Free homeowner tool
Attic insulation and roof vent calculator
How much insulation is up there, what it takes to reach R-30 or R-38, and how much soffit and ridge vent the attic should have. A flashlight and a tape measure at the hatch is all it takes.
Insulation
Enter the attic area to see where it stands and what it takes to reach the target.
Ventilation
The same attic area sets how much vent the roof needs.
Where the numbers come from
R-30 is the prescriptive ceiling minimum in the Florida Building Code, Energy Conservation volume, for climate zones 1 and 2 (all of Florida); R-38 is a common recommendation. The vent figures follow the Florida Building Code, Residential (R806.2): 1/150 of the attic floor in net free area, or 1/300 when 40 to 50% of it is high on the roof and the rest at the soffits. Editions change — verify with your city's building department.
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How the calculator works
- Step 1
Measure what is there
Depth times R-value per inch: blown fiberglass about R-2.5 per inch, batts about R-3.1, cellulose about R-3.5. Four inches of old blown fiberglass is roughly R-10.
- Step 2
Pick the target
R-30 is the Florida Building Code, Energy Conservation prescriptive ceiling minimum for climate zones 1 and 2. R-38 is a common recommendation and only a few more inches once the crew is there.
- Step 3
Size the vents
Net free area of 1/150 of the attic floor, or 1/300 when balanced with 40 to 50% of it high (ridge or off-ridge) and the rest low at the soffits — Florida Building Code, Residential R806.2.
- Step 4
Keep the two working together
Insulation blown into the eaves blocks the soffit vents and the attic stops breathing. Baffles at every rafter bay keep the intake open.
Thinking about the roof itself too? Check how much life it has left, or read about our blown-in insulation.
Insulation and ventilation questions
- How much insulation should a Florida attic have?
- The Florida Building Code's energy volume sets a prescriptive ceiling minimum of R-30 for climate zones 1 and 2, which is all of Florida, and R-38 is a common recommendation. Many older Florida attics have a fraction of that — four inches of settled fiberglass is around R-10. The tool shows the inches to add to reach either target.
- What is net free area?
- The open area a vent actually lets air through, after the louvers and screen — smaller than the hole it covers. Vent products publish it in square inches. The code sizes attic ventilation by net free area, which is why the tool reports it that way.
- Ridge vent or off-ridge vents?
- Ridge vent runs along the peak and works best on a shingle or metal roof with a long ridge. Tile roofs and hip roofs with short ridges usually use off-ridge vents — boxes or slant-backs cut high on the slope. Either way the exhaust should be 40 to 50% of the total, with the rest at the soffits.
- Can I add insulation over what is there?
- Usually, yes — new blown insulation goes over the old. Removal is for insulation that is wet, mold-damaged, rodent-contaminated or full of debris. Baffles at the eaves and a cover for the attic hatch are part of doing it right.
- Will more insulation hurt my roof?
- Only if it chokes the ventilation. A thick layer with blocked soffit vents traps heat and moisture under the roof deck, which shortens the roof's life. That is why we check the vents while we are in the attic.
- What does it cost?
- We price it at the free estimate, in writing, after looking in the attic — access, how much is up there now, baffles, the hatch and whether old insulation has to come out all change the number. The tool gives you the depth and vent quantities so the estimate starts from the right place.
Get a free attic estimate
Tell us roughly the square footage. We measure the depth, check the vents and put a flat rate price in writing.