How to Install Side-by-Side Double-Hung Windows

Understanding Side-by-Side Window Installation

Installing two double-hung window units adjacent to one another requires careful planning and precision execution. When properly executed, side-by-side window installations create a unified appearance while allowing each window to operate independently. This configuration is popular in modern homes where wide openings demand multiple window units rather than oversized single panes. Understanding the fundamental principles of adjacent window installation ensures structural integrity, proper water management, and long-term performance.

Preparation and Assessment of the Opening

Before beginning any installation work, thorough evaluation of the rough opening is essential. The opening must accommodate two separate window units while maintaining proper spacing and alignment. Measure the total width of the planned opening at three distinct points: near the top, at the middle, and along the bottom. This three-point measurement approach reveals any irregularities in the opening’s geometry. The opening width should be slightly larger than the combined width of both window units to allow for shimming and adjustment. Likewise, measure the height at both left and right sides, again at three locations to ensure consistency.

Check that the opening is truly square by measuring diagonally from corner to corner. These diagonal measurements should be virtually identical, with a maximum acceptable variance of 1/8 inch for window units up to 20 square feet. If the opening deviates from square beyond this tolerance, the window units may not align properly or may operate with difficulty. Inspect the sill for level and the vertical sides for plumb. Any deviation from true level or plumb must be addressed through shimming during installation rather than forced into the frame.

Gathering Materials and Tools

Successful window installation demands having the correct materials and equipment available before work begins. Essential items include:

• Two double-hung window units of matching size and style
• A mull or connecting bracket designed for side-by-side installation
• Shims for leveling and plumbing
• Exterior-grade sealant and backer rod
• Appropriate fasteners as specified by the window manufacturer
• House wrap or weather-resistant barrier material
• Flashing tape for water management
• A level (preferably 4-6 feet long)
• A square for checking plumb and alignment
• A power drill with appropriate bits
• A utility knife for cutting materials
• Measuring tape and pencil for marking

Verify that all fasteners match the manufacturer’s specifications for your particular window model. Using incorrect fasteners can compromise the window’s structural integrity and void the warranty. Have at least two people available for the installation process, as maneuvering and holding window units requires adequate support.

Preparing the Rough Opening

Before the window units can be installed, the rough opening must be properly prepared. If installing into new construction, ensure that wall framing is complete and any house wrap or weather-resistant barrier material is properly installed. The backing support material should be suitable for your regional climate and building codes. For replacement installations where existing windows are being removed, carefully extract the old window unit and remove any old sealant, flashing, or deteriorated framing materials. Clean the opening thoroughly to remove debris, dust, and old sealant remnants.

Apply a continuous bead of exterior-grade sealant around the interior perimeter of the opening where the new window frame will make contact. This sealant creates a moisture barrier that prevents water from entering the wall cavity behind the window frame. Leave 2-inch gaps every 8 inches where the sealant contacts the rough opening, particularly at the sill and near weep hole locations. These gaps are critical for allowing water drainage and preventing moisture accumulation.

Installing the Mulling System

The mull is the connecting component that joins two window units into a single integrated assembly. Typically, the mull is installed onto the vertical inside edge where the two windows will meet. Align the mull carefully on the stile (vertical frame member) of the first window unit. The mull should be positioned such that when both windows are installed, their frames align perfectly and the mull creates a continuous vertical line across the center of the combined opening.

Fasten the mull securely to the first window unit according to the manufacturer’s instructions. Most mulls are attached with screws spaced at specified intervals along the stile. Use a level to verify that the mull is perfectly plumb before tightening all fasteners. Once the first window is mulled, position the second window unit so that its vertical frame member aligns with the opposite side of the mull. This creates a sandwich effect where the mull bridges the gap between the two window units.

Positioning and Initial Fastening

With both windows prepared and the mull connected, the combined assembly is ready for installation into the rough opening. Place shims at strategic points along the rough opening sill to create a level base. Position the window assembly into the opening, resting the sill completely on the prepared shims. The sill must be level to prevent binding or misalignment of the sashes during operation.

With the combined window assembly resting in position, temporarily fasten the unit through the predrilled holes in the jamb at one upper corner. This temporary fastening holds the assembly in place while you complete alignment and shimming operations. If predrilled holes are not available, create them carefully using a 1/8-inch tapered drill bit with a 3/8-inch countersink to avoid splitting the frame. Temporarily fasten only at one location initially, allowing for adjustment before permanent fastening.

Achieving Proper Alignment and Level

After temporary fastening, verify that both window units are level, square, and plumb. Place your level on the sill to confirm that it sits level from side to side. A level sill prevents sag or bowing that could cause operational problems. Next, check that the vertical jambs are truly plumb using a 4-6 foot level placed against each side jamb. Shim as necessary behind the jambs to achieve perfect plumb. The head of the window assembly should also be level, running horizontally without any crown or sag.

Verify that the window measures the same width within 1/16 inch when measured at the top, middle, and bottom. Check diagonal measurements to ensure the assembly measures relatively square with a tolerance of 1/8 inch in either direction. Make small adjustments by tapping shims with a hammer and rechecking measurements frequently. Once the window assembly achieves perfect alignment in all planes, proceed with permanent fastening.

Fastening and Securing the Assembly

Permanent fastening occurs through the nailing fins or fastening flange that surrounds the window frame. Use approved fasteners specified by the window manufacturer, inserted at intervals matching their guidelines. Typically, fasteners are placed 3 to 7 inches from corners and then every 8 inches along the perimeter. For window units wider than 3 feet, fasten the head jamb at 24-inch maximum intervals using free-floating screws that allow slight movement while maintaining support.

When fastening side-by-side units, pay particular attention to fastening the mull connection points. Insert screws through the mull into the stile of the second window unit at intervals specified by the mull manufacturer, typically 4 inches from the top and 4 inches from the bottom, plus intermediate fasteners along the full length. Do not overtighten fasteners as excessive force can compress the vinyl frame and cause jamb bow, which impairs window operation. Hand-tighten fasteners until snug, then verify that the window still operates smoothly before finalizing.

Addressing the Sill and Water Management

The sill area requires special attention because it is the primary drainage path for water that penetrates around the window frame. If your window model uses a sill adapter, install it according to specific instructions provided by the manufacturer. Trim the sill adapter to adjust for variations in the sill plane, then align the vertical leg and fasten it with provided hardware. Install insulation on the sill in areas that will not be accessible after the window is fully installed. This insulation prevents thermal bridging and maintains energy efficiency.

The transition between window units at the mull location must be sealed to prevent water penetration. Apply exterior-grade sealant along the joint where the two window frames meet at the mull. This sealant creates a unified weatherproof barrier while allowing for normal expansion and contraction. Maintain the same sealant standards at the sill where water drainage is critical, leaving appropriate gaps for water to escape rather than accumulate.

Flashing and Weather Sealing

Proper flashing installation is one of the most critical aspects of weather-resistant window installation. If installing in new construction with house wrap, cut the house wrap in an X pattern from corner to corner at the window opening, then fold the flaps back to expose the opening. This allows flashing to be installed against the wall sheathing rather than over the house wrap. Apply flashing tape along each side of the window opening, extending from 3 inches below the bottom to 3 inches above the top. Split the difference so that half the flashing tape width adheres to the wall and half to the opening frame.

Install a flashing strip across the bottom of the window opening first, ensuring that it extends into the sill area while leaving drainage paths clear. Then apply flashing up each side, pressing it firmly to create a solid seal. Finally, apply flashing across the top, working to match up properly with the side flashing strips. Once all flashing is installed, fold the house wrap back over the window to cover the nail fins and fasteners. The house wrap should lap over the flashing at the top to direct water down and away from the window frame.

Completing the Interior Finish

After the window assembly is secured and flashing is complete, interior finishing work can commence. Before applying interior trim or closing walls, verify one final time that both windows operate smoothly and that the mull joint does not flex or move excessively. Caulk around the interior perimeter where the window frame meets the wall, using an interior-grade sealant. This seals the thermal envelope and improves energy efficiency. Leave the area around fasteners clear unless the window manufacturer specifies otherwise.

Interior trim installation can proceed using standard carpentry methods. The trim should frame both windows as a unified assembly, with the mull appearing as a continuous vertical element. Some installers choose to continue trim across the mull as if it were a single window, while others prefer to articulate the mull with distinct trim treatments on each side. Either approach is acceptable as long as the trim remains functional and aesthetically pleasing.

Frequently Asked Questions

Q: Can I install two double-hung windows without a mull?

A: Technically possible, but not recommended. A mull provides structural support at the critical joint where the two windows meet, ensures proper alignment, and creates a weatherproof seal between units. Without a mull, the windows may operate inconsistently and water infiltration becomes more likely.

Q: What is the maximum width for a mulled window assembly?

A: Most manufacturers recommend that combined window assemblies not exceed 8-10 feet in width. Beyond this dimension, structural support becomes challenging and operating the windows becomes difficult. Check your specific window manufacturer’s guidelines for maximum assembly dimensions.

Q: How much space should remain between the window frame and rough opening?

A: Typically 3/8 to 1/2 inch on all sides, though this varies by manufacturer. This space accommodates shims, sealant, and slight adjustment without forcing the frame into a non-square opening. Never force a window frame into an opening that is too small.

Q: Can I install side-by-side windows if the opening is not perfectly square?

A: Yes, with limitations. Openings with up to 1/8 inch deviation from square can usually be accommodated through careful shimming. Larger deviations may require adjusting the opening itself or selecting smaller window units that fit the available space.

Q: Should the mull be on the inside or outside of the window frame?

A: The mull is typically positioned on the interior side of the frame where it connects the two window stiles. This placement allows the exterior of the assembly to appear as a unified frame while the mull provides internal structural support at the critical joint.

Q: How long should installation take for a side-by-side window assembly?

A: A typical installation takes 2-4 hours including preparation, flashing, and securing. Complex scenarios such as difficult openings or challenging flashing situations may extend this timeline. Rushing the process often results in poor water management and future problems.