Okay, I understand the situation. You have a vanity light fixture that requires a standard 4" octagon box, and you're facing challenges mounting it in a finished wall due to the fixture's design and the presence of a PVC vent pipe blocking access to a stud. You're proposing using a backer board glued to the back of the drywall to provide support.Here's a breakdown of the considerations and a few potential solutions

Concerns and Issues with Your Proposed Solution:
Weight of the Fixture

How heavy is the vanity light? Drywall alone is not designed to support significant weight, and the backer board's ability to do so depends on several factors. Adhesive strength, drywall integrity, and the contact area of the backer board all play a role. Over time, vibration from closing doors and regular use can cause the adhesive to fail, especially with heavier fixtures.

Accessibility

The 8" length of the 1x4 backer board going through the hole might be difficult to manipulate and position precisely. It's also crucial to ensure you have good, even contact between the backer board and the drywall.

Code Compliance

Electrical codes prioritize safe and secure installations. While using construction adhesive for a backer is not explicitly forbidden in most residential codes, the security of the connection will be questioned upon inspection since the electrical box is not attached to a structural member.

Future Maintenance

If the fixture ever needs replacing or the wiring needs servicing, access to the box might be limited. Removing the fixture could potentially damage the drywall if the backer board isn't securely attached.

Safer and More Reliable Alternatives

Here are some alternative approaches that might be more suitable:1.

Careful Cutting & Inset Box (Best if Space Allows)

If you can manage to precisely cut the drywall on the fixture side to create a recessed area (even a 1/4" or 3/8" deep), this could allow the light fixture to sit closer to the wall. This may allow you to use a shallow rework box.

Benefit

Cleaner, more secure, and potentially eliminates the need for the backer board entirely (or reduces its weight-bearing requirements).2.

Hybrid Approach: Adhesive + Mechanical Fasteners (Recommended)

This is a combination of your original idea and a more secure method:

Prepare the Backer Board

Use a piece of wood that's larger than the electrical box – at least 6" x 6" for a standard 4" octagon box. Plywood or a solid wood board (like a 1x6) is a good choice.

Pre-Drill for Screws

Drill pilot holes through the drywall from the fixture side, spaced evenly around the box location. This will guide your screws into the backer board.

Apply Adhesive

Apply a generous amount of high-quality construction adhesive to the backer board where it will contact the drywall.

Position and Secure

Carefully thread the backer board through the hole and position it so that it aligns with the pilot holes.

Use Screws

Drive screws (wood screws or drywall screws) through the drywall and into the backer board. This is critical to ensure a strong, lasting connection. The adhesive provides initial bonding, and the screws provide mechanical reinforcement.

Install the Octagon Box

Mount the 4" octagon box to the backer board using screws.

Benefit

Significantly stronger and more reliable than relying solely on adhesive.3.

"Old Work" Box with Extra Support

Even if a standard flanged "old work" box doesn't fit, there are variations. Look for boxes with longer "ears" or wings that can grip a larger area of the drywall.

Reinforcement

Use shims between the back of the box and the wall to provide extra support and prevent the drywall from flexing.

Caulking/Adhesive

Apply construction adhesive around the edges of the box where it meets the drywall to further secure it and prevent vibration.

Benefit

Simpler installation if you can find a suitable box and reinforcement method.4.

Opening the Wall (Most Reliable but More Work)

If none of the other methods work, consider cutting a larger access panel in the wall behind the vanity to install a proper support structure.

Install Blocking

Add wood blocking between the studs to create a solid mounting point for the electrical box. This is the most secure option.

Patch the Drywall

Patch and repair the drywall after the blocking is installed.

Benefit

Provides a permanent, code-compliant solution that can handle the weight of the fixture and ensures easy access for future maintenance.

Drawback

Requires more work and drywall repair.

Important Considerations



Electrical Safety

Always turn off the power to the circuit before working with electrical wiring.

Wire Connections

Ensure all wire connections are made inside the electrical box and are secure. Use wire connectors (wire nuts) and properly ground the fixture.

Local Codes

Always check your local electrical codes for specific requirements regarding the installation of electrical fixtures.

Recommendation

I strongly recommend the

Hybrid Approach (Adhesive + Mechanical Fasteners) or the Opening the Wall method. While it requires a bit more effort, it will provide a much more secure and reliable installation, ensuring that your vanity light stays in place and meets safety standards. If you're comfortable with drywall work, opening the wall is the best approach.Before starting, carefully examine the weight and mounting requirements of your vanity light fixture. This will help you determine the appropriate level of support you need to provide. If you are unsure about any aspect of the installation, consult with a qualified electrician. Flag for review