The icicles are a symptom. The causes are the climate, the age of the houses, and the fact that Iowa's statewide energy code arrived long after most of them were built. Every number below is sourced.
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The U.S. Department of Energy's Building America program assigns every county to a climate zone for insulation and construction purposes. Polk County, along with Dallas, Warren, Madison, Jasper and Story counties, is listed as IECC climate zone 5, moisture regime A, in the Building America "Cold" region. Zone 5A is the same classification as Chicago, Cleveland and much of southern New England. It is the reason the code's ceiling insulation requirement here is R-49 rather than the R-38 required in warmer zones.
NOAA's 1991-2020 climate normals for the Des Moines International Airport station (USW00014933) give the specifics. January is the coldest month: average temperature 22.3 F, average daily low 13.8 F, average high 30.9 F. December through February averages 25.6 F. The station records about 6,180 heating degree days per year on a 65 F base, against roughly 1,070 cooling degree days, so the heating season dominates. On average there are about 128 days a year with a low at or below 32 F and about 10 with a low at or below 0 F. Normal snowfall is 36.5 inches per season, with February (10.2 inches) and January (9.4 inches) the snowiest months.
Snow on a roof insulates it. If the roof deck stays below freezing, the snow sits there until it sublimates or a warm day melts it from the top. If the deck warms above freezing from below, the bottom layer of snow melts and the water runs down under the snowpack until it reaches the eave, which overhangs the house and has no heat under it. There the water refreezes. Repeated over days, the ice builds into a ridge, water pools behind it, and it backs up under the shingles and into the wall and ceiling.
University of Minnesota Extension is direct about the cause: "It is primarily heat flowing from the house that causes the nonuniform temperatures of the roof surface leading to ice dams." The heat gets to the deck two ways. Conduction through thin insulation is one. The bigger one is air leakage: warm indoor air rising through openings in the ceiling plane into the attic. The Building America Solution Center notes that "when heated air from the house escapes into the attic, it can warm the underside of the roof deck, melting the snow on the roof above," and calls air sealing "the most important step."
Three conditions have to line up: snow on the roof, a roof deck warmed from below, and cold enough eaves to refreeze the melt. Des Moines supplies the first and third every winter. The second is a property of the house, and it is the only one you can change.
The Census Bureau's 2024 American Community Survey (1-year estimates) counts roughly 95,700 housing units in the city of Des Moines with a median year built of 1963. About 26 percent were built in 1939 or earlier. Adding the 1940s and 1950s, roughly 48 percent of the city's housing predates 1960. For Polk County as a whole, which includes the fast-growing suburbs, the median year built is 1983.
That matters because insulation practice changed dramatically over the century. Houses from the 1900s through the 1930s were often built with balloon framing and empty wall cavities; attic insulation, if any, was added later as a few inches of rock wool or cellulose. Postwar houses commonly got two to four inches of attic insulation and nothing in the walls or at the rim joist. Even 1970s and 1980s houses, built after the energy crisis, typically stopped at R-19 or so in the attic and left the rim joist and band uninsulated. None of these had a deliberate air barrier at the ceiling.
So the typical Des Moines house, statistically, is a 1960s ranch or a pre-war two-story with a leaky ceiling plane and shallow attic insulation, sitting in a cold-region climate with three feet of snow a season. That is the recipe.
Iowa adopted a statewide energy code based on the 2012 International Energy Conservation Code with state amendments; the rule passed March 12, 2014 and took effect June 1, 2014. For climate zone 5, the residential prescriptive requirements are R-49 ceilings, R-20 wood-frame walls (or R-13 cavity plus R-5 continuous), R-19 or R-15 continuous on basement and crawl space walls, and a tested air leakage limit of 4 air changes per hour at 50 pascals.
Two limits are worth knowing. First, the state code applies to new construction of one- and two-family homes but, in the state's own language, "not to renovation or remodeling of one and two family residences." There is no requirement to bring an existing house up to it. Second, local jurisdictions are permitted to adopt a newer edition of a nationally recognized energy code in place of the state's, so requirements can differ between Des Moines and a suburb or an unincorporated part of the county.
The practical upshot: any Des Moines house built before mid-2014 was never held to a tested air-leakage standard, and the large majority were built before any insulation standard at all. The code is still the best local yardstick for a retrofit. R-49 in the attic, a sealed ceiling plane, and R-15 to R-19 on the basement and crawl walls is what a well-insulated Central Iowa house looks like on paper.
Stack effect drives it. Warm air is less dense, so a heated house pushes air out through the top and pulls it in through the bottom. At the top: the attic hatch, the gaps around plumbing and wiring chases, top plates where drywall meets framing, recessed light housings, bath fan housings, the chimney chase, and open wall cavities in balloon-framed houses. At the bottom: the rim joist and sill plate, crawl space vents, gaps around the sewer and water service, dryer vents, and the garage-to-house connection.
The heat that leaves at the top is what melts the snow. The cold air pulled in at the bottom is what makes floors cold and rooms drafty and keeps the furnace running. Sealing one end without the other helps, but the pressure keeps working on whatever is still open. That is why a thorough scope treats the attic floor and the rim joist as the two halves of the same job.
MidAmerican Energy is the electric and natural gas utility for Des Moines and most of the metro, and it runs residential efficiency programs in Iowa. As of this writing, its Iowa home discounts and rebates page lists incentives for appliances, heat pump water heaters and smart thermostats, and notes that many equipment incentives have moved to an instant-discount program; it does not list an insulation or air sealing rebate. Utility programs change from year to year, so check the current page or call MidAmerican before assuming an insulation rebate is available. If one exists when you are ready, we will follow its documentation requirements.
Federal tax treatment of insulation has also changed in recent years. We do not give tax advice; confirm the current rules with the IRS or your tax preparer.
If your house grows ice dams, has frost on the attic sheathing, or has a second floor that is ten degrees off the first, the cause is almost certainly heat and air leaving through the ceiling plane, and the fix is in the attic, not on the roof. Heat cables and roof raking manage the symptom. Air sealing the attic floor, insulating to depth, keeping the soffit vents open and closing the rim joist address the cause.
Start by looking in the attic on a cold morning with snow on the roof. Dirty streaks in the insulation, frost on nails and sheathing, and bare spots that have melted through the snow on the roof above are the map. That is what an on-site assessment looks for, and it is what the scope is built around.
Questions
Yes. The DOE Building America county list puts Polk County, Iowa in IECC climate zone 5A, the same zone as Cook County, Illinois. Zone assignments are based on heating and cooling degree days and moisture, and Des Moines's roughly 6,180 heating degree days put it squarely in zone 5.
They can melt channels through the ice so water drains, which limits damage. They do not stop the heat loss that forms the dam, and they add to your electric use. University extension guidance treats them as a stopgap; the durable fix is sealing and insulating the attic floor.
No. The state energy code applies to new one- and two-family construction, not to renovation or remodeling of existing homes. The code's values are still the most useful local benchmark for deciding how much insulation a retrofit should aim for.
Tell us what the house is doing and we will come look at where the heat is going.
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