Blown Fiberglass for Herndon Attics, and Blanket-Fill for Walls Open to the Studs
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The typical Herndon house is a detached home from around 1983. Detached houses are 43.7 percent of the Town's units, and the 1980s and 1990s together account for 3,308 of them. The attics are big, plain and lightly insulated by current standards: thin batts, a few recessed cans, a bath fan that stops short of the roof. That suits blown fiberglass, which fills a space like that quickly and evenly.
Fiberglass has a second, narrower job in houses being opened up. When a kitchen or family room is stripped to the studs, BIBS puts fiber behind fabric in the open bays. We drive in from Sterling Road, Herndon Parkway and Sunrise Valley Drive, and the work covers Oak Hill and Franklin Farm addresses as well as the Town itself.
An attic day, from the hatch to the cleanup
| Stage | What happens | Why it matters |
|---|---|---|
| First look | We count recessed cans, trace bath fan ducts and check for vermiculite and cloth wiring | Anything that cannot be buried is found before fiber is on site |
| Sealing | Top plates, wire holes and chases over interior walls are sealed | ENERGY STAR says to seal leaks in the attic floor before adding insulation |
| Fans and lights | Fan ducts run to a roof or wall cap; non-IC cans get 3 inches of clearance or a swap | Insulation must not envelop a hot fixture or trap fan moisture |
| Baffles | One at every soffit vent | The code keeps a 1-inch space between insulation and roof sheathing |
| Blowing | Loose-fill to the target, depth rulers every few feet | R-49 takes roughly 17 inches of fiber; one chart lists 16.75 |
| Close-out | Insulated, weatherstripped hatch lid; hose and debris removed | The hatch is the largest hole left in the floor |
Fiberglass jobs, priced for a Franklin Farm-area attic and a stripped ranch
1980s detached house on a county parcel, 1,400 square feet of thin batts
The hatch is a standard pull-down stair, the old batts measure somewhere near R-13 and a bath fan duct lies on the floor. Once the sealing, the duct and the baffles are done, fiber goes down to R-49. At $0.90 to $1.60 per square foot the floor is $1,260 to $2,240. Because the parcel is unincorporated Fairfax County, Land Development Services is the office if a question ever comes up. One day.
1970s ranch gutted to the studs, kitchen and mudroom walls open
About 760 square feet of 2x4 exterior wall is exposed, with the plumber and electrician finished. BIBS fabric is stapled across the bays, a slit is cut for the hose, and fiber is blown to a firm, full cavity, good for roughly R-15. At $1.50 to $2.50 per square foot the walls come to $1,140 to $1,900. The house is inside the Town, where alterations need a permit, so the fabric stays uncovered until the Town inspector passes it. Then drywall covers it, because the fabric is never left exposed. Two days.
Blanket-fill versus dense-pack: same fiber, different walls
The two get confused. BIBS is fabric stapled over open bays, with fiber blown through a slit in the fabric, and it only works where the studs are bare. Dense-pack uses the same kind of fiber, driven into a closed wall through drilled holes at a density that depends on the product data sheet, with no fabric at all. A finished Herndon wall can take dense-pack, and in practice we usually choose cellulose for that job. A small open room is faster with batts, and a leaky rim joist is spray foam work.
Virginia's 2021 code asks for R-60 at the ceiling of new construction and additions, and accepts R-49 where it lies uncompressed at full height across the whole attic. For a house that already stands there is no required retrofit, so R-49 is a recommendation, and going to R-60 takes about 20 inches of fiber by one manufacturer's chart.
Which office, and what needs a permit
Insulation alone is not a listed permit trigger in Virginia, and the building official decides what is minor work. Inside the Town, Building Inspections at 777 Lynn Street, 2nd Floor, 703-435-6850, is the office, and its page requires permits for additions and alterations without mentioning insulation. Oak Hill, Floris, Franklin Farm, Folkstone and McNair use Land Development Services, 703-222-0801.
Additions, wall changes and wiring are permit work in both places.
Map notes and exterior review
Sugarland Run is its own census area. The W&OD Trail and Herndon station are landmarks, not permit boundaries. Inside the Herndon Historic District, exterior changes go to the Historic District Review Board, so blown attic insulation is fine by itself, but we ask first if a vent or siding work is involved.
Dominion Energy, Washington Gas and, in places, Columbia Gas of Virginia supply most properties here.
Herndon homeowners ask about old batts, baffles and BIBS
Can blown fiberglass go straight over the old batts in my Herndon attic?
Yes, if they are dry and not fouled. Old batts that are merely thin or flattened get loose fill over them, after the air sealing. Wet, moldy or rodent-damaged insulation comes out first. We also pull back any batt that covers a bath fan or a recessed can before sealing around it.
What separates BIBS from dense-pack fiberglass in a Herndon wall?
Fabric and access. BIBS needs bare studs, because the fabric is stapled across them before the fiber is blown in. Dense-pack goes into a finished wall through drilled holes with no fabric, at a higher density set by the product data sheet. Only one of them is possible once the drywall is up.
Does a house in the Herndon Historic District need sign-off before an attic is blown?
The Historic District Review Board looks at exterior changes, and most of them need a Certificate of Appropriateness first. An attic floor is interior work. If the job involves new roof vents, a hatch through a gable or anything drilled through siding, we ask Community Development at 703-787-7380 before starting.
Why do you install baffles, and what is the 1-inch rule?
Where there are eave vents, the code wants at least a 1-inch space between the insulation and the roof sheathing so air can flow from the soffit. Baffles hold that channel open. Without them, loose fill slides against the vent and blocks it, which invites moisture in the roof deck.
How deep would blown fiberglass need to be in a new Herndon addition over a garage?
New ceilings follow the 2021 code: R-60, or R-49 where it is uncompressed and full height across the whole attic, over the top plates included. By one manufacturer's chart R-60 takes about 20 inches and R-49 about 16.75, so the depth is checked with rulers before we leave.
More on attic options, and fiberglass pages near Herndon
For the other attic choices, read blown-in attic insulation in Fairfax County and spray foam versus cellulose. The Herndon hub page and the service area list have the rest.
Fiberglass pages nearby: Reston, Chantilly, Sterling, Great Falls and Oakton.
Other Insulation Options in Herndon
Fiberglass is not always the best fit for a Herndon house with closed walls, a damp roof deck or a hot attic full of ductwork. These pages cover the alternatives:
Spray Foam Insulation in Herndon | Batt Insulation in Herndon | Cellulose Insulation in Herndon
