Basement window frosting or drafty: what it means and what a replacement costs in Fort Collins

Frost on the inside of a basement window means single-pane or metal-frame glass that is colder than the dew point of the room, and a draft at the frame means the seal between the window and the buck has failed. Both are end-of-life symptoms on an original basement window. A modern unit installed in the existing opening runs $450 to $1,200 per window, no concrete cutting. If the room is a bedroom and the opening is short of egress size, the fix is an enlargement at $2,800 to $5,000 instead.

Check this first (five minutes on a cold morning)

Where is the frost? On the glass only, in a feathered pattern, means single-pane or a failed insulated unit. On the frame means an uninsulated metal frame. Between the panes of a double-pane window means the seal has failed and the unit is done regardless of the frame.
Hold a hand at the corners. Cold air moving at the corners or along the bottom means the window is not sealed to the buck, or the buck has shrunk away from the concrete.
Look at the buck. The wood frame around the window in the concrete. Soft, dark, or crumbling wood means water has been reaching it from the well.
Look in the well. Standing water, silt, or a rusted well is the cause of the rot. The flooding well page covers it, and it comes first.
Measure the opening. If the room is or will be a bedroom, the replacement needs to meet the city’s replacement-window rule: same operation and the same or larger clear opening, never less than 22 inches high, 20 inches wide, and 4 square feet.

What is usually wrong

What you see	Likely cause	Fix and cost
Frost on single-pane glass every cold night	Original window	Replacement $450 to $1,200
Fog or frost between the panes	Failed insulated unit	Replacement, same range
Draft at the corners, frame sound	Perimeter seal gone	Reseal and foam, or replace if the frame is metal
Draft plus soft buck wood	Rot from a wet well	Drainage $350 to $1,100, then replacement
Hopper window in a bedroom, under 5.7 sq ft	Window is also short of egress	Enlargement $2,800 to $5,000 or new cut $3,800 to $7,000
Frost on the concrete around the window, not the glass	Uninsulated foundation wall	Insulation to the city’s R-19 or R-13 plus R-5 standard, part of a basement finish

Why this happens in Fort Collins

The season is long and the nights are cold. NOAA’s 1991 to 2020 normals for the CSU campus station give Fort Collins 55.8 inches of snow a year, a median first fall freeze on October 6, a median last spring freeze on May 1, and only 159 days of growing season between them. An original single-pane basement window is below freezing on the inside face for a large share of that, and any basement with a shower, a dryer, or people breathing in it has enough moisture to frost it.

The city sets the bar for the replacement. Its Residential Basement Finish Guide requires that new and replaced windows be labeled and meet U = 0.28 maximum and SHGC = 0.35 maximum, and that uninsulated foundation walls be brought to R-19 continuous, R-13 between studs plus R-5 continuous, or R-19 between studs. Those numbers are why a basement window swap here is not a hardware-store hopper: the unit has to carry the label. The same guide requires a permit for a basement finish, and the city’s building permits page lists window replacements as permitted work, so the swap gets filed.

The well is usually the reason the buck rotted. The clay soils over the Pierre Shale under Fort Collins hold water, and a window that has spent twenty springs in a wet well has a frame that is done. Replace the drainage first, or the new window will follow the old one.

Replacement, done right

Pull the old unit and the failed buck. Set a new pressure-treated buck, which the city requires against concrete, anchored to the wall. Set a labeled vinyl slider or hopper that meets the U-factor, shim, foam, flash, and seal. If the room is a bedroom, the new unit’s clear opening cannot be smaller than the old one and must clear the 22 by 20 inch, 4-square-foot floor the city sets for replacements. If the old opening cannot get the room to code, we say so and quote the enlargement instead of installing a window that will fail the next inspection. The failed inspection page has that tape-measure check.

Window replacement in an existing opening is the one job on this site that runs through the winter. No excavation, no concrete, so the 30-inch frost line does not matter. The winter install post explains what waits for spring and what does not.

When to call

Send the form with a photo of the frost or the gap and one of the buck, and say whether the room is a bedroom. A window swap is usually a half-day visit. The egress cost calculator has a swap-only option that prices it on the page. We replace basement windows across Fort Collins, Loveland, Greeley, and Berthoud, and the pricing page lists every line.

Frequently Asked Questions

Why does frost form on the inside of my basement window?

Warm, moist basement air is hitting glass cold enough to freeze it. Single-pane and old aluminum-frame basement windows do that every night below about 20 degrees. Fort Collins has a long freeze season, with the median first fall freeze on October 6 and the last spring freeze on May 1 at the CSU station, so an old window frosts for months.

What does a replacement basement window cost?

$450 to $1,200 per window installed in the existing opening, no cutting. The city requires new and replaced windows in a basement finish to meet a U-factor of 0.28 or better and an SHGC of 0.35 or lower, and the units we install do. Cutting the opening bigger for egress is a different job at $2,800 to $5,000 or more.

Can I just seal it with plastic film for the winter?

Film stops the draft and reduces condensation, and it is a fair stopgap on a window in front of a storage room. It does nothing for frost on single-pane glass in a room someone sleeps in, and it is not a fix for water coming in around the frame.

Is a drafty window a sign the well is the problem?

Sometimes. A frame that has been sitting in a flooded well for years rots and racks, and the draft is the gap that opened. Check the well for standing water before replacing the window, because a new window set into a wet well is a short-lived fix.