R-value measures thermal resistance — how effectively a material slows heat transfer per inch of thickness. A higher R-value per inch means you need less material depth to hit your target, which matters when you are working within a wall cavity or a limited attic joist height.
But R-value per inch is only one factor. Cost, moisture performance, fire resistance, sound attenuation, and longevity all differ significantly across insulation types. This reference brings those factors together so Northern Virginia homeowners can compare options clearly.
R-Value Comparison: All Major Insulation Types
| Insulation Material | R-Value Per Inch | Fire Resistance | Moisture Resistance | Sound Insulation | Typical Cost Range | |---------------------|-----------------|-----------------|---------------------|------------------|--------------------| | Closed-Cell Spray Foam | R-6.0 to R-7.0 | Good | Excellent (vapor barrier) | Good | $0.90-$1.50/board ft | | Polyisocyanurate (Polyiso) Board | R-5.6 to R-8.0 | Good (with facers) | Good | Moderate | Moderate | | Extruded Polystyrene (XPS) | R-5.0 | Moderate | Excellent | Moderate | Moderate | | Expanded Polystyrene (EPS) | R-3.6 to R-4.2 | Moderate | Good | Moderate | Low | | Open-Cell Spray Foam | R-3.5 to R-3.9 | Moderate | Low | Good | $0.44-$0.65/board ft | | Blown-In Cellulose | R-3.1 to R-3.8 | Good (borate-treated) | Low | Good | $0.60-$2.30/sq ft | | Mineral Wool Batts | R-3.1 to R-3.4 | Excellent (1,000C+) | Good (hydrophobic) | Excellent | $4-$5/sq ft | | Fiberglass Batts | R-2.9 to R-3.8 | Good (non-combustible) | Good | Moderate | $1.90-$2.40/sq ft | | Blown-In Fiberglass | R-2.2 to R-2.9 | Good (non-combustible) | Good | Moderate | $0.50-$1.50/sq ft |
What the Numbers Mean for Your Attic
Virginia building code requires R-49 to R-60 for attic insulation in new construction in Climate Zone 4 — the zone covering Northern Virginia from Arlington and Fairfax through Loudoun County. Here is what that means in required depth for each material:
To reach R-49:
- Closed-cell spray foam: approximately 7-8 inches
- Blown-in cellulose: approximately 13-16 inches
- Blown-in fiberglass: approximately 17-22 inches
- Fiberglass batts: approximately 13-17 inches
Most existing attics in Northern Virginia have between 3 and 6 inches of insulation — equivalent to R-9 to R-20 — far below code minimums. The gap between existing conditions and the R-49 target is where the energy savings live.
Material Profiles: Choosing the Right Type for Each Application
Closed-Cell Spray Foam
The highest R-value per inch of any commonly available insulation. Closed-cell spray foam also acts as a vapor barrier and adds structural rigidity to the surfaces it is applied to. It is the standard choice for rim joists, crawl space walls, and any application where space is limited and moisture control is critical.
Best for: Rim joists, crawl space walls, metal building panels, locations where high R-value in minimal thickness is required.
Limitations: Highest cost per board foot. Requires professional installation. Off-gassing during application requires occupants to vacate during cure time. Not cost-effective for large open attic applications.
Blown-In Cellulose
The most common professional choice for attic insulation upgrades in Northern Virginia. Cellulose is made from 75-85% recycled paper treated with borate — a naturally occurring mineral that provides fire resistance, mold resistance, and pest deterrence. It distributes evenly around obstructions and installs quickly.
Best for: Attic floor insulation (open blow), existing finished walls (dense-pack), retrofit applications.
Limitations: Settles 15-20% over time. Can absorb moisture if exposed to water; must dry quickly to avoid mold. Not appropriate where the insulation will be in contact with persistent moisture.
Blown-In Fiberglass
Similar application to cellulose but with lower R-value per inch and less settling. Blown-in fiberglass is appropriate in attics where settling is a concern and some depth can be traded for stability.
Best for: Attic floors where depth is not constrained, retrofit addition over existing insulation.
Limitations: Lower R-value per inch than cellulose. Less effective at blocking air movement than dense-pack cellulose.
Fiberglass Batts
The most widely recognized insulation product. Batts are appropriate for new construction where stud bays are open and accessible, but they perform poorly when cut imprecisely — even small gaps and compressions significantly reduce effective R-value.
Best for: New construction walls and floors with accessible framing cavities.
Limitations: Requires precise fitting; gaps reduce performance disproportionately. Risk of sagging in attic applications over time. Does not stop air movement — must be paired with a continuous air barrier.
Mineral Wool (Rockwool)
The premium choice for fire resistance and sound attenuation. Mineral wool is made from volcanic rock or blast furnace slag and is naturally fire-resistant to over 1,000 degrees Celsius. It is also hydrophobic — it repels water rather than absorbing it — making it appropriate in applications where occasional moisture exposure is possible.
Best for: Fire-rated assemblies, party walls and interior sound insulation, mechanical rooms, applications requiring both thermal and acoustic performance.
Limitations: Higher cost than fiberglass batts. Heavier to handle during installation.
Rigid Foam Board (XPS, EPS, Polyiso)
Rigid boards are used where insulation must be installed over a continuous surface — exterior walls, basement walls, or under slabs — rather than between framing. Polyiso offers the highest R-value per inch of the rigid board options but performs slightly worse at very low temperatures. XPS is the most moisture-resistant and is appropriate for below-grade applications.
Best for: Continuous insulation layers on exterior walls, basement walls, under concrete slabs, cathedral ceilings with limited rafter depth.
Limitations: Requires protection from UV exposure. Must be covered with a thermal barrier (drywall or similar) when used in occupied spaces.
Matching Material to Application: Northern Virginia Quick Reference
| Location | Recommended Material | Target R-Value | |----------|---------------------|----------------| | Attic floor | Blown-in cellulose or fiberglass | R-49 to R-60 | | Rim joists | Closed-cell spray foam | R-15 minimum | | Crawl space walls | Closed-cell spray foam or rigid foam | R-15 minimum | | Basement walls | Rigid foam board plus batts | R-15 minimum | | Exterior walls (retrofit) | Dense-pack cellulose | R-13 minimum | | Cathedral ceiling/roof deck | Closed-cell spray foam | Application-specific | | Interior sound walls | Mineral wool batts | Acoustic performance |
Get the Right Material for Your Home
EcoGuard Insulation installs blown-in cellulose, blown-in fiberglass, and closed-cell spray foam in attics, crawl spaces, and rim joists throughout Northern Virginia — serving Arlington, Fairfax, McLean, Reston, Herndon, and surrounding communities. We will assess your current conditions, recommend the right material and depth for your application, and install to code. Contact us to schedule a free estimate.