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 new homes with crawl spaces or unconditioned basements, I typically recommend redefining the thermal enclosure to include the crawl/basement space. In cases where the floor system must be insulated, I only see two options: apply spray foam insulation to the floor, or encapsulate the joist system by installing and sealing a layer of rigid foam to joist bottoms, in which case the air gaps above the cavity batts becomes irrelevant. This approach is typically impractical due to interference from ducts & pipes running below the joists

I too would typically suggest insulating around the perimeter of the crawlspace but if somebody already has the floor insulated I don't want to reinvent the wheel. Radon is also a very common issue here too so I'm hesitant to reestablish the thermal boundary without a radon test and subsequent abatement system.

Sprayfoam is a great performance product but unfortunately extremely cost prohibitive. The foamboard option you mentioned is also typically impractical because of time involved and mechanical/plumbing/electrical issues involved as you mentioned. At that point I feel like it would be the same cost to fill the whole cavity and secure in place at the bottom like I mentioned. There would be more material cost but way less man hours to do the job.

Agreed, having gaps above the insulation is not optimal. This is a situation where I lament the lack of overlap between new home construction and existing home performance work. 

A cost effective method for insulating framed floors (over crawl spaces or garages) in HP work is to dense pack these spaces with cellulose, but in new construction (production building that is) this is never done and I'm not sure why.  This would negate the need to "hold up" the insulation and ensures that there are no gaps inside the cavities. Most builders I work with here in Wisconsin will have  a couple of inches of foam sprayed first and then make up the balance with fiberglass. 

Are you builders just being cheap by not using full depth insulation batts?

I agree about enclosed cavities with cantilevered areas and finished garage ceilings. I always suggest dense packing all those cavities as well to ensure properly aligned thermal boundaries. At least 90% of the time the insulation is resting on the bottom of the drywall or plywood effectively doing nothing.

Robin wrote: "the insulation is resting on the bottom of the drywall or plywood effectively doing nothing."

Not true, as long as the exposed cavity sidewalls are insulated and the cavity is tight!

As Joe points out, this configuration is ideal from a comfort standpoint, and is only less efficient when compared to a fully filled cavity with the same continuous foam beneath the joists. Moreover, depending on joist depth and climate, it's certainly not a given that whatever efficiency benefit might accrue from dense packing the cavities would be cost justified.

On the other hand, dense packing without a continuous layer of foam is clearly less efficient than with the foam. If this is what you're advocating, then I would argue you'd be better off putting less insulation in the cavities and adding a layer of rigid foam so that you get to the same effective assembly R-value.

You're speaking along the lines of new construction and I'm talking about retrofits. When retrofitting these areas the least invasive method would be a drill and fill.

My mind runs back to this article from Joe Lstiburek

BSI-064: Bobby Darin and Thermal Performance

Seal and insulate the Rim Joists first and then add continuous insulation on the underside of the joists. In the Kansas City Market, we have mechanicals in crawlspaces and generally opt for a conditioned crawlspace.

We also see this issue in Garage ceilings quite often and end up blowing insulation in to completely fill the cavity.

Some one citing Joe Lstiburek.  Good work.  

I think most of us jump to assumption that might not be true.  The insulation does not have to fill the cavity to be effective. BUT or IF or ONLY WHEN. First of all we are talking about horizontally installed insulation, where the still are issues are generally present to keep the R-value. Second, we are assuming that the air beyond the insulation is still or sealed.  I see rim joists mentioned and agree that there are more issues in those locations and transitions.

Lastly, Lets praise them for getting some of it right.  If they install the vapor barrier on the wrong side of the floor, we could be sealing in structural failure causing moisture.

Lastly, every inch of insulation in the floor is less valuable than that same inch on any other surface of the house.  Now R-16 to R-19 on the floor might be better than going R-48 to R-51 on the ceiling.  Over insulating a floor is not stopping the ice caps from melting or changing your utility bill by a dime/sqft a year.   

Good discussion. 

No but my argument here is that if the insulation isn't correctly installed in direct contact with the conditioned space then you might as well not even have it at all. The floor is essentially uninsulated in these instances and it's regrettably extremely common. I agree that you need to work your way from the top down in a house with both air sealing and insulation but this is a pretty widespread issue that should have an easy enough fix but I can't think of anything other that what I've already mentioned or reestablishing the whole air and thermal boundary which also comes with its own issues.

This insulation may be installed correctly. There are way too many variables in play to give you the "right answer".  Yes, un-faced bats in joist cavities is not pretty, but is probably the best one size fits all answer for insulating. 

Yes un-faced bats in horizontal joist cavities do provide thermal insulation. Assuming the joists are non conductive, basically not metal, the assembly will have the rating of the insulation. Voids are bad, but don't ruin more than about 100% greater area than the voids. 

This would probably be the insulation that I would detail. There are flame spread, moisture, structure issues that other insulation types and installations. I know this looks like a cop out or band-aid, but it isn't wrong. Sloppy work doesn't help. Spray foam is a superior installation, but there is no way for the plumber to fix that foam when he needs to snake a drain, or touch a trap. The batt CAN be removed and reinstalled. 

I agree with you here as well but my concern is looking at this from a retrofit perspective. Is there not a better way to address this issue other than filling the whole 10 or 12" cavity? I ask because inevitably I'm going to have a client request a bid for me to remedy their situation and I dread having to manually staple up their existing R19 flush with the floor. Without hearing otherwise from anybody else in the know I would lean towards adding a R13 or 19 batt (depending on joist height) and using twine and staples at bottom of floor joist.

When encapsulation, or foam insulation isn't an option... See if the installers can use the supports for metal framing (metal framing is thinner as I-joists are), or there's this great product I saw once called an X-hanger. It is 2 tiger supports bound together in the middle so the legs can reach out to become shorter without flopping around. This product is not widely available.



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