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Insulation in floor joists of an encapsulated crawlspace?

meganwood | Posted in Energy Efficiency and Durability on

Hi! We’re currently in the planning/permitting phase of building an high performance home in the Southeast. We’ve employed a green building specialist to help model our house to find the proposed hers rating and put into effect all the awesome things we’ve been reading about on gba. But our contractor (general, traditional) is giving us some push back about one of his recommendations. The green building specialist recommended we put rock wool insulation in the floor joists above our encapsulated crawlspace based on his computer modeling of our home. Our contractor said that’s unheard of and with an encapsulated crawlspace we should use no insulation. We’re puzzled because wouldn’t we want to utilize the generally cooler temperature of the ground since we’re in the South? And not worry as much about having cold floors in the mild winter? HELP!

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  1. GBA Editor
    Martin Holladay | | #1

    If you insulate your crawl space walls, bringing your crawl space into the conditioned volume of your house, there shouldn't be any need to install any insulation on your crawl space ceiling.

    Here is a link to an article with more information: Building an Unvented Crawl Space.

    If I were you, I would ask your green building specialist a few more questions about why he or she recommends installing insulation between the joists. I can't imagine that there would be energy savings -- certainly not enough to justify the cost of the extra insulation.

  2. Andrew_C | | #2

    I suggest that you use the Search function at the top of the page to look for Martin's recent article on crawlspaces: "Building an Unvented Crawl Space". Once you've read it, you can look at the numerous links to similar articles that are listed on the left-hand side. That should give you all the information that you need.

    On to the opinions...assuming you really want a crawlspace (?), encapsulating it is a start. But you almost certainly want to insulate the walls of the crawlspace instead of the bottom of your floor. And you really don't want moisture laden air to be able to reach the floor you've insulated, so if you do insulate the floor, you shouldn't use air permeable insulation (fiberglass or mineral wool) by itself.
    Better way to think about it: you've encapsulated the crawlspace, it should be inside the conditioned airspace of your house.

    If you're still in the design phase and you can't eliminate the crawlspace (by making it a basement or a slab), at least make it tall enough that you can easily move around down there. A rat slab (2-3") of concrete will make everything forever more pleasant, and a few permanent lights as well.

    But start with the referenced article, and then read some of the links.

    Good luck with your project.

  3. Andrew_C | | #3

    I see that I was so slow in typing that Martin responded first. In which case, my answer is..."what he said".

  4. meganwood | | #4

    Thank you! I did read the article first, I didn't see anything about insulating the crawlspace ceiling in it but I wasn't sure if it was a given or not. I will definitely ask about why this was recommended.

  5. LucyF | | #5

    I spent many hours crawling under the house we built insulating the floor joists. It is extremely difficult to do correctly. Our house was built on piers rather than a crawlspace so we had to insulate under the floor, but only someone highly motivated (or short of money and had to do it themselves) would do it right.

    As Martin says, it is far easier to insulate the crawlspace walls well. And in terms of the area to insulate, the area around the house is much less than the area of the floor so it should be less expensive. Andrew is correct that you want the crawlspace ceiling to be high enough to maneuver. That's always a good idea.

  6. dsmcn | | #6

    Make sure your access door is large enough to get sheets of rigid insulation through it, if you plan to insulate the walls. If part of it is below grade, the well around the access also needs to be large.

    Also, be sure to make the door airtight as well, with threshold and weatherstripping just like the entry door, and a latch that will compress the door against the gaskets. And why not insulate it with some rigid foam? A piece of 1/2 plywood scrap cut to fit and painted, and closed with a hasp, is not adequate for a sealed conditioned space.

  7. Expert Member
    Dana Dorsett | | #7

    The exact location / deep subsoil temps are an important factor on whether you'd want to insulated the floor of the crawlspace or not, but there is no reason to insulated between the joists if the crawlspace walls are insulated. If your subsoil is between 65F and 73F you are in the Goldilocks zone (most of US climate zone 2), and would be better off being "earth coupled", with no insulation on the crawlspace floor, with the foundation walls insulated down to the footings:

    Depending on your performance requirements and thermostat settings you can expand that range a bit, but at ground temps above 78F or below 58F there is an energy use advantage to at least some insulation on the crawlspace floor.

  8. LucyF | | #8

    Where is your general location in the Southeast? I'm in Upstate SC.

  9. meganwood | | #9

    We're in Mcclellanville, right outside out of Charleston ;)

  10. Expert Member
    Dana Dorsett | | #10

    In McClellanville, your deep subsoil temps are in the 67-68F-ish ideal range. Unless you're going to PassiveHouse extremes on low energy use, it's better to earth-couple the house to the thermal mass of the soil rather than insulate against it.

    Insulate the foundation walls all the way down to the footings, install a ground vapor retarder, and a non-structural 2" "rat-slab" to protect it. Don't bother insulating under the slab, and don't insulate between the floor joists.

  11. LucyF | | #11


    I am always interested in questions from women since there aren't too many on this forum. I am also impressed with the quality of questions from women readers because it is often obvious they have done a lot of homework before they posted a question. (I'm not saying anything about the questions from men, often they are very technical and well researched as well).

    Your question is a very good one and shows that you do have a good idea of what makes a high performance home. Dana's answer, as always, is excellent. The only thing I would add is that you have to worry about termites and carpenter ants, so if you using foam you are at risk of termites being able to tunnel through the foam to your home.

    If I were to do my retrofit on my house over, I would use Foamglas at the base where the insulation touches the ground and foam above that. Here is the link to the Foamglas site

    A very good article about it is on this website:

    Another idea I have is to give your contractor a membership to this site. There is a small risk to that should you ask questions as to whether what he (or she) is doing is correct. But really you have to be transparent about that anyway.

    And then finally, in my opinion you have to be onsite during the construction. Contractors may not like it, but you have to look at the quality of the work and keep up with it, especially as the building envelope is completed.

    I was heavily involved in building a house for my brother. My contractor is good friend of mine, a great guy that I've known for over 30 years, a very reliable, honest caring person. BUT he didn't really know much more than code about energy efficiency matters. The house itself was built by 2-3 carpenters which means it was built slowly. So if I disagreed with what was happening such as pointing out a major thermal bridge that could be avoided or useless blocking, I could intervene before it was too late.

    Most homes, however, are constructed by different crews - the foundation crew, the framing crew, the drywall crew, the insulation crew, the finish carpenters, etc - which makes it much more difficult to maintain quality like you want. The framing crew is of particular importance because you have to know that sealed under the sole plate or the bottom piece of wood for the wall framing, etc, etc. You have to be there to make sure that it done. Or make sure that your contractor ensures that it is done.

    I also like the idea of holding your contractor to a specific blower door test result. Pick whatever number you think is reasonable - 0.6 ACH50 for a passive house, 1 for a very good house, 2 for still well above average house, etc. Do a blower door test when the house is framing is complete and then again when plumbing, electrical, duct penetrations have been added. The contractor is responsible for guaranteeing these results. Any corrections that have to be done, have to be done as the contractors responsibility (in other words, he or she has to pay for them).

    I don't think you asked for all of this advice, I hope that's ok. I laughed when I saw that you're in SC as well. Have fun building your home. I had a blast. If I had more money and someone to build a home for, I would build another one.

  12. meganwood | | #12

    Wow, thanks so much for the responses.

    Martin, Andrew:

    We are planning on insulating the crawlspace using rigid foam. David McNeely: Additionally the energy efficiency consultant also mentioned making sure the door had a good seal and was insulated with rigid foam. Seems like good advice.

    It would be a 3ft crawlspace with blocks. However, due to code (i have not looked up the code, we are subject to 2012 IRC) I'm told we have to have a 4" above soil inspection gap as well as a 4" gap below floor joist termite inspection gap. So we can't completely insulate it. It was mentioned that we make sure we sill seal it when that part happens.

    Dana - We asked our contractor to price a rat slab, since that is what we originally wanted (having read the GBA article referenced). I'm unsure of the cost vs just having some heavy plastic on the ground. We plan on getting a take-off sometime next week and we'll see what the cost impact might be. There was a good article either here (on gba) or on another building site called "the good enough house" which really represents our thinking We don't have the funds to go "all out" (otherwise I probably would be trying to go to passivehaus standards) and we are really trying to pick the things that would get us the best bang for our buck in making the house more energy efficient as well as usable to us for a long time to come. (We plan living here for a LONG time). That said, maybe the rat slab makes the cut. It seems like the definite way to go, just have to see how the take off shakes out.

    Martin - regarding some questions on the insulated floor joints - the energy consultant we are using did a HERS index generation based on the data we provided (SHGC / Ufactor / Sizes of windows, placement of house etc), Wall insulation etc etc. According to the model, he said adding the Floor insulation definitely lowered the HERS index - I cant remember the actual point reduction, but it was not insignificant. Maybe I should ask him if the model included the subsoil temperatures? He mentioned it would help more in the winter.

    Lucy - thanks for your comments. It seems building a house makes you wise to many things. We are learning so much just going through the paces. Regarding foamglas are you saying if we just go with a rigid foam termites and other things burrow/tunnel into it? We are required to have that termite inspection gap to see the tunnels. From what I've read though, termites don't typically tunnel more than a few feet up although I'm sure there are exceptions. Given our location, it was difficult to find contractors much less energy efficient contractors that would come out to our location and build. In the end we ended up picking a local contractor because they seemed willing and open-minded when it came to some of the stuff we were asking for and had no qualms about us hiring a 3rd party for verification and blower door tests. Given our contractor would probably not be well-versed in energy efficient building, and we are not well-versed in building at all, we decided to hire this energy consultant who is also a contractor that could do on-site inspections, do the manual-j calcs and the blower door tests as you suggested. Putting the ach in the contact is definitely a good idea we'll look at doing. Given all this, I can't tell if your "I had a blast" was sarcasm or not :) There is so much conflicting information, it makes me a little crazy.

    So in summary:
    - Does not having a rat slab, and the 4" gaps inspection gaps, and 1" of rigid foam (according to models, 2" in our climate was breaking the law of diminishing returns) change any opinion on insulating floors?
    - Is it likely the model is wrong or doesn't have some variable?

    Thank you all for your informative inputs. It is not taken for granted and very much appreciated.

    - Megan

  13. GBA Editor
    Martin Holladay | | #13

    There are a couple of possibilities here. It's possible that there is a glitch in the HERS software that overestimates the benefit of adding insulation between your floor joists, or underestimates the contribution of heat from your crawl space floor. No software is perfect.

    It's also possible that the software is accurate. I would ask your consultant how many dollars per year (according to the energy modeling software) would be saved by adding the insulation between the floor joists. Then I would get an estimate for the cost of installing the insulation, and make a simple payback calculation. I'm guessing that the payback period is many decades -- although if the software is faulty, a different answer is possible.

  14. synergytodd | | #14

    If you encapsulate the crawlspace use a minimum of 2" of closed cell foam on the interior side of the exterior foundation walls and rim joists/band joists. Make sure you leave a 2"-3" termite viewing strip where the concrete meets the sill plate.

  15. Dana1 | | #15

    A metal sill gasket that extends all the way to the interior of the wall insulation & finish is a good termite barrier, and less of a thermal performance hit than a 4" inspection gap.

    A rat slab that extends all the way to the foundation (or partially under it) with metal flashing sealing any concrete to concrete seams would also be a good termite barrier, especially if that flashing were copper, which is toxic to termites.

    It's highly unlikely that insulating the floor saves energy if the crawlspace walls are up to IRC code minimums, and may in fact increase energy use by a modest amount. But at only 1" of wall foam it has to be XPS to meet the R5: minimums:

    It's probably both cheaper and more effective to use 2" of EPS (R8-R8.4) on the crawlspace walls, and nothing between the joists.

    Building Science Corp suggests that R10 on the walls and R5 under the slab might still be cost-effective in climate zone 3, per Table 2, p.10 of this document:

    They also recommend a continuous R20 for the floor in zone 3 if it's an exposed floor (vented crawlspace or pier foundation.)

    An uninsulated slab with R8 walls and no floor joist insulation splits the difference between the IRC and Building Science Corp's starting point for long term cost effectiveness rationale. The "additional" cost of the R8 foam instead of the code-min R5 is about 25 cents per square foot. In a 4' deep crawl space would then be about a buck per foot of perimeter. So for a 50' x 50' 2500' house that adds up to $200 cost adder. I doubt you'd be able to insulate between the joists for anywhere near that little. By contrast, putting an inch (R4) of EPS foam under the rat slab would run an additional 35-40 cents per square foot, x 2500', or about a grand. With 67-68F subsoil temps there's no real payback on a grand's worth of sub slab foam here, but there IS for the additional R3-R4 of wall foam- the walls see much bigger daily and seasonal temperature differences than the crawlspace floor.

    At your subsoil temps you get a wee bit of "free" sensible cooling during the cooling season, and it's effectively no heat load during the heating season. (In a 70F room buoyancy stratification alone will make the floor temps 68F or colder.) The crawlspace temperaure will be below the outdoor air's dew point for some of the summer, but as long as you give it even a sip of air-conditioned air on a regular basis it won't have a mold problem.

  16. BendStar | | #16

    I'm building a cabin in the panhandle of North Florida that will be used maybe 1X per month and the winters are very mild. Between December and February it gets below 40 only a handful of times and typically warms up into the 70s quickly. I've considered a vented crawl space, but it seems like a closed crawl space will help considerably with the high humidity that often accompanies 90 degree temps in the summer. I'm thinking of coupling that with a dehumidifier.
    My question I really need the rigid insulation on the inside face of the CMU? Could I fill any open CMU cells with insulation to help?

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