Does attic insulation thickness affect invader entry?

It's an understandable mix-up: insulation and pest exclusion both live in the same physical space — the attic — and both get discussed in the same breath as 'sealing up the house for winter.' But they solve different problems. Insulation slows heat transfer through a surface; it does nothing to close a gap an insect can crawl or fly through. A gable vent with torn screen behind six inches of insulation is exactly as open to entry as one with twenty inches of insulation behind it, because the insect never has to pass through the insulation at all — it goes straight through the torn screen and into the open attic space above or beside the insulation layer. Thickness and R-value describe how well the material traps air; they say nothing about whether the vents, soffits, and cracks around that material are sealed.

Mississippi State Extension is specific about where attics actually fail: missing or torn gable-vent screen, poorly screened or improperly installed soffit vents, and cracks around eaves, fascia, and soffits. UK Entomology lists attic vents and chimneys among the common points stink bugs exploit to get inside a structure. None of these fixes involve insulation at all — they involve screening (ideally 20-mesh or finer), caulk, and sealing utility penetrations where cables, pipes, and vents pass through the building envelope. A homeowner comparing two otherwise-identical attics, one thickly insulated and one thinly insulated, will see the same entry rate if both have the same unsealed gable vent — and a well-sealed, thinly insulated attic will outperform a poorly sealed, heavily insulated one every time.

This deserves the fuller second half, because 'insulation doesn't matter' overstates the correction in the other direction. Once insects are already inside an attic or wall void, insulation becomes real harborage: Iowa State documents cluster flies sheltering under insulation, and boxelder bugs and other overwintering insects simply pick a side of the insulated wall to sit against, using the material's insulating properties the same way they'd use bark or a rock crevice outdoors. So insulation is a factor in where insects sit once they're in, and it has zero effect on whether they get in the first place. Upgrading attic insulation is worth doing for energy-efficiency reasons — U.S. DOE's Building America program directly connects air-sealing work (which happens alongside many insulation projects) to keeping pests out — but the pest benefit comes from the air-sealing that often accompanies an insulation job, not from the insulation material itself.