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.

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Do you have a link or source you could provide? I've not heard of this product before, thanks.

We also learned from Joe that a gap between the floor and the insulation leads to warmer floors.

I wonder if these houses use less energy because the floors are more comfortable.

I only glanced at the mentioned article as I'm pressed for time but will have to circle back and read the whole thing later. This is still a bad justification for leaving the issues I'm seeing as is. With the way the floor insulation is installed it is also barely there and any disturbance from moisture, critters, gravity (on the correct timescale), other lazier people working the trades who need access but don't bother putting insulation back - will send batts down to the floor and now you have entire sections of bare subfloor exposed to the elements. When you have bare sections like that then that entire floor cavity for its entire length is compromised and exposed to the elements. These issues are always in vented crawlspaces and get plenty cold. I've been able to verify this visually when in the crawlspace and also with the blower door and thermal camera at the end of the audit.

Hi Robin,

I assume you are referring to unfaced fiberglass batts.  If that's the case, the most common problem I have seen over the years is the incorrect sized batt, (width).  The fact is, the spacing for a 16" or 24" on center assembly, regardless if it is a wall, floor or rafter, is not the same for dimensional lumber, (2x) and manufactured joists, (TJI, BCI, open web, etc.).  When installed on the correct layout spacing, dimensional lumber on 16" centers has a clear cavity space of 14-1/2" inches. The standard 15" wide batt is designed to be friction fit and supported with drywall or other approved method of support.  Similarly, a 24" layout assembly has a 22-1/2" space thus a 23" wide batt is used.  For manufactured lumber using the same layout spacing the clear cavity space is wider because the thickness of the manufactured lumber is not the same as 2x lumber.  That spacing when  laid out correctly yields a clear cavity space of 15-5/8" and 23-1/4" respectively.  

Regardless of the support technique used, the incorrect, (narrower)  batt width allows an air gap on each side of the manufactured lumber and, as we all know, air infiltration is not our friend and reduces the effectiveness of thermal resistance.  

The problem is many fold, 1) manufacturers only list the wider sizes for metal framing, they do not publish this information for use with manufactured lumber, 2) the insulation distributors are not necessarily aware of this and unfortunately do not keep stock in these widths resulting in 3), the installers not being aware as well.  

As far as supports go, lighting rods, tiger teeth, etc are not necessarily the best choice for support specially in a floor above a crawl space.  Rodents and other warm blooded creators, (humans excepted, I think) love to keep warm and find their way above the insulation where the warmth is just below the condition space floor.  Their weight can easily dislodge lighting rods, etc. and create and cavity void.  Best practice is to diagonally lace across the bottom of the joists using a material that will not easily dislodge, rot or come loose when properly installed.  

Check out the graphic I put together explaining the cavity width example of 16" on center layouts.  


I'm not speaking of the width of the batts but rather their placement with the air gap above the batt, between the batt and subfloor. Critters absolutely love these nooks as you mentioned and inevitably their presence coupled with gravity always makes batts drop. When you speak of diagonally lacing material I'm assuming you are talking about nylon twine and staples like I mentioned? This still has the same problem of the 4 or 6" air gap above batts though and we're in the same spot. Not trying to sound cross here either, I'm just still scratching my head on coming up with best SOP.

There are several really simple solutions to Insulate Rim Joist.  1st Remember You must use unfaced Batts if it is exposed in the living areas because of Fire Code regulations. I Air Seal the Rim joists, then I have used Unfaced Batts turned sideways, wrapping them onto the joist 3 to 4 inches (C pattern ). I use strips of Thermo -Ply or Tiger Teeth or just staple it to the Joist. By the way, I have seen "Tiger Teeth" for 24 "OC joist layouts at Insulation Suppliers and Home Depot. Another way is to cut Foil board to closely fit and glue and foam it into place.

For Crawl Spaces I would only Close it, by Sealing and Insulating the walls AFTER installing a Moisture Barrier.   If there are extreme Moisture Problems, I would only use Spray Foam on the bottom of the floors

For Cavities such as above Garage Ceilings with living space above, Only Recommend Dense Pack.

Yes we're on the same page here too. Ideally insulating the perimeter is best thing to do so long as there aren't significant moisture issues but where I am (Colorado) radon is very prevalent so at point you're also trying to sell somebody on radon testing and then likely mitigation system vs. leaving vented crawlspace as is and trying to get insulation on floors correctly installed.

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

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.


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.



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