Insulation around Microwave hood vent

Submitted by RatArmy t3_yhr6ww in DIY

Last Winter, my first as a homeowner, I noticed my newly installed microwave was letting in a lot of cold air. In the small space in between the back of the microwave and the outside wall, cold air was just pouring through; like a small waterfall of frigged air. I forgot about it all Summer, like and responsible adult, and I want to address it before Winter hits.

There is a gap between the outside wall and the interior wall with insulation, but the area directly around the hood vent is open to the outside wall with no insulation on it. The vent is very short as it goes outside directly behind the microwave.

Can I safely put insulation against the vent? My only concern is a fire if the vent gets hot, but I'm pretty sure the hood vent doesn't get nearly hot enough, but I'm not 100% sure. I was thinking that spray can foam. I've seen cans list the products Service Temperature, should I be going for the higher temps? I've seen some cheaper ones list 194°F (90°C) and the more expensive ones list as high as 240°F (115°C) Should I be going for the highest? Is there a risk of fire, or am I over thinking it?

Lastly, there seems to be no sealant around the exterior vent, likely also contributing to the air getting in. I'll have to seal that. My plan is to use some sort of silicone sealant, probably DAP.

Below are a couple of pictures of the microwave vent both interior and exterior.

Interior Vent 1

Interior Vent 2

Exterior Vent 1

Exterior Vent 2

Exterior Vent 3

Update:
Spent some time last night researching different insulation brands / types. Come to find out, I have this in my basement. GE Insulation Foam

The previous owners left a good amount of similar cleaning & gardening supplies, and a bunch of paint cans. The spray can is not that old either, it has a production date on it.

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Comments

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Teamfreshcanada t1_iuflpz5 wrote

You can safely insulate against this vent, it's not a fire hazard. Spray foam will insulate and seal against drafts as well

For outside, use a silicon sealant rated for outdoors.

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RandallBagsyFoot t1_iug8rrn wrote

HVAC guy here. Looks like the Microwave vent was added either as a renovation or as a last minute fix during construction. It should be buried in the studying, not sitting atop it. That vent won't reach high temperatures at all, so any sealant is fine. Depending on your climate, insulating the vent with pink fibreglass could be a good idea. #1 to prevent condensation dripping down the wall cavity and growing mold and #2 for heat loss.

***Ah, "newly installed microwave" I see.

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RatArmy OP t1_iuhpo5r wrote

Yeah the microwave itself is new that I'm putting in. But there was an older microwave there and the vent was previously installed.

When I took the old one down, that's what I noticed that the vent was basically uninsulated.

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GhostofDan t1_iufhqra wrote

Definitely put some insulation in there. It looks like there's some now, but got pushed aside during installation, and was never replaced.

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RatArmy OP t1_iufl9cz wrote

Yea their is some in the wall, just not right around the vent. It's closer on the sides, but the top and bottom has much more empty space.

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hicow t1_iugc5e4 wrote

Did they knock the hole through the drywall with a hammer?

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kavisiegel t1_iuhor56 wrote

Spray foam it with low expansion foam (sometimes called low expansion foam) then come back the next day and trim it flush to the wall and the inside of the vent.

Then use some foil flashing tape from the pipe to the wall.

Then put some thick self stick foam gasket / door seal on top of the foil flashing tape

All 3 together gets you insulation, an air seal from the outside to the inside to prevent air blowing through the gap between the drywall and the vent, plus a seal between the vent and the microwave, to prevent air blowing through the damper and then down behind the microwave

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RatArmy OP t1_iui3k84 wrote

After looking at my set up, I'm not sure if the stick foam would fit around the vent without impeding it. Unless I found some thinner stuff. But I'll poke around.

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RatArmy OP t1_iuia8wu wrote

I assume non foil flashing tape will suffice? My local store has a bunch of 3M flashing tape, non foil I think.

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decalsocal1 t1_iuhq2ha wrote

Non- expanding foam is sufficient. There is no temperature concern for the exhaust. The vent outlet at the back of the microwave keys to the wall vent metal so don't over do the foam to the point that the vent metal is deformed. Clean up excess foam when it has dried. BPI certified general contractor.

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Mattums t1_iuii5yf wrote

After our vent was installed by our contractor’s assistant, we noticed a lot of cold air even with the insulation and silicone they used. Eventually I found that the outside vent flap wasn’t closing properly and it was noticeable on cold, windy days. I bought a vent with a magnetic flap that doesn’t rely on springs to close and it works really well now.

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RatArmy OP t1_iuhrxcs wrote

Thanks for all the helpful response folks. I really appreciate them.

Spent some time last night researching different insulation brands / types. Come to find out, I have this in my basement. GE Insulation Foam

The previous owners left a good amount of similar cleaning & gardening supplies, and a bunch of paint cans. The spray can is not that old either, it has a production date on it.

Unless this brand is complete garbage, I guess I'll use this around the interior vent. I'll have to get some sealant for around the vent's exterior myself. Can't find everything in the basement I suppose.

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FLTDI t1_iufjuu6 wrote

I'd use spray foam. Either temp range, is fine for exhaust air.

It'll help, but that vent isn't insulated at all, thin sheet metal has essentially no R value. I wonder if you can find a better version?

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RatArmy OP t1_iufm0x2 wrote

You mean changing out the vent itself? I suppose I could look into that. I'll have to see how it's secured in.

I'll look into it.

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Timbucktoooooo t1_iug4tte wrote

Don’t waste your time looking for an “insulated” vent. There’s no such thing. Just insulate around it inside the wall and make sure there’s a door on the outside part of it that keeps the air and critters from coming in but opens when the fan is blowing out.

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