Fiberglass vs. Spray Foam vs. Rigid Board Insulation: Which Should You Use?

Fiberglass vs. Spray Foam vs. Rigid Board Insulation: Which Should You Use?

Not all insulation is created equal. The right choice depends on the application, climate, budget, and construction type. Each of the three major insulation categories — fiberglass batts, spray foam, and rigid board — has unique strengths and ideal use cases. Here's a practical comparison to help you spec the right product for every part of the building envelope.

Fiberglass Batt Insulation

Fiberglass batts are the most widely used insulation product in residential construction and the baseline for most new builds.

 

• R-value: R-3.0 to R-3.8 per inch; pre-cut to 2x4 (R-13/R-15) or 2x6 (R-19/R-21) framing
  
• Cost: Lowest cost per square foot of any insulation type
  
• Installation: DIY-friendly; friction-fit between studs and joists
  
• Pros: Affordable, widely available, fast to install in open framing
  
• Cons: Doesn't seal air gaps; performance drops with poor installation; susceptible to moisture in some applications
  
• Best for: Framed wall cavities, floor joists, attic floors, cost-sensitive projects
  

Spray Foam Insulation

Spray polyurethane foam (SPF) is the highest-performance insulation option and the only product that simultaneously insulates and air-seals.

Open-Cell Spray Foam

 

• R-value: ~R-3.5 to R-3.7 per inch
  
• Soft, spongy texture; vapor permeable
  
• Lower cost than closed-cell; best for interior walls and sound control
  

Closed-Cell Spray Foam

 

• R-value: ~R-6.0 to R-7.0 per inch — highest of any insulation
  
• Rigid, adds structural strength to wall assemblies
  
• Acts as vapor barrier; excellent for basements, crawlspaces, and coastal/cold climates
  
• Highest upfront cost but may reduce other system costs (smaller HVAC, no separate vapor barrier)

 

Spray foam best for: Attic decks (unvented assemblies), crawlspace walls, rim joists, complex geometries, and high-performance building envelopes.

Rigid Board Insulation

Rigid foam boards are used where thinner, high-performance insulation is needed — typically on exterior walls, under slabs, or in basements.

 

• XPS (Extruded Polystyrene): Blue/pink boards; R-5 per inch; moisture-resistant; great under slabs and on foundation walls
  
• EPS (Expanded Polystyrene): White boards; R-3.6 to R-4 per inch; most affordable rigid option; good for continuous insulation layers
  
• Polyisocyanurate (Polyiso): Foil-faced; R-6.5 per inch; best thermal performance; commonly used on roofs and commercial walls
  

Rigid board best for: Exterior wall sheathing layers, basement walls, garage ceilings, under concrete slabs

Quick Reference: Which to Use Where

 

• Wall cavities (new framing): Fiberglass batts or open-cell spray foam
  
• Exterior walls (continuous layer): Rigid XPS or EPS board
  
• Attic floor (vented attic): Blown fiberglass or blown cellulose
  
• Attic rafters (unvented): Closed-cell spray foam
  
• Crawlspace walls: Closed-cell spray foam or rigid foam
  
• Rim joists: Closed-cell spray foam (most effective)
  
• Under slab: Rigid XPS board
  

Pro Tip

For production builders, the most cost-effective approach is usually fiberglass batts in wall cavities plus spray foam at rim joists, penetrations, and the attic hatch — capturing the majority of air-sealing benefits at a fraction of the full spray foam cost.