Fiberglass Insulation in Rockville: Blown-In Attic Fill for Newer Homes, BIBS for Gutted Walls
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About 13,800 of Rockville's homes, nearly half, were built in 1980 or later: 1,961 in the 1980s, 2,283 in the 1990s, 5,195 in the 2000s and 3,875 in the 2010s, according to the Census (ACS 2020-2024). Many of those are townhouses and apartments, but the newer houses and townhouses share a trait that suits blown fiberglass, which is a broad, simple attic floor with few obstructions.
From Twinbrook to King Farm: what a neighborhood's age suggests
| Area | When it was built | First question about the attic |
|---|---|---|
| East Rockville, Twinbrook | 1940s to early 1950s; Twinbrook from 1948 | Flattened batts, and are the walls empty? |
| Hungerford | 1950s and 1960s | Thin fill, kitchen gut with open studs? |
| Woodley Gardens, College Gardens | Mid-1960s to early 1970s | Uneven depth, cans and fans |
| Rockshire, Fallsmead | 1960s to the 1980s | Patchy fill, thin near the eaves |
| King Farm, Fallsgrove | Late 1990s and 2000s; Fallsgrove plan approved in 2000 | Depth may be fine, so we measure before selling anything |
These ranges come from the City's neighborhood plans. Within any of them a previous owner may have added or removed insulation, so the hatch has the last word. Crews reach these streets by Wootton Parkway, Shady Grove Road and West Gude Drive.
The arithmetic behind R-49 in blown fiberglass
Blown fiberglass delivers about R-2.2 to R-2.9 per inch. The chart we use lists minimum installed depths of 13.25 inches for R-38, 16.75 for R-49 and 20.00 for R-60, plus 22.2 bags of 32 pounds per 1,000 square feet for R-49, and it reports negligible settling. A bare 1,900-square-foot floor therefore takes about 42 bags. Air sealing comes first, because fiberglass filters air instead of stopping it, and baffles at the soffits hold open the 1-inch airway. ENERGY STAR recommends R-49 to R-60 for attics in this region. Existing homes have no required level, while new work follows the 2021 energy code, which the City of Rockville adopted effective May 29, 2024.
When the fiberglass is BIBS, and when we use something else
BIBS, the Blow-In-Blanket System, stretches breathable fabric across open studs and blows fiberglass behind it, reaching about R-15 in a 2x4 wall and up to about R-23 or R-24 in 2x6. Drywall must cover the fabric. That limits it to new construction and gut renovations. Finished walls are a dense-pack cellulose job, plain open cavities are cheaper in batts, and an overhang or rim joist with air leaks needs spray foam.
Two fiberglass projects, scoped for Rockville houses
2000s house in Fallsgrove with a 1,900-square-foot attic and uneven blown fill
Fill that looked fine from the hatch is thin over the rear bedrooms and thicker near the ridge. We seal the wire holes and top plates, cover the recessed cans that are not IC-rated, add baffles at the soffits and blow a top-up that levels the whole floor to R-49. At $0.90 to $1.60 per square foot, 1,900 square feet costs $1,710 to $3,040. The job takes one day and the hose runs through a standard hatch.
Gutted 1950s house in Hungerford with 2,200 square feet of exterior wall open
Demolition is finished and the new wiring has passed rough-in. BIBS goes into the 2x4 bays at $1.50 to $2.50 per square foot, so $3,300 to $5,500, and the garage ceiling adds $1,000 to $2,000, for $4,300 to $7,500 overall. That sits inside our whole-wall BIBS range of $3,000 to $7,500 for the walls alone. Fabric is stapled, slit, filled until taut and trimmed flush before drywall. The house predates 1978, so we ask your remodeler who handles lead-safe practices. Two days.
Two questions that set the permit path
First, is the parcel inside the City? If so, Inspection Services at Rockville City Hall (240-314-8240) handles it; if not, Montgomery County's Department of Permitting Services in Wheaton (240-777-0311) does, and a Rockville mailing address proves nothing. Second, is any wall or wiring changing? A gut renovation is permit work under either office, and BIBS cavities go in on the inspection schedule. Fiber blown over an existing attic floor is usually not a permit item, but the office decides, so we confirm for your parcel first.
Associations, districts and utilities
A King Farm or Fallsgrove association may have rules of its own, and whether it does is a matter for its documents, which we cannot read for you. Attic fiber is not an exterior change, so the City's Historic District Commission, which reviews exterior alterations in local districts such as King Farm Farmstead, is not involved. Pepco and Washington Gas serve most houses, and Pepco's Maryland Home Performance with ENERGY STAR program works only through participating contractors, so confirm terms with Pepco and do not count on a rebate from us. Vermiculite, if found, stops the job.
Fiberglass Answers for Rockville Homeowners
How many bags of blown fiberglass fill a 1,100-square-foot townhouse attic?
On a bare floor, one manufacturer chart calls for 22.2 bags of 32 pounds per 1,000 square feet to reach R-49, which is about 24 bags for 1,100 square feet. That assumes a minimum installed depth of 16.75 inches. If the floor already holds insulation, fewer bags are needed, and the estimate shows the depth we will add. We leave rulers in the attic so you can verify it.
My Fallsmead walls are finished. What is the fiberglass option for them?
Dense-pack, meaning holes are drilled and the fiber is blown in at a set density with no fabric. The figure depends on the product: the Department of Energy cites 1.6 pounds per cubic foot, and manufacturers publish 2.2 to 2.3 for their own dense-pack fiberglass. BIBS is not an option, since it needs open studs. We more often use cellulose for closed walls, and the estimate says which we recommend.
Why does R-49 need more inches of fiberglass than of cellulose?
Blown fiberglass gives about R-2.2 to R-2.9 per inch, while settled cellulose gives about R-3.2 to R-3.8, so cellulose reaches the same rating with roughly 3 to 4 fewer inches. Fiberglass makes up for it by staying put: one manufacturer data sheet reports negligible settling. Where a roof slopes low near the eaves, the extra height of fiberglass matters, and baffles keep it from blocking the vents.
What will blown fiberglass cost in a Rockville townhouse versus a larger house?
The price follows the floor area. Blown fiberglass to R-49 is $0.90 to $1.60 per square foot, so a 1,100-square-foot townhouse floor is $990 to $1,760 and a 1,900-square-foot house floor is $1,710 to $3,040. Fiberglass batts in exterior walls are $1.00 to $1.80 per square foot, and BIBS is $1.50 to $2.50. A garage ceiling is $1,000 to $2,000.
Do recessed lights and bath fans change a fiberglass job in a newer Rockville house?
Yes, both get dealt with before any fiber is blown. A can that is not rated for insulation contact needs 3 inches of clearance, a cover or replacement with an airtight IC-rated fixture. A bath fan that ends in the attic gets a duct to the outdoors. Skipping either one leaves moisture or a fire concern buried under new insulation, so they are on the checklist for every attic.
Further reading, and fiberglass pages beyond Rockville
For the cost and performance side, see the fiberglass versus spray foam comparison and spray foam against cellulose. The Rockville hub page has our local services in one place, and the service-area page covers the region.
Fiberglass elsewhere in Montgomery County: Potomac, Bethesda, Kensington and Silver Spring.
Other Insulation Options in Rockville
Fiberglass is a poor match for finished walls and leaky rim joists. If that describes your Rockville house, start with one of these instead:
Spray Foam Insulation in Rockville | Batt Insulation in Rockville | Cellulose Insulation in Rockville
