Maryland doesn’t have one insulation answer. It sits across two IECC climate zones, 4A and 5A, and the levels ENERGY STAR recommends shift depending on which side of that line a house sits. Most of the state’s 24 counties fall in zone 4A; a smaller group sits in 5A, where the recommended floor insulation jumps higher.
Which climate zone Maryland is in
Under the 2021 International Energy Conservation Code (IECC), Maryland’s 24 counties split into two climate zones: 22 counties fall in zone 4A, and 2 counties fall in zone 5A. That’s a county count, not a population share, so don’t read it as “almost the whole state.” A couple of counties in the higher-demand zone could still hold a meaningful chunk of Maryland’s households, and the reverse is also possible. The point of the split is that there’s no single statewide answer to How Much Insulation a Maryland home needs.
A climate zone number describes how much heating and cooling demand a location typically carries over a year, with higher numbers meaning colder winters. The letter that follows the number describes moisture, not temperature: A stands for moist, B for dry, and C for marine. Zones 7 and 8 don’t carry a letter at all. That letter matters here because the insulation table groups zones together by letter combinations, and the exact zone assignment changes which row of recommendations applies to a given house.
| IECC Climate Zone | Number of Maryland counties |
|---|---|
| 4A | 22 of 24 |
| 5A | 2 of 24 |
This is the piece worth sitting with: two counties in Maryland are classified in a colder zone than the rest of the state, and that reclassification isn’t cosmetic. It changes the recommended floor insulation level, as the next section shows. Because this page can’t tell an individual reader which zone their own county falls into, the right move is to check the county against the official IECC zone table or the ENERGY STAR zone map before buying material. Guessing based on general geography, or assuming a whole state shares one zone, is exactly how homeowners end up buying the wrong R-value.
It’s also worth remembering that these zone boundaries are set by county, not by a formula on temperature averages or degree days. A location can have brutal winter demand and still sit in the same zone as a milder county nearby, because the zone assignment is a code designation, not a live measurement. That’s a separate question from how much heating demand a given place actually carries year to year, which is covered further down this page.
The insulation levels that apply here
ENERGY STAR publishes a retrofit table for existing wood-framed homes, based on the 2021 IECC’s residential provisions. The table has a header that’s easy to misread: it lists insulation levels for attics in two conditions, uninsulated and already holding 3 to 4 inches, plus a separate column for floors. Three numbers per zone, but only two of them describe the attic. The third is the floor.
| Climate Zone | Attic if UNINSULATED | Attic if you already have 3-4 inches | Floor |
|---|---|---|---|
| Zone 1 | R30 | R25 | R13 |
| Zone 2 | R49 | R38 | R13 |
| Zone 3 | R49 | R38 | R19 |
| Zones 4A and 4B | R60 | R49 | R19 |
| Zones 6, 5, and 4C | R60 | R49 | R30 |
| Zones 7 and 8 | R60 | R49 | R38 |
For Maryland, two rows matter, not one. Counties in zone 4A follow the “Zones 4A and 4B” row: R60 for an uninsulated attic, R49 if there’s already 3 to 4 inches in place, and R19 for the floor. Counties in zone 5A follow the “Zones 6, 5, and 4C” row instead, which recommends the same attic levels, R60 or R49, but jumps the floor recommendation up to R30. Attic targets don’t change between Maryland’s two zones. Floor insulation does, and the difference is not small: R19 versus R30 is a meaningfully thicker layer under living space.
A homeowner in one of the 22 counties in zone 4A and a homeowner in one of the 2 counties in zone 5A are not shopping for the same floor insulation, even though their attic targets match. Averaging these two rows into a single “Maryland number” would get the floor recommendation wrong for whichever zone got averaged away. These levels apply to retrofitting existing wood-framed buildings. They’re not new-construction code minimums, and they don’t translate into a specific number of inches, since depth depends on the material and manufacturers print that figure on the packaging, not on this table.
Sealing comes before insulating
ENERGY STAR treats air sealing and insulation as two parts of one project, and its own guidance sequences attic air sealing before attic insulation. That order isn’t arbitrary. Insulation slows heat moving through a material, but it does nothing to stop air moving around it. Lay batts or blown-in fill over an unsealed gap around a chimney chase or a recessed light, and the insulation hides the leak instead of closing it. The house still loses conditioned air through that gap; it’s just harder to spot afterward.
The practical order of a whole-house insulation project runs like this:
- Seal air leaks in the attic first, since this is where stack effect pulls the most conditioned air out of a house.
- Add attic insulation to the levels above once the sealing is done.
- Seal and insulate the rim joist, an area often skipped entirely in older homes.
- Address the floor over a crawlspace or unconditioned basement.
- Move to walls last, since wall work is typically the most disruptive and the least likely to be needed first.
Skipping straight to attic insulation without sealing first is one of the more common mistakes homeowners make on this kind of project, and it’s why ENERGY STAR frames the two steps together rather than as separate line items. The published savings estimate for this work, covered in the last section of this page, applies to sealing and insulation done together, not to sealing alone. For the mechanics of each step, the national insulation guides on this site cover attic air sealing, rim joist work, and crawlspace insulation in more depth than a single state page can.
What the winter here actually asks for
At Baltimore Washington International Airport, NOAA’s 1991-2020 climate normals put the annual heating degree day total at about 4,484, against roughly 1,322 cooling degree days. Heating degree days measure demand, not temperature: the figure adds up, for every day of the year, how far the average temperature sat below 65°F. A higher total means more fuel burned to hold a home at a comfortable temperature, all else being equal. At roughly 4,484 heating degree days, Maryland’s reference station carries a heating season that’s real but not extreme, well short of what a northern New England or upper Midwest station would show, and well above what a Gulf Coast station would record.
That demand runs through specific equipment. EIA’s Residential Energy Consumption Survey for 2020 shows 64% of Maryland homes using a furnace as their main heating equipment, 19% using a central heat pump, and 7% using a steam or hot-water boiler. On fuel, natural gas covers 49% of homes, electricity 41%, and fuel oil or kerosene 7%; propane’s figure wasn’t reported, which means the estimate was suppressed as unreliable, not that no propane furnaces exist in the state.
A state running mostly on furnaces through ductwork carries a different insulation priority than one leaning on boilers and radiators. Ductwork routed through an unconditioned attic is its own heat-loss pathway, and no amount of ceiling insulation fixes air leaking out of a duct seam before it ever reaches a room. With 64% of Maryland homes on furnaces, and presumably a large share of those pushing air through ducts, that’s a real gap in a lot of attics across the state.
Given that mix, a reasonable order of priorities for a Maryland home looks like this:
- Seal and insulate the attic to the level matching the county’s zone (see the table above).
- Check attic ductwork for leaks and insulation gaps, especially in homes on furnace heat.
- Address rim joists and floors over crawlspaces or unconditioned basements.
- Consider wall insulation only after the above steps are done.
The duct guides on this site go into more detail on sealing and insulating ductwork that runs through unconditioned space, which matters more in a furnace-heavy state like Maryland than it would in a state where boilers and radiators dominate.
What the work is worth
ENERGY STAR’s own estimate: “EPA estimates that homeowners can save an average of 15% on heating and cooling costs (or an average of 11% on total energy costs) by air sealing their homes and adding insulation in attics, floors over crawl spaces, and accessible basement rim joists.” Both numbers matter together. Fifteen percent applies to heating and cooling costs specifically; eleven percent applies to total energy costs, a broader denominator that includes things like water heating and appliances.
These are averages drawn from energy modeling of a typical existing U.S. home, not a guarantee for any specific Maryland house. The modeling also names exactly where the work happens: attics, floors over crawl spaces, and accessible basement rim joists. It doesn’t cover walls, windows, or doors, so stretching this figure to a whole-house renovation overstates what the estimate actually measured.
It’s also worth keeping this number separate from two others that sound similar but aren’t the same claim. ENERGY STAR’s program landing page mentions “up to a 10% savings on your annual energy bills” for sealing and insulation work, which is a different, capped figure on a different denominator. And the Department of Energy’s commonly cited 10% figure has nothing to do with insulation at all; it refers to savings from a thermostat setback of 7 to 10°F held for eight hours a day. Three real numbers, three different claims, and only the 15%/11% figure from ENERGY STAR’s methodology applies to the sealing and insulation work described on this page.