For most homes with a vented attic, the insulation belongs on the attic floor. Loose fill or batts go over the ceiling, and the attic above stays vented and unconditioned. That's the standard, lower-cost approach, and it's how most existing attics are insulated.
Insulating the roof instead is a different design, not just insulation in a different spot. When insulation goes on the underside of the roof deck, usually spray foam, the attic's vents are sealed and the attic becomes part of the home's conditioned space. That makes sense in specific cases, most often when the air conditioning equipment and ductwork live in the attic.
The key point: an attic should be designed as one or the other. Stapling insulation between the rafters while the soffit and roof vents stay open doesn't create either assembly correctly.
Two Different Assemblies
Attic floor insulation (vented attic)
In a vented attic, insulation covers the attic floor, and outside air moves in through soffit vents and out through ridge or gable vents. ENERGY STAR describes that airflow as key to a durable attic: in summer it carries superheated air out of the attic, while the insulation keeps the heat from moving down into the house.
This approach depends on two details:
- Air sealing the ceiling before insulating, so house air isn't leaking up into the attic.
- Keeping the eaves open. Rafter vents (baffles) let air flow past the insulation.
Roof insulation (unvented attic)
The Building America Solution Center describes an unvented attic as one where the ventilation openings are eliminated. The insulation, air barrier, and moisture control all move up to the roof deck. The soffit and roof vents are closed and sealed, and insulation is installed below the roof deck (usually spray foam) or above it (rigid insulation, typically during a reroof).
The attic then sits inside the insulated envelope as a tempered space. Huntsman, a spray foam manufacturer, notes that an unvented attic generally stays within about 10 to 12°F of the living space below.
When Insulating the Roof Makes Sense
Ducts and HVAC equipment in the attic
This is the most common reason. Department of Energy Building America research found that ducts in unconditioned, vented attics increase heating and cooling energy use by about 10%. It also found duct air leakage commonly exceeds 20% of conditioned airflow. With the insulation at the roofline, any duct leakage stays inside the conditioned space instead of escaping into a hot attic.
If your air handler and ducts are in the attic, it's worth comparing a roofline approach with an attic-floor approach that includes sealing and insulating the ducts.
Complicated or shallow attics
Low-slope roofs, tight eaves, and attics crowded with ducts and framing can be hard to insulate deeply on the floor. Spray foam at the roof deck avoids some of those limits.
Using the attic
If you want to use the attic for conditioned storage, or it already has finished space, insulating at the roofline brings that space inside the envelope.
When the Attic Floor Is the Better Choice
- The attic is a standard vented attic with room for deep insulation.
- The ducts aren't in the attic, or they can be sealed and insulated where they are.
- Budget is a major factor.
- The roof is older or has leak issues that need attention first.
For most San Antonio homes looking to add insulation, the attic floor is where the conversation starts. Our attic insulation service covers floor-level insulation and the air sealing that should come with it.
How to Insulate an Attic Roof
Roofline insulation is a professional job. The general approach, per the Building America Solution Center:
- Inspect the roof deck for leaks or damage and repair them first. Spray foam manufacturers such as Johns Manville note foam needs a clean, dry, sound surface.
- Close and seal all attic vents, including soffit and roof vents.
- Install the insulation below the roof deck (spray foam) or above it (rigid insulation during reroofing).
- Meet the code minimum for the insulation R-value in your climate zone. Codes set minimum R-values for the air-impermeable insulation used in these roof assemblies.
Spray foam also means the household leaves during installation. EPA guidance says occupants should stay out until the foam has cured and the space has been ventilated, with re-entry timing set by the product manufacturer. Our spray foam insulation service covers foam type, thickness, and what to expect.
What about insulating attic roof rafters with batts?
Putting batts between the rafters of a vented attic, with the vents left open, doesn't create a sealed roof assembly. Outside air still moves through the attic and around the batts, and the floor below stays the real boundary. Cathedral ceilings with finished drywall are a separate case and have their own ventilation and insulation details. If you're considering rafter insulation, have the whole assembly evaluated first.
Roof Insulation Cost vs. Attic Floor
Roofline insulation generally costs more. Our insulation cost calculator uses these planning ranges, based on published installed-price data for the San Antonio market (checked September 2026). They're estimates, not quotes:
| Approach | Planning range per sq ft of attic floor |
|---|---|
| Blown-in fiberglass or cellulose on the attic floor | $1.00 to $2.50 |
| Spray foam at the roofline | $2.00 to $4.50 |
Roofline jobs are priced by the roof surface, which is larger than the floor on most roofs, and by foam type and thickness. Sealing vents, roof repairs, and handling existing floor insulation add to the scope.
Common Mistakes
- Insulating both the floor and the roof without a plan, leaving an attic that isn't clearly vented or unvented.
- Insulating rafters but leaving the vents open.
- Blocking soffit vents with floor insulation in a vented attic.
- Foaming a roof deck that leaks or will be replaced soon.
- Skipping air sealing with floor insulation.
Frequently Asked Questions
Can you insulate both the attic floor and the roof?
An attic works best as either a vented space with floor insulation or an unvented space with roof insulation. When an attic is converted to unvented, what happens to the existing floor insulation depends on its condition and the design. The installer should explain the plan.
Is roof insulation better for a San Antonio home?
It depends on the house. Roofline insulation is most compelling when the ductwork and air handler are in the attic. For a standard vented attic without ducts, well-sealed floor insulation to the right depth is usually the practical choice.
Does insulating the roof keep the attic cooler?
Yes. That's the main difference. With insulation at the roof deck, the attic is inside the insulated envelope, so it stays much closer to indoor temperatures than a vented attic.
Do I still need attic ventilation if I insulate the roof?
No. In an unvented attic, the vents are sealed as part of the design. Ventilation matters when the insulation is on the floor, and those vents must stay open.
The Takeaway
Attic floor insulation and roof insulation are two different ways to build an attic:
- Floor insulation keeps the attic vented. It's simpler and costs less, and it works for most homes.
- Roof insulation seals the attic and brings it inside the home. That's most useful when the air conditioning equipment and ducts are up there.
Pick one design and build it completely, and have the roof and ducts evaluated before deciding. Get an Estimate and we'll look at both options for your attic.
References
- U.S. Department of Energy, Building America Solution Center: Unvented Attic Insulation, Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck, and Controlling Moisture in Unvented Attics, Code Compliance Brief. basc.pnnl.gov
- U.S. Department of Energy: Unvented, Conditioned Attics, Building America Top Innovation. energy.gov
- ENERGY STAR: A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR. energystar.gov
- Johns Manville: JM Corbond Closed-cell and Open-cell SPF. jm.com
- Huntsman Building Solutions: Benefits of Unvented Attics, Design Note BES-101. huntsmanbuildingsolutions.com
- U.S. Environmental Protection Agency: Vacate and Safe Re-entry Time for Spray Polyurethane Foam Application. epa.gov