Fiberglass Insulation in McLean: Blown Attics and BIBS Walls for Bigger Houses
Free McLean Estimate
Book Phone Consultation
Not every McLean attic is a mid-century problem. The census table shows 2,528 homes from the 1980s, and about 5,300 from 1990 onward, out of 17,833. Newer houses are more likely to have broad, simple attic floors, and a broad simple floor is where blown fiberglass is cheapest to install and easiest to get right. Older houses in the same town are different, which is why we size each job from the hatch, not the zip code.
Crews travel the Georgetown Pike and Lewinsville Road corridors and come in from Dolley Madison Boulevard. We quote houses near Chain Bridge Road, Langley Oaks, Chesterbrook Woods and Dolley Madison Estates the same way. Those are names of places we serve, not claims about when their houses were built.
Blown fiberglass in a big open attic
Blown fiberglass delivers about R-2.2 to R-2.9 per inch, which puts R-38 to R-49 at roughly 13 to 17 inches. The chart we cite calls for a minimum of 13.25 inches for R-38, 16.75 for R-49 and 20.00 for R-60, and notes that the fiber shows negligible settling, so the depth we blow is the depth that stays. The air sealing comes first, because fiberglass filters air rather than stopping it, and baffles go in at the eaves. Gray streaks on old fiberglass mark where house air has been leaking through; they are not mold and not a flaw in the material.
BIBS for walls that are still open
BIBS stands for Blow-In-Blanket System. A breathable fabric is stapled across bare studs, a slit is cut, and fiberglass is blown in until the fabric is taut. Expect about R-15 in a 2x4 wall and up to about R-23 or R-24 in a 2x6. The fabric is never left exposed; drywall or another code-compliant covering goes over it. BIBS therefore belongs to new construction and gut renovations. A finished wall is drilled and dense-packed, without fabric, and that is the work described on our cellulose page.
Where fiberglass is the wrong choice: a cluttered attic floor where cellulose settles into the gaps, a plain remodel cavity where batts are cheaper, and a rim joist or cantilever that needs the air seal only spray foam provides. New houses and additions follow the 2021 code, with R-60 at the ceiling and R-49 accepted at full, uncompressed height. An older house nobody is altering has no mandatory level.
Two fiberglass projects, scoped for McLean
Two-story house from the 1980s with a 2,100-square-foot attic floor
The old fill is patchy, roughly R-19 and thinner near the eaves, and a bath fan hose ends in it. Several can lights are not IC-rated. We seal the wire holes and top plates, duct the fan outdoors, cover the cans, ring the soffits with baffles, and blow a top-up to R-49 across the whole floor. At $0.90 to $1.60 per square foot, 2,100 square feet costs $1,890 to $3,360. One day, hose through the hatch. If the attic proves to hold vermiculite, we stop and you call an asbestos professional.
Rear addition to a 1970s house, framed in 2x6, with a low roof above
Framing and rough-in are finished. About 900 square feet of 2x6 wall gets BIBS at $1.50 to $2.50 per square foot, or $1,350 to $2,250, and the 500-square-foot addition ceiling gets blown fiberglass at $450 to $800. The scope totals $1,800 to $3,050. The additions are permit work, so BIBS cavities go in on the county's inspection schedule and drywall follows. Walls take a day, the ceiling half of another.
Who inspects and who permits
Additions, new houses and gut renovations are permit work in McLean, and the office is Fairfax County Land Development Services, 12055 Government Center Pkwy, Suite 324, Fairfax, VA 22035 (703-222-0801). Open-wall fiber goes in on the county's schedule. Blowing fiberglass over the floor of an existing attic is not one of the listed permit triggers, but the building official has the final say, so we confirm with LDS before beginning.
Overlay, association and utility notes
Langley Fork, McLean's only county historic overlay of its own, centers on the Georgetown Pike and Chain Bridge Road crossroads. Its board reviews exterior alterations that need a building permit, which attic fiberglass is not. We do not speak for homeowners' associations. Dominion Energy and Washington Gas serve the area, and Dominion's Home Energy Evaluation lists attic insulation, though no dollar amounts are published and we offer no figure.
Fiberglass Questions From McLean Owners
How many bags does an R-49 fiberglass attic take?
One manufacturer chart lists 22.2 bags of 32 pounds per 1,000 square feet for R-49, at a minimum installed depth of 16.75 inches. A bare 2,400-square-foot McLean attic floor therefore takes about 53 bags. Depth varies by product, so the bag label for the fiber we blow governs. We leave depth rulers in the attic so the result can be checked against the label.
What keeps blown fiberglass from drifting when wind blows through the soffits?
Baffles. One goes in every soffit bay, which holds open the 1-inch airway under the roof deck and keeps loose fill from blowing back from the eaves or blocking the vents. The ventilation rule for our zone is 1 square foot of vent per 150 of attic floor, or 1 per 300 when 40 to 50 percent of the vent area sits high in the attic. Baffles go in before any fiber does.
Is BIBS worth it for a McLean addition, compared with batts?
BIBS fills every gap around wiring and boxes because the fiber is blown behind fabric, reaching about R-15 in a 2x4 wall and R-23 or R-24 in a 2x6. Batts cost less where the framing is plain and regular. BIBS pays for itself in bays full of pipes, wires and odd blocking. Either way the framing must be open, and the fabric must be covered by drywall.
Which recessed lights can sit under blown fiberglass?
Only cans rated for insulation contact, known as IC-rated, can be buried. A can that is not rated needs 3 inches of clearance on all sides, a rigid cover built around it, or replacement with an airtight IC fixture. Covering a hot, unrated fixture with loose fill is how many attics end up with both a fire concern and a large air leak, so this gets resolved before we blow.
What does fiberglass cost across a whole McLean house?
Figure blown fiberglass to R-49 at $0.90 to $1.60 per square foot, so a 2,000-square-foot attic floor runs $1,800 to $3,200. BIBS in walls is $1.50 to $2.50 per square foot and a whole-house BIBS job is $3,000 to $7,500. Fiberglass batts in exterior walls run $1.00 to $1.80 per square foot, and a garage ceiling is $1,000 to $2,000. Each area appears as its own line item.
Keep reading, and fiberglass pages around McLean
The honest fiberglass and spray foam comparison and our piece on spray foam in Great Falls estate homes explain where foam earns its higher price. The McLean hub page collects our local services, and the service-area page covers the wider region.
Fiberglass work elsewhere on this side of the Beltway: Tysons Corner, Great Falls, Vienna, Falls Church and Arlington.
Other Insulation Options in McLean
Big attics and open walls are where fiberglass works best. Closed walls, cluttered floors and leaky rim joists are better served by one of these:
Spray Foam Insulation in McLean | Batt Insulation in McLean | Cellulose Insulation in McLean
