Getting solar? Insulate the attic first

Solar panels are a long commitment for a roof. The Department of Energy says most systems keep at least 80 percent of their original output for 25 years or more, and for all that time the panels, rails and wiring sit between you and the top of your attic.

This page is about the order of work around a solar install: what to settle on the roof and in the attic before the array goes up, and what to ask the installer so the attic isn't the casualty. It makes no claims about savings, system size or billing. Those are questions for your installer and your utility.

Why the order matters

Three things change once panels are up.

• The roof is harder to work on. The DOE is plain about it: "During a roof replacement, the solar panels and mounting system will need to be removed to complete the work." The same logic applies on a smaller scale to anything that needs the roof surface: moving a vent, adding a ridge vent, fixing flashing around a pipe.
• The attic gets new wiring. Depending on the system and the house, the installer's conduit may run across the roof, down an outside wall, or through the attic. Every route through the attic is something future attic work has to work around.
• More people walk through the attic. Installers sometimes go into the attic to locate rafters, check framing for the mounts or run conduit. Every footstep across loose-fill insulation flattens it where it lands.

None of that is a reason to avoid solar. It's a reason to do the attic work first if it's going to be needed in the next few years anyway.

Step 1: the roof itself

Start with the roof's age and condition, before any panel layout is drawn. The DOE's guide to choosing an installer says to "ask your installer if your roof needs repair prior to installation," and also to ask who is responsible if the roof gets damaged or a leak develops. Get those answers in writing.

If the roof needs replacing, do it before the panels. In climate zones 4 and 12, which include San Jose, the Santa Clara Valley cities, Concord, Walnut Creek and the Tri-Valley, a reroof can bring in California's cool-roof rule, and the attic is part of the way around it. Our reroof guide covers that rule and the order of trades. One detail from the Energy Commission's manual is worth knowing if you're considering solar shingles: in a zone 12 example, an integrated photovoltaic roofing product still has to meet the cool-roof requirements, and only the active photovoltaic area is exempt.

Step 2: vents before the array layout

Roof vents and solar panels compete for the same space. The DOE suggests that if you're getting the roof repaired before installation and have protruding vents, you can ask whether they can be moved to a spot where there won't be panels.

Before the layout is final, it's worth looking at three things:

• Vent area. California's Residential Code sets a minimum net free vent area for vented attics, and the attic still has to meet it after any vents move. Moving a roof vent shouldn't quietly reduce how well the attic breathes.
• Ember resistance. In the Bay Area's hillside and wildland-edge neighborhoods, a vent you're moving anyway is a good candidate to replace with an ember-resistant vent. Doing it later means working around the array.
• Bathroom and kitchen exhaust. Fans should vent outdoors through a proper roof or wall cap, not into the attic, where they add moisture. If a fan duct ends in the attic, fix it now, while the roof cap can go wherever it needs to.

Step 3: air sealing and insulation

Then the attic floor. Air sealing closes the gaps around top plates, wiring holes, plumbing vents and light fixtures. It has to come before insulation, because it can't be reached afterwards.

Insulation comes last, and ideally after anyone else who needs to be in the attic has finished. That's the whole sequencing question in one sentence: if the solar installer will be running conduit or checking framing from inside, it's better that they do it before fresh blown-in insulation goes down, or that the plan says clearly who restores the depth afterwards.

If the installer's work is entirely on the roof and outside walls, insulating first is fine. Just ask.

A checklist for the installer's site visit

Bring these to the site visit. They're about the attic, not the sale.

1. Will anyone need to go into the attic? For what, and where?
2. Where will the conduit run, roof surface, exterior wall or attic? If through the attic, along which path?
3. Does the roof need any repair before installation, and who is responsible for leaks or damage afterwards?
4. Can any vents be moved out of the array area, and will the attic still meet its vent area afterwards?
5. If insulation is disturbed, who restores it, and to what depth?

If the panels are already up

That doesn't rule out attic work. Air sealing and blown-in insulation happen from inside, so the array is usually no obstacle. What we'll want to know is where the solar conduit and any junction boxes are, so the crew works around them and doesn't bury anything that needs to stay reachable. Roof-side work, like adding vents, gets harder, because the vent may need to go where the panels are.

The Bay Area angle

Two local quirks are worth a mention. In foggy climate zone 3, from San Francisco down the Peninsula's bayside and across to Oakland and Berkeley, winters are damp, and moving or reducing vents for an array matters more than people expect, because weak ventilation is one of the common causes when an attic shows condensation or mold. In hot inland zones 4 and 12, attic work in summer is miserable work, and doing it before the install avoids squeezing it into a July afternoon later. The sequencing is the same; the urgency is different.

Next steps

If you have a solar proposal, book a free inspection before you sign it, or send us photos of the attic and the proposed panel layout. Tell us whether a reroof is planned and where the installer expects to run conduit, and we'll suggest the order that leaves the attic in one piece.