How to insulate the space under a southern Maine house that has no full basement: the floor or the walls, keeping pipes from freezing, dealing with ground moisture first, and what Efficiency Maine pays, mobile home underbellies included.
A crawl space under an addition, a porch room, a cottage or a mobile home is where the coldest floors in a house come from. In Efficiency Maine's description of a drafty house, outside air enters near the bottom and escapes near the top, and a crawl space open to the weather sits at the very bottom of that loop.
The first decision is where the line between inside and outside should run: under the floor, or down the crawl space walls. After that come water, pipes and access. This page sets out both approaches against the Maine code and ENERGY STAR, the frozen-pipe question, the mobile home underbelly rebate, and why a seasonal camp does not qualify for the program.
There are two ways to put a crawl space on the right side of the insulation.
Pick one: insulating the floor and then sealing the vents leaves a space that is neither heated nor vented. Which side the pipes are on usually decides it, along with whether the ground stays dry. We look at both before recommending either.
Maine's statewide code, on the 2021 IECC since April 7, 2025, gives each route a number for climate zone 6:
| Approach | Maine code, zone 6 | ENERGY STAR retrofit |
|---|---|---|
| Floor over the crawl | R-30 | R30 |
| Crawl space walls | R-15 of continuous board, an R-19 cavity fill, or R-13 in the cavity with R-5 of board | Add R15 sheathing or R19 batt |
Those figures are written for new building. On an existing house they are the benchmark, and what fits is set by the joist depth, the height of the crawl and how the walls are built. The state applies its code to renovations and makes meeting it the owner's job, so a written proposal should say which figure it is aiming at and why.
Efficiency Maine files a crawl space with the basement: its checklist applies the basement rules to "basements, crawlspaces, or other unconditioned space directly below a conditioned living area", and a crawl that already holds better than R-5 of insulation falls outside the rebate.
A crawl space is close to the ground, and the ground is wet for part of every year. Under Efficiency Maine's rules, any open soil or pooled water found down there outside a sump has to be pointed out to the owner ahead of the job. Its air sealing page lists musty smells drifting up from below, and spider webs in a crawl space, as signs that air is moving through it, and says damp has to be sorted out before or during the sealing.
So the order on a crawl space job runs: find where water gets in and stop it; cover or deal with exposed earth; seal the rim joist and the openings around pipes, wires and ducts; then insulate. Insulation laid in a damp crawl holds the moisture against the framing.
Efficiency Maine also argues against the easy fix. Its myths sheet says a dehumidifier is not the answer to damp crawl spaces and mustiness, and that a vendor should find the moisture source instead. The air sealing page covers the sealing step.
The Portland Jetport averages 6.7 nights a year at or below zero, by NOAA's 1991–2020 normals, and 144 nights at or below freezing. A water line running through an open, vented crawl space is outdoors on those nights.
Efficiency Maine's insulation page is direct about it: pipes freeze and burst when they run through uninsulated spaces that are not heated, and while heat cable or heating the space lowers the risk, insulating the space can be the better answer. Its myths sheet adds that a plumber can repair a frozen pipe, but weatherizing is how you stop it freezing in the first place.
This is where the floor-or-walls decision earns its keep. Insulate only the floor above a crawl full of plumbing and the pipes end up colder than before, below the insulation and cut off from the house's heat. On a rebated job the installer has to leave any vulnerable pipes in the work area protected against freezing, which in a crawl usually means insulating the walls so the pipes stay inside.
The three counties hold about 14,700 mobile homes in the Census count, roughly 5 percent of all homes, with the largest numbers in places such as Wells and Brunswick. Under a mobile home the space that matters is the underbelly: the insulated, closed-in layer under the floor, which tears, sags and lets in animals over the years.
Efficiency Maine treats it as its own zone. For 500 square feet or more insulated it pays $3,000 to any owner, $4,000 to a verified moderate-income household and $5,000 to a low-income one, under a lifetime cap of $5,600 per mobile home, with air sealing paid on top. The underbelly has to start at an effective R-11 or less, and the checklist accepts three methods: the underbelly filled to capacity with blown fiberglass, two inches or more of closed-cell foam on the subfloor, ducts and pipes, or two inches or more of rigid board fastened to the joists and sealed. Skirting insulation does not count. A mobile home on a foundation is handled as a basement instead, and a home can claim one of the two, not both.
Some houses here are closed for the winter. In Phippsburg, the town research for this site found nearly as many seasonal homes as year-round ones.
Efficiency Maine's insulation rebates are for space heated the entire heating season, so a camp that is drained and shut in October does not qualify, whatever its crawl space looks like. If the plan is to start living in it year-round, the insulation is usually part of a larger job: water lines brought inside the insulation, the floor or the crawl walls insulated to the code's numbers, and a heat source added. That is also the point where the town's code office may want a permit, so ask before the work starts.
For a camp that stays seasonal, the useful work is protective rather than thermal: keep the ground dry, keep animals out of the floor, and make sure the plumbing drains fully.
If plumbing, ducts or a furnace are down there, insulating the walls and sealing the rim joist usually keeps them warm. If the crawl is empty and stays vented, insulating the floor above can be simpler. Mixing the two rarely works.
New construction in zone 6 calls for R-30 in the floor above the crawl, or for the walls an R-15 board, an R-19 cavity fill, or R-13 in the cavity with R-5 of board. On an older house those are the benchmark rather than a fixed target.
Yes, under its basement zone, which takes in crawl spaces and other unheated space directly under a heated room. A crawl already insulated beyond R-5 does not qualify, and the work has to follow its checklist.
For 500 square feet or more of underbelly, $3,000 for any owner, $4,000 at moderate income and $5,000 at low income, under the table that took effect October 1, 2026, with a lifetime limit of $5,600 per mobile home.
Not if it is shut for the winter. Efficiency Maine pays only on space that is heated the entire heating season, so a seasonal camp has to become a year-round, heated home first.
Crawl space 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: Crawl spaces under the basement rules; R-5 and R-11 maximums; underbelly methods; pipes; ground and water notice · Underbelly and basement zone rebates; heated-season rule; lifetime limits · Rebate table for upgrades completed on or after 10/1/2026 · Frozen pipes and air entering low · Moisture before sealing; musty smells and spider webs · Frozen pipes and dehumidifier myths · Zone 6 floor and crawl space wall R-values · ENERGY STAR floor and crawl wall recommendations · Portland Jetport nights at or below 0F and 32F · Mobile homes by town and county (B25024) · Maine code applies to renovations. Codes, programs and prices change; confirm the current rule with your city before you rely on it.
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