Blown Fiberglass for Chantilly Attic Floors and BIBS for Open Walls
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Along Route 50 the Chantilly roofline is mostly 1980s, and under those roofs sit wide, plain attic floors that are easy to reach. That is the situation loose-fill fiberglass suits best: a level layer over the whole floor in a day, nothing cut, nothing fitted. By the census count, 4,609 of the 7,598 homes here are detached, so this is our most common fiberglass job in Chantilly.
Fiberglass has a second use, and it is narrower. When a house is opened to the studs, blow-in blanket fills the bays behind fabric. Finished walls do not qualify. On the Loudoun side of the Chantilly mailing name, in South Riding, Stone Ridge and Arcola, the attics are the same job under a different inspector. Wherever the house stands, the order of work is the same: measure what is there, fix what is wet or loose, seal the air leaks, then add the fiber.
Pick the method by the part of the house
| Part of the house | Method | What it delivers |
|---|---|---|
| Attic floor, open and dry | Loose-fill fiberglass | About R-2.2 to R-2.9 per inch; one manufacturer chart lists 13.25 in for R-38 and 16.75 in for R-49, with negligible settling |
| Walls open to the studs | BIBS, fabric stapled over the bays | About R-15 in a 2x4 wall, up to roughly R-23 in a 2x6, then covered with drywall |
| Finished walls | Not BIBS; dense-pack through drilled holes | Density depends on the product data sheet; we usually use cellulose here |
| Basement wall in new work | Code prescriptive path | R-13 in the cavity or R-10 continuous |
Attics crowded with wiring or a floored walkway are easier with batts, and finished walls are a job for cellulose. For a sealed roofline instead of a vented attic, see spray foam; our fiberglass versus spray foam comparison is plain about the trade-offs.
What it costs, starting from the hatch: a South Riding attic and a stripped kitchen near Walney Road
Detached house in South Riding, 1,800 square feet of floor and a compressed builder blanket
The existing batts measure about R-19 after years under storage boxes. The plates and wire holes get sealed, the bath fan duct is extended to a roof cap, baffles hold a 1-inch airway at each soffit, and non-IC cans are boxed or replaced. Then fiber goes down to R-49, about 17 inches deep. At $0.90 to $1.60 per square foot, 1,800 square feet is $1,620 to $2,880. One day, and because the house is in Loudoun County any inspection question goes to Building and Development.
1980s colonial off Walney Road, kitchen and family room taken to the studs
About 900 square feet of 2x4 exterior wall is open, with the electrician's rough-in finished. BIBS fabric is stapled across each bay, a slit is cut for the hose, and the fiber is blown to a full cavity. At $1.50 to $2.50 per square foot that is $1,350 to $2,250. Moving walls and wiring are permit items, so the fabric stays uncovered until the Fairfax County inspector signs off, and the drywall crew follows. Two days: fabric first, fiber second.
Where blown fiberglass is the wrong answer in Chantilly
- A floored attic or a walkway of plywood over the joists. Fiber under boards cannot be blown evenly, so a crossed course of batts does better there.
- A wet attic or a stained roof deck. Moisture is repaired before anything new goes in, and the old insulation that got wet is removed.
- Ducts or an air handler in the attic. Those are better brought inside the thermal envelope with foam than buried under fiber.
- A finished wall where someone suggests BIBS. The system needs open studs, and we will say so plainly rather than sell it.
Offices and order of work
Insulation by itself is not a listed permit trigger in Virginia, and the building official decides what counts as minor work. Chantilly proper answers to Fairfax County Land Development Services, 12055 Government Center Pkwy, Suite 324, 703-222-0801. A ZIP 20152 address answers to Loudoun Building and Development, 703-777-0220.
ENERGY STAR advises sealing the attic floor before adding insulation, so we start with the plates, the pipe and wire penetrations and the chases over interior walls. New framing follows the 2021 Virginia code.
Getting the map right
Route 50 is the name Lee Jackson Memorial Highway has carried since July 5, 2023, and Route 28 is Sully Road. Westfields Boulevard and Stonecroft are business addresses rather than subdivisions. We pass the Udvar-Hazy Center and Ellanor C. Lawrence Park on the way to jobs.
The Sully Historic Site lies within a county historic overlay, where permit-requiring exterior work goes to design review. Blowing an attic is interior work, but we look up the parcel before anything goes through an exterior wall.
Depth, BIBS and recessed-light questions from Chantilly homeowners
How deep should blown fiberglass be in a Chantilly attic to reach R-49?
About 17 inches for one common product, and the chart for another lists 16.75 inches. The answer shifts with the product, so we read the bag label, blow to that figure and leave depth rulers standing. ENERGY STAR recommends R-49 to R-60 for Zone 4, which is guidance for an existing house, not a mandate.
Could the blow-in blanket system work on my Chantilly house if the drywall is already up?
No. The fabric has to be stapled across exposed studs, so BIBS is limited to new framing or walls opened during a renovation. Closed walls that are empty are dense-packed through small drilled holes instead, usually with cellulose in our work.
Is blown fiberglass or a second layer of batts better over my thin attic?
If the floor is open and clear, blown fiberglass is quicker and cheaper and fills the corners. If the floor is partly boarded, or wiring runs make blowing messy, laying a crossed course of batts is easier. We walk the attic first and recommend whichever suits the framing.
Do recessed lights limit how much fiberglass can go around them?
They do when the fixtures are not rated for insulation contact. A non-IC can needs 3 inches of clear space all around, so we build a rated cover over it or replace the fixture with an IC-rated one. The energy code also requires recessed lights in the thermal envelope to be airtight and IC-rated.
What happens to the attic fan and gas flue when you blow fiberglass?
The flue keeps its required clearance, and a dam or cover goes around any fan opening so fiber cannot fall in. If a gas furnace or water heater sits below, we suggest a combustion safety test after air sealing. Columbia Gas of Virginia serves parts of Chantilly, so check which utility reads your meter.
Reading that helps you choose a depth, and fiberglass pages nearby
Before you settle on a number, read the R-49 guide for Zone 4A and what R-value means. The Chantilly hub page and the service area list show the rest of what we do locally.
Fiberglass pages for nearby towns: Centreville, Herndon, Sterling, Fairfax and Oakton.
Other Insulation Options in Chantilly
Not every Chantilly house wants loose fill. A damp attic, closed walls or a cold basement rim might be better served by one of these:
Spray Foam Insulation in Chantilly | Batt Insulation in Chantilly | Cellulose Insulation in Chantilly
