Foam vs. Fiberglass in Texas Heat: Real-World Pros, Cons, and Costs for Your Attic

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If your attic bakes every summer and your AC runs nonstop, you’re not alone. In San Antonio, attic temperatures can reach 130°F on a sunny day. The right insulation slows heat flow, seals air leaks, and helps control moisture—keeping your home comfortable and your energy bills in check. This guide explores the differences in insulating properties when considering Foam vs. Fiberglass and compares open-cell spray foam, closed-cell spray foam, and blown-in fiberglass insulation. You’ll find pros, cons, costs, and when each type makes the most sense for your home.

Quick Definitions

  • Open-cell spray foam: A softer foam that offers excellent air sealing and sound control.
  • Closed-cell spray foam: A dense foam with higher R-value per inch and strong moisture resistance.
  • Blown-in fiberglass: Loose fibers blown across the attic floor to reach a target R-value.

All three can perform well when installed correctly. Your best choice depends on your goals, budget, and attic layout.

Performance Per Inch, Explained

  • Open-cell foam: R-3.6 to R-3.8 per inch. Usually applied 5–7 inches thick to the roof deck in conditioned attic setups.
  • Closed-cell foam: R-6 to R-7 per inch. Best for tight spaces and moisture-prone areas.
  • Blown-in fiberglass: R-2.5 to R-2.7 per inch. Typically installed at 14–16 inches deep to reach R-38.

In San Antonio, attics often target R-38 to R-60 depending on home size and goals.

Foam vs. Fiberglass for Air Sealing and Comfort

  • Spray Foam (Open & Closed Cell): Expands to fill gaps, seal air leaks, and block dust and humidity swings.
  • Blown-in Fiberglass: Offers thermal insulation but requires separate air sealing for best results. Seal top plates, attic hatches, recessed lighting, and plumbing penetrations first.

If you’re looking for maximum comfort and energy efficiency, spray foam wins on total envelope control. Fiberglass paired with air sealing can deliver excellent results at a lower upfront cost.

Moisture Performance in Humid Heat

  • Open-cell foam: Vapor permeable. Can allow drying, but needs to be designed carefully to avoid condensation in rooflines.
  • Closed-cell foam: Vapor and air resistant. Ideal for moisture-prone areas like rim joists and roof decks.
  • Fiberglass: Doesn’t stop air or moisture by itself. Works best when paired with proper sealing and ventilation.

Sound Control

  • Open-cell foam: Strong sound absorption. Great for noisy roads or under metal roofs.
  • Closed-cell foam: Moderate soundproofing.
  • Fiberglass: Good sound dampening when applied deep enough across the attic floor.

Typical Installed Costs in San Antonio

Blown-in Fiberglass (R-38 to R-60):

  • Small attics (800–1,200 sq ft): $1,200–$2,400
  • Mid-size attics (1,200–2,000 sq ft): $1,800–$3,500
  • Large attics (2,000–3,000 sq ft): $3,000–$5,500

Open-Cell Spray Foam (Roof Deck):

  • $3–$5 per sq ft of roof area
  • Best for conditioned attics and sound control

Closed-Cell Spray Foam:

  • $4–$7 per sq ft
  • Ideal for tight cavities or moisture-sensitive areas

Rebates through CPS Energy can significantly reduce final costs. Geo-Insulation helps customers apply these rebates directly to invoices.

Downsides and Common Concerns

Spray Foam:

  • Higher upfront cost
  • Needs professional application
  • Attic behavior changes; HVAC adjustments may be needed
  • Requires careful moisture management

Fiberglass:

  • Doesn’t seal air leaks
  • Uneven coverage reduces effectiveness
  • Must be paired with proper sealing and ventilation

Most Effective Insulation for Texas Heat

  • Best for overall performance: Spray foam at the roof deck
  • Best value: Blown-in fiberglass with attic floor air sealing

Open-cell spray foam is great for sound and value; closed-cell offers moisture resistance and structural strength. Fiberglass remains a strong budget-friendly option when installed with air sealing.

When Each Option Is the Smarter Choice

Choose open-cell foam if:

  • You want a conditioned attic
  • Sound control is a priority
  • Your HVAC is in the attic

Choose closed-cell foam if:

  • You need high R-value per inch
  • Moisture control is critical
  • You’re dealing with hard-to-reach framing

Choose blown-in fiberglass if:

  • You prefer a vented attic
  • Budget is a concern
  • You want fast installation with good value

Hybrid approaches are also common—using closed-cell foam in tricky areas and fiberglass elsewhere.

Why Work With Geo-Insulation

With Geo-Insulation, you’ll work directly with owner Tom Porter. He inspects each attic personally, documents the job with photos, and provides honest, no-pressure quotes. You’ll also benefit from:

  • Clean installation
  • Guidance on submitting rebate paperwork
  • Responsive follow-up if anything needs attention

Next Steps

Ready to stop the heat from pouring into your home?

  • Start with a free attic inspection
  • Receive an itemized quote with rebate info
  • Get a custom plan that fits your home and budget

Visit Energy Star’s Insulation Guide to learn more about insulation basics, or contact Geo-Insulation today to see how we can help you stay cooler, longer, and for less.

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