Ductwork in the attic? Roof deck or floor foam for Columbus homes

Plenty of Columbus-area houses built from the 1990s on hide their second-floor heating and cooling equipment in the attic. If yours does, the standard advice to pile more insulation on the attic floor misses the largest loss in the house. This guide compares the two ways to insulate that attic, with the tradeoffs that matter in Central Ohio.

Why ducts in the attic change the math

Take a late-1990s two-story in Westerville with a basement furnace for the first floor and a second air handler in the attic for the bedrooms. In January that attic sits close to outdoor temperature, and on a July afternoon a vented attic under dark shingles runs far hotter than the air outside. The air handler cabinet, the supply plenum, and every flexible duct run live in that space. Each leaky joint and each thin duct jacket trades heat with air at the wrong temperature, all day long.

Insulating the attic floor protects the bedrooms from the attic but leaves the equipment outside the insulated envelope. Insulating the roof deck instead pulls the whole attic, equipment and all, inside.

How to tell what you have

You can answer most of this from the hatch with a flashlight. Look for a metal cabinet with a filter slot and a condensate line, which is an air handler or a furnace. Follow the ducts: round flexible ducts wrapped in a silver or black jacket, or rectangular metal trunks, running to the ceiling registers of the rooms below. Check whether a return duct pulls air back from the upstairs hallway. Then look at the thermostat setup; two thermostats, one upstairs and one down, usually mean two systems, and the upstairs one often lives in the attic. Photograph what you find, including the equipment label, before anyone quotes the work.

Option A: seal and insulate the attic floor

What it involves. Seal the ceiling plane from above: top plates, the chimney chase, plumbing and wire holes, can lights, bath fan housings, and the hatch. Seal duct joints and the boots where supply registers pass through the ceiling. Then add depth with open-cell foam or blown cellulose toward the Ohio code value of R-49 for a new attic, and keep soffit vents open with baffles.

Where it wins. Lower cost per square foot, fewer roof warranty questions, no change to how the attic dries, and good results when ductwork is minor or can be relocated.

Where it falls short. The equipment still sits in an unconditioned attic. Duct sealing helps, but ducts buried in loose fill are hard to service, and every later repair disturbs the insulation.

Option B: move the insulation to the roof deck

What it involves. Foam goes against the underside of the roof sheathing and the gable walls, down to the top plates at the eaves, and the soffit and ridge vents are closed so the attic is no longer ventilated. The Residential Code of Ohio permits this unvented attic under section 806.5. In Climate Zone 5, the foam against the deck must be a Class II vapor retarder or be coated to become one, and if batts or loose fill are added beneath the foam, the foam layer has to reach at least R-20.

Where it wins. The air handler and ducts sit inside the heated and cooled space, so duct leakage stays in the house. Attic temperature follows indoor temperature instead of outdoor extremes, and heat that used to warm the roof deck and feed ice dams largely stops escaping.

Where it falls short. More foam at a higher price per board foot, more surface area because the roof is larger than the ceiling, shingle warranty language to check, and a deck that can no longer be inspected from inside. The old floor insulation often comes out so the attic can work as part of the house.

A quick way to compare

• Air handler or furnace in the attic with long duct runs: the roof deck usually wins.
• Only a bath fan duct or one short run: the floor usually wins.
• Shingles near the end of their life: plan the roof first, then decide, since an unvented deck is easiest to get right just after a re-roof.
• Finished rooms with sloped ceilings up there: those rooms already follow the roofline, so a roof deck approach often simplifies the whole job.
• Tight budget: seal the floor and the ducts now, and keep the roof deck on the list for the next roof.

What each option costs in Central Ohio

Attic work in the Columbus area generally falls between $2,800 and $8,500, and the two options sit at different ends of that range for the same house. A floor job on a simple attic is mostly sealing labor plus a moderate amount of material. A roof deck job is mostly foam, and thickness drives the bill: roughly 3 inches of closed-cell to meet the R-20 condensation layer when batts go underneath, or roughly 7 inches of closed-cell if foam alone has to reach R-49. Open-cell with a vapor retarder coating is the lower-cost roof deck route when rafters are deep. The pricing FAQ shows how board feet turn into dollars.

Questions to settle before either job

Is the roof sound and the deck dry? Are the bath fans ducted outdoors? Is there knob-and-tube wiring or vermiculite up there? Will anyone store things in the attic, which affects the fire barrier over the foam? Does the house have a natural-draft gas water heater that needs a draft check after sealing? With those answered, either option can be done well. For the moisture side of the unvented choice, read can spray foam cause roof rot, and for the full service description see attic spray foam.