Spray Foam vs Fiberglass Insulation: Which Is Better for a Conversion Project?

Choosing between spray foam and fiberglass insulation for your garage, container, or van conversion? Here is an honest comparison of cost, performance, and when each type makes sense.

The Insulation Decision Every Conversion Project Faces

Whether you are converting a garage, shipping container, van, or shed, insulation is one of the most important decisions you will make. The two most common options are spray foam and fiberglass batts, and each has real advantages and disadvantages depending on your project type.

Spray Foam Insulation

Spray foam comes in two types: open-cell and closed-cell. For conversion projects, closed-cell spray foam is generally preferred because it provides both insulation and a vapor barrier in one application.

Pros:

Highest R-value per inch (R-6 to R-7 per inch for closed-cell) — critical when wall depth is limited, like in vans and shipping containers. Creates an airtight seal that eliminates drafts and condensation issues. Acts as a vapor barrier, preventing moisture from reaching the wall structure. Fills irregular gaps and curves perfectly, which is essential for van conversions and metal structures. Adds structural rigidity to thin walls.

Cons:

Expensive — professional installation costs $1.50 to $3.50 per square foot, or roughly $2,000 to $4,000 for a typical garage. Requires professional installation with specialized equipment. Cannot be easily removed or modified after curing. Off-gassing concerns during installation require vacating the space for 24 to 48 hours. Some DIY spray foam kits exist but results are often inconsistent.

Fiberglass Batt Insulation

Fiberglass batts are the pink or yellow rolls you see at every hardware store. They have been the standard residential insulation for decades.

Pros:

Affordable — costs $0.50 to $1.50 per square foot including installation, or roughly $300 to $800 for a typical garage. Easy to install yourself with basic tools. Widely available at any hardware store. Can be removed and replaced if needed. No off-gassing concerns after installation.

Cons:

Lower R-value per inch (R-3.2 to R-3.8 per inch) — requires thicker walls to match spray foam performance. Does not create an air seal — requires separate vapor barrier installation. Absorbs moisture if it gets wet, losing its insulating properties and potentially causing mold. Does not conform to irregular shapes well, leaving gaps around pipes, wires, and curves. Not suitable for metal structures like shipping containers or vans without additional moisture management.

When to Use Spray Foam

Spray foam is the better choice when:

You are converting a shipping container. Metal containers create extreme condensation issues. Spray foam bonds directly to the metal and prevents moisture from reaching the steel, which would cause rust. Fiberglass in a container is a recipe for mold and corrosion.

You are converting a van or bus. The curved, irregular surfaces of a vehicle make fiberglass batts impractical. Spray foam (or rigid foam board cut to fit) conforms to the shape and prevents condensation on the metal skin.

Wall depth is limited. If you only have 2 inches of wall cavity, spray foam gives you roughly R-12 to R-14, while fiberglass would only give you R-6 to R-8. In a garage where you want to maximize usable floor space, this matters.

You are in a climate with extreme temperatures. The airtight seal of spray foam dramatically reduces heating and cooling costs compared to fiberglass.

When to Use Fiberglass

Fiberglass is the better choice when:

You are on a tight budget. Fiberglass costs one-third to one-half the price of spray foam. For a basic garage gym or home office where climate perfection is not critical, fiberglass gets the job done.

You are converting a wood-framed structure. Garages and sheds with standard 2x4 or 2x6 framing are designed for fiberglass batts. The wall cavities are the right size, and moisture management is straightforward with a poly vapor barrier.

You want to do it yourself. Fiberglass batt installation is a true DIY project. Measure, cut, friction-fit, and staple. No special equipment needed beyond a utility knife and staple gun.

You might modify the space later. Fiberglass is easy to remove if you need to access wiring or plumbing behind the walls.

Other Options Worth Considering

Rigid foam board (XPS or polyiso): A great middle-ground option that offers better R-value per inch than fiberglass (R-5 to R-6.5 per inch), can be cut to fit irregular spaces, and is DIY-friendly. Popular in van conversions and as continuous insulation on the exterior of garage walls.

Mineral wool (Rockwool): Similar to fiberglass batts but with better fire resistance, sound dampening, and moisture resistance. Costs slightly more than fiberglass. Excellent choice for garage recording studio conversions where sound isolation matters.

Bottom Line

For most garage conversions on a reasonable budget, fiberglass batts with a proper vapor barrier are the practical choice. For shipping containers, vans, and any metal structure, spray foam or rigid foam board is the clear winner. And if budget is no concern and you want the best performance in any structure, closed-cell spray foam is hard to beat.

Use our cost calculator to see how insulation choice affects your total project cost, or browse our conversion guides for specific insulation recommendations for each project type.