Cold Air and Microwave Vent

[Curly Qsawn]

I replaced the microwave above the stove a few years back. The vent runs through a soffet to the outside. I am getting a lot of cold air back through the vent. which isn't a new problem, but I'm now getting more concerned about it. The inside of the MW is absolutely frigid and the wall cabinets between the MW and the outside wall are very cold too. Where should I start to trouble shoot what could be going on and what are some probable fixes? Any advice is appreciated.

[atgcpaul]

I have the same situation underneath my kitchen sink that is on an exterior
wall. I can't get behind the sink cabinet to look, but I suspect that the wall
behind my sink is not finished with drywall and sits against the wall studs.
(Since buying this house, I have noticed some builder shortcuts) I know in
my house, there was no insulation in a skinny stud bay. I also suspect there is
no insulation behind my sink.

Maybe they neglected to insulate behind your MW. Also (this one's a guess)
maybe your house is actually "breathing in" from your MW vent. Double check
this, but maybe it's possible to use a dryer type vent that closes after the vent
turns off.

Paul

[mrojec]

When we put in the vent for our range hood some years back the exterior fitting had a spring loaded door that stays closed unless the fan is on. When the fan is switched on the force of the air opens the door and exhausts the kitchen fumes. I wanna say we got it at Home Depot or Lowe's. I can't remember precisely. That seems to be happening a lot lately...
Mark

[Cheeky]

I had a similar problem before I replaced the kitchen exhaust.

3 things that will make a world of difference:

insulate the entire run of the duct, and make sure all connections are properly secure (use zip ties, tape etc).

spray foam insulation around unsealed openings (ie soffet and drywall openings)

use an exhaust grill with a substantial flapper damper (you won't be able to pick these up at HD or Lowes, most likely you will have to get it online or a nice shop).

[crokett]

I just discovered that problem in my house this week. Why it took 2 winters to notice I have no idea. In any case, I talked to a friend of mine who owns an HVAC business. Range hoods are supposed to come with dampers. Mine doesn't have one, so next option is a damper in the vent pipe. You can either put one in-line (in the cabinet) or you can add one to the end of the pipe.

[Curly Qsawn]

Thanks all. I think I may need to go back through all my instructions for the microwave since it is the also the vent for the stove. Never thought I got much suction through it anyway. Perhaps I did not follow the instructions (naw, never happen!). I have also found this vent (http://www.batticdoor.com/dryerventseal.html) in case anyone else is looking around for a fix. I also suspect there is no insulation in the soffet where the vent pipe runs, but that will have to wait until a kitchen makeover to remedy.

[Jeffrey Schronce]

I have the same situation underneath my kitchen sink that is on an exterior
wall. I can't get behind the sink cabinet to look, but I suspect that the wall
behind my sink is not finished with drywall and sits against the wall studs.
(Since buying this house, I have noticed some builder shortcuts) I know in
my house, there was no insulation in a skinny stud bay. I also suspect there is
no insulation behind my sink.

Maybe they neglected to insulate behind your MW. Also (this one's a guess)
maybe your house is actually "breathing in" from your MW vent. Double check
this, but maybe it's possible to use a dryer type vent that closes after the vent
turns off.

Paul

Better check that wall out. Pipe freeze can occur when the wall is uninsulated, even in a heated home.