Blown Fiberglass for Great Falls' Wide Attics, and BIBS for Gutted Wings
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An attic floor in a big Great Falls house is a bag-count problem before it is anything else. One manufacturer's chart for blown fiberglass calls for 16.75 inches at R-49, which is about 22 bags of 32-pound product for every 1,000 square feet. A 2,400 sq ft floor is roughly 53 bags on the truck. The houses here are mostly 1970s to 1990s detached homes, with a median build year of 1985 in the Census Bureau's 2024 estimates, so large attics over the main block and low slopes over bonus rooms are the norm.
Crews reach them by Georgetown Pike, by Leesburg Pike near Dranesville and by Old Dominion Drive. We measure attic by attic, because a house that has been added onto can have three separate roofs at three different depths.
Depth numbers for a blown fiberglass attic floor
| Target | Installed depth, one maker's chart | Where the target comes from |
|---|---|---|
| R-38 | 13.25 inches | A partial step; below the usual recommendation |
| R-49 | 16.75 inches | ENERGY STAR's goal over 3 to 4 inches of old fiber; new code accepts it only at full uncompressed height |
| R-60 | 20.00 inches | Ceiling figure in the 2021 Virginia code for new work; ENERGY STAR's goal for a bare floor |
Blown fiberglass delivers about R-2.2 to R-2.9 per inch, and the maker reports negligible settling, so the depth we mark is the depth you keep. The bag label you actually buy sets the final numbers. Before any fiber goes down we seal the top plates, set baffles so the eaves keep a 1-inch air gap, and cover or replace recessed cans that are not IC-rated, since insulation must stay 3 inches clear of those.
BIBS is for bare studs only, so it splits a Great Falls house in two
- Open walls in a gut renovation or an addition. Fabric is stapled across the studs and fiber is blown in through a slit, giving about R-15 in a 2x4 wall and up to R-23 or R-24 in a 2x6 wall. The fabric is then covered with drywall and never left exposed.
- Closed walls. These take dense-pack fiberglass, a separate drill-and-fill method packed to the product data sheet, between roughly 1.6 and 2.3 pounds per cubic foot depending on the fiber. Many owners choose cellulose here instead.
- Flat attic floors. Loose-fill over the ceiling below, as in the table above, with no fabric and no holes.
Who decides what on a Great Falls fiberglass job
| Question | Who answers |
|---|---|
| Does insulation alone need a permit? | The county building official, through Land Development Services (12055 Government Center Pkwy, Suite 324, Fairfax 22035, 703-222-0801). It is typically not a listed trigger, and we call before we start. |
| An addition, basement finish, wall change, wiring or asbestos? | A county permit. Our wall visit is booked ahead of the drywall date. |
| Exterior work near Colvin Run Mill? | The Architectural Review Board (703-324-1380), only for exterior alterations that need a building permit. The district boundary is unverified, so we check the parcel. |
| Does Georgetown Pike add rules? | No zoning overlay carries that name; the Pike's Byway and National Register status describes the road. |
| Which utility program might apply? | Your electric bill tells you. Dominion's Home Energy Evaluation lists attic and wall insulation among its measures, the amounts are not published, and we promise none. |
Two fiberglass scopes: a 2,600 sq ft attic and a gutted primary-suite wing
1987 colonial with flattened R-19 batts over the main block
The floor is about 2,600 sq ft, reached by a pull-down stair, with a bath fan duct ending in the insulation and several non-IC cans in the kitchen ceiling below. The crew seals the plates, ducts the fan outdoors, sets baffles, boxes the cans and blows fiberglass over the batts to R-49. A bare floor would take about 58 bags, and the old R-19 cuts that, with the bag label setting the final count. At $0.90 to $1.60 per sq ft the job is $2,340 to $4,160, done in a day.
1986 house with the primary suite and kitchen wing down to studs
The exterior walls are 2x6, about 1,150 sq ft in all. Once rough-in is finished, BIBS fabric goes over each bay and fiber is blown through a slit, reaching about R-23 to R-24 against the R-15 the 2021 code lists for a new wood wall. BIBS at $1.50 to $2.50 per sq ft is $1,725 to $2,875. The drywall crew cannot start until we finish, so that visit is scheduled first.
Fiberglass answers for wide attics, gutted suites and county paperwork
How many bags of blown fiberglass does a 2,400 sq ft Great Falls attic take for R-49?
One manufacturer's chart lists 22.2 bags of 32-pound product per 1,000 sq ft for R-49, so a bare 2,400 sq ft floor needs about 53 bags at 16.75 inches deep. Existing batts reduce that, and the label on the bag actually used governs, so we confirm it on the truck. We mark the depth across the floor before blowing and check it again at the end.
Our 1990s house has a vaulted great room and flat attic above the bedrooms. Is one fiberglass plan enough?
No, these are two different jobs. The flat attic over the bedrooms takes loose-fill blown to a marked depth. The vaulted ceiling has closed, sloped bays, where loose-fill cannot be laid and BIBS fabric cannot go on, so it needs dense-pack or another material, decided by a scan and a test hole. We quote the two areas as separate lines so one does not hide the other.
We are taking our 1986 primary suite down to studs. What has to happen before BIBS goes in?
The wiring, plumbing and ductwork must be roughed in, and any permit inspection the county requires at that stage done. Then fabric is stapled over each bay, fiber is blown through a slit and the wall is covered with drywall, because the fabric is never left exposed. Tell us the drywall date as early as you can, since there is no way to add BIBS after the boards go up.
Does a Great Falls mailing address tell us which county office handles our permit?
Not reliably. Great Falls is unincorporated Fairfax County, and Great Falls Park itself uses a McLean mailing address, so Great Falls, McLean and Herndon addresses can all sit in the same county. Land Development Services handles permits for all of them. We did not check every street, so we confirm your parcel with the county before scheduling.
Do the recessed lights in our two-story foyer limit how deep the fiberglass can go?
Yes, unless the fixture is rated for insulation contact. Code keeps insulation at least 3 inches from a can that is not IC-rated, and newer energy rules expect cans inside the thermal boundary to be IC-rated, airtight and sealed to the ceiling. We swap or box the old ones before blowing so the depth stays uniform across the floor.
Fiberglass near Great Falls, and the guides behind these numbers
The R-value guide explains the numbers above, and the fiberglass versus spray foam comparison is candid about both. Browse the Great Falls hub and all service areas. Fiberglass pages for nearby houses: McLean, Vienna, Herndon and Reston.
Other Insulation Options in Great Falls
Loose fiberglass suits the open stretches of a Great Falls attic, not every cavity. For a closed wall, a floor over the garage or a cold dormer slope near Riverbend Park, the pages below may fit better:
Spray Foam Insulation in Great Falls | Batt Insulation in Great Falls | Cellulose Insulation in Great Falls
