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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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.

@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.

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.

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.

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