I am considering building 400 watt metal halide up lighters for church. I can get some 400 watt MH high bays with flat glass prismatic diffusers. I would be hesitant to use them up side down for fear of overheating the glass. I am open to any suggestions you may have, however I would prefer a base-down lamp and maybe a remote ballast. This would be used indoors. Thanks.

Do you have a pic of the area you're lighting?

No, but it is a part of a church that has a 70 feet ceiling.

I think that the MH would be fine, just make sure that the glass cover will resist lamp explosions even though it is rare. Also you could use lamps that are rated for open air, normally labeled /O, for added protection versus using the /E lamps. The /O lamps have a special protective coat so that it blocks the flying glass shards from the explosions.

The glass cover is original to the fixture. What I am worried about is the fact that it is not designed to be operated in an up side down configuration.

I'd run the fixture reguular and then upside down for an hour or so. Measure the temperatures after each run, making sure to let it cool down completely. How old are the fixtures?

The fixtures were originally installed in my university's gymnasium between 1995 and 1998. They are now for sale at a local salvage yard for 30 dollars each. As far as I know, they were all working when they were taken down. The temperature differences is a good way to measure them, I might try that.

Either way, it may not hurt to add varnish. Just warn the ballast up and spray so then the varnish will get into the nooks and crannies between coils.

Are they going to be outdoors ? If so, rain falling on the glass may well shatter it

Anyway, you can install them "normal" and do the rest of the work with mirrors

They are to be installed indoors, so the rain issue is insignificant. I thought about mirrors, but I will have to experiment with that...

I think it's a good idea with the mirrors. If you still want to mount them upside down just do the temperature measurements. If you need to you could just neatly drill vent holes or cut slots out at the top of the fixture.

But then it will fill up with cooked insects....

You could put a metal screen like that used with windows.

Are these lamps High bay ones meant for ceiling mount? I have many lights like that and run them with the ballast on the floor with no problems.400W MH and HPS(use the Philips Master color bulbs in them)and a 450W Pulse start MH Widelight from the old Michlen tire factory.The lights work just fine--with the optical above the ballast--as "upside down" the ballast runs cool.The lights I have came from contractors replacing the older lights with flourescent ,LED,or pulse start MH.

They are in fact highbays meant to be suspended from the ceiling. They have a prismatic flat glass factory installed on the reflector. There is a gap between the top of the reflector and the ballast housing, though. Upside down would be the best way to use them as up lighters, though. They have been used for years with standard M59 lamps (not open rated). I don't have pictures of them and I can't get them until the weekend as the salvage yard is only open on Saturday and on Sunday.