Fiberglass in Kensington: Blown Attic Floors by the Bag, and BIBS Where Walls Are Open
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Fiberglass is the workhorse material for the house that dominates this area: a detached 1950s or 1960s rambler, Cape or split-level with a simple attic floor. In the 20895 postal area about 78 percent of homes are detached and the median was built in 1958, which means plenty of thin, matted batts from the original build and attics of a size one crew can cover in a day. Inside the Town the picture is more varied, with about half the units in houses. We also install BIBS, the fabric-held blown fiberglass that only works where wall studs are open, and we see both jobs between Connecticut Avenue and Knowles Avenue.
Who answers what, by Kensington address
| If your address is | Office that answers | What it means for blown fiberglass |
|---|---|---|
| Inside the Town of Kensington (about half a square mile; Town Hall can confirm a lot) | Town Hall, 301-949-2424, plus Montgomery County permitting | The Town Code asks for a permit to alter a building or part of one and lists no interior exemption. Nothing official mentions insulation, so you ask before we schedule. |
| North Kensington, South Kensington or another unincorporated 20895 address | Montgomery County Department of Permitting Services, 240-777-0311 | Blown attic fiberglass is usually not a permit item. The building official decides, and we confirm before we start. |
| A Kensington Parkway address | Depends: Kensington, Chevy Chase View and North Chevy Chase all use the street name | We check the parcel, never the street name. |
| Exterior work in the Kensington Historic District | County historic preservation staff, 301-563-3400, after any Town review | Interior blowing is exempt from the Historic Area Work Permit. New siding or other exterior changes are not. |
The numbers we use on a Kensington attic floor
- Depth. One manufacturer's chart asks for 13.25 inches installed for R-38, 16.75 inches for R-49 and 20.00 inches for R-60.
- Bags. The same chart lists 22.2 bags of 32 pounds per 1,000 square feet for R-49, so a 1,250-square-foot floor takes about 28 bags.
- Per inch. Blown fiberglass delivers about R-2.2 to R-2.9 per inch, and cellulose needs roughly 3 to 4 inches less for the same R.
- Headroom and venting. Baffles hold a 1-inch gap under the roof at each eave vent, and recessed cans that are not IC-rated get a box with 3 inches of clearance.
- Gas appliances. If a furnace or water heater sits in or near the space, a combustion safety test follows the air sealing.
The maker calls settling negligible, but we measure depth at every few rafters rather than trust the bag. ENERGY STAR suggests R-60 for a bare attic floor and R-49 where 3 to 4 inches already lie there; for an existing house those are recommendations, and the energy code applies to permitted new work and additions.
Two fiberglass jobs, priced for Kensington houses
1958 rambler in North Kensington: a 1,250-square-foot attic under thin R-11
The old batts are matted and dusty-gray along the plates, which marks where house air has been leaking through. Blown fiberglass to R-49 at $0.90 to $1.60 per square foot comes to $1,125 to $2,000 for the floor, about 28 bags. Access is a pull-down stair in the hall ceiling, so the hose runs through the house and floor protection goes down along that path. We seal the top plates and wire holes, run the bath fan duct to the outside, set baffles at the soffits and box two old cans. One day for two people.
Rear kitchen and den opened to the studs behind a 1950s rambler near Knowles Avenue
The new addition framing is up, the rough wiring is in, and 700 square feet of 2x4 exterior wall stands open. BIBS runs $1.50 to $2.50 per square foot, so $1,050 to $1,750. Fabric is stapled over the studs, fiberglass is blown in through a slit, and the fabric must be covered by drywall, so the board waits for us. Filled 2x4 bays reach about R-15. The addition's permit belongs to the remodel, and we schedule after the framing inspection. Two days keep the drywall crew moving. None of this applies to a closed wall: that is dense-pack, with no fabric.
When we steer a Kensington job away from blown fiberglass
A closed plaster wall in the older blocks around Howard Avenue takes dense-pack cellulose through drilled holes. One small room with open studs is easier with batts cut to the bay. Air leaking past a rim joist is a spray foam problem. Loose fiber also does not belong over vermiculite or knob-and-tube wiring; EPA says to assume vermiculite contains asbestos and leave it alone.
Fiberglass Questions Kensington Owners Bring Us
I have a Kensington mailing address. How do I know whether Town Hall is involved in my blown-fiberglass job?
Ask Town Hall, because a ZIP code will not tell you. The Town is about half a square mile inside a far larger 20895 postal area, and most addresses with a Kensington label are outside it. Town Hall (301-949-2424) can confirm whether your lot is inside. If it is, ask about a permit. If it is not, the County is the only office.
How many bags does a 1,250-square-foot Kensington attic take, and what does it cost?
For R-49 plan on about 28 bags of 32 pounds, going by one maker's figure of 22.2 bags per 1,000 square feet. At $0.90 to $1.60 per square foot that is $1,125 to $2,000. A garage ceiling is priced separately at $1,000 to $2,000, and the estimate lists every area as its own line.
Will the blown fiberglass cover the recessed lights in my rambler's hall ceiling?
Only if they are rated for insulation contact. Insulation must stay 3 inches from a can that is not IC-rated, so we build a box around each old fixture that keeps the fiber clear, or you can replace it with an IC-rated light. We also seal around each one, since an old can leaks house air into the attic.
We are building a rear addition off a 1950s rambler. Is BIBS right for the new walls?
BIBS suits open stud walls, so yes for the addition, and no for the old walls, which are closed. We blow after framing, rough wiring, plumbing and mechanical work are done and after air sealing, and before drywall, since the fabric has to be covered. Your addition's permit comes from the project, and we work around its inspections.
Is R-60 worth it for a 1950s Kensington attic, or is R-49 enough?
ENERGY STAR suggests R-60 for a bare floor and R-49 where 3 to 4 inches are already there. R-60 needs about 20 inches of blown fiberglass, against 16.75 for R-49, so headroom near the eaves often decides it. Nothing requires either number in an existing house. We will tell you how much depth your roof pitch allows before you choose.
Fiberglass reading, and the same service in nearby towns
Two guides compare materials directly: the complete spray foam and fiberglass guide and Maryland attic insulation options. The Kensington page lists local services, spray foam in Kensington covers air sealing, and the service areas page shows where crews work.
Fiberglass pages for neighbors: Bethesda, Silver Spring and Rockville.
Other Insulation Options in Kensington
Loose fiberglass suits a clear attic floor, not a plaster wall near the Kensington MARC station or a rim joist that leaks air. The other options are here:
Spray Foam Insulation in Kensington | Batt Insulation in Kensington | Cellulose Insulation in Kensington
