Spray Foam vs. Fiberglass Insulation in Tampa, FL

R-Value Tells You Half the Story

R-value measures how well a material slows heat moving through it. It says nothing about air leaking past it. Florida's energy code treats those as two separate things on new homes: R-38 in the ceiling, plus a blower door test showing the house leaks no more than 7 air changes per hour at 50 pascals.

Fiberglass can reach whatever R-value you pay for. What it can't do is close the gaps around the attic hatch, light fixtures, wire holes and duct boots, and those gaps are where cool air escapes and attic air gets pulled in. A deep fiberglass blanket over a leaky ceiling is a good fishing cooler with the lid propped open. The walls are fine. The ice still melts.

Open-cell foam is considered air-impermeable at typical thicknesses, and so is closed-cell, so either one handles insulation and air sealing in the same pass.

Attic Heat and the Ducts Living Up There

Here's the part most comparisons skip. In a Fort Myers field study, the Florida Solar Energy Center measured a dark-shingle attic that reached 110°F on a typical July day and ran 40°F above the outdoor temperature at the peak hour. The same report points out that Florida ducts are almost always in the attic, and that attic heat soaking into them can push cooling loads up as much as 30 percent during peak summer periods.

Fiberglass on the attic floor shields the rooms below, but the ducts and often the air handler stay in that heat. Foam sprayed under the roof deck moves the whole attic, ducts included, inside the insulated envelope, which the Department of Energy's Building America guide calls an unvented, conditioned attic. In the Fort Myers study, the home with 5 inches of open-cell at the deck had the coolest attic of the group, maxing out at 83°F.

The payoff was real but modest: a 6 to 11 percent cut in cooling energy, with little change at peak. Leaky ducts and the insulation on the ceiling swing results like that a lot, so we look at both before we quote anything.

Humidity Is the Tampa Tiebreaker

Summer dew points across Florida typically sit in the 65 to 75 range, and Tampa's record is 86°F. A vented attic breathes that air all day. When it meets a surface colder than its dew point, it leaves water behind, and in an attic the coldest surfaces are usually the supply ducts.

Fiberglass doesn't stop that air. It just sits in it. Sealing the roof deck with foam keeps most of that muggy outside air out of the attic to begin with. One caution: open-cell foam is vapor-open, so a sealed open-cell attic still needs a way to shed moisture. Our open-cell vs. closed-cell breakdown explains which foam fits which roof.

What Each Option Costs

These are typical installed ranges. Your number depends on the area sprayed, the thickness and how easy the attic is to work in.

• Blown-in fiberglass or cellulose top-up: roughly $1.00–$2.80 per sq ft
• Open-cell spray foam: roughly $1.50–$3.50 per sq ft ($0.60–$1.60 per board foot)
• Closed-cell spray foam: roughly $3.00–$5.00 per sq ft ($1.30–$3.10 per board foot)
• Removing old attic insulation first: about $1–$2 per sq ft
• Small jobs: minimum charges of roughly $1,000–$2,000 are common

Two things widen the gap. A sloped roof deck has more surface than the floor under it, so a foamed attic covers more square feet. And thickness costs money: R-38, the code level for ceilings, takes about 10.6 inches of open-cell foam, while blown-in gets there cheaply because the attic floor has room. Our spray foam cost guide has the full breakdown.

When Fiberglass or Blown-In Is the Smarter Buy

Honestly, foam isn't the right answer for every attic. A blown-in top-up usually makes more sense when:

• The ducts are tight and insulated, and the real problem is a thin layer on the attic floor
• There's room to reach R-38 and the existing insulation is clean and dry
• The budget covers blown-in across the whole attic but foam over only part of it
• The roof is near the end of its life, since a foamed deck can't dry toward the attic if a leak starts
• You're counting on a utility rebate tied to ceiling insulation

On rebates: if Duke Energy Florida serves your address, as it does across much of the Zephyrhills area, its ceiling upgrade pays up to $800 when the existing insulation averages R-19 or less, you finish at R-38 or better, and Duke's free Home Energy Check recommends the work. TECO pays $0.16 per square foot for each R-11 added after a pre-approval attic inspection. Whether a foam project qualifies today is something we'll check with TECO for you.

Using Both in the Same House

It doesn't have to be one or the other. When we foam a roof deck, clean fiberglass on the attic floor can often stay. In modeling for the Florida Building Commission, FSEC's worst case for an R-20 foam roof with no ceiling insulation used 18 percent more HVAC energy, and FSEC recommended keeping at least R-11 on the ceiling unless the ducts are very tight and the roof foam reaches R-23. Leaving good insulation in place also skips the removal charge.

If the old stuff is matted, damp or fouled by rodents, it comes out first. Either way, our Tampa crew will tell you which route we'd take and why, and you can book a free estimate to get it in writing.