Why the basement is where a southern Maine house loses its warm air, how the rim joist, the foundation walls or the basement ceiling get insulated, what wet walls and radon change, and what Efficiency Maine pays for the basement zone.
In a southern Maine house the basement is usually where the cold air comes in. Efficiency Maine's myths sheet says incoming air generally enters through basements, garages and crawl spaces, and its insulation page describes the loop: cold air in at the bottom, warmed, and out through the top floor and the attic.
That makes the basement the other end of every attic job, not a separate project. This page covers the rim joist and sill, the choice between insulating the foundation walls or the basement ceiling, old and damp walls, bulkheads, radon and combustion safety, and how Efficiency Maine's basement zone works. Nine in ten homes in the three counties were built before Maine had any state energy code, so there is no reason to assume a basement was ever insulated.
Warm air rises and leaves through the top of a house, and the same amount has to come in somewhere. In most houses here it comes in through the basement: around the sill, through the rim joist, past the bulkhead and through every gap where a pipe, wire or duct goes up into the first floor. Efficiency Maine lists musty smells traveling up from the basement among the signs of air leakage, and says incoming air through basements and crawl spaces can bring poor air with it.
The other reason is age. Efficiency Maine points out that houses built before June 2010 were not held to any state energy efficiency code. In the three counties we cover, more than half of all homes were built before 1980 by the Census Bureau's count, and in Portland 46 percent were built before 1940.
A basement job also helps the floors upstairs, the pipes and the boiler or furnace room, which is why it is often the second step after the attic.
The rim joist closes off the ends of the floor joists where the frame of the house rests on the foundation. Between the sill, the rim and the subfloor there are seams on every side, and in an old house they have had a century to open up. It is the single leakiest strip in most basements and the cheapest one to fix.
Efficiency Maine's checklist folds it into the foundation wall: the insulated area runs all the way up to the first floor's framing, and it names the sill and band joist as part of it. The usual method is closed-cell foam, or foam board cut to fit each bay and sealed at its edges, which stops the air and insulates in one step. Batts stuffed into the bays do neither well.
Wires and pipes that pass through the rim, and the dryer vent, get sealed around as the work goes. A dryer, bath or kitchen fan that exhausts into the basement or a joist bay has to be rerouted straight outside before a rebated job is signed off.
For basement walls in climate zone 6, Maine's code, the 2021 IECC since April 7, 2025, gives three ways to comply for new work: an R-15 layer of continuous board, a framed wall filled to R-19, or an R-13 framed wall backed by R-5 of board. ENERGY STAR's retrofit advice for zones 5 to 8 is the same idea: add R15 of insulating board or an R19 batt.
Efficiency Maine is more specific about rebated work:
Board needs its seams and edges sealed to work as an air barrier; foam does that as it goes. Either way, the face of it usually needs a fire-rated covering in a usable basement, which the spray foam page explains.
A foundation may be laid stone, concrete block or poured concrete, and each behaves differently. A poured wall that stays dry is the easy case. A stone or block wall that weeps in March is not, and insulation over it traps the water against the wall.
Efficiency Maine's position is that a moisture problem gets fixed first or as part of the job, never covered over, and a rebated basement job starts with the installer warning the owner about any wet floor or bare dirt beyond the sump. Its myths sheet argues against relying on a dehumidifier for a damp basement, and for finding the source of the moisture instead.
So a wet foundation gets a water plan first: gutters, downspouts and grading outside, and drainage or a sump inside where it is needed. Once the wall is dry, foam or sealed board can go on. Where it will not stay dry, insulating the basement ceiling instead of the walls can be the safer route, as long as the pipes below are dealt with.
Sometimes the better line between warm and cold runs across the basement ceiling rather than down the walls: a basement that will never be used, one with a stubborn water problem, or a space under a heated addition. Efficiency Maine's basement zone allows it, since the zone covers floors that separate heated space from unheated space, a basement ceiling among them.
Its checklist wants the joist bays dense-packed full, or lined with foam two inches deep or more. Maine's code figure for a floor over unheated space in zone 6 is R-30.
The catch is everything left below. Water pipes, drain lines, a boiler, a water heater and ductwork all end up on the cold side, and pipes that never froze before can. Pipes put at risk this way have to be protected before a rebated job is finished, which can mean insulating them or rethinking the choice. In most houses with a furnace or boiler in the basement, insulating the walls keeps the whole basement just warm enough and is the better trade.
A bulkhead, the slanted outside door down to the basement, is a large hole in the envelope. Maine added its own rule to the energy code for new work: where a bulkhead is installed, a wall and a vertical door go at the bottom of the stairs, or where the bulkhead meets the foundation, and get air sealed. In an existing house the same idea, an insulated door at the foot of the stairs, closes one of the biggest leaks in the basement.
Tightening a basement also changes how air reaches the boiler or furnace and how radon moves. New one- and two-family houses built to the 2021 codes must have radon mitigation, by the state code office's account, and Efficiency Maine will not sign off a job unless a radon system exists or a test has been suggested. The same paperwork calls for a working CO alarm wherever a fuel-burning appliance shares the house, and a spillage test on open-flue appliances once the work is done, with every exhaust fan running.
Efficiency Maine pays on the basement as one zone: the foundation walls, or the floors over unheated space, or both, counted together. A basement zone of 500 square feet or more is worth $2,000 to any homeowner; a household verified as moderate income gets $3,000 for it, and a low-income household $4,000. A smaller basement, 250 to 499 square feet, earns half of each figure. Sealing all the accessible basement leaks adds $100, $150 or $200 by the same tiers.
The fine print: the basement has to begin no better than R-5; the work is done by one of the program's registered vendors, with a blower door test before and after wherever one can be run; and the claim reaches Efficiency Maine within half a year of the finish date. Those amounts are for basements completed on or after October 1, 2026, and the program says it can change them. Efficiency Maine's home energy loans also cover insulation and air sealing, with the terms on its loan page.
If the furnace, boiler or plumbing is in the basement and the walls stay dry, insulating the walls usually works better, because everything down there stays above freezing. A ceiling job suits an unused, damp basement with nothing that can freeze.
New construction in climate zone 6 calls for an R-15 board, a framed wall filled to R-19, or R-13 framing backed by R-5 of board. Efficiency Maine's rebate wants two inches or more of foam or foam board, running from two feet below grade up to the floor framing.
Yes, once it is dry. A stone or block wall that weeps needs its water handled first, inside and out, because insulation over a wet wall holds the moisture against it.
It can change how air moves through the house. Efficiency Maine's rules for a rebated job expect either an existing radon system or advice to test, and testing before and after is a sensible step.
For a basement zone of 500 square feet or more, $2,000 to any homeowner, $3,000 to a moderate-income household and $4,000 to a low-income one, plus $100 to $200 for sealing its air leaks, if the basement begins no better than R-5.
Basement insulation across greater Portland and southern Maine, from the Casco Bay islands to the York County coast and the Sebago Lake towns. Each city has its own page with the permit rule and code edition that apply there:
Sources: Basement requirements: R-5 maximum, 2 feet below grade, sill and band joist, 2 inches, floor cavities, pipes, fans, radon, CO, spillage, blower door · Basement zone and air sealing rebates, eligibility · Rebate table for upgrades completed on or after 10/1/2026 · Air entering low; moisture · Basements, garages and crawl spaces; pre-2010 houses; dehumidifiers · Musty smells; moisture before sealing · Maine bulkhead amendment R402.4.7 · Zone 6 basement wall and floor R-values · ENERGY STAR basement wall advice · Radon systems for new one- and two-family homes · Homes by year built (B25034) · Home energy loans. Codes, programs and prices change; confirm the current rule with your city before you rely on it.
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