonium sulfate. Over sixty-plus years, those chemicals can lose some effectiveness, wich explains why it feels a bit like tinder now. It’s not necessarily an immediate fire hazard just sitting there, but it’s definitely not up to modern safety or thermal standards.

I’ve worked on plenty of these older houses and the R-value of that old wood fiber is pretty dismal by now. It tends to settle and compact til it’s basically just a layer of heavy, organic dust. If you just blow new insulation over the top, you’re burying a layer of material that isn't doing much work and is likely full of decades of debris. Plus, it makes any future electrical work a total nightmare because it's so messy and heavy.

The best move is usually to suck it all out with a high-powered vacuum. This gives you a chance to **air seal** the attic floor properly. You want to fill all the gaps around light fixtures, top plates, and plumbing stacks with spray foam while the joists are exposed. You can't do a real air sealing job with that old gunk in the way. Once it’s clean and sealed, blow in a fresh layer of R-49 or R-60 cellulose. You’ll notice a massive difference in your comfort levels that you won't get by just layering over the old stuff.

Wear a high-quality P100 respirator if you’re going up there to move it around. Even if it’s just wood, that dust is incredibly fine and will wreck your lungs after a few minutes of exposure.

That's wild, I had no idea they used actual wood fibers back then. If I decide to pull it all out like you suggested, is that somethig a homeowner can realistically rent a vacuum for and do over a weekend, or is it a total nightmare? I’m also a bit worried about the house getting too 'tight' if I air seal everythign perfectly. I’ve heard horror stories about moisture and mold in these old PNW houses once they stop breathing. Is that a real risk with these older builds?

Renting a vacuum is a beast of a job for a DIYer. Those rental units often clog on the bigger wood chunks, so be ready for a long, dusty weekend. Regarding the 'too tight' concern, that's a common myth. You want the living space sealed tight to save on heat, but the attic itself needs to breathe. As long as you don't block your soffit vents with the new insulation, you'll have enugh airflow to prevent mold. It's about controlling where the air goes, not just letting it leak through your ceiling joists.