GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How to properly insulate a vented crawl space?

David Kenesson | Posted in Energy Efficiency and Durability on

From the articles I’ve read on this website I know most people prefer a basement, slab on grade, or a sealed crawl space to a vented crawl space. Unfortunately a basement isn’t an option due to high ground water and apparently in Idaho (where the new build is taking place) the vented crawl spaces out perform the sealed crawl. There aren’t any slab on grade homes in the area and our builder isn’t familiar with them, so we are trying to avoid it if possible.

So it sounds like we’re stuck with a vented crawl, but all the insulation info I see is about SEALED crawl spaces. Does anybody have any advice for properly insulating a VENTED crawl space? The walls of the house will be in the R35 neighborhood and the roof should be around R50 so I want to insure the floors are in the same league as the rest of the house. There will probably be traditional duct work and water pipes in the crawl space. The house is in climate zone 6 in eastern Idaho.

There is so much conflicting info about crawl spaces I don’t know where to begin, any advice would be helpful. Thanks.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. charlie_sullivan | | #1

    High ground water and a vented crawl space doesn't immediately sound to me like good combination. And I'm not sure I can think of a climate condition that would lead to a vented crawlspace outperforming a sealed crawlspace. But It's a pretty long way between the opposite corners of Idaho and I think the space between includes a pretty wide range of altitudes so there must be a reasonably wide range of climates in there. And one of those might be a scenario I haven't thought about before. Could you offer a zip code or more description of the climate? And does the idea that vented crawlspaces perform better in in Idaho come from a source worth telling us about, or is it just what your heard people saying?

    If I did want to insulate above a vented crawlspace, I'd think about I joists to reduce thermal bridging, and either make them extra deep so the thermal bridges would not conduct as much heat (and allow room for more insulation), or use mineral-wool boards underneath to get a layer of continuous insulation without thermal bridging.

  2. David Kenesson | | #2

    Thanks for the response. The house will be built near Warm River, ID 83420. The article on this website "Building an Unvented Crawlspace" led me to an article in Home Energy Magazine, both articles seem to suggest that vented crawlspaces work well in dry, western climates. Also, all the homes being built in the area that don't have basements have vented crawl spaces, but this may just be because builders have been building the same way for 30 years. Would you suggest doing a sealed crawl space?

  3. morganparis | | #3

    With R35 walls and R50 roof you are clearly aiming for a high performance building enclosure, in which case it makes little sense to run ductwork and plumbing in a crawl space open to the atmosphere. Creating a well insulated unvented crawl is not difficult or expensive, the technology is well established and even if builders in your area are unfamiliar it's not going to be hard to learn. The multiple benefits of a clean, dry, conditioned mechanical space with easily accessible plumbing and electrical under the whole of the house should be easy to appreciate.

  4. user-4053553 | | #4

    Pier foundation?

  5. charlie_sullivan | | #5

    I'm not sure I've read the very same articles, but my understanding is a little different--my understanding is that is a humid climate, a vented crawlspace is likely to create a serious problem, whereas in a dry climate, either can be OK. But just because it's OK in your climate doesn't mean it has any advantages.

    Your climate has warm days and cool nights in the summer, and low summer humidity, and low dew point temperatures year 'round. That means you really don't need to worry about moisture problems for most building types and locations. However, building in an area with water table could result in some moisture so a vapor barrier on the ground and protection against moisture wicking up the foundation walls are details to pay attention to regardless of whether the crawl is vented.

    Given that you have both options, how do you decide? The main advantage of unvented is that a given investment in insulation can get you to a higher overall insulation level, because the ground isn't as cold as the outside air, so the ground doesn't need much insulation, and the walls are small so you can buy super-thick insulation there for much less money than it would cost to put insulation that thick on the floor...and you lose less heat because the area is smaller.

    The secondary advantage is you then have a reasonably warm space where you can run pipes without them freezing and perhaps even run ducts if you want an ERV or HRV system.

    Hmm, "Warm River". Does that mean you have hot springs around? Are there opportunities for geothermal heating?

  6. GBA Editor
    Martin Holladay | | #6

    If you plan to install any ducts in your crawl space, it certainly needs to be sealed and conditioned.

    If you go ahead with your plan to vent your crawl space, here is a link to an article that describes how you should insulate your floor assembly: How to Insulate a Cold Floor.

  7. Brian Knight | | #7

    I'm unfamiliar with common practices in your area but skeptical that builders only doing vented crawlspaces are achieving the total R values you are expecting. Suggest getting third party blower door testing as it's usually a better measurement of overall energy performance.

  8. Expert Member
    Dana Dorsett | | #8

    Air sealing a vented crawlspace is a lot tougher than a conditioned crawlspace. Deep subsoil temps in that area average about 45F, but proximity to local volcanic zones can be more than 20F warmer than that. You may be able to better estimate your actual subsoil temps from county or state information resources.

    It's a dry enough climate that venting the crawlspace is not a humidity problem the way it can be 1500+ miles east of ID.

    The total amount of insulation required to hit a high performance level can be higher with a vented crawlspace than a conditioned crawlspace, particularly if you're under a local volcanic warm spot. An expose floor in a pier foundation or highly vented crawlspace needs to have a whole-wall R value comparable to your R35 walls, but a sealed conditioned crawl can hit the same performance levels with about half that for foundation wall-R, plus1/4 that for slab- R (assuming you're not over a warm spot.)

    To hit R35 whole-wall floor with 2x10 joists would require R30 between the floor joists and R5-R7 of continuous insulation covering the under side of the floor joists, but that's not a sufficient R-ratio to protect against moisture accumulation in a zone 6B climate. It really needs to be treated more like the roof ratio to hit adequate dew point control, which would be R20 in the cavities, with R20 continuous insulation on the under side (or at least R25 cavity, R16 continuous if it's a minimally-vented crawlspace.) It may be cheaper (and easier, and certainly more reliable) to pour the foundation stemwalls in a minimalist 2" + 2" or 2.5" + 2.5" EPS symmetric insulated concrete form, with 2" of EPS under a vapor barrier & 2" rat-slab. Depending on the height of the crawlspace and the total floor area it may even be less foam (including the ICF foam.) But that would allow you to run the electrical, plumbing & mechanicals in the crawlspace with much less concern about high quality air sealing of the floor penetrations.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |