Both work. Both are sold at every home improvement store in Harris County. The difference that matters is not the R-value printed on the bag, it is how much of your attic floor actually ends up covered, and that is where these two part company badly.
The Short Version
| Blown-in | Batt | |
|---|---|---|
| Form | Loose fibers blown through a hose | Pre-cut rolls or panels |
| Fits around obstructions | Yes, flows into every cavity | Only if cut precisely by hand |
| Typical R per inch | About 2.2 to 3.8 | About 3.1 to 3.8 |
| Install speed on an attic floor | Fast | Slow |
| Best use | Attic floors | Walls, floors, new construction |
Why Blown-In Wins on an Attic Floor
Look at your attic floor. It is not a clean rectangle. It has joists at irregular spacing, cross-bracing, wiring runs, plumbing stacks, ductwork, can light housings, and a hundred awkward corners where a roof deck meets a top plate.
Batt insulation is rectangular. Every one of those obstructions is a place where somebody has to stop, measure, cut, and fit a piece of material by hand while crouching in a 140 degree attic. Guess what happens in practice. Batts get compressed to squeeze past a duct, which destroys their R-value. Corners get left uncovered. Small gaps get skipped.
Those gaps matter more than people expect. Research on this is consistent: even a small percentage of uncovered area disproportionately undermines the performance of the whole assembly, because heat takes the easiest path available. An R-38 attic with gaps does not perform like R-38.
Blown-in material does not have this problem. It is loose fiber carried by air, so it flows around and into everything and settles at a consistent depth. On a retrofit attic floor, that is the entire argument.
The compression trap
An R-30 batt squeezed into a smaller cavity does not deliver R-30. Compressing insulation reduces the trapped air that makes it work. This is the single most common way DIY batt installations quietly underperform, and it is invisible once the job looks finished.
Where Batt Still Makes Sense
Batts are not bad material, they are material for a different job. They are excellent in open stud walls before drywall goes up, in floor joist bays, in garage ceilings, and anywhere the framing is regular and accessible. In new construction, where a crew can install batts into clean open framing before anything else is in the way, they perform well and go in fast.
An existing Houston attic with thirty years of accumulated ductwork and wiring in it is not that job.
Cellulose or Fiberglass?
Once you have chosen blown-in, there is a second choice. Both are common in Houston.
- Blown fiberglass is spun glass fiber. It does not absorb water readily, it does not settle much over time, and it is not food for anything.
- Cellulose is recycled paper treated with borate for fire and pest resistance. It has slightly better R-value per inch and packs into awkward cavities well. It does settle somewhat over the years, so a good installer accounts for that by blowing to a greater initial depth.
Either performs well when installed to the right depth. The installer matters more than the material.
What Actually Determines Your Result
Here is the part no product comparison will tell you. The choice between blown-in and batt is worth less than three other things:
- Whether the attic was air sealed first. Insulation slows heat conducting through a surface. It does nothing about air pouring through an unsealed can light housing. Skip this and you will notice very little difference no matter what you installed.
- Whether it was installed to full depth everywhere, including out at the eaves where it is uncomfortable to reach.
- Whether the soffit vents were kept clear. Blown-in material pushed into the soffits blocks attic ventilation, which in Houston humidity is how you get a moisture problem later. Proper installs use baffles.
Get those three right and either material will serve you for decades. Get them wrong and the best product on the market will disappoint you.
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