Cutting a 14” square hole in your new van to install a vent and fan has been covered extensively on various forums, Antoine and Isabelle’s FarOutRide Transit blog, and elsewhere, but I’ll write up my version anyway.

We decided on a MaxxFan 7500 because it can stay open while driving, although we may not need that much air movement in a Promaster. If I were to start over, I’d seriously consider Accrete’s approach with a G&C vent, since that would allow more roof space for solar panels.

I decided to use Hein’s Promaster Adapter rather than improvising a solution to the roof corrugations. Thanks to vanlifedestiny who sent me a partial tube of Window Weld, which attached it securely, but was extremely viscous and difficult to apply. I suspect that it didn’t like being open for two weeks, so when I install windows I’ll get a new tube.

Cutting out the 14” square was much easier than I expected. The 12” Greenlee Slug Buster made short work of the corners, although cutting so close to the roof corrugations made for some slightly messy holes. (I wouldn’t necessarily bother with a Slug Buster if you don’t have one already - a step drill will probably be almost as clean)

slug buster

Hole from Slug Buster

hole from slug buster

Taping a plastic bag below the hole before cutting the edges with a jigsaw worked well. I tried to leave a small tab at each corner to avoid vibration on the last edge, and that mostly worked. Experimenting in the middle of the panel with cutting across the corrugations was helpful to get a feel for how the sheet metal behaved when cutting.

A fresh smooth-cut metal blade in my cheap Skil jigsaw left fairly clean edges. I worked them a bit with a deburring tool and file, but there wasn’t much to clean up. For the windows I might consider an electric shear instead - we’ll see. They did produce a lot of shavings though, even trying to capture them with plastic underneath wasn’t completely successful.

cutting with jigsaw

Unfortunately I didn’t have any appropriate metal primer, and starting this project on Christmas Eve meant no quick run to Lowes was possible. I don’t expect any water infiltration, and based on MsNomer’s report the promaster sheet metal is pretty rust-resistant. We’ll see whether we regret that later.

I decided to add 1” aluminum bar reinforcement a la Dave Orton’s install, which is probably unnecessary, but was a quick addition with a few cuts on the miter saw and 16 holes on the drill press. To avoid galvanic corrosion, I added nylon washers between the stainless steel screws and the aluminum (again probably unnecessary).

I oversized the holes in the adapter and roof by 116” and preinstalled the screws in the aluminum + flange + butyl tape sandwich, which made installing on the roof much easier.

aluminum + flange + butyl tape sandwich aluminum reinforcement

I have not yet added any waterproofing beyond the Window Weld and Butyl Tape, but I expect I’ll use some non-sag Dicor - a bead at the adapter-roof join, and some more on the screws. First I need to decide what to use on the inside of the roof, to support the inner flange. Currently considering plywood, Celtec or HDPE, but I’m not in a huge rush.

The final product looks pretty much like any other MaxxFan 7500 install, except for the missing waterproofing.

any other MaxxFan 7500 install