Crawl Space
Insulation
in Maryland

Batt insulation — the fiberglass rolls or batts stapled between floor joists — was the standard recommendation for crawl spaces under the old vented-crawl-space model. The premise was that insulating the floor above a vented crawl space would keep the living area warm while allowing the crawl space itself to vent freely to outside air. That approach has two problems in Maryland.

First, batt insulation has no vapor barrier function. It does not stop moisture movement. In a Maryland crawl space, where summer humidity is 70–80% outdoors, moisture vapor diffuses freely through fiberglass batts. The batts absorb moisture over repeated humid seasons, lose R-value as moisture content increases, and create a damp microenvironment on the face of the batt that is ideal for mold growth. Over time, the mechanical fasteners (staples or wire rods) corrode in the damp environment and the batts fall, leaving sections of the floor entirely uninsulated.

Second, batt installation between floor joists is incompatible with a sealed, conditioned crawl space. Once you seal the foundation vents and condition the crawl space air, the space itself is part of the conditioned building envelope. Insulating the floor above a conditioned space wastes that conditioning — the crawl space air is at house temperature and humidity, and the floor insulation above it serves no function. The insulation belongs on the foundation walls, not the floor joists, in a sealed system.

In a sealed and conditioned crawl space, insulation is installed on the foundation walls — not the floor joists. The wall insulation keeps the crawl space air temperature consistent with the rest of the house, which reduces condensation risk and minimizes the load on the dehumidifier. Rigid foam board (typically XPS or polyiso) and closed-cell spray foam are both appropriate for foundation wall application in Maryland's conditions.

Rigid foam board is cut to fit the foundation wall surface and mechanically fastened or adhered. Seams are taped. For a standard 8-foot crawl space wall, a single layer of 2-inch rigid foam achieves R-10, which exceeds Maryland's Climate Zone 4 requirement of R-10 for crawl space foundation walls. Closed-cell spray foam achieves higher R-values per inch and provides an integrated air and vapor barrier — it's appropriate for walls with irregular surfaces or where air sealing is a priority.

In some crawl spaces — particularly those with irregular foundation geometry or stub walls — a combination approach is used: spray foam at the rim joist and irregular sections, rigid foam on flat wall surfaces. OBW specifies the approach after assessing your crawl space geometry.

Maryland falls in IECC Climate Zone 4 (with parts of western Maryland in Zone 5). Under the current adopted energy code, crawl space foundation walls require minimum R-10 continuous insulation or R-13 cavity insulation. For comparison, a typical 2-inch layer of XPS rigid foam is R-10 and a 2-inch layer of closed-cell spray foam is R-12 to R-14 depending on formulation.

The 'continuous' designation is important. R-10 continuous means R-10 with no thermal bridges — no breaks in the insulation layer where framing or fasteners conduct heat. Rigid foam on the interior face of the wall achieves this. Batt insulation between studs on a framed interior wall does not — the stud framing at 25% of wall area is uninsulated and conducts heat freely, reducing the effective R-value significantly.

OBW quotes insulation to meet or exceed current energy code requirements for your county. If your home is subject to an energy rating or HERS score review — which can be required for major renovation permits — we can provide documentation of the installed R-value.

In most sealed crawl space conversions, yes. Leaving batt insulation in the floor joist bays of a conditioned crawl space is unnecessary and can be counterproductive. The batts absorb moisture even in a dehumidified space, and over time they contribute to elevated humidity in the lower portion of the crawl space air. Removal is included in OBW's crawl space insulation quotes when existing batt is present.

The exception is very new batt insulation in excellent condition in a crawl space with no moisture history. In those cases, OBW may recommend monitoring the insulation's condition through the first summer after sealing rather than removing immediately. If the hygrometer readings indicate the dehumidifier is maintaining target RH without issue, the batts may be left in place short-term. This is an assessment made during the inspection, not a blanket recommendation.

Batt removal is straightforward but labor-intensive and requires proper disposal — fiberglass batts with mold growth are regulated waste in Maryland. OBW includes removal and proper disposal in the project estimate.

Crawl space wall insulation pricing depends on the linear footage of foundation wall and the insulation method specified. Rigid foam board is typically $3–$6 per square foot installed. Closed-cell spray foam runs $5–$9 per square foot installed. For a typical 1,000 square foot crawl space with 8-foot walls and 150 linear feet of foundation perimeter, that translates to roughly 1,200 square feet of wall area.

Crawl space insulation is almost always quoted as part of a complete encapsulation system. The insulation component of a full encapsulation quote is typically $2,500–$6,000 depending on wall footage and method. As a standalone service — if you already have a sealed and dehumidified crawl space — the same range applies. OBW provides itemized quotes so you can see the insulation cost separately from liner, drainage, and dehumidifier components.

Batt removal, if needed, is quoted as a separate line item and typically adds $500–$1,500 depending on quantity and disposal requirements. OBW does not bundle batt removal into the base insulation price — you see what each element costs before any work is scheduled.