Nail fin, flush fin, block frame or insert? Choosing a window installation type

Every window line is sold in one or more installation types. Pick the wrong one and you either tear off siding you meant to keep or lose glass area you didn’t need to.

Type	How it installs	Best for
Nail fin	A flange nailed to the sheathing, then flashed into the weather barrier	New construction and full re-siding
Flush fin	A fin set back to sit flush with stucco or other cladding	Stucco and remodels where the exterior finish stays
Block frame	A frame with no fin, set on shims and fixed with straps or screws	European-style and Passive House windows in new construction, masonry, and full-frame replacement
Insert	A complete unit set inside the existing window frame	Fast replacement without touching trim or siding

Rules of thumb

• If the siding is coming off, use nail fin and flash it properly.
• If the siding stays and the old frame is sound, use an insert.
• With stucco, use flush fin, or an insert to avoid cutting stucco back.
• For European, tilt-turn and Passive House windows, plan for block frames from the start.

Block frame installation

Block frames have no fin. The unit sits on load-bearing blocks under the frame corners and mullions, is fixed to the structure with steel straps or frame screws, and is sealed to the wall’s air and water barriers with tapes or membranes. Under it goes a sloped sill pan that drains to the outside.

Two things to settle before framing:

• Where the window sits in the wall. With exterior insulation, setting the window in line with the insulation (often on a plywood buck or bracket) cuts heat loss at the frame edge. Setting it at the sheathing plane is simpler but loses more heat around the frame.
• The rough opening. Block frames need room for shims, the pan and the tape on every side. Use the gaps on the maker’s installation drawings, not the gaps you would use for a nail-fin window.

Many crews used to nail-fin windows haven’t installed block frames before. Get the manufacturer’s installation instructions to the installer early, and ask whether the maker offers site support for the first units.

Replacement: insert or full-frame

An insert keeps the old frame and trim and slips a new unit inside. It is fast and tidy, but it loses some glass area and depends on the old frame being sound and square. A full-frame replacement removes everything down to the rough opening, so you can fix rot, flash properly and keep the glass size. Use full-frame wherever the old frame is damaged, leaks, or when the new window is a block frame or nail fin.

Questions to settle before ordering

• Is the siding or cladding coming off, staying, or being replaced later?
• What is the wall assembly, and where is the insulation layer?
• Who supplies the sill pan, tapes and fasteners: the window maker or the installer?
• Does the installer have the maker’s instructions for this installation type?

This guide is a draft and should be reviewed by a credentialed author before publication.