Fiberglass Insulation in Potomac: Blown Attic Depth, Bag Counts and BIBS for Open Walls
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Blown fiberglass suits Potomac for a simple reason: size. A large detached house can have an attic floor of 2,000 square feet or more, and loose-fill covers it at a lower price per square foot than anything else we install. About 71 percent of homes here are detached and the median was built in 1977, so plenty of attics still hold thin batts from the original build. The other fiberglass job is BIBS, a fabric-retained blown system that only works where studs are open, as in additions and gut renovations. We see both from River Road to Travilah and Avenel.
Depth and bag numbers for a large Potomac attic
| Target R-value | Minimum installed thickness (one maker's chart) |
|---|---|
| R-38 | 13.25 in |
| R-49 | 16.75 in |
| R-60 | 20.00 in |
That chart lists 22.2 bags of 32 pounds per 1,000 square feet for R-49, so a 2,600-square-foot attic floor takes about 58 bags. The maker calls settling negligible, yet we still measure depth across the floor rather than trust a label. Blown fiberglass gives about R-2.2 to R-2.9 per inch, and cellulose needs roughly 3 to 4 inches less depth for the same R, which matters where headroom is tight. ENERGY STAR suggests R-49 where 3 to 4 inches are already there and R-60 for a bare floor, and nothing requires either in an existing Maryland home.
Who decides what on a Potomac fiberglass job
| Question | Who answers it | What we do |
|---|---|---|
| Is a permit needed? | Montgomery County Department of Permitting Services, Wheaton, 240-777-0311 | Insulation alone is usually not a permit item; we confirm with DPS for your address first |
| Is the parcel inside Rockville? | City of Rockville, 240-314-8240, only for parcels within city limits | We check the parcel, because 20850 and 20878 mailing addresses occur near Potomac |
| Is exterior work planned on a historic site? | County Historic Preservation Commission, 301-563-3400 | Interior blowing needs no Historic Area Work Permit; exterior work on a Master Plan site does |
| Do covenants apply? | Your homeowners association, not the County | You check the covenants; the County does not enforce them |
| Is there anything unsafe in the attic? | EPA guidance | Assume vermiculite is asbestos and leave it alone; no fiber over knob-and-tube wiring |
Access is the other practical question on a big house. The hose runs from the truck to the hatch, so floors are covered only along that path, but stored boxes have to come out of the attic first because nobody can blow around them. We lay walk boards over the joists, and we keep the old batts in place and blow over them unless they are wet or have been chewed by animals, in which case they come out before anything goes on top.
Two fiberglass projects, scoped for Potomac
1970s house near Potomac Village: 2,600 square feet of attic over two wings, plus the garage ceiling
The old R-11 batts are matted and gray-streaked along the plates where air has leaked through. Blown fiberglass to R-49 at $0.90 to $1.60 per square foot is $2,340 to $4,160, and the garage ceiling adds $1,000 to $2,000, so the job is $3,340 to $6,160. Both wings have their own hatch, and the low-pitch wing means a long hose run over the joists. We seal the top plates, extend bath fan ducts to the outside, set baffles at the soffits to keep a 1-inch vent space and mark depth at every rafter. It takes about 58 bags and one day for two people.
Second-floor suite on a 1990s house off Glen Road: open 2x6 walls
The new framing is up and the rough wiring is in, leaving 2,000 square feet of open wall. BIBS runs $1.50 to $2.50 per square foot, or $3,000 to $5,000. Fabric is stapled across the studs, fiberglass is blown through a slit, and the fabric has to be covered by drywall, so the board stays off until we finish. Fully filled 2x6 bays reach about R-23 to R-24. The addition is permitted under the builder's plans, and we keep the day to two so drywall is not held up. Nothing here applies to a closed wall: that is dense-pack, which uses a different density and no fabric.
Where fiberglass loses to something else in Potomac
For a finished wall the choice is dense-pack cellulose through drilled holes. A small open room, such as a single-wall remodel, is easier with batts cut to fit. Anywhere air is the real problem, at a rim joist or an overhang, spray foam does what loose fiber cannot.
Fiberglass Questions Asked by Potomac Owners
Will blown fiberglass block the soffit vents in my Potomac attic?
It should not. Baffles go in at the eaves before any fiber is blown, which keeps a 1-inch space between the insulation and the roof sheathing at each vent, the clearance the model residential code asks for. Afterward we check each bay from the hatch, and bath fan ducts are extended outdoors so moisture does not end up in the fiber.
We are framing an addition near Travilah. When should the fiberglass go in?
After framing, rough wiring, plumbing and mechanical work are finished and air sealing is done, and before drywall. BIBS fabric has to be covered by a finish material, so the board cannot be left off. Your builder's permit with Montgomery County covers the addition, and we coordinate the day so the drywall crew is not held up.
Why is the old insulation in my Potomac attic gray and black in streaks?
That is filtration soiling. Air leaking from the house carries dust through the insulation and the fibers trap it, so the streaks map where the leaks are. It is not mold and not proof that fiberglass cleans your air. The fix is air sealing the plates and penetrations before new fiber goes on.
What does blown fiberglass cost for a 3,000-square-foot attic in Potomac?
At $0.90 to $1.60 per square foot, a 3,000-square-foot floor is $2,700 to $4,800 for R-49. A typical 1,200 to 1,500 square foot attic runs $1,200 to $3,000, and a garage ceiling is $1,000 to $2,000. The estimate is free and lists each area separately.
My house has a Rockville mailing address. Which office handles my permit questions?
The mailing address does not decide it. Parcels inside the City of Rockville go to the City, and other Potomac-area parcels, even with a 20850 or 20878 address, go to Montgomery County's Department of Permitting Services. We do not know which side your lot is on until we check it, and we do that before we start.
Fiberglass reading, and nearby towns with the same service
Two posts compare the options head to head: fiberglass versus spray foam, compared honestly and spray foam in Bethesda and Montgomery County. The Potomac page lists every local service, and the service-area page shows where crews travel.
Fiberglass pages for neighbors: Bethesda, Rockville, Kensington and Great Falls, Virginia.
Other Insulation Options in Potomac
Fiberglass is not the answer to every Potomac attic, wall or rim joist, and a crew coming from Falls Road to your door should carry the right material. The alternatives are here:
Spray Foam Insulation in Potomac | Batt Insulation in Potomac | Cellulose Insulation in Potomac
