Installation

A window is only as good as its installation. Our windows are engineered to last for decades, and that depends on setting them in the right place in the wall, anchoring them to the structure correctly, and sealing them continuously to the air and water barriers. This page sets out our recommended installation practice for GlasFëns F-Series fiberglass and A-Series hurricane windows and doors. For membranes, tapes, sealants and substrate-specific details, see Air Barrier Interface.

Installation guidance and support

GlasFëns supplies windows and doors; installation is carried out by your contractor or installer. Our team includes specialists experienced in installing European-style and direct-set windows, and we support your installers from start to finish: shop drawings and installation details, review of your wall sections, guidance on anchoring, sealing and flashing, and guidance on adjusting sashes, doors and hardware after installation. Our project-specific shop drawings and installation instructions always take precedence over the general guidance on this page.

Where we set the window: in the insulation layer

We recommend installing our windows within the insulation layer of the wall, not on the outer face of the sheathing. Current research and Passive House practice confirm this is the best position:

• Less heat loss at the perimeter. The joint between window and wall is a thermal bridge. Studies of insulated walls find that moving the frame from the face of the structure into the insulation layer cuts the installation heat loss by roughly two-thirds or more, and with insulation lapped over the frame it can approach zero. The best results come from a position partway into the insulation, not at either face.
• Warmer interior surfaces. Setting the frame in the insulation raises interior surface temperatures at the frame by several degrees, reducing condensation and mold risk, especially at the sill.
• Insulation overlap. Where possible, lap the exterior insulation over the outside face of the frame by at least ¾ to 1½ in (20 to 40 mm). It protects the frame and further reduces heat loss.
• Better architecture. A nailing fin fixes the frame and glass at the outer face of the sheathing, so the window sits nearly flush with the cladding and the façade looks flat. Setting the window back into the opening, as traditional masonry and timber buildings always did, creates a reveal and a shadow line that give the wall depth, and it shelters the frame and glass from sun and rain. The recessed window reads as part of the architecture rather than applied to its surface.

A recessed window has one obligation: the exposed exterior reveal and sill must be waterproofed. Every recessed installation needs a sloped sill extension or flashing with end dams and a drip that carries water past the cladding, and a sill pan under the window.

Three ways to attach a window

Nailing fins have a long history on American houses, and installers know them well. Commercial window installers, by contrast, have long used through-frame anchoring and clips, and anchor clips have been standard for decades on storefront, masonry and retrofit work. The longer, bent European-style straps have become more common in North America over the past 15 years with the growth of Passive House and imported windows. Even with flexible fins, many manufacturers call for added clips or through-frame screws on larger units and at higher design pressures, because the fin alone cannot carry the loads.

Our recommendation

For F-Series fiberglass windows and doors, we recommend direct set, through-frame anchoring as the standard method, with anchor clips where the wall requires them.

• Direct set places the window exactly where it performs best, is installed from the inside, makes both the exterior and interior seals continuous and easy to inspect, and allows windows to be replaced later without disturbing the cladding.
• Use anchor clips where screws cannot reach solid structure, chiefly when the window sits outboard of the structure in thick exterior insulation without a buck, and at heads that must allow deflection or shrinkage, such as steel framing with a deflection track and multi-story wood framing. In these cases use slotted or low-profile clips and plan the interior seal around them.
• A-Series hurricane windows are available with aluminum nailing fins, suited to wood-framed houses and low-rise buildings. In multi-story and impact-rated applications, follow the anchorage in the product's Florida Product Approval or Miami-Dade NOA exactly; the tested anchor type, size, spacing and edge distance are part of the approval.
• F-Series fiberglass windows do not use nailing fins. Our frames are designed to be set within the wall and anchored through the frame or with clips.

Direct set: how to install

1. Prepare the opening. Air barrier and WRB wrapped into the opening, sill pan installed with back dam and end dams, blocking in place at anchor locations. See Air Barrier Interface.
2. Prepare the frame. Pre-apply the interior airtight tape and, where detailed, the exterior membrane tape to the frame on the bench. Clean bonding surfaces with isopropyl alcohol.
3. Operable windows and doors: open or remove the sash or panel to reach the frame. Fixed windows: remove the interior glazing beads, secure the glass so it cannot move or fall, fasten the frame, then reinstall the beads and check the glazing seals. For large fixed units where removing glass is impractical, ask us about anchoring with clips instead.
4. Set the window on load-bearing setting blocks or shims under the jambs and mullions, at the depth shown on the drawings.
5. Plumb, level and square the frame, and check the diagonals.
6. Shim and fasten. Place a hard shim directly behind every fastener so the frame is not bowed. Use the fastener type, size and spacing on the shop drawings. Spacing depends on window size, design pressure and substrate, and on engineered or code-approved projects it is set by the anchorage calculation. As a guide: low-wind, low-rise sites: 4 to 6 in (100 to 150 mm) from each corner and mullion and no more than 12 to 16 in (300 to 400 mm) on center, with at least two fasteners per side. High-wind, coastal or tall buildings: no more than 4 in (100 mm) from corners and typically 6 to 12 in (150 to 300 mm) on center, as engineered. For comparison, a tested impact casement installation at a design pressure of +50/−65 psf calls for through-frame anchors no more than 4 in from corners and about 9 to 10½ in on center into concrete. Do not overdrive; the screw should hold the frame, not pull it.
7. Keep fasteners out of drainage. Do not fasten through the sill drainage chamber or the sill pan. Where a sill fastener is required, fasten above the pan and seal the head.
8. At heads that will move, such as multi-story wood or steel with deflection, do not fasten the head rigidly. Fasten the jambs and sill, and leave a movement gap at the head closed with a flexible seal.
9. Check operation, reveals and hardware before sealing.
10. Seal: connect the exterior (primary) seal on all four sides, including the sill, as described under Sealing the sill below; insulate the shim space; complete the interior (secondary) air seal all the way around.
11. Cover fastener heads so they are protected and concealed.

Anchor clips: how to get them right

• Use the clip type, gauge and fasteners on the shop drawings. Clips must be corrosion-resistant, typically stainless or galvanized steel, and isolated from dissimilar metals.
• Fasten clips to the frame first, on the bench, then to the structure. Use slotted connections where movement is expected.
• Interior seal around clips: apply the interior airtight tape to the frame before the clips, so the clips pass over it, and then seal the tape to the substrate with small patches or split-liner tape at each clip. Alternatively, keep clips on the exterior side of the interior seal line.
• Interior finish: notch the drywall or furring at each clip, or use a return detail with enough depth to cover them, so the clips do not telegraph through the finish.

Nailing fins (A-Series)

• Install the sill pan and wrap the opening first. Apply sealant behind the jamb and head fins only (leave the sill fin free to drain or use a gapped bead), set the window and fasten through the fin slots per the approval.
• Tape the jamb fins, then the head fin, shingle-lapped, then lap the WRB over the head flashing.
• On multi-story buildings, plan access and sequence: fins must be installed from outside before the cladding goes on.

Sealing the sill: two schools of thought

Few installation questions start more arguments than whether to seal the bottom of a window on the exterior.

• The North American rule: leave the sill open. Common U.S. practice for flanged windows, taught by ASTM E2112, building scientists and many builders and manufacturers, is to tape or seal the jamb and head flanges but leave the bottom flange untaped, or sealed only intermittently. The reasoning: water that gets past the window or its flashing, through a corner joint, a mullion or a failed jamb seal, lands in the sill pan and must be able to drain out. Seal the bottom and the pan can become a bathtub.
• The European practice: seal all the way around. European windows, installed to the German RAL installation guideline and similar standards, are sealed on all four sides in three levels: an airtight, vapor-tighter seal inside, insulation in the middle, and a weather-tight but vapor-open seal outside. The window itself is designed to manage water. Rain that enters the frame's drained chambers exits through weep slots onto a sill board or sill extension, so no water is expected between the frame and the opening.

Both approaches have merit. Leaving the sill open drains incidental leaks, but it lets outdoor air flow into the shim space beneath the frame, all the way to the interior seal. That washes out the insulation at the sill, chills the inner edge of the frame and the interior seal, and on thermally broken aluminum it bypasses the thermal break.

Our recommendation for GlasFëns windows: seal the frame on all four sides, and keep the sill pan drained below it.

• Our frames are European designs with internal drainage and weep slots. Water entering the frame is drained to the outside by the window itself, through the weeps and over the sill extension.
• Seal the frame continuously to the air barrier and WRB at the jambs, head and sill, with tape or fluid-applied membrane and neutral-cure silicone. Where an extruded aluminum or composite sill extension is used, tuck it under the frame and seal the frame to the sill extension, with the extension's end dams sealed to the jamb membranes.
• Keep the sill pan as a second line of defense. The pan beneath the sill extension must still slope to the exterior and have a deliberate outlet, for example a gapped or intermittent seal at the front edge of the pan below the sill extension's drip, so that any water reaching the pan can drain. Keep the outlet small and below the frame, so outdoor air does not reach the frame or the interior seal.
• Make the interior seal continuous on all four sides; it is the primary air barrier connection.
• Field test the first installed windows with water before the detail is repeated across the building.

For flanged A-Series windows in wood-framed buildings, follow the product approval and common U.S. flange practice, with a drained sill.

Primary and secondary seals

We design every installation with two lines of defense:

• Primary seal (exterior): the weather seal, continuous on all four sides, made with tape or fluid-applied membrane from the frame to the air barrier and WRB, and a backer rod and low-modulus, neutral-cure silicone joint where the frame meets the cladding, substrate or sill extension. It sheds water and stops wind-driven rain, while the drained sill pan below catches anything that gets past it.
• Secondary seal (interior): the airtight, vapor-retarding seal from the frame to the interior air barrier or finish, continuous on all four sides. In cold climates it should be tighter to vapor than the exterior seal, so any moisture in the joint dries outward.

Between the two, the shim space is filled with low-expansion window foam or mineral wool. Sealant selection, adhesion testing on our painted fiberglass frames and compatibility with membranes are covered in Air Barrier Interface.

Handling, protection and handover

• Delivery and storage: store upright on blocking, under cover, off the ground, with crates or racks secure. Do not lay glazed units flat or stack them against each other without padding.
• Protect during construction: keep protective film and covers on until finishing trades are done, and remove film within the period the manufacturer recommends; film left in sun becomes difficult to remove. Protect frames and glass from mortar, plaster, paint, welding spatter and cleaning chemicals such as masonry acid.
• Final adjustment: after interior finishes, check and adjust sashes, hinges and locking points so every window and door operates smoothly and closes tightly. Tilt-and-turn and lift-and-slide hardware should be adjusted by a trained installer; we provide adjustment guidance on request.
• Testing: on multifamily and commercial projects, field test a sample of installed windows for water and air (AAMA 502 or ASTM E1105) early, before the detail is repeated across the building.
• Handover: provide owners with care, cleaning and hardware maintenance instructions and register the warranty. See Warranty.

Questions about a specific opening or wall type? Send us your wall sections and we will mark up the recommended installation. Contact us.

General guidance only. Project shop drawings, the product's installation instructions, applicable code approvals and the design professional of record govern each installation.