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Cellulose or fiberglass to insulate floors above encapsulated crawlspace?

agurkas | Posted in General Questions on

Year ago I put in some major sweat, blood, and tears to turn most disgusting crawlspace on the East Coast to something I remove shoes to walk on now. Radon levels are down too. I got all 1st and 2nd floor walls densepacked. MIni-splits are doing great. But I have one room above the crawlspace with hardwood and floors are really cold. This also happens to be room my in-laws come to stay (aka “free daycare”) couple times each month and my mom-in-law really hates how cold the floors are. Temprature in the crawspace hovers between 47 and 57. Moisture runs about there too, since I have ventilation between it and the large basement next to it.

So the big question:
Insulweb the 8″ deep floor joists and try to blow in cellulose or go the route of adding R19 of the pink stuff?

Whole basement is something like 1200sq ft (walls insulated) and crawlspace area is only 300sq ft.

Just want to give warm(er) floors for my in-laws, since they are so kind to help us out with our kid.

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  1. GBA Editor
    Martin Holladay | | #1

    You didn't say whether you have air-sealed and insulated the crawl space walls. If you haven't done that yet, that would be the first step. Those two steps alone might be all you need to do.

    If you have, in fact, already air-sealed and insulated the crawl space walls, I'm surprised that your crawl space is so cold. What is your climate zone?

    If you decide to insulate the floor joists above the crawl space, I think that the best way to do that is with a layer of foil-faced polyisocyanurate fastened to the underside of the floor joists. Tape the seams with a foil tape, and pay attention to airtightness at the perimeter. I think that 2 inches of polyiso would be good, and 3 or 4 inches of polyiso would be even better.

  2. agurkas | | #2

    I am in Boston, so zone 5.
    Yes, everything is air sealed, since I had radon problem. Walls haven't been insulated yet, since I am still trying to figure out if I should cut larger entry into the crawlspace or cut the Polyiso sheets into smaller pieces to get them in.
    Crawlspace walls are CMU and basement walls are concrete. There is usually 5+ degree difference between crawlspace and the basement, but it is likely because of boiler, furnace, and cutwork losses. Crawlspace has none of those and is ventilated with a fan in the 2'X3' opening between the two.

  3. Expert Member
    Dana Dorsett | | #3

    It would have been easier/better to put 2" of closed cell foam on the foundation walls before installing the full-on vapor barrier. (Water under the bridge.)

    Code min floor insulation for zone 5 is R30, and that's possible in 2x8 framing with rock wool. You can get R30 Roxul designed for 2x8 framing at the big blue box store chain in MA, probably at the orange store too, if you can't find it through other distributors for less. At box store pricing you're looking at about $500 in material for a 300 square foot floor.

    You can make a bottom-side air barrier with housewrap and be done with it. Or...

    After you insulate that floor with there will be a bit of temperature-striping effect from the thermal bridging of the 2x8s. If you cut some 4" wide strips of 2" thick polyiso and cap-nail them centered on the underside of the joists you will have a fairly effective thermal break (more than doubling the effective R value of the framing fraction) rendering the striping less prominent.

  4. agurkas | | #4

    I looked into closed cell foam. I am just too sensitive to that smell. I can tell when place has been sprayed. Still looking for how I will insulate the band joist, since I don't want to spray and cut and cobble method may not be too effective, since I can't get rigid insulation 100% flush with the band joist, since it used to be baloon framed and then someone filled it in with concrete and bricks to close off the "vermin highway".

    I actually have Roxul bats you mentioned, since I was planning to use it for another project, but chose to go later the cellulose route.

    If I am putting in housewrap, why no blow that in with cellulose? Even rental machines have enough pressure to provide for enough density in the bays. Am I wrong?

  5. Expert Member
    Dana Dorsett | | #5

    Using housewrap as a blowing mesh would be fairly pillowy, and there would be some settling over time, but it's do-able. At 7-1/4" milled 2x8 depth that would come up around R26-R27-ish, 10% shy of code-min. If it's a full-dimension 8" you'd be at R29-R30, pretty much meeting the letter of code, but it would have some cold-striping issues.

    If you put up some 2-3" wide 1" or 1.5" thick polyiso edge strips up before putting tacking up the housewrap, and install 1x furring perpendicular to the joists 16" o.c., through screwed to he joists to keep the housewrap from sagging/pillowing excessively it would meet or beat code performance, and the cold striping would be much subdued.

    When blowing with the 2" hose, cut a 3-4" slit in the housewrap about 3 furring steps away from the outer wall (about 4') , push the hose in until it hits the wall then back it out 8-12", and blow until it stalls (a single stage blower will surely stall), back it out bit to get it flowing, blow until it stalls, until you're right up to your hole. Then go to the next bay. When you've packed all the ends on one side bay, go to the other end, and pack those before moving on to fatten up the density in the middle. That ensures maximum air retardency & leak plugging at the ends. Tape the slits right after you finish blowing there, to keep from filling the crawlspace with cellulose as you go. Otherwise you'd be getting some blow by when filling the adjacent bays. Use a cut off sleeve from an old sweatshirt to hold the hose at the slit when blowing to limit the blow-back in your face, and mark the hose with a wrap of tape about a foot from the end so you know when you're getting close.

    You may have to experiment around a bit before you get rhythm, but it should go pretty quickly once you've worked out the kinks. Don't go by the coverage sheet numbers on the bags, go by the nominal weight. Buy enough material for 3lbs density, even though you probably won't get more than 2.7 2.8lbs density out of it with this technique. For 300' of floor area with ~8" deep joist + strips milled joists, at 3lbs density that's about 550 lbs. of cellulose, but you'll probably be returning ~50lbs maybe even 75lbs depending on the condition of the blower.

  6. agurkas | | #6


    Does it have to be housewrap for air barrier or would insulmesh be just fine, since cellulose is an air barrier?

    I was thinking I could use leftover FoamSealR Sill Plate Gasket (or even cut strips of XPS) and put those in between floor joists and the furring strips I would put down across the joists. Then either glue insulmesh on those or staple the housewrap.

    I do have bunch of leftover Roxul AFB 3", which is something like R12, but probably that insulation is not a fit for this project.

    Reason it is 47 there today is because the CMU walls of the crawlspace are that temperature. Basement walls are insulated, so basement is running 57 today.

  7. GBA Editor
    Martin Holladay | | #7

    We agree: Your crawl space is cool because its walls are uninsulated.

    Neither InsulWeb nor cellulose is an air barrier. InsulWeb works very well as a membrane to enclose cellulose, but it is air-permeable and vapor-permeable. While dense-packed cellulose insulation reduces air leakage, it is not an air barrier.

    After you have finished installing cellulose behind the InsulWeb, you can add an air barrier to the underside of the floor joists. I strongly suggest that you use foil-faced polyiso as your air barrier.

  8. Dana1 | | #8

    Since you have a 2' x 3' hatch, you may be able to ge 2' x 8' sheets in there(?) . If you cut halfway or a bit more through a 1" or 1.5" sheet of polyiso from one side you can usually fold it without tearing the other facer, which would leave the full 4' x 8' air & vapor barrier on one side.

    Another possibility is non-perforated fan-fold 1/4" XPS siding underayment, It typically comes in 4' x 50' sheets with a thin vapor retardent layer of some other plastic that works very well with a number of tapes.

    The crawl space will drop a few degrees after you insulate the floor above, but I'd be surprised if it averages below 40F, which would be a freeze danger for your drains and any potable plumbing, so monitor it. You may have to insulate the crawlspace walls if it's too cool down there.

  9. agurkas | | #9

    I get it why you are vocal about thermax. Yes, ignition barrier is important, especially when there are two bedrooms above.

    Awesome suggestion re folding. Haven't even though about that.

    It all really opens up 3rd choice: 2" of polyiso on the walls of the crawlspace (they are 5' tall most spots and cold air is hitting 3'+ from the other side) to get those temperatures up and then do 2" on the ceiling of the crawlspace.

  10. Expert Member
    Dana Dorsett | | #10

    A key factor for insulating the walls is making sure you don't ruin your vapor barrier's seal. To hit code min would require 3" of polyiso on the walls, not 2. It's easier to do BEFORE the vapor barrier is installed, or concurrent with it's installation. You could put 3" of polyiso and hold it in place with 1x furring through screwed to the foundation wall with 5" masonry screws, but it's a bit awkward.

  11. agurkas | | #11

    Hmmm. Every installation I have ever seen had XPS or ISO attached over the vapor barrier.

    Factory 2nds thermax polyiso runs about $20 per sheet in 2". Recycled 1.5" without thermax runs $12.

    I know there is (expensive) ignition barrier paint or you can put drywall over it (which is a no no in crawlspaces). Is there like another ignition barrier available? In my kneewalls contractor densepacked joists and put some kind of foile-faced paper. They said that was what state was OK with for ignition barrier in kneewalls.

  12. Expert Member
    Dana Dorsett | | #12

    If you install the cheap 2" stuff with 1x furring then install cut'n'cobbled fire-rated 3/4" Thermax between the furring you should be good to go, and you'd be at code. The 1x furring is thick enough to qualify as the ignition barrier, and so is the Thermax.

  13. jay443 | | #13

    I'm going to glob onto this question because I'm doing the same thing with my crawlspace.

    Are all foil-faced rigid foam boards created equal in terms of ignition? For example, each of the big box stores sells their own brand of foil-faced insulation. But if it's not Thermax, can it still be considered an ignition barrier? Is that a local code decision?

  14. GBA Editor
    Martin Holladay | | #14

    Q. "Are all foil-faced rigid foam boards created equal in terms of ignition?"

    A. No.

    Q. "If it's not Thermax, can it still be considered an ignition barrier?"

    A. Ignition barriers are defined by most building codes. In sections R314.5.3 and R314.5.4, the 2009 IRC defines an ignition barrier as one of six permissible materials: 1 ½-inch-thick mineral fiber insulation; ¼-inch-thick wood structural panels (e.g., plywood); 3/8-inch particleboard; ¼-inch-thick hardboard; 3/8-inch-thick gypsum board; or corrosion-resistant steel having a base metal thickness of 0.016 inch. Presumably, code officials will also approve thicker versions of any of the six listed materials as ignition barriers.

    To get a material approved as equivalent to a prescriptive ignition barrier, the material must meet the “specific approval” criteria set forth in IRC section 314.6. (This section lists the type of fire testing that is required.) To get an assembly approved as an alternate assembly, the assembly must be tested by a laboratory according to criteria set out by the ICC Evaluation Service in Acceptance Criteria 377, Appendix X.

    For more information, see Thermal Barriers and Ignition Barriers for Spray Foam.

  15. agurkas | | #15


    Looking more at this issue I found Naiton Fiber claiming that 1.5" of cellulose is an ignition barrier. So netted cellulose - for 2X8 joists additional inch and a half furred out (furring strips on 1" spacers accross the joists) sound like should be just fine.

    Here is doc I am referring to

  16. GBA Editor
    Martin Holladay | | #16

    Yes, my article on ignition barriers mentions the use of cellulose insulation as an ignition barrier. In your case, that solution will work -- as long as your crawl space stays dry.

  17. agurkas | | #17

    GE Geospring hybrid water heater will be of great help, especially if I duct it to exclusively draw air from that space. Crawlspace is also at the highest spot, compared to the rest of the basement, so even in wet spring (like last year), sump pit I built there was dry as a bone, when the other sump pits were being pumped 24X7

  18. agurkas | | #18

    So I decided to use 1" spacer I made out of XPS I had left over. I will attach strapping to that every other joist (running them at 90-degree angle to the joists) and then staple Tyvek housewrap to it, which I will tape for airtightness. Nice bonus is that PEX will be nicely covered with couple of inches of cellulose, which should insulate them in very rare case temps in the crawlspace drop.
    Humidity control will be via intake duct that is going to 80-gallon GE Geospring (there is a duct kit for those), which should nicely keep that space bone dry.

  19. Expert Member
    Dana Dorsett | | #19

    By drawing air for the GeoSpring from the crawlspace it'll guarantee an air exchange too, which would limit the temperature difference a bit, albeit at a small cost in heat pump efficiency should the crawlspace be cooler than the basement.

  20. agurkas | | #20

    Wall that connects crawl space with the basement used to be basement outside wall. So there are two 2'X3' openings between the two. That should allow for nice circulation.

  21. agurkas | | #21

    One detail question just came up. My rim joist is one of these double ones with 2X4 between 2X8s. It is balloon framed from sill to 2nd floor and most areas are filled with cellulose, when walls were densepacked.
    4 inches up the rim joist and everything down from that is 20mil radon tight poly. So there is not moisture coming up the CMU wall into crawlspace. Rim joist seems to show no previous moisture problems. Lastly I will be doing superinsulating of the exterior, so I will have at least 3" of polyiso.

    So now the question is if I should staple housewrap that is going to keep all the densepack cellulose all the way to the bottom of the rim joist or stop short of the rim joist and staple it up to the floor and down the road fill in that rim joist area with spray foam? It is too irregular for cut and cobble method, since at some point the open areas of the rim joist were filled with brick and cement to keep vermin out.

  22. GBA Editor
    Martin Holladay | | #22

    I don't recommend insulating rim joists with cellulose, because cellulose is vapor-permeable and air-permeable.

    Looking back at Comment #2, I notice that you are in Boston. In that climate, it's easy for the cold rim joist to accumulate moisture and mold if you install an air-permeable and vapor-permeable insulation on the interior side of the rim joists.

    I vote for spray foam on the rim joist.

  23. agurkas | | #23

    That is big bummer. Was hoping to avoid foam. Would one inch of spray foam work? I am thinking about the minimum I can spray I can just ventilate out while spraying and not cause smell in the house.

  24. Expert Member
    Dana Dorsett | | #24

    Cut'n'cobble foil faced polyiso in there, using can-foam to seal it in place. That minimizes the amount of foam involved.

  25. agurkas | | #25

    I already had a post back in March about this. Martin did not think it was a good idea, since how irregular certain areas were. Pipes may be gone, but the brick balloon frame infill isn't.
    Went ahead and ordered 200bf of foam. Will ventilate out the crawlspace for 24 hours after applying and that should take majority of the possible stink out.

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