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I've mostly finished my 1966 F100 project. I live in Oregon where it is very wet at times. The inside of the truck is very wet; water dripping from the ceiling. Looks like it's all condensation. What might be the reason for this? I don't want to put the carpet and headliner in until I get this solved.
Is your truck outdoors year round? If it is garaged, or even outdoors, just keep a low wattage heater in it. I did this when I lived under the redwoods where it’s moist all the time and downright wet in the winters.
Most likely a lack of insulation. On the other cars they might be better insulated or you just can’t see the condensation or the weatherstripping is better.
I'd look into a brush, spray on product like lizard skin. It is touted as a thermal and acoustic insulator. There are other "roll-on" insulation mats but something put on with a brush or spray would have better coverage me thinks. I have lizard skin in my truck but condensation isn't a issue here in North Texas.
I have no affliction to lizard skin, just used it.
Does your truck live outside?
The condensation is probably a result of the warm moist air in the cab contacting the cold steel of the cab, mostly roof be my guess.
You probably see it on the windows as well?
Your answer is in your question. You haven't put your headliner in yet.
2 things are at play here. First is the cold temperature of the steel roof, and second, the warmer moist air in the cab. By installing the headliner you create an air pocket on the inside of the roof that is closer to the outside air temperature which reduces condensation. Also, with the headliner installed you're preventing the moist air inside from getting to the roof thus preventing condensation. More insulation is not necessarily the answer but rather keeping a break between inside and outside surfaces is. Think about it, the inside of the windows are wet but not the inside of the doors. Why? Because of the air space within the door. How about the center roof support brace? I betcha there's no condensation there. Same reason.
And yes, even here in Arizona it can sometimes be a problem.
**edit: I attempted to photograph but near impossible to see. This morning, underside of roof (no headliner) frosted. Currently 31° with 96% humidity
Last edited by AZSCAWPION; Jan 6, 2026 at 09:22 AM.
I've mostly finished my 1966 F100 project. I live in Oregon where it is very wet at times. The inside of the truck is very wet; water dripping from the ceiling. Looks like it's all condensation. What might be the reason for this? I don't want to put the carpet and headliner in until I get this solved.
Recently moved to OR myself and currently experiencing my first rainy season. I can’t say I’ve noticed any condensation in the cab, but I will say pretty much everything under the hood that isn’t painted is starting to rust. Our trucks are stored in a garage, but the 66 has been acting as our dd for a few weeks out of necessity. Man I can’t wait for spring and six months of zero rain.
I moved to Oregon from California 17 years ago. Found out quickly that it was a good idea to run a dehumidifier in my garage/shop. Tools, etc were beginning to rust.
Upon further investigation, I discovered that rain water is getting into the cab. So far I haven't found the source, but it seems to be somewhere in the windshield/cowl area. Can anyone tell me where water is supposed to end up that enters the grille on top of the cowl just in front of the windshield?
Water that enters the cowl grille should flow left and right to the sides and down the cowl pillar to this small drain - one on either side - in front of the doors.
In the picture the front fenders are off making it easier to see the drain. They can get plugged up. And, of course, if there is rusting inside of the cowl area on top water can come in there or if it's rusted through toward the inside in the cowl pillars. You can poke a chopstick or gently use a screwdriver up into those drain holes while running water into the cowl to flush it out some. While running the hose watch to see if you are getting any noticeable water inside the cab.
Another place to check is the wiper shafts that stick through the cowl below the windshield. Originally, they put a leather gasket on the shafts before they were installed but that leather piece can disintegrate which can allow water to seep in around one or both shafts.
C1TB-17A475-B - SEAL - WINDSHIELD WIPER PIVOT SHAFT TO COWL TOP
Thanks, this is VERY helpful.
Might you be able to point me to a process for removing the fenders? I see bolts, but I don't know if there are hidden bolts and other stuff has to come off first.
You don't have to remove the fenders to get to those drains. I had mine off when I took the one picture and it does make it easier to see in a picture, but you can get to the drains just about as easy with the fenders on as off.
If you do want to pull your fenders sometimes it's easier to pull the front clip as a whole - fenders, inner fenders, grille, core support, etc. - sounds like more work but it is less bolts. Separating the grille from the fenders and lower valance can be tricky in good conditions due to rust, spinning U-nuts and things are pretty cramped in some areas. The four fender bolts that aren't as obvious are the ones under the back lower edge of the fenders on each side and then the special one on each side that you get to from a small opening inside the cab just in front of each door in the kick panel area. See the yellow oval with 02449 - follow the arrow from it and you'll see the area.
The cover plates only apply to 1964-1966 F100/250 2WD Custom Cab (81B) trucks and possibly 1966 F100 4x4 Custom Cab as well. But if there is no cover then you should be able to look into that area and see the bolt head. That one and the one underneath can often spin inside of the clip rather than come out smoothly.
