Air Seal Around a Window Frame — Casing Off, Gap Sealed, Trim Back
The window locks tight and the glass is clear, but cold air pours out around the casing and under the stool.
Reply within one business day. Or call / text 763-204-2099.
To air seal around a window frame, the interior casing comes off, the packed fiberglass in the gap between the window and the framing comes out, and the gap is sealed with low-expansion window foam or backer rod and sealant before the trim goes back. JMB Builders does it across the Twin Cities metro on windows that are sound but leak air around the frame. Priced per window, quoted from photos, reply within one business day.
- Priced per window
- Casing removed and reset or replaced
- Low-expansion foam or backer rod and sealant
Every window sits in a rough opening a little bigger than the window, with a gap of a quarter inch to an inch all the way around. That gap is behind the interior casing. In a lot of houses it was stuffed with fiberglass batt when the window went in. Fiberglass slows heat but does not stop air, and on a windy January day air moves straight through it, out past the casing and under the stool.
That is why some drafty windows do not get better with new weatherstrip or even new sashes. The sash is sealed; the frame is not. You can tell by feel: air at the edge of the sash is the window, air at the casing joint or along the stool is the gap.
The fix is to open it up. The casing and, where needed, the stool and apron come off carefully. The old fiberglass comes out. The gap gets low-expansion foam made for windows and doors, which does not bow the frame, or backer rod and sealant where the gap is narrow or the frame is sensitive. Then the trim goes back on, or new trim if the old splits coming off. Painting is quoted as a separate line — or leave it primed and caulked and paint it yourself.
Get a written quote for this job
Three photos and a sentence is enough: a reply within one business day, or ask for a free in-home estimate. Or text the photos to 763-204-2099.
What you supply
- A photo of each drafty window from inside, with the casing in view
- Where you feel the draft: at the sash, at the casing, or under the stool
- Whether you want the old trim reused or new casing on the windows
- Clear access to the windows inside: furniture and window treatments moved
What JMB supplies
- Interior casing, and stool and apron where needed, removed and labeled
- Old fiberglass and loose debris pulled from the gap
- Low-expansion window and door foam, or backer rod and sealant, in the gap
- A check that the window still opens and locks after the gap is sealed
- Trim reinstalled, nail holes filled and joints caulked, or new casing to match
- A note on any window where the sash, not the frame, is also leaking
- Photos of the finished windows in your job folder
How it is priced: per window
Quoted in writing from your photos, with a reply within one business day. What moves the number:
- Window size and how much trim has to come off
- Reusing the old casing vs. new casing when old trim splits or is painted in
- Stool and apron that have to come off to reach the bottom gap
- Drywall-return windows where the seal is reached by cutting and patching drywall
- Painting or staining the trim afterward, quoted as its own line
- Number of windows done in the same visit
Tell us where you feel the draft. Air at the sash and air at the casing are different jobs, and the photos help us quote the right one.
How long it takes
Most windows take one to two hours each with trim reused; a room of three or four windows is usually a day.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized reply within one business day. Prefer a look in person? A free in-home estimate is always an option.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Reset the existing trim
Casing comes off clean, the gap is sealed, and the same casing goes back on with holes filled and joints caulked. Best for stained trim you want to keep.
New casing after sealing
When old trim splits, is painted to the wall in many coats or you want a new look. The gap is sealed and new casing is installed and caulked.
Drywall-return windows
No casing to pull. The drywall at the edge of the window is cut back, the gap sealed, and the return patched and taped. Painting is quoted as a separate line.
Air Seal Around a Window Frame — Casing Off, Gap Sealed, Trim Back: questions, answered
Why is cold air coming around my window trim?
The gap between the window frame and the wall framing is not sealed. It is often stuffed with fiberglass, which does not stop air. Wind pushes air through that gap and out past the casing and under the stool. Sealing the gap behind the trim stops it.
How do I tell if the draft is the window or the frame?
Lock the window and feel along the sash edge, then along the casing and stool. Air at the sash edge is weatherstrip or sash fit. Air at the casing joints or under the stool is the gap around the frame.
Should you use spray foam around a window frame?
Use low-expansion foam made for windows and doors, not regular expanding foam. Regular foam can bow the frame so the sash binds. In narrow gaps or around some frames, backer rod and a bead of sealant is the better seal.
Do you have to remove the trim to seal around a window?
To seal it properly, yes. Caulking the outside edge of the casing hides the draft for a while, but the gap behind it stays open. With the casing off, the old fiberglass comes out and the gap is sealed its full depth.
Can my old window trim be put back after air sealing?
Usually. We pull it carefully and label the pieces so each goes back where it came from. Trim that splits, or that is painted to the wall in thick coats, may need new casing, and we tell you on the quote which windows that is likely on.
Will air sealing around the frame fix condensation on my windows?
It can help on the frame and casing, where cold air was cooling the surface. Condensation on the glass itself is mostly about indoor humidity and the glass. Sealing the gap is about drafts and cold trim first.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners and the builders we work with. Counts come straight from our job log; no customer names.
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Read morePage reviewed October 9, 2026 · JMB Builders LLC, MN license BC809005 · All windows pages
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