Can I rent a blower for blown-in insulation?

Yes — most big box home improvement stores (Home Depot, Lowe's) loan blowing machines free of charge when you purchase a minimum number of bags, typically 10 bags or more. The machine handles both fiberglass and cellulose. Plan on 1–2 hours for an average attic with a helper feeding bags at the hopper.

What depth should I blow insulation to for R-49?

For cellulose at R-49 the installed depth is approximately 12.3 inches. For fiberglass the installed depth is approximately 15.5 inches. Install depth markers (rulers) every 10–15 feet across the attic floor so you can check depth while blowing. Account for settling — cellulose settles about 15% and fiberglass about 5% over time.

Should I use cellulose or fiberglass blown-in insulation?

Cellulose has a higher R-value per inch (R-3.2–3.8 vs R-2.2–2.7 for fiberglass), so it achieves the same R-value at less depth. It also provides better air blocking because its denser particles fill gaps. Fiberglass is slightly lower cost per bag, lighter, and easier to blow. Both perform similarly when installed correctly. Cellulose is made from recycled paper and is more eco-friendly.

Do I need to air seal before blowing in insulation?

Yes — air sealing is the most important step before adding insulation. Seal all top-plate gaps, plumbing and electrical penetrations, recessed lights, and attic hatch perimeters with foam or caulk. Insulation slows conductive heat transfer but does not stop air infiltration. Air sealing first can double your energy savings compared to insulation alone.

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Blown-In Insulation Calculator

Bags, installed depth & cost for fiberglass or cellulose — attic, wall & cathedral ceiling

— Bags needed

— Installed depth

— Material cost

— Area covered

Project Inputs

Application

Area to insulate

sq ft

Target R-value

Custom R-value

Existing R-value (0 if none)

Material

Climate zone

Cost per bag (optional)

$/bag

Results

— bags

— Area (sq ft)

— Installed depth

— Bags needed

— Material cost

— R-value added

— Total R-value

Bags Needed at Each R-Value — 1200 sq ft

Cellulose vs. Fiberglass Cost Comparison

R-Value	Cellulose bags	Cellulose cost	Fiberglass bags	Fiberglass cost

IECC 2021 Recommended Attic R-Values

Climate Zone	States / Regions	Min Attic R
1–2	Hawaii, S. Florida, S. Texas, Gulf Coast	R-30 to R-38
3	N. Florida, Carolinas, Georgia, Texas, Arizona, LA	R-38
4	Mid-Atlantic, Tennessee, Pacific NW, N. California	R-49
5	Ohio, Indiana, Illinois, New England, Colorado	R-49
6	Minnesota, N. New England, Montana, Wyoming	R-49 to R-60
7–8	N. Minnesota, Alaska	R-60

Higher R-values in zones 5–8 reduce energy bills significantly. Many utility rebate programs pay $0.05–$0.15 per sq ft for attic insulation upgrades.

Cellulose Coverage Specs (manufacturer data)

R-Value	Depth (in)	Sq ft per bag	Bags / 1000 sq ft
R-30	7.5"	40.6	25
R-38	9.5"	32.0	32
R-49	12.3"	24.8	41
R-60	15.2"	20.0	50

Fiberglass Coverage Specs (manufacturer data)

R-Value	Depth (in)	Sq ft per bag	Bags / 1000 sq ft
R-30	9.5"	57.4	18
R-38	12.0"	45.3	23
R-49	15.5"	35.0	29
R-60	19.0"	28.5	36

Air Seal First

Before blowing any insulation, seal all penetrations with foam or caulk: top-plate gaps, plumbing boots, recessed light boxes (use IC-rated covers), attic hatch perimeter, and wiring pass-throughs. Air sealing delivers the greatest energy savings and should never be skipped.

Install Ventilation Baffles

At every rafter bay between trusses, install cardboard or foam baffles from the soffit vent up at least 12 inches into the attic. Baffles maintain the air channel from soffit to ridge and prevent insulation from blocking soffit vents — blocked vents cause moisture problems and void manufacturer warranties.

Set Up Blower & Hose

Rent a blowing machine free with purchase of 10+ bags at most home improvement stores. Set up the machine on the ground near a window — run the 150 ft hose up through the attic hatch. Use the hose extension to reach far eaves without walking on drywall. Have a helper below feed bags into the hopper at a steady pace.

Install Depth Markers

Cut plastic rulers or marked wooden stakes and staple them to joists every 10–15 feet across the entire attic floor. Mark the target depth in inches clearly. You will verify depth while blowing without having to stop. Manufacturers require depth markers for warranty coverage.

Blow from Eaves to Hatch

Start blowing at the far eaves and work backwards toward the attic hatch — you will always be stepping on unfilled joists, not newly-installed insulation. Hold the hose 12–18 inches above the surface. Overlap passes slightly to avoid thin spots. Check depth at markers frequently and add passes as needed.

Safety & PPE

Wear an N95 or P100 respirator, safety glasses, work gloves, and long sleeves. Attics are hot and dusty — bring water, take breaks, and have a spotter below. Never step between joists on drywall. Use plywood walkboards or walk only on joist tops. Illuminate the full attic with portable work lights before starting.

Pro Tips

Install a temporary attic hatch dam (a box around the hatch opening) so insulation doesn't fall into the living space when the hatch is opened later.
For cellulose over existing fiberglass batts, the batts do not need to be removed — blow cellulose directly on top.
Fiberglass settles ~5% over time; cellulose settles ~15%. Over-install by the appropriate amount to compensate, or use the manufacturer's initial fill depth (listed on the bag).
Do not insulate over recessed light cans unless they are rated ICAT (Insulation Contact, Air Tight). Standard cans are a fire hazard if covered.
Check for vermiculite in existing insulation before disturbing it. Vermiculite from Libby, Montana may contain asbestos — have it tested before any work.

How to Use This Calculator

1Enter the area to insulate in square feet — measure your attic floor between the joists, or wall cavity area.

2Enter your existing R-value (check existing insulation depth × R/inch, or enter 0 for an uninsulated space).

3Set your target R-value from the IECC minimum for your climate zone. The calculator shows the additional R-value needed above your existing insulation.

4Select insulation type — cellulose (R-3.8/in, recycled content, denser) or fiberglass (R-2.5/in, lighter, easier to blow). Cellulose requires fewer bags for the same R-value.

5Enter bag price and review the total bags, installed depth, and material cost. Big-box stores typically loan blowing machines free with 10+ bag purchase.

Key Terms

Blown-In Insulation — Loose-fill insulation installed using a blowing machine that forces material through a flexible hose. Ideal for attic floors, hard-to-reach cavities, and retrofitting existing walls.

Cellulose — Blown-in material made from recycled newsprint treated with fire retardants. R-3.8/inch. Denser than fiberglass — provides better air blocking and requires less depth for the same R-value.

Fiberglass (Blown) — Loose glass fiber insulation blown into attics or walls. R-2.5/inch. Lighter than cellulose, non-combustible, and inorganic (won't support mold). Requires more depth to reach high R-values.

Coverage Chart — The label on every bag of blown-in insulation shows the number of bags per 1000 sq ft at each R-value and installed depth. Always follow the manufacturer's chart — it accounts for material density and settling.

Settling Factor — Blown-in insulation compresses over time. Cellulose settles ~15%; fiberglass ~5%. Manufacturers print the initial installed depth needed to achieve the target R-value after settling — install to that depth, not the final depth.

ICAT Rating — Insulation Contact, Air Tight. A rating required for recessed light cans that will be covered by insulation. Standard (non-ICAT) cans must not be covered — leave a gap or install an ICAT-rated fixture before insulating.

Frequently Asked Questions

How many bags of blown-in insulation do I need for a 1200 sq ft attic at R-49?

For cellulose at R-49 in a 1200 sq ft attic you need approximately 49 bags (40 lb bags covering ~24.8 sq ft each). For fiberglass at R-49 you need approximately 35 bags (25 lb bags covering ~35 sq ft each). The calculator above computes the exact quantity based on your area and existing R-value.

Can I rent a blower machine for free?

Yes — most big box home improvement stores loan blowing machines at no charge when you purchase 10 or more bags. The machine handles both fiberglass and cellulose. Plan on one to two hours for an average attic with a helper feeding bags at the hopper below. Return the machine when finished and unused bags can typically be returned for a refund.

Should I choose cellulose or fiberglass blown-in insulation?

Cellulose achieves the same R-value at a shallower depth (R-3.8/inch vs R-2.5/inch for fiberglass) and provides better air blocking due to its denser particles. Fiberglass is slightly lighter and easier to blow but requires more depth. Cellulose costs more per bag but fewer bags are needed. Both perform well — cellulose is the more eco-friendly choice as it is made from recycled paper.

Can I add blown-in insulation over existing batt insulation?

Yes. In attics, blown-in insulation can be installed directly over existing fiberglass batts without removing them. Lay any new batts perpendicular to the joists first, then blow cellulose or fiberglass on top. If existing insulation is wet, moldy, or contains vermiculite, remove and dispose of it first. Never cover blocked soffit vents — always install baffles first.

What R-value does my attic need by climate zone?

IECC 2021 minimum attic R-values: Zone 1–2 (Hawaii, S. Florida): R-30 to R-38. Zone 3 (Carolinas, Texas): R-38. Zone 4 (Mid-Atlantic, Pacific NW): R-49. Zone 5–6 (Midwest, N. New England): R-49 to R-60. Zone 7–8 (Alaska): R-60. Many energy efficiency programs recommend exceeding code minimums for maximum savings — especially in zones 5 and above.