You already know the space needs insulation. The open question is what kind. Fiberglass is the familiar choice, and closed-cell spray foam is the one people keep hearing about. Before you buy either, it helps to compare the things that actually decide how the finished room feels: heat flow, air leakage, moisture, installation and total project cost.
The short answer
Fiberglass is a budget-friendly way to fill standard, open stud cavities, but it does not stop air movement on its own, so air sealing has to be done separately. Closed-cell spray foam insulates and air seals in one step, and it conforms to gaps, penetrations, rim joists, metal and masonry. It costs more per square foot of coverage and takes more preparation. Many projects get the best result by using both.
Insulated isn't the same as air sealed
Insulation slows heat moving through a material. Air sealing stops air from moving around it. They are two different jobs.
A fiberglass batt is a mat of fibers with air trapped between them. It slows conductive heat flow well when it is installed without gaps, but air can still pass through the fibers and around the edges of the batt, especially where wires, pipes and outlet boxes interrupt the cavity. That leakage is why a wall can be "insulated" and still feel drafty.
Closed-cell spray foam expands as it is applied and bonds to the surfaces it touches. That lets it fill irregular gaps and seal around penetrations while it insulates. If you are trying to fix drafts, hot and cold spots, or an uncomfortable garage, the air-sealing difference usually matters as much as the R-value number.
Fiberglass vs. closed-cell spray foam, side by side
R-value measures resistance to heat flow. The higher the R-value, the greater the insulating power. Always compare R-value at the same thickness.
| What to compare | Fiberglass batts | Closed-cell spray foam |
|---|---|---|
| R-value per inch | Roughly R-3 to R-4 per inch, depending on density (for example, R-13 in a 3.5-inch 2x4 wall) | Higher per inch. The closed-cell foams sold on Chem2Go list about R-5.66 per inch; check each product page for its exact value |
| Air sealing | Not an air barrier; needs separate caulk, foam or membrane | Helps air seal as it insulates by expanding into gaps and bonding to surfaces |
| Irregular spaces | Must be cut and fitted; gaps around wires, pipes and odd framing are common | Conforms to rim joists, penetrations, corners and uneven surfaces |
| Moisture | Performance drops if it gets wet or compressed | Dense cell structure resists water absorption; vapor-control role depends on thickness and product specs |
| Installation | Fast in open, standard-size cavities; gloves, long sleeves and a dust mask recommended | More prep: surface cleaning, ventilation, PPE and following the kit instructions closely |
| Material cost | Lower per square foot | Higher per square foot; the value depends on how much air sealing and moisture resistance the space needs |
| Often a good fit for | Large, dry, standard stud and joist bays | Rim joists, garages, metal buildings, sheds, workshops, RVs and hard-to-fit areas |
Compare the whole project, not just the material
The cheapest material is not always the cheapest project. Before choosing, answer these five questions:
- What kind of space is it? A standard interior wall and a metal pole barn are very different jobs. Fiberglass needs framing to hold it; spray foam can bond directly to metal, wood and masonry.
- Where is the air coming in? If the problem is drafts at the rim joist, top plates or around pipes, you need air sealing either way. Count that cost when you price fiberglass.
- Is moisture a factor? Basements, crawlspaces and metal buildings prone to condensation call for a material that tolerates moisture well.
- How much depth do you have? In shallow cavities, a higher R-value per inch can matter more than price per square foot.
- What does the finished project really cost? Add up material, separate air sealing, tools, safety gear and your time, not just the price of the insulation.
When fiberglass makes sense
- You are insulating large, open, standard-size stud or joist bays.
- The space is dry and air sealing is already handled or will be done separately.
- Upfront material cost is the main deciding factor.
When closed-cell spray foam makes sense
- Drafts and air leaks are the main complaint, not just temperature.
- The space has irregular gaps, penetrations or odd framing that batts can't fit well.
- You are insulating rim joists, a garage, workshop, shed, metal building or RV.
- Depth is limited and you want more R-value per inch.
- The area sees occasional moisture or condensation.
The hybrid approach: seal first, then fill
You don't always have to choose one. A common approach is to use spray foam where air sealing matters most, then use fiberglass for bulk fill:
- Seal the rim joist, top and bottom plates, and every pipe, wire and duct penetration with spray foam.
- Fill the remaining open cavities with fiberglass batts.
This puts foam where it does the most work and keeps material costs down in simple cavities. Check your local building code and the product instructions for layering and covering requirements before you start.
Before you close the wall
Once drywall goes up, fixing a missed gap gets expensive. Walk the space first:
- Check where the walls meet the floor, ceiling and rim joist, the most common leak points.
- Look at every outlet, switch box, pipe and vent that passes through the wall.
- Make sure insulation fills the cavity without gaps or compression.
- Confirm how the insulation will be covered. Foam plastic insulation generally has to be separated from living and occupied spaces by an approved thermal barrier, such as 1/2-inch drywall. Your local code and the product instructions have the final say.
Compare spray foam options at Chem2Go
Chem2Go carries several trusted spray foam brands in one place, so you can compare by project instead of by label:
- IXIRPRO for contractors and serious DIYers who work to a PRO standard, with bulk and pallet options for bigger jobs.
- MagicChems complete kits for sealing and insulating with the tools included in one box.
- Y'allCanDIY for approachable weekend projects.
Next step: figure out how much you need. Our guide How Much Spray Foam Do I Need for My Garage? walks through the math, or browse all spray foam insulation.
Frequently asked questions
Is spray foam better than fiberglass?
It depends on the project. Closed-cell spray foam offers more R-value per inch and helps air seal as it insulates. Fiberglass costs less per square foot and works well in open, standard cavities when air sealing is handled separately.
Does fiberglass stop air leaks?
No. Fiberglass slows heat transfer, but air can move through and around it. Gaps and penetrations need to be sealed separately with caulk, foam or another air barrier.
Can I use spray foam and fiberglass together?
Yes. A common method is to seal gaps, plates and penetrations with spray foam, then fill the remaining cavity with fiberglass. Check local code and product instructions for your specific assembly.
Can I spray foam over existing fiberglass?
Spray foam needs to bond to a clean, solid surface, so it should not be sprayed onto fiberglass batts. Remove or pull back the batts, apply foam to the framing and sheathing, then reinstall or replace the fiberglass if you are using both.
Which is cheaper, fiberglass or spray foam?
Fiberglass usually has a lower material cost per square foot. For a fair comparison, add the cost of separate air sealing and your time to the fiberglass price, then compare total project cost.