I looked at a large new house today, and saw what I see in almost every house... the fiberglass batt insulation installed under the subfloor between the floor joists has an air gap above it. This house has ~12" I-joists and R-30 insulation, and a vented crawl space. The insulation has a 1-2" gap above it. There is gypcrete and hydronic radiant on top of the subfloor, so the delta T is quite high.
I mention this issue to the owner wherever I see it with the suggestion that the insulation should be pushed up in full contact with the subfloor and more material added below it to completely fill the joist cavity depth. I normally don't attempt to quantify the impact of the air gap but say something to the effect of "this is significant and should be addressed".
In this case, I'm pretty sure I'm going to trigger a call to the builder to do something about the issue. I'd like to be able to be a little more specific about how big an issue this is. Can anyone point to facts and figures, articles, publications, studies, anything that might shed some light? Thank you in advance.