Roofing services
Roof insulation: where it goes decides who installs it
Roof insulation can sit in three places: on the attic floor, under the roof deck, or above the deck beneath the roofing.
Written by HyreRoof Research Desk Primary-source research and fact checking
The short answer
Roof insulation can sit in three places: on the attic floor, under the roof deck, or above the deck beneath the roofing. Each placement makes a different kind of roof, vented or unvented, and each is done by a different trade, so the first question is where, not how thick.
What decides a roof insulation job?
ENERGY STAR lists attic air sealing ahead of attic insulation; insulation does not stop air leaks.
The roof stays cold and ventilated above it.
That is a different design under IRC R806.5, not an upgrade to a vented one.
It is done at reroof, under the new covering.
What should be checked first?
Around lights, pipes, chimneys and the hatch.
Depth, type and whether any is wet or compressed.
Whether intake and exhaust work, if the attic stays vented. See roof ventilation.
They argue for bringing the attic inside.
Above-deck insulation only makes sense at a reroof.
What happens during the work?
Gaps between the house and attic sealed before anything covers them.
For a vented attic, baffles at the eaves keep the code air space open.
Blown, batt, spray foam or rigid board, depending on placement.
Unvented designs close vents; vented ones leave them clear.
New edge trim and longer fasteners through the boards into the deck.
Where can the insulation go?
| Placement | What it means for you |
|---|---|
| Attic floor (vented attic) | Blown or batt insulation over the ceiling. Usually an insulation contractor’s job. Must keep at least 1 inch of air space at the vents. |
| Under the roof deck (unvented) | Spray foam or other insulation against the underside of the sheathing, bringing the attic inside. Vents are closed. Moisture rules in R806.5 apply. |
| Above the deck (unvented) | Rigid boards on the deck, under the roofing. Standard on flat roofs; on sloped roofs it raises the roof edge and needs new trim. |
What it costs
Sourced roof insulation prices are pending until we hold at least 12 dated quotes, so this page gives no figure. These drive the number. Add it to a reroof in the roof cost calculator or compare bids with the estimate comparison tool; see the cost guide.
| Factor | Why it changes the number |
|---|---|
| Placement | Attic-floor work is the simplest; above-deck work rebuilds the roof edge. |
| Material | Spray foam, rigid board, blown and batt insulation differ widely in material and labor. |
| Air sealing | A separate line on careful bids; skipping it lowers the price and the result. |
| Old insulation removal | Wet, contaminated or compressed insulation comes out first. |
HyreRoof publishes no national average price for this work. Regional labor, disposal and material rates move it far enough that a single figure would mislead.
What to ask before you sign
Which leaks will you seal?
Where this goes wrong
Warm, moist air keeps reaching the cold roof deck through the insulation.
Spray foam under the deck with vents left open mixes two designs that each need the other closed or open.
Insulation pushed into the eaves closes intake and can lead to ice dams.
Your inquiry
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A person reads it and replies by email, free and with no obligation. Matching is not live yet, so this is not a quote. If water is coming in now, call a local roofer directly and check their license first.
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Questions
Should a roofer or an insulation contractor do this?
Does more insulation stop ice dams?
How much insulation does code require?
Written and audited by
HyreRoof Research Desk
Primary-source research, data analysis and fact checking
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Sources & retrieval dates
Information on this page is general and may vary by location, building code, roof design, insurance policy and your own circumstances. It is not insurance, legal or engineering advice.
HyreRoof is not a roofing contractor: we do not repair, replace or inspect roofs, and we do not supervise or warrant roofing work. This page explains the work so you can judge a quote and a contractor.