Blown-In Attic Insulation

A cold upstairs bedroom, frost on the inside of the attic hatch, icicles hanging off the eaves: that’s usually heat going up through the ceiling. Blown-in insulation is one of the most straightforward fixes there is. Loose insulation blown across the attic floor makes one continuous blanket over the joists and around wires, pipes and framing, with none of the gaps you get fitting batts around obstacles.

Fiberglass or cellulose

“Blown-in” is how it goes in, not what it is. The two common materials:

• Fiberglass blowing wool: glass melted and spun into fibers. It’s light, so it doesn’t load the ceiling, and it’s less dusty to install.
• Cellulose: mostly recycled newspaper, usually treated with borate for fire and pest resistance. It’s heavier and dustier to blow, and it packs densely, which makes it the usual choice for dense-packing closed wall cavities.

Either one does the job in an open attic when it goes in at the right depth. Pick by what’s already up there and what you’d rather work with.

WBC’s blown-in insulation

Product	Coverage per bag
JM Climate Pro B-7700 fiberglass blowing wool	77 sq ft at R-30
Blown-in insulation	73 sq ft at R-30

That coverage number is the one that matters. It tells you how much attic one bag covers at a given R-value when it’s blown to the depth on the bag’s chart.

How much R-value, and how many bags

R-value is resistance to heat flow; the higher, the better a layer holds heat in. How much you need depends mostly on climate, and Montana is cold-climate country: energy-efficiency guidance for climates like ours commonly recommends R-49 to R-60 in an attic. Your local code sets the minimum for new work; for an upgrade, R-49 or more is a sensible goal.

To figure bags:

1. Measure the attic floor in square feet: length times width of the insulated area.
2. Pick your R-value and find the coverage per bag at that R-value on the bag’s chart.
3. Divide the area by the coverage, and round up.

At R-30, a 1,000-square-foot attic takes about 13 bags of Climate Pro (1,000 ÷ 77) or 14 of the 73-square-foot bags. For a higher R-value, the chart on the bag gives a smaller coverage per bag, so you’ll need more.

Two things to watch. Blow to the depth on the chart, not by eye. Loose fill settles a little, and the charts already allow for it; if you stretch the bags over more area, you get a thin layer and cold spots. Count your existing insulation. Dry, clean old insulation can stay, and you only buy for the R-value you’re adding. If there’s roughly R-19 of old insulation up there and you’re going to R-49, figure bags for about R-30, not R-49.

Prep before the blower runs

Insulation slows heat; it doesn’t stop air. Warm air leaking into the attic through gaps carries heat and moisture with it, and neither fiberglass nor cellulose seals those gaps. Do these first:

• Seal the holes around plumbing vents, wiring, the attic hatch and the top plates of walls.
• Fix wet spots. Roof leaks and condensation have to be corrected before you bury them.
• Install baffles at the eaves so the insulation doesn’t block the soffit vents. Your attic needs that air moving to stay dry and keep ice dams down.
• Keep it clear of recessed lights and chimneys where the fixture or code calls for clearance.
• Mark a path and keep access to the hatch, so you’re not walking through and crushing the finished layer.

In Kootenai country, especially around Libby, pre-1990 homes may have vermiculite insulation that can contain asbestos. It’s pebbly, gray-brown or silvery. If you find it, leave it alone and have it assessed before anyone works in that attic.

Walls

Closed walls in an older house can be filled through holes drilled in each cavity, most often with dense-packed cellulose. That’s a different job from blowing an attic, packed much tighter, and worth planning carefully, including making sure water isn’t getting into those walls.

The Pick

For an attic top-up, JM Climate Pro fiberglass blowing wool or the 73-square-foot-per-bag blown-in insulation, enough bags for your attic’s area at your target R-value from the bag chart. Seal the air leaks and set the baffles first.

Frequently Asked Questions

How many bags of blown-in insulation do I need?

Divide your attic floor area by the coverage per bag at the R-value you’re adding, from the bag’s chart, and round up. At R-30, a 1,000-square-foot attic takes about 13 bags of Climate Pro.

Is fiberglass or cellulose better?

Both work in an open attic at the right depth. Fiberglass is lighter and less dusty; cellulose is denser and is the one used to dense-pack closed walls.

Can I blow new insulation over old?

Usually, if the old insulation is dry, clean and not crushed. Fix leaks and moisture first.

Do I need to air seal before insulating?

Yes. Insulation doesn’t stop air, and air leaks carry heat and moisture into the attic. Seal the holes first.

How is blown-in different from spray foam?

Blown-in is loose fiber laid across the attic floor. Spray foam is sprayed on as a liquid and expands into a solid layer that also seals air. They’re different products and different jobs.