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I found SECTION R408.3 of the International Residential Code which says that unvented crawl spaces are only allowed if one of the following is true:

  1. There is exhaust ventilation fan running continually
  2. The crawl space is air conditioned along with the rest of the house

Notably, this means that if you have a fully dehumidified crawl space, code would not allow it to be sealed off completely! This goes against a lot of recommendations, for example this one from buildingscience.com:

The crawlspace is either “vented” and “not conditioned” and connected to the “outside”…or…the crawlspace is “not vented” and “conditioned” and connected to the “inside”.

And by "conditioned" later it is clarified that dehumidification counts:

You have to provide air change between the crawlspace and the house or you have to provide dehumidification.

If conditioning crawlspaces with air from the house is not desirable install a dehumidifier. The dehumidifier handles the moisture control.

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  • Think because most floors above crawl space are not air tight. If any fumes build up in crawl space they can seep up to living areas, or explosive fumes do not escape enough. Both conditions in section allow air to be changed. – crip659 Mar 18 at 19:57
  • So, code requires you to dehumidify the neighborhood? Why even install a dehumidifier with your encapsulation then? – brentonstrine Mar 18 at 19:59
  • Crawl spaces aren't usually dehumidified. Fresh air ventilation is usually adequate. – isherwood Mar 18 at 21:07
  • Also if crawl space is sealed, no vents, then most likely moisture will build up to high levels. Most wood or steel beams/joists will tend to rot/rust a lot faster(10,20 years) instead of lasting for a long time. Venting can be as little as two holes on opposite walls. A dehumidifier usually costs much more running than having a small fan and one or two holes. Your power company will send thank you cards for using a dehumidifier in crawl space. – crip659 Mar 18 at 21:20
  • @isherwood this question is about encapsulated crawl spaces, which are often dehumidified. – brentonstrine Mar 18 at 22:10
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The reality of code is that it is written by a group of people who are influenced by tradesmen, lobbyist groups, government organizations, etc. Is the international residential code always going to allow for all acceptable solutions to all scenarios in all locations where it applies? Not by a long shot. They pick and choose methods, sometimes get things right, sometimes wrong (and hopefully fixed in a future revision as has historically been the case with a lot of requirements for basement insulation techniques).

If some new doohicky comes along where a company stands to make a lot of money from a code change to allow the use of the doohickey, a company can lobby the council in charge of the code (or if it’s a large company, they may have a sitting member on the council) and possibly get their special interest addressed.

Without official published reasoning behind the code being the way it is, the best we can do is speculate.

So, right or wrong, here is my speculation:

I think this scenario is uncommon enough to date where the council has currently settled on an “all unfinished” or “all finished” approach to the treatment of crawl spaces. You could make a similar argument about requirements for attic spaces.

I think one of the primary reasons could be that a standalone dehumidifier like you are suggesting is a small appliance not usually installed by a builder as part of new builds where air conditioning equipment is (toaster vs oven kind of thing). Since the dehumidifier is not built in, you could move out and take your dehumidifier with you and then the next homeowner would be none the wiser that a dehumidifier is required. Therefore, I think the requirements are in place to ensure the space will be addressed passively by vents or addressed actively by something expected to be a permanent fixture of the home that you or a future homeowner will notice if it stops working (a homeowner would likely notice if the furnace or air conditioning fails).

It is what it is for now, but if you or the wifi dehumidifier lobbyist group wants to go to bat, you could see a change in the next revision of the code itself.

Another option, depending on your plans and locality, you could always request special dispensation from the Authority Having Jurisdiction (e.g. local building department) and they might grant you an exception.

Also, you didn’t quote code verbatim and I do think there might some wiggle room for using a zoned air conditioning approach where the crawl space could technically be a different air conditioning zone. Again, this is open to interpretation and the Authority Having Jurisdiction has final say.

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  • Huh, so maybe with a WiFi-enabled dehumidifier that notifies me if it goes out, I'd be ok? – brentonstrine Mar 19 at 18:21
  • @brentonstrine Ask your AHJ. They will have the answer. You could also write to the IRC code council and request an exception for newfangled wifi dehumidifiers (since code is usually working off of bleeding edge technology). – statueuphemism Mar 19 at 18:57
  • Also, does that dehumidifier tell you via wifi when it has lost power (e.g. breaker tripped). – statueuphemism Mar 19 at 19:00
  • I think the app has a "deadman switch" e.g. if it hasn't received contact for a certain amount of time it assumes there's a problem. – brentonstrine Mar 19 at 23:44
  • Interesting thought about it being removable. Obviously some are more removable than others--but even the ones that need to be professionally installed only require a screwdriver to remove since they're plugged into an outlet and aren't connected to anything else (except maybe a length of duct. – brentonstrine Mar 20 at 12:21

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