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Community and Q&A

Sizing HVAC Equipment With Poorly Conditioned Crawlspace

finePNW | Posted in General Questions on

I’m in the Seattle, WA area (Climate Zone 4 coastal) with a minimally ventilated crawlspace (~1 ft2 vent area per 1500 ft2 floor space), with poly vapor barrier on the floor, R30 insulation on the crawlspace ceiling, and R12 rigid foam around about 2/3 of the foundation walls. The air handler, HRV, and all ducts (*mostly* separate between HRV and Air Handler) will all live in the crawlspace. Ducts will be hard metal wrapped in R8.3 insulation (3″ foil backed fiberglass). I’ve considered encapsulating the crawlspace and not insulating the crawlspace ceiling, but my building dept. says I’d need to put R10 as a 2ft perimeter covered in cement slab on the inside of the crawlspace to make this work… can this possibly be true? Are there any less expensive 2018 IRC-approved options?

Anyways, I’m not finding any options on the CoolCalc Manual J tool that fit this condition, and am trying to understand how to size my Heat Pump w/central air handler (no wall units) setup. Given the partial insulation and minimal ventilation, I feel like assuming ducting is in an “open crawlspace” is going to lead me to oversize, but am nervous that choosing “closed crawlspace” could lead me to undersize. Switching between the two puts me right on the line for a 32 kBTU/hr Mitsubishi hyper heat.

Is oversizing as a “buffer” poor practice here? Any ideas or rules of thumb?

Difficult crawlspace encapsulation requirements; concerns about duct losses in vented crawlspace.

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  1. walta100 | | #1

    If you stop using the word encapsulate and decide you will fully “condition” the crawlspace to more or less the same temp and humidity as the rest of your home you will stop looking for the free lunch of the risky semi conditioned space and suddenly your manual J calculations become clear the crawlspace is a room just like any other room you enter the values and it gives you the load.

    “my building dept. says I’d need to put R10 as a 2ft perimeter covered in cement slab on the inside of the crawlspace to make this work… “

    The insulation requirement sounds right but the part about being cover in concrete sounds wrong.


    1. finePNW | | #8

      I'm not sure what this "free lunch" business is. Our building inspector explained what we wanted to do as 2 separate things: 1) "encapsulate" crawlspace by closing all natural vents and adding mechanical ventilation and 2) "condition" by either facilitating mechanical air exchange with the habitable space or adding a separate conditioning element in the crawlspace. All I'm trying to do is convey their statements & conditions and understand how to size my HVBAC according to the fact that my budget can't accommodate meeting the city's stated conditions for properly conditioning the space under our permit (slab-protected 2ft perimeter of foam insulation laid on the crawlspace floor). The other information below has been quite helpful, so thank you for getting the conversation going.

  2. Expert Member
    Michael Maines | | #2

    Spaces that aren't fully indoors or fully outdoors are impossible to model accurately, moisture-related issues often occur and energy is usually wasted. I agree with Walta that you should fully condition you crawlspace, especially since you have equipment down there. The 2018 IRC requires R-15 continuous insulation on crawlspace walls and no insulation on the crawlspace floor. ( Even in CZ6 we aren't required to insulate crawlspace or basement floors. Maybe there's a local amendment or a stretch code he's referring to? You can ask him to cite the code reference. It might be a good idea to insulate the floor but in your climate zone it probably doesn't make a huge difference.

    1. finePNW | | #25

      Ah, I just realized it was you responding. Love your book -- I gave it to my contractor (once he became interested in the topic) and my plans examiner said he was familiar with the concept of a "Pretty Good House" when I told him what I was trying to do. So thanks for everything! Do you ever get out to the PNW for conferences, events, etc.?

      1. Expert Member
        Michael Maines | | #29

        I'm glad the book was helpful! And it's great to hear that even building inspectors are hearing about the concept.

        I love the PNW but don't get out there often. We're overdue to visit my wife's uncle in Seattle but he's coming here to Maine for a visit in a few days so it will be at least a couple years before we get out there again.

  3. walta100 | | #3

    Michael, I don’t read code books for a living, what is the part I painted red trying to tell us if not that the perimeter of the foundation requires 2 feet of R10 in zone 4?


    1. Expert Member
      BILL WICHERS | | #4

      If you read the rest of those code book pages, the implication is (from R402.2.10) that that "slab" R value requirement is for slab on grade floors.

      R402.2.11 "Crawl Space Walls" implies that insulation is needed only on the wall, as long as it can go at least 24" down vertically below grade. If it can't go that far down the wall (i.e. the footing is shallow), then it would need to extend the rest of the way inwards. That section also specifically mentions "exposed earth", and talks only about the need for a vapor barrier. That makes me think the intention is that if you can insulate the crawlspace walls from the upper level floor down at least 24" below grade, then you don't need any insulation on the floor of the crawlspace itself.

      You may try pointing this out to your local inspectors. If they are following the IRC, they shouldn't be requiring the extra insulation. They may have local codes though that are over and above what the IRC requires. Just be careful if the inspector starts doing the "not in MY city" stuff though, which usually means they're making stuff up.


      1. finePNW | | #9

        Thanks Bill. We do not have 2 feet of stemwall and/or footings, and I imagine we don't want to dig down past the footings even if vertically as my understanding is that weight is distributed at a 45 degree angle from the bottom of the footing. Is that right?

        The 2ft-inward of perimeter foam on the floor is totally doable, but they seem very uncomfortable with me protecting that foam with anything other than a slab. Do you know if there are other commonly used means of protecting ground-laid foam insulation? Is it crazy to attempt to use mineral wool (laid on top of our 10 mil poly vapor barrier) instead of foam, and without any protection?

    2. Expert Member
      Michael Maines | | #6

      Bill is correct; I used to think that "slab" meant any crawlspace or basement slab but that's not the case, it's only for slab-on-grade designs. I wish it was more clear and it took me a long time to understand what they really meant.

      1. Expert Member
        BILL WICHERS | | #7

        If the code book was clear, they wouldn’t be able to sell their companion code explanation books :-D

        The electric code is similar, constantly referencing other sections of the code for exceptions. I think that the footnote for the “slab” part here really should mention “slab on grade” or some such to clarify that it’s for that type of “foundation”, and not just for slabs in general.


        1. finePNW | | #20

          Thanks Bill,

          Believe it or not, that was almost verbatim the discussion we had, except I did not have a list of absurd things to suggest... got any? I'll try again!

          I think my only viable option at this point is to find a local expert or a university-related person (or something), get their opinion, and then directly site them at "open counter hours" down at the planning dept. The plans office seems to (rightfully, in most cases, I'm sure) treat every conversation as if their immediate interpretation is correct or else you need to pay an architect to write up plans and spend 6 weeks in queue to get a more in-depth answer.

          They did basically say "you don't need to do anything for your existing crawlspace, but in the addition, the crawlspace must have X,Y,Z (including rat slab perimeter)." The problem is we're already poured and dried in, so it's like we're an existing structure, but the code rightfully still applies as if we are a new structure (to that half of the house that is addition). Perhaps this 2ft depth / floor insulation makes more sense to you in that context? Or maybe the plans examiner is still just likely wrong. I'm just hoping to nail this down before we decide to spend money insulating the crawlspace ceiling and keeping a vented crawlspace -- I'd much rather spend that money insulating the crawlspace walls and adding mechanical exhaust to properly condition the space.

          1. Expert Member
            DCcontrarian | | #21

            My interpretation of the "2 foot depth" is that it means 2 foot width -- you need R10 for 2 feet around the perimeter.

            I get why he's saying it needs a rat slab. Touching the ground most people will use some form of polystyrene, either XPS or EPS. Polystyrene is highly flammable, it needs to be covered with something fireproof. Concrete is usually the most practical choice. So it follows logically from needing insulation.

          2. finePNW | | #22

            Reply to #21 (DC Contrarian),

            That's a good point. Perhaps I should look into alternative fire-protective coverings. I will do some research later tonight, but any idea off the top of your head if foil-backed polyiso typically requires the same fireproof protection? If I went with mineral wool, I wonder if it would require any protection (other than the 10 mil poly ontop of [or below?] which it would be laid.

            And I think you're right about width here. If not depth, then width -- or depth as far as possible + remainder as width, as I've gathered from another comment here.

          3. Expert Member
            DCcontrarian | | #23

            There is a brand of polyiso called Thermax which is fire-safe. Usually the problem is that polyiso isn't rated for ground contact.

          4. finePNW | | #24

            Thanks DC. I'll shoot them an email.

          5. Expert Member
            BILL WICHERS | | #27

            A rat slab serves no real purpose if just around the perimeter. The insulating skirt along the ground doesn't do much either once you're down near the frost line.

            I think your plan examiner is just not correct on this, and is not interpreting the code correctly. You could try calling up a state-level building guy, then ask the same question. If the state guy says "naw, you don't need insulation on the ground here", then ask the state guy to call up the local guy and educate them. That would solve your problem here. Just make sure to do it in a nice way.

            Crazy situations would be thinks like insulating basketball courts, garage slabs, carport slabs, pretty much any other slab you can think of that wouldn't normally need any insulation at all.


      2. finePNW | | #11

        Thanks for these clarifications Michael and Bill. Unfortunately, the plans examiner seemed pretty focused on the "slab" wording, but was amenable to me drawing and submitting an alternative approach for "protection" of a 2ft strip of insulation laid on the ground at the perimeter... but we're so many permitting delays deep at this point, I'd like to find an approach that allows me to just do it and have the inspector examine it and approve (ie., something fairly bullet proof in its relation to the code). I'm thinking my next steps should be considering a non-foam insulation for this 2ft on-floor perimeter of insulation and contacting EPS/XPS/Polyiso manufacturers and asking about viable protection materials for such a scenario.

        1. Expert Member
          Michael Maines | | #16

          Is it required to have concrete on the ground? It isn't in the IRC or anyplace that I've worked. No slab = no requirement, if the plan reviewer is going to be pedantic.

          You can buy the code commentary which should spell out what they really mean. I have not done so. I've actually tried (through the i-codes) but for some reason they don't make it easy.

          1. finePNW | | #18

            Interesting. So, if I've got this straight, you're suggesting I can condition and mechanically vent my crawlspace so long as the crawlspace walls are adequately insulated, even if there is no below-grade insulation (there are no exposed below-grade walls in my crawlspace) and no insulation in the crawlspace ceiling?

            The plans examiner was clear about 2' perimeter of rat slab over insulation... but I don't see specific language to this effect in IRC and, to be honest, he pointed at the word "slab" and said 'this is the minimum requirement and all footnotes are simply for weird exceptions and don't apply to your request to condition your crawlspace'. As Walter points out, Table R402.1.2 seems to suggest 2' depth of insulation is required, though. Is that not your interpretation?

          2. Expert Member
            BILL WICHERS | | #19

            Reply to post #18:

            I think the plan examiner is confusing the "slab", that is supposed to mean a "slab on grade", with any generic concrete "slab". If that was the case, you'd need to insulate under the edges of the slab floor in your garage, the slab on a backyard basketball pad, etc. etc. etc. Maybe point that out in a nice "are you SURE that's what this means, because it would also then apply to these other [insert list of ridiculous situations] then." kind of way.

            Dirt floor crawl spaces are pretty common, and there is no good way to insulate them without opening yourself to other issues (thermal/ignition barriers for foam, etc.). Normally only the walls are insulated, the floor just gets a vapor barrier of at least 6 mil (although 10+ mil is much better).


  4. freyr_design | | #5

    Seattle specifically has a number of amendments pertaining to the codes that cover these areas. I would link directly to the code snippets but unfortunately UpCodes isn’t letting me (usually does). Here are some links to the general chapters.
    ^look at 1202.7
    ^this ammends the standard irc definition for unvented requirements.

    I believe they would consider a conditioned crawlspace a plenum crawlspace (even though it’s not exactly that?) as they removed the conditioned section of code and replaced with a exhaust vent requirement. It doesn’t seem like a slab is a required feature, though aggregate and radon vent are.

    1. finePNW | | #10

      Thanks! Very helpful. Unfortunately, half of this project is new construction, which I believe takes the "plenum" approach off the table.

  5. walta100 | | #12

    I think hand digging the perimeter of an existing crawlspace to 24 inches seems almost impossible making the 24 inches of horizontal insulation the only real option.

    Is their code language that would require the horizontal foam to be covered in concrete as the inspector is asking for?

    “Thanks! Very helpful. Unfortunately, half of this project is new construction, which I believe takes the "plenum" approach off the table.”

    Why do you think this is true? I think you will find code will require you to condition the crawlspace or vent it nothing in between. Conditioned crawlspaces are treated like plenum because they will have supply and return registers and in the event of a fire the smoke from the crawlspace will enter the living space.


    1. finePNW | | #13

      "Is their code language that would require the horizontal foam to be covered in concrete as the inspector is asking for?"
      I have not been able to find it -- I'm just going off of the plans examiners' statement, which in my experience with the City on this project has been more important than code given they simply say "well, precedent of our department's interpretation is X, even if X is not strictly stated or IRC overrules it." I could dig further without a potential alternative (aside from "no; I don't need to protect it") in hand at the risk of more delays.

      "Why do you think this is true?"
      From section M1601.5 (M1601.5 Under-Floor Plenums): "Under-floor plenums shall be prohibited in new structures. Modification or repairs to under-floor plenums in existing structures shall conform to the requirements of this section."

    2. finePNW | | #14

      "I think you will find code will require you to condition the crawlspace or vent it nothing in between."
      Interesting. The city indicated mechanical ventilation was OK regardless of conditioning via WAC 51-51-0408 R408.3, though conditioning required mechanical ventilation. Perhaps this is not in effect until Oct 29, 2023 and your interpretation of the Code is currently true?

      EDIT: Actually, allowance for unvented unconditioned crawlspace does seem to be allowed under the current state of things in WA:

      1. freyr_design | | #15

        Seattle and Washington only allow for the mechanical exhaust option, the irc also has an option to provide conditioned air to the space or dedicated dehumidifier. The mechanical continuous ventilation does not seem like a good option as it will draw and exhaust interior air, it would be better to just have a vented crawlspace, no?

        To my knowledge conditioned crawlspace are not treated like plenums generally, as plenums have a whole other combustion requirement.

  6. finePNW | | #17

    For other folks in the PNW, this is an interesting article: Attached for posterity, since a lot of links in old posts eventually end up dead. Abstract below.

    In the discussion, they state "Modeling on these homes suggests that the energy performance impacts of conditioning the crawlspace (thereby bringing the ducts within conditioned space) is minimal, and only when supply ducts in the crawlspace are quite leaky (Lubliner et al. 2007)." and "There also seems to be confirmation that the heat loss even from tight, well-insulated R-8 (0.71 W/m2K) ducts has a warming affect on the crawlspace temperatures that helps to reduce the relative humidity. While one would expect an energy penalty from this heat loss from the ducts, eliminating the ducts from some crawlspaces could increase the wetting potential." Interesting indeed!

    Ducting in the crawlspace will also, intuitively, bring more of the nasty crawlspace air into the house: "The tracer gas data seem to confirm the intuitive idea that there is more air movement between the crawlspace and the house when ducts are located in the crawlspace. This could have a negative impact on indoor air quality by providing a pathway for pollutant sources in the crawlspace."

    And, of course, without true ventilation, Radon values go up quite substantially.

    Vented and conditioned crawlspaces in the marine and cold climates of the Pacific Northwest are evaluated. Temperature, relative humidity, wood moisture content, dew point, radon levels, and air change rates were monitored for up to 2 years. Four new homes in each climate zone were constructed to International Residential Code standards. Homes were configured with vented and conditioned crawlspaces, with and without duct systems located in the crawlspaces.

    Testing showed that roughly 40% of the house air entered from the vented crawlspace. Power-venting a conditioned crawl- space reversed the stack effect, to where less than 6% of the house air was entering from the crawlspace. Air change rates between the crawlspace and the outside were roughly 4 times higher in vented crawls, providing significantly higher dilution rates for potential pollutant sources.

    Temperatures were higher and more stable in the conditioned crawls; wood moisture and relative humidity were lower. While more variable, conditions in the vented crawlspaces rarely reached dew point or exceeded acceptable levels. Radon levels in the conditioned crawlspaces were roughly three to ten times the level of the vented crawlspaces.

    Implications for indoor air quality, duct placement in crawlspaces, and overall performance in heating-dominated climates are discussed."

  7. walta100 | | #26

    Let me see if I understand you don’t like your old fashion vented crawlspace because it is too inefficient and cold and the new plan is to power vent the crawlspace by install a blower suck the warm moist air out of the crawlspace and blow it outdoors? Seem to me the blower will move more air than the old vents ever did and 100% of the warm air expelled will be replaced thru the gaps in the buildings envelope with cold air from outdoors. Yes that option is in the code but it seems like only a fool would select it.


    1. finePNW | | #28

      You don’t understand. Apologies if I haven’t made it clear enough.

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