Best Insulation Materials for Garage Conversions in Los Gatos Climate

Why Insulation Is the Most Important Decision in Your Garage Conversion

When Los Gatos homeowners plan a garage conversion, discussions usually start with finishes, flooring, and kitchen layouts. But the single decision that most affects long-term comfort, energy costs, and code compliance is insulation.

A garage is an unconditioned space. The walls are typically uninsulated single-layer framing with exterior siding or stucco. The ceiling may be open rafters or a thin layer of plywood. The concrete slab sits directly on the ground. Converting this shell into a comfortable living space means creating a thermal envelope that keeps heat out in summer and warmth in during winter.

Los Gatos sits in California Climate Zone 4, characterized by a Mediterranean climate with hot, dry summers that regularly exceed 90 degrees and cool, wet winters that dip into the low 40s. This dual-season swing makes insulation performance critical in both directions. The wrong insulation choice leads to a space that is stifling in August and cold in January, with HVAC costs that never stop climbing.

This guide compares the four most common insulation materials for garage conversions, explains how each performs in the Los Gatos climate, and breaks down what California’s Title 24 energy code requires for your project.

Title 24 Energy Code Requirements for Los Gatos

Before comparing materials, you need to understand the minimum performance standards your insulation must meet. California’s Title 24 Building Energy Efficiency Standards apply to all garage conversions because you are converting an unconditioned space into a conditioned (heated and cooled) habitable room.

The 2025 Title 24 standards, effective January 1, 2026, set the following prescriptive insulation minimums for Climate Zone 4:

• Walls (2x4 framing): R-15 cavity insulation
• Walls (2x6 framing): R-21 cavity insulation
• Ceiling/roof: R-22 minimum with wood framing (or U-factor of 0.043)
• Supply and return ducts: R-6 minimum, R-8 recommended for Climate Zone 4

These are prescriptive minimums. Your project’s Title 24 energy report may call for higher values depending on the overall energy budget, which factors in window efficiency, HVAC equipment, lighting, and the building envelope as a whole.

A Title 24 compliance report must be prepared by a certified energy consultant and submitted with your permit application to the Town of Los Gatos. Your insulation contractor and general contractor both need a copy of this report to ensure installation matches the specified values.

Comparing Insulation Materials for Garage Conversions

Four insulation materials dominate the residential market for garage conversions. Each has distinct strengths and trade-offs. The right choice depends on your wall framing depth, budget, performance goals, and whether moisture control is a concern.

Insulation Comparison Table

Material	R-Value Per Inch	Cost Per Sq Ft	Air Sealing	Moisture Resistance	Fire Rating	Best For
Fiberglass Batts	R-3.0 to R-4.3	$0.40 - $0.70	Poor (needs separate air sealing)	Low (absorbs moisture)	Non-combustible	Budget projects with standard framing
Closed-Cell Spray Foam	R-6.0 to R-7.0	$1.50 - $4.00	Excellent (self-sealing)	Excellent (vapor barrier at 2”+)	Good with thermal barrier	Maximum performance in tight spaces
Rigid Foam Board (XPS/Polyiso)	R-5.0 to R-6.5	$0.75 - $1.50	Good (with taped seams)	Good to Excellent	Varies by type	Continuous exterior insulation, thin walls
Mineral Wool (Rockwool)	R-4.0 to R-4.2	$0.80 - $1.50	Moderate (dense fit)	Good (hydrophobic)	Excellent (fire-resistant to 2,150F)	Fire-prone areas, soundproofing

Fiberglass Batts: The Budget Standard

Fiberglass batt insulation has been the go-to choice for residential projects for decades, and for good reason. It is widely available, easy to find installers for, and costs roughly half of any other option.

R-value performance: R-3.0 to R-4.3 per inch, depending on density. A standard R-15 batt fits a 2x4 wall cavity (3.5 inches deep). An R-21 batt fits a 2x6 cavity (5.5 inches deep). Both meet Title 24 prescriptive minimums for Los Gatos.

Where it works well: Fiberglass performs adequately in standard garage conversions where the framing is clean, the wall cavities are uniform, and the installer takes time to cut batts precisely around electrical boxes, pipes, and framing members. Kraft-faced batts include an integrated vapor retarder.

Where it falls short: Fiberglass does not seal air leaks. In a garage conversion, where the original construction was never designed for an airtight envelope, gaps around the sill plate, rim joist, window framing, and utility penetrations allow conditioned air to escape. Without meticulous air sealing using caulk, foam sealant, and gaskets at every penetration, fiberglass insulation underperforms its rated R-value by 20 to 40 percent in real-world conditions.

Fiberglass also absorbs moisture. If your garage has any history of water intrusion, whether from rain, irrigation overspray, or ground moisture wicking through the slab, wet fiberglass loses its insulating properties and can promote mold growth behind the drywall.

Cost for a typical 450 sqft garage: $180 to $315 for materials. Installed cost with labor runs $600 to $1,200. Add $500 to $1,000 for proper air sealing work.

Closed-Cell Spray Foam: Maximum Performance

Closed-cell spray polyurethane foam (SPF) is the highest-performing insulation available for residential wall cavities. It is applied as a liquid that expands and hardens, filling every gap, crack, and irregular space in the wall cavity.

R-value performance: R-6.0 to R-7.0 per inch. Just 2.5 inches of closed-cell foam in a 2x4 wall cavity delivers R-15 to R-17.5, meeting or exceeding Title 24 requirements while leaving room in the cavity. In a 2x6 wall, 3 inches of spray foam provides R-18 to R-21 with space to spare.

Where it excels: Spray foam is uniquely suited to garage conversions because it addresses the three biggest challenges of converting an unconditioned shell: thermal performance, air sealing, and moisture control. It fills irregular framing cavities that batts cannot conform to. It bonds directly to studs, sheathing, and concrete, eliminating air gaps. At 2 inches or more, closed-cell foam qualifies as a Class II vapor retarder, eliminating the need for a separate vapor barrier.

For Los Gatos garages with older, irregular framing or any moisture history, closed-cell spray foam is the most reliable path to a comfortable, code-compliant envelope.

Where it falls short: Cost is the primary barrier. Spray foam requires specialized equipment and certified installers, and the material itself is more expensive. It also requires a thermal barrier (typically 1/2-inch drywall) to meet fire code when used in occupied spaces, though this is standard practice for any finished wall.

Open-cell spray foam is a lower-cost alternative at R-3.5 to R-3.6 per inch, but it does not provide the same moisture resistance or structural rigidity as closed-cell. For garage conversions in Los Gatos, closed-cell is the better investment.

Cost for a typical 450 sqft garage: $2,500 to $5,500 installed for walls. Ceiling adds $1,500 to $3,500 depending on accessibility and thickness.

Rigid Foam Board: Versatile and Space-Efficient

Rigid foam boards come in three main types: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso). Each offers different R-values and moisture characteristics, but they share a common advantage: high R-value in a thin profile.

R-value performance: EPS delivers R-3.8 to R-4.4 per inch. XPS provides R-5.0 per inch. Polyiso leads at R-5.7 to R-6.5 per inch (though its performance drops in cold temperatures, which is rarely a concern in Los Gatos).

Where it works well: Rigid foam is excellent as continuous exterior insulation over existing sheathing, which eliminates thermal bridging through the studs. It is also useful inside the wall cavity when combined with batt insulation for a hybrid approach. In garages where the wall depth is limited, 1 inch of rigid foam on the interior face of the sheathing combined with fiberglass batts in the remaining cavity can achieve high R-values without furring out the walls.

For garage ceilings with limited depth between the rafters and the desired ceiling height, rigid foam boards maximize insulation in a thinner profile than batts.

Where it falls short: Rigid foam boards must be cut precisely to fit each cavity, and the seams between boards need to be sealed with tape or spray foam to prevent air leakage. The installation is more labor-intensive than rolling out batts. Some types (XPS and polyiso) have higher global warming potential than fiberglass or mineral wool due to the blowing agents used in manufacturing.

Cost for a typical 450 sqft garage: $750 to $2,000 for materials depending on type and thickness. Installed cost runs $1,500 to $3,500.

Mineral Wool (Rockwool): Fire Resistance and Sound Control

Mineral wool insulation, commonly sold under the Rockwool brand, is made from spun basite rock fibers. It has gained significant market share in the Bay Area, particularly in areas concerned about wildfire risk.

R-value performance: R-4.0 to R-4.2 per inch, slightly better than fiberglass batts. An R-15 mineral wool batt fits a 2x4 cavity. An R-23 batt fits a 2x6 cavity, exceeding the Title 24 minimum of R-21.

Where it excels: Mineral wool’s standout properties are fire resistance and sound attenuation. It is non-combustible and maintains its structural integrity up to 2,150 degrees Fahrenheit. For Los Gatos homeowners converting a garage attached to their primary residence, mineral wool adds a fire-resistant buffer between the new living space and the rest of the home. Its dense fiber structure also provides superior soundproofing compared to fiberglass, which matters if the converted space will be a bedroom, home office, or music studio adjacent to a driveway or street.

Mineral wool is hydrophobic, meaning it does not absorb water. If exposed to moisture, it dries out and retains its R-value. This is a meaningful advantage over fiberglass in garage conversions where the moisture history of the walls is uncertain.

Where it falls short: Mineral wool is denser and stiffer than fiberglass, which makes it slightly harder to cut and fit around obstacles. It costs roughly twice as much as fiberglass batts. Like fiberglass, mineral wool does not seal air leaks on its own; separate air sealing work is required.

Cost for a typical 450 sqft garage: $360 to $675 for materials. Installed cost runs $800 to $1,800. Add $500 to $1,000 for air sealing.

Los Gatos Climate Considerations

Los Gatos experiences a Mediterranean climate that creates specific insulation demands. Understanding these seasonal patterns helps you prioritize the right material properties.

Summer Heat Management

Los Gatos summers bring extended stretches above 85 degrees, with peaks above 95 degrees. A west-facing or south-facing garage wall absorbs solar radiation all afternoon. Insulation with a high R-value per inch is essential on sun-exposed walls to keep the interior comfortable without oversizing the air conditioning system.

Closed-cell spray foam and rigid foam boards perform best here because they combine high R-value with air sealing that prevents hot outdoor air from infiltrating the building envelope. Adding a radiant barrier (reflective foil) under the roof decking can further reduce heat gain through the ceiling.

Winter Moisture Control

Cool, wet winters bring rain and elevated humidity. The temperature differential between the warm interior and the cool exterior creates conditions for condensation inside the wall cavity. Insulation that resists moisture (closed-cell spray foam, rigid foam, mineral wool) outperforms moisture-absorbing fiberglass in this season.

If you choose fiberglass, proper vapor retarder placement and thorough air sealing are non-negotiable. Moisture that reaches the cold side of the wall and condenses on the sheathing will degrade fiberglass and create mold risk over time.

Wildfire Smoke and Air Quality

Los Gatos, like much of the Bay Area, faces periodic wildfire smoke events that degrade outdoor air quality. A well-sealed building envelope, which spray foam insulation inherently provides, reduces smoke infiltration into the living space. Fiberglass and mineral wool batts, without supplemental air sealing, leave gaps that allow smoke particles to enter.

The Hybrid Approach: Combining Materials

Many experienced contractors in the Bay Area recommend a hybrid insulation strategy for garage conversions. This approach uses higher-performance (and higher-cost) materials where they matter most and more affordable materials where the return on investment is lower.

A common hybrid strategy for Los Gatos garage conversions:

• Exterior walls: 2 inches of closed-cell spray foam against the sheathing, with fiberglass or mineral wool batts filling the remaining cavity depth. The spray foam handles air sealing and moisture protection while the batts add cost-effective R-value.
• Ceiling: R-22 or higher fiberglass or mineral wool batts between the rafters. Ceilings are easier to air-seal than walls because there are fewer penetrations, so the spray foam premium is less critical here.
• Garage door wall replacement: When the garage door is removed and replaced with a new framed wall, use the full depth of the new framing for high-performance insulation. This is typically the largest wall area and the easiest to insulate well because it is built from scratch.

This hybrid approach typically costs 30 to 40 percent less than full spray foam while achieving 80 to 90 percent of the thermal performance.

Insulation and Your Garage Conversion Budget

Insulation typically accounts for 5 to 10 percent of the total garage conversion budget. For a $150,000 project, that is $7,500 to $15,000 depending on the materials chosen.

Insulation Strategy	Estimated Cost (450 sqft garage)	Title 24 Compliance	Performance Level
Fiberglass batts + air sealing	$1,100 - $2,200	Meets minimum	Good
Mineral wool batts + air sealing	$1,300 - $2,800	Meets or exceeds	Good to Very Good
Rigid foam + fiberglass hybrid	$2,000 - $4,500	Exceeds	Very Good
Closed-cell spray foam (walls) + batts (ceiling)	$3,500 - $7,000	Exceeds	Excellent
Full closed-cell spray foam	$4,000 - $9,000	Exceeds	Premium

These numbers cover materials, labor, and air sealing. They do not include the Title 24 energy report ($500 to $1,500), which is required regardless of which insulation you choose.

Spending more on insulation saves money on two fronts. First, a better-insulated envelope allows a smaller, less expensive HVAC system. Second, monthly energy costs are lower for the life of the space. In Los Gatos, where PG&E rates continue to climb, these savings compound meaningfully over 10 to 20 years.

How Custom Home Approaches Insulation for Garage Conversions

At Custom Home, insulation decisions are made during Phase 1 design, not improvised during construction. Our process works like this:

1. Site assessment: We evaluate your garage’s existing wall construction, framing depth, sun exposure, and any moisture indicators to determine the right insulation strategy.
2. Title 24 energy report: Our energy consultant models the full building envelope and recommends insulation values that achieve code compliance cost-effectively.
3. Material specification: We specify exact insulation materials, thicknesses, and installation details in the construction documents. There are no vague references to “insulate to code.” Every wall, ceiling, and floor assembly is detailed.
4. Locked-in pricing: The insulation scope is included in the Phase 1 construction estimate, so you know the exact cost before committing to Phase 2 construction.

This approach eliminates the common scenario where a homeowner discovers mid-project that the insulation allowance in their budget is insufficient for the materials the energy report requires.

Making Your Decision

For most Los Gatos garage conversions, the decision comes down to budget and priorities:

• Tightest budget: Fiberglass batts with thorough air sealing. Meets code. Requires disciplined installation.
• Best value: Hybrid approach with spray foam on walls and mineral wool or fiberglass in the ceiling. Strong performance at a moderate premium.
• Maximum comfort and efficiency: Full closed-cell spray foam. Highest R-value, best air sealing, built-in vapor protection. Worth the investment for full-time living spaces, especially rental ADUs where tenant comfort and low utility bills matter.
• Fire safety priority: Mineral wool throughout. Non-combustible, excellent soundproofing, good moisture resistance.

Whatever material you choose, proper installation is more important than the material itself. Poorly installed spray foam with gaps and voids performs worse than carefully installed fiberglass batts with meticulous air sealing. Work with a contractor who understands building science, not just pricing.

Start Your Los Gatos Garage Conversion

Insulation is one piece of a complex puzzle that includes structural assessment, Title 24 compliance, permitting, HVAC design, and finish selection. Getting the insulation right from the beginning sets up every other system for success.

Custom Home (CSLB #986048) handles Los Gatos garage conversions from design through construction, including insulation specification, Title 24 compliance, and inspections. Our two-phase process gives you complete plans, material specifications, and a locked-in price before construction begins.

Contact Custom Home to schedule a free site assessment for your Los Gatos garage conversion.