Why You Should Spray Foam Bonus Rooms Above Garages

Homeowners often struggle with temperature control in rooms built above garages — commonly called bonus rooms. These spaces are notorious for being freezing cold in the winter and unbearably hot in the summer. While many assume insulation is already in place, the material used in many builds—fiberglass—falls short of solving the problem.

Recently, we were called out to a six-year-old home where the bonus room had become unusable due to extreme temperature swings. The issue? Standard fiber insulation in the garage ceiling below simply wasn’t cutting it. That’s because fiber insulation, while common, is not an air barrier. It allows air to move through it freely, which means heat and cold can easily pass from the garage ceiling into the room above.

The Real Problem With Fiber Insulation

In most cases, when builders install insulation in garage ceilings, they use fiberglass batts. While this material has been around for decades, it’s not designed to stop air movement. It merely slows down thermal transfer. So when vented air—either hot or cold—makes its way into the joist spaces through soffits or rim joists, it travels over the insulation and directly impacts the subfloor of the bonus room.

This air leakage is what causes those rooms above garages to be so uncomfortable. You can think of it like wearing a sweater on a windy day without a windbreaker. Sure, you’re technically covered, but you’re still going to feel that chill.

Spray Foam: A Real Air Barrier

To solve this problem properly, we removed sections of drywall from the garage ceiling to expose the joist cavities. From there, we pulled out the old fiberglass insulation and replaced it with closed-cell spray foam. Unlike fiber, spray foam insulation acts as a full air barrier. It’s fully adhered to the underside of the subfloor above, meaning there’s no space for air to sneak in and compromise the room’s temperature.

Spray foam not only insulates — it seals. This means it blocks airflow entirely, keeping outside temperatures from reaching the living space above. In the home we worked on, the transformation was immediate. The foam created a continuous, gap-free barrier that locked the comfort in and kept the discomfort out.

Do It Right the First Time

One of the most important takeaways from this job is this: it costs far more to fix insulation problems after the fact than to do it right from the beginning.

If spray foam had been used when the house was built, the homeowner would have saved money and hassle in the long run. Instead, they had to pay for drywall removal, insulation removal, spray foam installation, and then the added cost of repairing and repainting the ceiling. That’s a significant investment — all because the original builder chose a cheaper, less effective material.

Homeowners aren’t the only ones paying the price here. Builders and contractors who take shortcuts ultimately damage their reputation and client satisfaction. The cost difference between fiberglass and spray foam during construction is minimal when compared to the cost of rework. Choosing spray foam from the start is not only better for the homeowner — it’s smarter business.

A Challenge to Builders

We encourage homebuilders to reconsider outdated practices. Spray foam insulation isn’t new — it’s tried, tested, and proven. In any home design where living space sits directly above an unconditioned garage, spray foam should be the go-to material.

Yes, spray foam may have a higher upfront cost compared to fiber. But the long-term benefits — improved comfort, energy efficiency, and peace of mind — far outweigh that initial investment. More importantly, it avoids the kind of invasive, expensive retrofits like the one we performed.

The Bottom Line

If you have a bonus room above your garage that’s too hot or cold, there’s a good chance fiberglass insulation is to blame. The best solution is to replace it with closed-cell spray foam insulation, which seals out the unwanted air and keeps your space livable year-round.