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My house was built in 1976. I recently replaced the range hood and discovered that the piece of rectangular ducting that runs inside the cabinet is not connected to the outlet vent on the outside of the house. It effectively vents into the attic space under the roof. That attic space is not used for anything, and in fact it is not even accessible.

My contractor quoted 650$ to install the duct, which is not surprising given the difficulty accessing that space.

What are the pros and cons to installing the duct vs. leaving it as is? Out of curiosity, was it considered a standard practice in the 70's to skip a 10-foot section of ducting and let the cooking fumes dissipate in an attic space?

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    When I think crawlspace, I think under the house. So this is venting into the attic? – JPhi1618 May 6 at 19:54
  • Consult your favorite dictionary, or follow this link: merriam-webster.com/dictionary/crawl%20space I suppose, attic is a better word. English is not my first language. – user443854 May 6 at 20:01
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    The ICC Code requires all exhaust fans to discharge to the exterior of the building and cannot be discharged to the attic or crawl space. (BTW, I’d use ridged steel duct.) – Lee Sam May 6 at 21:21
  • If you need a 10-foot section of duct to do this then it sounds like your attic is large enough to require an access door. My guess is there is access either from outside or it's been covered with drywall. – Platinum Goose May 7 at 19:20
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Regardless of if this was ever standard, it's a bad idea. I lived in a 60's built house and the dryer vented under the house from a hole in the floor. The lint and moisture issues there were pretty bad to say the least.

The same issue is happening in your attic (crawlspace?). The moisture in the exhausted air can cause problems for sure. The air can also contain grease and other contaminants that will build up and cause... general grossness.

It's true that if you don't use the hood very much, it might not be a big deal, but if you want to to be done right, it needs to be in a duct from the hood, all the way until it exists the house through a wall or the roof.

  • The "grossness" argument does resonate. I am definitely inclined to do this right, but what gave me a pause was my wife asking: "if it's been like this for 40 years, why do you want to do it now?" – user443854 May 6 at 20:13
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    Honestly, it will depend a lot on the usage of the hood. Some people are going to cook dishes that will have more oily residue and the duct (or lack of duct) can get gross. If you're using the proper filters on the hood, that helps as well. The moisture can be an issue because it can't be filtered out, but if you're not using it a lot, the space will have time to dry out between uses if it ever gets damp in the first place. I assumed you wanted to fix it and needed reasons why. If it's working for you, I'm not sure I can justify spending > $600 to "fix" it. – JPhi1618 May 6 at 20:19
  • @user443854 Do note that the "general grossness" includes things that have food value to various things that you don't want infesting your house. This can be anything from mold to insects to rodents. How much is going to be available will depend on what's cooked over time. – Makyen May 7 at 4:06
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Not standard and never should be ducted into a confined space. The duct should exit the home with the shortest possible path and least bends possible, the contractor should of course have the proper license and insurance and the signed contract should specify the completion date and penalties for cost and time overruns.

  • Well, strictly speaking it is not confined. There is a vent. If this is prohibited by the building code, I would like to gain some understanding of the reasoning behind it, and what would happen if that was not followed, e.g. cooking oil deposits accumulating over the years and creating fire hazard. – user443854 May 6 at 20:07
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    I've never seen an exhaust hood vented into the attic, but yes, the grease would cause "general grossness" and could contribute to a fire, mold growth, or insect infestation. The water vapor would also condense and over time lead to mold growth and wood rot. Your exhaust hood should definitely be vented outside. Side note... check your bath fan too; they need to be vented outside for the same reason. – Eric Simpson May 6 at 20:12
  • Talking of licensing and all that, what sort of contractor/license should we be looking for when wanting to put a vent in the roof? A roofing contractor? – Zac Faragher May 7 at 2:26
  • U.S. fire departments respond to an average of one home fire every 88 seconds. Between 2012 and 2016, U.S. fire departments responded to an average of 355,400 home structure fires per year. These fires caused 11,670 civilian injuries, 2,560 civilian deaths, and $6.5 billion in direct damage. On average, seven people per day die in U.S. home fires. Cooking is the leading cause of home fires and home-fire injuries. Cooking (21%) is the leading cause of home fires and home-fire injuries. Cooking (21%) and heating equipment (19%) were each involved in one in every five home-fire deaths. – Bob the builder May 7 at 23:59
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Whatever you do, do not vent into an attic space under any circumstance. Gross grease aside, the humidity from your cooking will migrate into your attic space and condense, potentially causing moisture damage or mould, especially if your attic isn't vented. Why not block up your vent further up if you have access, and run a new duct venting to an adjacent wall?

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