Condensation (?) inside kitchen vent

[RBPhilip]

As I've been remodelling I've been upgrading various features of the house. This summer when the roof was off I added a vent pipe into the kitchen, to be connected to the over-stove microwave when the kitchen got to that state.

Well, yesterday I installed the cabinets and today the microwave goes in. But I discovered a small puddle of water in the cabinet where the vent terminates.

It's an unbroken 4" pipe, nicely sealed and capped outside the house and it's clear (drops remaining in the pipe) that the water has come from inside the pipe.

Is this likely just because the (open) pipe was allowing warm, moist air from inside the house to rise into the pipe sticking above the roof into the 5 degree air? I'm guessing that when the microwave is hooked up and everything is more sealed that this won't be happening..

Or, have I done something stupid with my installation, perhaps? It really is just a strait piece of galvanized pipe sticking out of the roof (nicely sealed, as mentioned) with a cap...

Rob

[jdkimes]

Probably condensation. Since it sounds like you don't have the vent fan from the microwave attached yet a lot of the air from the kitchen was probably just drifting up there and the cold air from the outside and in the attic made it condense on the duct.

Make sure you wrap insulation around that vent duct in the attic and you might think about getting an exhaust fitting that has a damper. Some string around the insulation will help keep in on the duct or look for some insulation specifically made for goind around a 4" duct.
Once you get the microwave vent fan on that will keep most air from drifting up and the roof damper will keep some of the cold air out of the duct and when the fan is on the insulation will help keep the moisture from condensing until it gets outside.

[jadnashua]

I assume that the microwave goes over the stove and doubles as a vent fan? Usually, the internals of the thing has a damper on it. If not, then you should install one. The installation instructions should indicate. I'd put the damper down as low as possible to prevent convection currents. It doesn't hurt to insulate the pipe in the attic as well. I agree - the moisture is probably from condensation. It is possible that some is from moisture getting around the vent cap, too, especially if it has been windy.

[RBPhilip]

Thanks gents.

The microwave (above the stove) does have a baffle. There really isn't any attic space - above the microwave is about 5' of pipe - 1' in the ceiling, 1.5' going through two roofs and 2.5 feet above the roof.

I'm betting that now the microwave is installed there won't be much head rising up the pipe. Next summer I'll install a baffle at the top, too...

Rob

[daveydo]

You must insulate the pipe in the attic and anywhere else it comes in contact with cold air. What would concern me about this type of installation is that the pipe is vertical or less than 90 degress to the horizontal
The pipe is exposed to cold air on the outside. It will give up its heat to anything colder that touches it, such as air. Unlike a pipe making a graded horizontal exit thru a soffit or wall, the vertical pipe being exposed outside the building has a tendency to cool the air as it passes thru and create condensation which due to gravity will run back inside.