Every single time I do an audit where the house has I joists instead of regular floor joists I see this issue. Either the insulation batts are installed with no tiger teeth at all and the batt is resting on the bottom of the joist or the installers DID use tiger teeth but those are in turn resting on the bottom of the I joist with the insulation on top. Even though I joists are still 16" OC the actual width between the wood members is greater than with typical floor joists. Tiger teeth are 16" wide and aren't long enough to properly support anything and in turn rest on the bottom of the I joist. Here in Colorado R19 is pretty standard and typical I joist heights are either 10 or 12". This results in an approximate 4 or 6" airgap above the batt and extremely ineffective or totally pointless insulation. What I'm getting at is what do most professionals recommend to remedy this? If tiger teeth aren't the right width then are people typically using twine and staples to get the R19 flush with the floor? Otherwise are people getting an additional R13 or 19 batt to fill the entire cavity and using twine and staples on the bottom of the joist to secure them in place? Does somebody manufacture oversized tiger teeth for this type of application that I'm unaware of?

Any input is appreciated as I see this all too often.

You need to be a member of Building Performance Community to add comments!

Join Building Performance Community

Email me when people reply –


  • Please don't install non-fire code safe foam or any insulation (please read the printed notice on the kraft and foil faced FGB) in a crawlspace or basement that can be accessed from the living space as it's just not safe! If there is a single object stored there it's generally considered occupiable space and must meet thermal (fire) barrier standards. It's so easy to install densepack cellulose in the assembly by covering the bottom chord of the joist with either insulweb or a code approved vapor barrier (think mento+), etc. Sure, it's overkill on the R-value but the ease of install and fire resistance of the assembly is worth it in IMHO.
  • Mr. Henry,

    I built houses with those joists and found the building science training I had been getting at the time was difficult to align with the practices I had been taught. So I found a better way, I think.

    I also used 1/2 inch EPS but I cut and installed it vertically to replace the wire supports. I cut it to rest on the bottom ledger of the I-joist on both sides and push the insulation up snug to the floor. I also cut and installed the insulation tight to the sides, no gap. At times this meant buying larger widths and trimming to fit the cavities. All the scrap insulation I created went into the attic before the cellulose blow.

    Hope this helps.

  • I'm new to this so I could be wrong, and I'd appreciate feedback on how I'm wrong.

    I agree that: 1) insulation supported on "tiger teeth" with an air gap above it does practically no good; 2) the insulation must be in contact with the bottom of the subfloor to be effective. The problem is found not just in I-beam type joists, but conventional wood joists - the wire supports (tiger teeth) fail over time. That was my experience in the previous house I lived in, and in my current house. I'm reworking my own crawlspace now with a new method, and trying out my new method on another house. I will monitor it closely to assure there aren't problems that develop over time.

    The first step is to seal the crawlspace (including the band joists); there's plenty of info here about doing that, so I won't go into it. The real problem is what to do with the insulation if it's already there, installed with wire supports (the conventional way). My solution is to support it with 1/2" EPS sheet material, with film facing (one side is radiant barrier, the other side just a clear film. This material is available from the usual suppliers, and is typically used for continuous insulation. It has an R-value of about 2. I cut the sheets to a width about 1/2" wider than the joist spacing, then push it into the gap with an upward arch to force it against the joists on either side, similar to how the wire supports are supposed to work (but don't). Picture attached.

    The labor to do this is not too bad, although it does take some time because you can't just cut all the pieces ahead - the joist spacing typically varies on the order of 1/2", so the pieces will be too tight for the smaller gaps, and will fall out of the larger gaps. I'm getting the "hang" of it (pun intended) and it's starting to go faster. I leave about a half-inch gap between EPS sheets, which I cut across the narrow dimension (4x8' sheets cut to ~4x16" for 16" joist spacing). The gap will allow any moisture that makes its way into the space (from whatever source, someday it will happen) to diffuse out. Unless you have a big plumbing leak, then the insulation will have to be replaced.

    The added R=2 has minor benefit (but it's a plus). The goal is to eliminate, as much as possible, the air circulation through the fiberglass. You could also eliminate it by boxing it in with continuous rigid foam board, as in some of the solutions mentioned here, but that would be a good bit more expensive.

    I'm curious about the comments re: no vapor barrier backing on the batts. Every installation I've seen (and the ones I'm doing) have the backing. Is the "fire code" reference in regard to insulation open to space that's intended for human occupancy?

    I'd love to see comments from the experienced people here.


    • @Stanley, your strategy for holding batts against the subfloor is interesting. Much better than wire insulation compressors! That 1/2 EPS you're using is relatively inexpensive and since you're not going for full encapsulation, it's not very time consuming. Also, your method would not be as impacted by interfering pipes and ducts since you're not going for an air barrier.

      Regarding facing on batts... Virtually every type of batt is available without kraft facings, although a given SKU may not be stocked at a given location depending on demand. If I'm not mistaken, the IRC only requires vapor retarders in wall assemblies and only then in CZ 5-8 and Marine 4.

      The issue with fire codes is that kraft paper facings (and even some foil facings on batts) do not meet the 'flame spread' and/or 'smoke developed' requirements of the IRC. In particular, Section R302.10.1 requires insulation facings to meet these requirements unless concealed and in contact with drywall or other surface separating cavities from living space.

      • I think the kraft facing would meet the requirements of "concealed" and "in contact" with "other surface" if the batts are supported to maintain contact. Not if there's a 5" air gap above them! Another reason to do something about these legacy installations.

    • I'm likely biased but I feel like the paper backing on batts is a bit of a joke and won't do much in regards to retarding any vapor transfer. I usually go for unfaced with whatever I do but I don't have to deal with moisture like many other climates would as it is extremely dry around my area.

      • I agree with you - if the batts are just stuffed into a joist cavity without attaching the edges of the vapor barrier backing to anything, the vapor will just sneak around the inevitable gaps and it's really doing nothing for you. They might as well not be there - but they're already there, and I don't think it is value-added to tear the batts off the backing before putting it back in the gap. If I were starting fresh, I would go with unfaced insulation. I should have clarified, the work I'm doing is retrofit where the insulation is already in place.

        Regarding the 5-6" air gap above the insulation - my opinion is it renders the insulation almost totally ineffective. I assumed R=3 for an R-19 batt at the bottom of the joist gap with 5" air above it. Just an assumption, but I took a little data which seems to back it up. My temperature measurements weren't well correlated enough to get an exact R-value out of it, but the indications are the R-value of the batt is roughly the same as the floor decking (total wood layer between air spaces). The reason it's bad is that there are lots of air gaps between the batts and the joists for air to flow into and out of that 5" air gap. Besides the loose fit to the I-beams, there are gaps around all the plumbing and wiring penetrations, and general bad fits in irregular-shaped cavities.

        One problem I encountered is where two I-beams overlap, leaving a ~2"-wide gap between the webs. These are air escape routes to and from the air gaps, and I've got to seal them up. Or better yet, stuff them with insulation and then seal them up. Ugh, there goes another 4 hours on the project!

        Filling the 5" air gap with another layer of fiberglass wouldn't be a bad idea, and maybe it would compete well with the cost of the EPS sheets I suggested - then just hold it all up with webbing stapled to the bottom of the joists. But I like the prospects of the EPS for cutting way down on air flow into and out of the fiberglass. Not that much air will be moving, once the crawlspace is sealed.

        Overall, I believe the crawlspace sealing will accomplish most of the needed improvements in the thermal envelope, and the insulation won't be doing a whole lot. The system might not be much less efficient if the subfloor insulation were just gone. That's what my analysis shows. Especially if the stem walls are insulated around the perimeter.

        • In order to make the air gap above the insulation work effectively, the framing must be airtight at the joist end as well as have a continuous air barrier, and preferably some continuous insulation below the joists. You are essentially creating a very tiny encapsulated crawlspace.

          I believe this method would be the most cost-effective if the job conditions are reasonable.

  • I think there is a code requirement (IECC) that talks to what you are asking about. Starts on page 32 at this link - http://media.iccsafe.org/Annual/2016/IECC-Residential-and-Multi-Family-Real-World-Applications.pdf 

    • Thank you I will look that over later

This reply was deleted.
Blue Valley Heating and Cooling and Blake Giesbrecht joined Building Performance Community
4 hours ago
Diane Chojnowski liked Leslie Jackson's blog post Ongoing Online Events, especially Training
23 hours ago
Josie Pivec’s blog post was featured
This post complements our Field Notes newsletter, which features quarterly updates on CEE's research projects. Sign up for Field Notes to get this information in your inbox. Geotargeted Distributed Clean Energy InitiativeBackground: Non-wires altern…
23 hours ago
Diane Chojnowski liked Josie Pivec's blog post Field Notes Spring 2020
23 hours ago
Kristina Frunze posted a blog post
Coronavirus has gripped much of the world and particularly the United States, which experts are warning could become the disease’s new epicenter.This has prompted wide-reaching shutdowns of businesses deemed non-essential by federal and local author…
Scott Katznelson replied to Scott Katznelson's discussion Closed Crawlspaces
"Thanks. "
Eric Miller replied to Dav Camras's discussion Designing airflow in large low load homes
"Hi there
A suggestion that might be a bit outside-the-box: might this be a situation resolved by the old paddle bladed ceiling fan technology?
Eric Miller replied to Scott Katznelson's discussion Closed Crawlspaces
A number of years ago, David Hales (noted building science guy in the NW) took a long look at this subject. I hope you will find this document useful:
Brett Little posted videos
NKaplan commented on Building Performance Journal's blog post Our First Issue is Complete!
"This is great news! Congratulations! I really looked forward to getting the Home Energy Magazine and I'm equally exited to learn what's in this one. Thank you for moving forward!"
Emily Ambrose posted a blog post
- By Cory Allyn, Energy Circle Content Writer
Energy Circle is continuing to provide information and resources to HVAC, home performance, and solar companies that are looking for suggested operating practices during and after the coronavirus/COVID-1…
David Heslam replied to Haley Keegan's discussion Remote Training Opportunities
Earth Advantage has developed a number of online trainings over the past few years. Currently, most of the online content is geared to the real estate industry. Largely, it's material designed to allow them to better understand the value of e…"