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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
An uninsulated attic hatch is a hole in an R-49 or R-60 ceiling. The energy code asks for the hatch to be weatherstripped and insulated to the same level as the attic around it, and for a dam to hold back loose-fill. Enter the hatch size and your climate zone to get the layers of rigid foam to glue to the lid, the sheets to buy, the weatherstrip and the dam height.
With these inputs:
| Result | Value |
|---|---|
| Foam layers on the hatch | 6 × 2 in |
| 4 × 8 sheets | 1 (6 pieces per sheet) |
| Weatherstrip | 8.7 ft |
| Dam around the opening | 8.7 ft long, at least 24 in tall |
What the code asks for. Section R402.2.4 of the 2021 International Energy Conservation Code says access doors from conditioned space to attics and crawl spaces must be weatherstripped and insulated to a level equivalent to the surrounding surfaces. Where the attic has loose-fill, a wood-framed or equivalent baffle, retainer or dam must be installed so the insulation can't spill into the house and keeps its installed R-value. Vertical access doors may instead meet the door (fenestration) requirements for the climate zone. Matching R-49 or R-60 with foam takes a thick stack — 9 to 11 inches of polyiso — so glue the layers to the top of the hatch lid with foam-safe adhesive and make sure the hatch frame can carry it and still close. Rigid foam boxes and zippered hatch covers are sold as alternatives; check their rated R-value against your attic. Seal it, then insulate it. A hatch leaks more air than it loses heat through the lid. Fit foam or rubber weatherstrip around the stop the lid rests on, so the lid compresses it when closed, and add a latch or weight if the lid lifts in wind. Then add the foam.
Target R = 2021 IECC attic level (R-30 zone 1, R-49 zones 2–3, R-60 zones 4–8) or your attic's R · Layers = ceil(target R ÷ (R per inch × board thickness)) · Pieces per 4 × 8 sheet = floor(48 ÷ width) × floor(96 ÷ length), best orientation · Weatherstrip and dam length = hatch perimeter; dam height = target R ÷ loose-fill R per inch
| Zone (attic R) | 2 in polyiso | 2 in XPS | Blown fiberglass depth |
|---|---|---|---|
| 1 (R-30) | 3 layers (6 in) | 3 layers (6 in) | 12 in |
| 2–3 (R-49) | 5 layers (10 in) | 5 layers (10 in) | 19.6 in |
| 4–8 (R-60) | 6 layers (12 in) | 6 layers (12 in) | 24 in |
22 × 30 in hatch. Computed with this calculator; R per inch as listed in the board options and the DOE loose-fill midpoint.
Weatherstrip the stop the lid sits on, glue layers of rigid foam to the top of the lid until it matches the attic's R-value, and build a dam around the opening if the attic has blown insulation. A 22 × 30 in hatch in zone 5 (R-60) needs six layers of 2 in polyiso.
Yes. The 2021 IECC (R402.2.4) requires attic access doors to be weatherstripped and insulated to the level of the surrounding surfaces, with a dam to hold back loose-fill.
At least as tall as the insulation around it. R-60 of blown fiberglass at about R-2.5 per inch is 24 inches deep, so the dam has to reach that; for blown cellulose at R-3.5 per inch it is about 17 inches.
Rigid foam board — polyiso has the most R per inch (R-5.7 for roof-rated board), XPS is R-5.0 per inch. Batts on a hatch tend to fall off and leave gaps; foam glued down stays put.
Usually one: a 4 × 8 sheet cuts into six 22 × 30 in pieces, so six layers come from one sheet with almost no waste.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.