CRL T-Vent windows are the upgrade Karlee pushed for and I dragged my feet on — twice. First on the style (T-vent vs sliding), then on the count (both sides vs one). Eight years in, both calls were correct. Both calls would have been cheaper to make from day one.
If you’re building a full-time ProMaster and you’re running a roof fan, the windows are how the fan actually moves air. A sealed van with one fan has nothing to pull from. T-vent windows on both walls turn the cabin into a cross-flow channel, and the difference is the kind you feel the first hot night.
Quick Verdict
Skip if: you’re committed to a fully stealth exterior with no visible windows, or your van is a weekend rig where buttoned-up sealing is fine for short stays.
Are T-Vent windows the right call for your ProMaster?#
Tap the situation that sounds like you.
🌴 Full-time, hot or humid climates — Florida, Gulf, desert summers. Both sides T-vent
This is exactly what these windows are for. In Florida summer the only thing that makes the van livable at night is real cross-flow — air pulled in one wall, exhausted out the other through the fan. Both sides T-vent is the configuration that works. One window is half the system. We learned this the slow way.
🔨 Building from scratch — haven't cut walls yet. Do it now
Cutting windows during the build is dramatically easier than retrofitting later. Walls are open, you can see your framing, you can place the cuts cleanly. Retrofit means working around insulation, paneling, and electrical that's already in. If you're at the build stage and already planning a roof fan, install the windows in the same phase. Future-you will not regret it.
🔒 Stealth-first build — visible exterior matters. Tradeoff to weigh
Honest answer: T-vent windows are visible from the outside. They look fine — clean glass, factory-style fit — but they do change the silhouette. Our ProMaster is black, and the windows still let it pass for a work van in most parking situations, but it's no longer a blank panel. If your stealth strategy depends on zero windows, skip these and lean harder on the roof fan and door venting.
🪟 Already have one window, debating the second. Add the second
I had one T-vent on the sliding-door side and thought I was done. Karlee said both. I pushed back. She was right. Cross-flow is the entire point — air has to enter from one side and exit from the other for a fan to move it. With one window, the fan creates a pressure differential and gives up. With two, you have a working channel. The second window is the cheaper of the two upgrades because the cutting confidence is already built.
What 8 years with these windows actually taught me#
T-vent style vs sliding — Karlee's first right call +
Sliding windows look simpler and you see them more often in van builds. But sliding windows leave a screen in the open path the whole time the window is open — debris, rain, bugs. T-vent windows crank outward like a top-vent awning, so the glass itself becomes a small overhang. You can leave them open in light rain. The crank mechanism feels more like a fixed window operating part-time than a sliding window operating full-time. Eight years and zero issues with the T-vent crank.
I pushed for sliding because they looked easier to install. Karlee pushed for T-vent because she'd thought through the use cases I was going to be the one annoyed by. T-vent was the right call.
Both sides, not just one — Karlee's second right call +
I tried to save money and effort by installing one window. The thinking was: roof fan + one window = cross-flow. The reality: a roof fan with one window pulls air from any leak it can find — door seals, cab gaps, the kitchen vent — and creates almost no actual circulation. The cabin felt stuffy on hot nights even with the fan on full.
The day we cut the second window on the driver's side, the difference was immediate. Air actually moved through the van. The fan started doing the job it was rated for. Two T-vent windows + a MaxxFan is the configuration that works. One window is a half-system that costs more in suffering than the second window costs in dollars.
The windows are how the roof fan actually works +
Reframe before buying: don't think of the windows as a separate ventilation upgrade. Think of them as the air-intake half of your roof fan system. A 900 CFM fan rating assumes the fan can pull 900 CFM from somewhere — and a sealed van can't supply that. The fan's real-world performance is gated by your air-in path. Windows on both walls open that path wide.
If you're budgeting and you already bought the fan, the windows are not optional accessories — they're the second half of the same purchase.
Cutting walls is a one-way decision — measure twice +
The install isn't bad, but the cuts are permanent. Take time on placement: clear the inside of any framing, electrical, or insulation runs you don't want to hit. Mark the cut on both interior and exterior. Use a metal cutting blade, not a generic jigsaw blade — wrong blade chews the steel and leaves jagged edges that won't seal cleanly.
Sealant is the other do-it-once thing. We used butyl tape behind the flange and a marine-grade sealant around the perimeter. Eight years of weather, no leaks. Don't rush the sealant. A leak around a window is way harder to fix than around a roof fan because the water tracks down the wall through the insulation before you see it.
Specs (for reference)#
Glass cranks outward like a top-hinged awning. Stays partially open in light rain. Different mechanism from sliding windows — better for full-time use because the open glass acts as a small overhang.
One window opens. Two windows on opposite walls create cross-flow. For full-time use with a roof fan, both sides is the configuration that works — one is a half-system.
Requires cutting through the van wall. Easier during the build than as a retrofit. Sealing is the make-or-break — leaks track down through insulation before you see them.
Frequently asked#
T-vent or sliding window — what's the difference for full-time? +
T-vent cranks outward, so the glass becomes a small awning when open. Sliding windows leave a screen in the open opening with no overhead protection. For full-time van life, T-vent's ability to stay open in light rain matters more often than people predict. Karlee called this one before we installed and I learned it the slow way.
Can I get away with one window if I have a roof fan? +
You can — for occasional use in mild conditions. For full-time use in hot climates, no. The fan needs an air-in path on the opposite side of the van for cross-flow to work. We tried one and it wasn't enough. The second window made the system actually work.
How hard is the install? +
Cutting through the van wall is intimidating but the process is well-documented. Hardest part is committing to the cut location — measure inside framing carefully. Use a proper metal-cutting blade. Take your time with the sealant and let it cure fully before testing for leaks. If you can install a roof fan, you can install these.
Do they affect stealth? +
Yes — T-vent windows are visible from the outside. The look is clean and factory-fitted, but the silhouette changes from "blank panel van" to "van with windows." Our black ProMaster still passes for a work van in most parking situations. If your stealth strategy depends on zero visible windows, these aren't the right upgrade.
The upgrade that turned our roof fan into a real ventilation system