It would be hard to find someone who didn't think you should encapsulate. At least in my area.

Thank you. As someone who only has experience with basements, I wasn’t sure if this is a gimmick that only makes the crawlspace look nicer, or if it is something that is essential. Sounds like even if a gravel floor and a vent are code, that it is not best? Any additional comments are appreciated.

The term "encapsulation" can mean a number of things. I prefer the term "conditioned" crawl space to differentiate it from a vented or "unconditioned" crawl space (plus it's consistent with the language in the International Residential Code.)

The prescriptive code gives a number of ways to meet the requirements for a conditioned crawl space. With respect to whether you should construct a conditioned crawl space, consider the size of the addition relative to the total area of the 1st floor, and whether the rest of the foundation is basement, vented crawl space or conditioned crawl space.

Thank you. 1st floor 2000 sq ft with full basement (half of it finished). Addition will be 300 sq ft.

I don't really know what encapsulate means, but I have a craw space under a room that was converted from a one car garage. When they built the room they put vents on the east and west side. It has been a convenient way for mice to get into the space. I wish they had not put those vents in, but rather opened up the crawl space to the existing basement for ventilation.

The crawl under our kitchen, laundry, and sunroom is gravel. No heat. So those spaces are freezing in the winter.

Since the crawl space foundation of the addition would communicate with the basement foundation of the main home, I'd favor constructing it as a conditioned crawl space.

Thanks for sharing your experiences. I am not sure about the crawl space communicating with the rest of the basement; would that mean a hole would be cut in existing foundation to allow air flow between the two?

Encapsulate and treat as a conditioned basement. Sealed, insulated, air supply & return. See https://www.buildingscience.com/documents/published-articles/pa-built-wrong-from-start/view

While installing a return air path to return air from the crawl space to the air handler is one prescribed method in the 2018 IRC for a conditioned crawl space, I'd advise against it. The last thing you want to do is to introduce anything picked up in the crawl space (soil gases, mold spores, insects or their droppings) back into your HVAC system so it can be circulated throughout your home. The supply-air-only option is the better one.

Received clarification from builder. Will install crawl space access door from existing basement, with vent in door, so finished basement at that location can help. Can also supply air duct into crawlspace. Suggested concrete floor with vapor barrier underneath as alternative to encapsulating. Will not vent to outside. Thoughts?

We've been building homes with conditioned crawl spaces since 1999. We supply conditioned air to the crawl space, but with no return. The last thing you want to do in my opinion (and the current IRC allows it) is to return air from a crawl space to a living space. Think about it: you'll return whatever soil gases (e.g., radon), insect parts, dust, mold spores, etc. that might be in the crawl space so it can be breathed by the home's occupants.

It's not really possible to pressurize a crawl space if you supply the code-required air flow of 1 cfm/sq. ft. of floor area (a 2,500 sq. ft. 1st floor footprint would only require 50 cfm which is the flowrate of the smallest bath vent fan on the market.) Code requires a vent to the exterior, but we've found a vent to the exterior to be unnecessary. Check with your local building official.

The concrete floor is not necessary; just make sure you've got a good, heavy-duty vapor retarder.

I prefer the term "cellar" -- and you'll want some crawling headroom, if at all possible. so it is easy to do what is necessary to install and maintain wiring and plumbing and heat/ac and to cover the soil in the area with material that will prevent moisture from the soil in the cellar adversely affecting the house.

Not a pro but in my (albeit limited experience) air flow is essential and especially designed concrete block sized windows/screens/vents built into the concrete block exterior cellar wall can often provide this without any need to blow air into the cellar. (Extreme temperatures may be the exception) There are, however, fans designed for providing air flow in the cellar and having an outlet in the cellar that can be controlled -- turned on/off -- from an upstairs switch, not just for a fan but also lighting as needed (such as one installed by the electric panel), would be a good idea. A fan that can be changed as needed over time rather than a built in system costly to change would be my personal preference ... but then, I'm not a pro.

The crawl space of my addition is built to code with insulated walls and vapor barrier under floor. It's dry as a bone, unlike the rest of the house that has had a wet floor during all of the recent rains. I put one HVAC supply register into the addition to meet code, but have never opened it as it doesn't seem to need it.

Thanks for all input. It looks like we will proactively waterproof with sump pump, then have a vapor barrier and a gravel floor. Architect said no need to install additional barrier over gravel. Will have air supply in crawlspace, with option to install dehumidifier later if it is too humid.