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.

Microwave Installation & Venting

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

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

• ATTIC CONDENSATION CAUSE & CURE
• ATTIC MOISTURE or MOLD INSPECTION

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.

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

• Extend the house duct down to connect to the exhaust adapter on the microwave.
• Seal the duct joints using [duct] tape.

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.

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:

• Un-vented microwave exhaust fans (and other re-circulating range hood fans) typically require an additional filter specified by the manufacturer
• The vent run length and diameter cannot exceed the capacity of the vent fan in the vented appliance or it not only won't work, it may be unsafe. Bliss points out that for kitchen vent systems
  
  For good performance, the total equivalent duct length, not counting the wall or roof cap, should not exceed about 30 feet. Equivalent duct lengths for common fittings are shown in Table 6-15.
  
  For example see BATHROOM VENT DUCT LENGTHS
• The vent materials must be approved for the appliance. I doubt that you'd want to nor be permitted to use plastic flex vent for a microwave oven. And we want to avoid a vent installation like the one shown in our photo.
• The vent must terminate outside the building, not in an attic and not in a crawl space (photo at left).
• The vent should be sloped so that moisture condensing in the duct drains out of the duct to the exterior, not back into the building where it may create a mold problem or water damage
• The vent termination should prevent animal entry as well as back-drafts.
• Insulation or insulated vent material may be appropriate for some installations to improve performance and to reduce condensation.  
• Take a look at our clothes dryer venting discussion for a very similar topic. CLOTHES DRYER VENTING for additional suggestions
• Watch out: Vertically-vented appliances from kitches may be a fire hazard, pointed out by Steve Bliss at KITCHEN VENTILATION DESIGN
• Follow the laundry dryer equipment manufacturer’s guidelines and safety standards such as those published by the National Fire Protection association (NFPA), the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) and the local code authorities.

Kitchen Exhaust Fan Companies

• Amana www.amana.com Pop-up and traditional range hoods
• Bosch www.boschappliances.com Chimney style, downdraft, and traditional range hoods, stainless steel and colors
• Kitchenaid www.kitchenaid.com Retractable downdraft, slide out, island, and traditional range hoods
• Broan-Nutone LLC www.nutone.com Nutone downdraft, island, and traditional range hoods
• Dacor www.dacorappl.com Commercial-style, stainless-steel pop-up and overhead range hoods; hood liners for custom canopies
• Gaggenau www.gaggenau.com/us Chimney-style, pop-up, slide-out, and island range hoods in stainless-steel, aluminum, and glass
• Jenn Aire/Maytag Corp www.jennaire.com Pop-up, under-cabinet, wall, soffit, island, and fans and hood liners for custom canopies
• Thermador www.thermador.com Updraft, downdraft, and island-range hoods
• Wolf Appliance Co. www.wolfappliance.com Pop-up, chimney-style, wall, and island-range hoods in stainless steel

...

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