May 15, 2026

Colorado's freeze-thaw cycle: why winter leaks become summer mold

How the spring freeze-thaw pattern unique to the Front Range produces mold in attics and upper-floor wall cavities that homeowners don't notice until July.

A meaningful share of the mold remediation work we do in Colorado Springs starts with a phone call in late July. The homeowner describes the same scenario: there’s a smell upstairs, or a stain on a bedroom ceiling, or worsening allergies for whoever sleeps in that room. They have no idea where the moisture came from. The roof looked fine all winter. There were no visible leaks during spring melt. The summer monsoon hasn’t been heavy yet.

What they’re describing, almost always, is the result of the Front Range freeze-thaw cycle that ran four months earlier.

What the cycle actually does

Pikes Peak winters are not constant cold. They cycle. A typical December–February at our elevation looks like: snow falls, sits at single-digit temperatures for three to five days, then a chinook wind pushes daytime highs into the 50s for an afternoon, then the temperature drops back into the teens overnight. That pattern repeats, sometimes ten or twelve times across a winter.

For a roof, that’s the worst possible loading. Each cycle does this:

1. Snow on the upper roof melts under midday sun
2. Meltwater runs down the slope toward the eaves
3. At the eave — where there’s less heat loss from the house below — the meltwater hits a colder surface and refreezes, forming an ice dam at the gutter line
4. Subsequent meltwater backs up behind that ice dam
5. The standing water finds the path of least resistance, which is often up under the shingles, past the flashing, into the underlayment
6. Once it’s past the shingle layer, it sits there until temperatures rise enough to evaporate it

In a well-sealed modern roof, the underlayment catches it and the water reaches no further. In an older roof, a roof with damaged flashing, or a roof with marginal underlayment, the water reaches the sheathing — and from there it can drip into the attic or wick into the top plate of a wall.

The homeowner sees none of this. There’s no visible leak inside the house, because the water never reaches a drywall surface. It pools quietly in attic insulation, on top of a kitchen ceiling, or inside a wall cavity around the second-floor windows.

Why the mold doesn’t show up until summer

Mold needs three things to grow: moisture, organic material to feed on, and a temperature range that supports it (roughly 40°F to 100°F, with the sweet spot in the 70s and 80s).

In late winter, the leaked water is sitting in cold materials. The temperature is too low for mold to take off. As outdoor temperatures rise through April and May, the attic and wall cavities warm up and the moisture is still there. By June, you have ideal conditions: temperatures in the 70s, residual moisture in materials, organic material everywhere (paper-faced drywall, wood sheathing, cellulose insulation).

The mold establishes during May and June. It starts producing detectable amounts of spores during late June and July. By the time a homeowner notices the smell or the upper-floor allergies, the colony is six to ten weeks old and well-established in materials they can’t see.

What we typically find on a “summer attic” call

The most common pattern, in order of frequency:

• Attic-sheathing mold on the north or shaded side of the roof. Where the snow took longest to come off. Usually concentrated near roof valleys, around chimney flashings, and at the eaves.
• Wall-cavity mold in upper-floor exterior walls. Particularly around second-floor windows where flashing meets siding. The mold is invisible from inside the house until it pushes through the drywall, which can take a year or more.
• Insulation contamination. Cellulose blown-in insulation that got wet during the original event holds enough moisture to seed widespread surface colonization.
• HVAC system involvement. If the upstairs return air is in an affected wall, the spores get distributed system-wide, which is why some homeowners notice the smell throughout the house even when the source is one specific roof valley.

The remediation looks different than ground-floor mold

For attic and upper-cavity work, the containment strategy is different. We can’t seal off “the attic” the same way we can seal off a single basement room. Containment goes at the attic access hatch, with HEPA-filtered negative-air pulling out of the attic and exhausting through a window or roof port. Removal of affected sheathing happens in segments, and reinstallation usually needs a small roofing trade to coordinate.

Cost range for a typical summer attic remediation in Colorado Springs sits between $3,500 and $9,000, depending on how much sheathing has to come out and whether the leak source has been fixed. (If the original roof intrusion path is still live, we won’t do the remediation until that’s addressed — putting clean drywall back over an active leak just gives us both a callback in eight months.)

What you can do now (in May) to prevent this in July

If you don’t already know whether you had ice damming last winter, the time to look is now, before everything dries out. Check:

• The attic, after a moderately warm day in May. Look for staining on sheathing, particularly near the eaves and around any roof penetration. If you smell anything organic up there, that’s a sign.
• Upper-floor windows from inside. Look at the top corner of the trim where it meets the wall. Any staining, paint bubbling, or softness in the drywall edge is worth investigating.
• The ceiling under any roof valley. Especially north-facing valleys. A faint discoloration is often the first visible sign that water reached the drywall over the winter.

If you find anything, the call now is to a roofer first (find and seal the path) and then to us for an assessment of whether mold has begun.

When to call us

If you’re already noticing a smell, an upper-floor allergy pattern, or a visible stain, the right next step is an assessment. Call (719) 782-8899 and describe what you’re noticing. Most assessments happen within 24 to 48 hours and there’s no charge for the visit.

Questions? Call (719) 782-8899 or send a message.