Microwave oven installed over a range top (C) InspectApedia Venting a Microwave Oven
     

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Microwave oven vent installation suggestions: this article describes the venting options for built-in microwave ovens and microwave oven-vent systems typically installed above a stovetop or range. We discuss the importance of venting to the exterior, choice of vent components and materials, and we refer to microwave oven manufacturers' installation instructions.

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Microwave Installation & Venting

Reader Question: how can I stop cold drafts from the microwave oven vent I routed into the attic?

Kitchen exhaust fan vent outside terminatinon - Nutone (C) Daniel Friedman

Sir, I have a installed microwave oven that is vented by an 8 inch sheet metal pipe into my attic space (no cap). how can I insulate it to stop letting my micro wave from getting all cold ? Thank you in advance for any help offered. - D.M. 1/28/2014

Reply: microwave oven vent to exterior wall: vent kits

I don't think what you described sounds like a good design - you are venting moisture into an enclosed attic space, asking for condensation, mold, lost insulation value and related trouble. Examples of this sort of problem are at

Our photo at left illustrates a side-wall vent termination for a kitchen exhaust fan. This same duct arrangement would suit a microwave oven - vent system.

[Click to enlarge any image]

If your microwave oven requires venting to the outdoors (I'd review the manufacturer's installation requirements) you would be better actually route it outside using the materials and sizes and routing recommended by the microwave appliance manufacturer.

For example a typical installation guide for a GE brand microwave describes mounting the appliance to a building wall (securing its mounting support to wall studs). The company gives three types of microwave installations:

  1. Outside top exhaust using a vertical duct vented microwave.
  2. Outside back exhaust using a horizontal duct vented microwave
  3. Recirculating, non-vented or ductless microwave installation. This installation requires the addition of a manufacturer-supplied charcoal filter accessory kit.

None of these recommended installations would approve what I'd call "pseudo-venting" of the microwave via a vertical duct but terminating inside the structure.

Microwave oven installed over a range top (C) InspectApedia

But other than the two very general parts sketches provided by the manufacturer (adapted and shown here) the installation instructions are silent about the duct materials, sizing, length, and routing.

One could guess that's because it's pretty obvious that while the microwave may have come set up for top exhaust venting (there are instructions for converting various parts to vent out of the back of the microwave), in a retrofit installation it is going to be hell to vent a microwave out through a first floor kitchen, up through a building, and outside through a roof, soffit or gable wall. Obviously in new construction it's easier to install vent ducting before the ceilings and walls have been finished.

Actually the microwave installation instructions I reviewed presumed that the vertical duct was already in place before the microwave was mounted. The instructions simply state

If that vent has to pass through your attic but terminates outdoors, perhaps down through a soffit or out through a gable end wall to avoid having to make a hole in the roof, the exit opening will need an exhaust-air-operated vent closure (as is used on dryer vents) not only to keep cold drafts from backing down into the occupied space but also to keep critters out of the vent system.

The top-vented microwave example given assumes that the vent runs straight up through the roof (see the roof cap in the sketch above). But an alternative worth considering if it does not make the vent run too long is to vent across the attic floor and down and out through a soffit or out through a gable end wall.

Microwave oven installed over a range top (C) InspectApedia

For any exhaust vent that is routed through a cold or cool space (like your attic) you will want to pay attention to these considerations as well:

Impoper dryer booster fan installation (C) D Friedman

Kitchen Exhaust Fan Companies

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