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.

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You want an airspace between the subfloor and the insulation.  It keeps the floor warmer.  Read these two articles by the Building Science Corporation for an in depth explanation. 



Adding an airspace changes the temperature gradient, as noted in Joe's doc (linked by Ethan, below), thus making the floor slightly warmer, with slight increase in heat loss. However, the real issue here is encapsulation, or lack thereof. 

Without 6-side encapsulation, the effective R-value for fibrous insulation between floor joists is reduced, even for a Grade 1 install, assuming that's even possible. In my experience, the typical installation of floor batts is worse than Grade 3 due to interference from wires and plumbing, and the use of compression wires to hold the batts to the floor, which further reduces effective R-value. I would posit the typical install has less than half the rated R-value of the batts. I know this has been studied, but I can't point you anything specific. Sorry.

Exactly  David.  And then there is the fact the energy codes require the insulation be "in permanent contact" with the sub-floor.

Yes & no - newer codes added in "Exception: As an alternative, the floor framing-cavity insulation shall be in contact with the topside of sheathing or continuous insulation installed on the bottom side of floor framing where combined with insulation that meets or exceeds the minimum wood frame wall R-value in Table R402.1.2 and that extends from the bottom to the top of all perimeter floor framing members."

Where is this written? please and thank you. As, its written you would have to do cont. insulation on the underside of the floor in order to be able to ignore the "6 sided substantial contact code"?

2018 IECC --- just think, you do have 6 sided encapsulation unlike an attic which see's more of a Delta T then you will see here. Also, in most cases most builders never put anything underneath (crawls especially, some basements) so you still don't have 6 sided

Im specifically thinking of floors over garage. Builders here often use rigid insulation under the FOG. Or, most out builders do blown insulation to meet the substantial contact rule. That's what we typically see in my area. Last "crawl space" i've seen was more a 6ft basement.


Assuming the band / rim is sealed what benefit is there to direct contact with the sub floor?  Isn't the point of "encapsulation" to reduce / remove air movement?  Even with direct contact convective looping can occur if the density isn't high enough so that isn't it.  I think we over think the "encapsulation" too much without a full understanding of how fibrous insulation works.  That being said I've seen many a floor where cold outside air was able to move over the top of the insulation and cause problems.  Usually a tilers bathroom floor that just HAD to be cantilevered over the first floor.

My retrofit work: Dense packing a 2-3’ perimeter for air seal, while keeping air space can keep water lines and toes warm

Water lines should simply not be located in problem areas and if the floor is properly insulated how can the floor be a different temperature than the room temperature?  This likely cannot be accomplished with fiberglass---but like I said "properly."

Charles wrote: "if the floor is properly insulated how can the floor be a different temperature than the room temperature?"

Any time you have a steady-state delta-T across an assembly, the indoor surface will be cooler (or warmer) than room temperature. Otherwise how could their be any heat loss? The R-value of the "air film" adjacent to the surface determines the temperature gradient through the assembly and thus plays a role in the surface temp.

You want to install enough insulation that the room temp line is within the insulation---not outside it.  That is why I said "properly."  The current codes are not interested in "properly" IMHO.



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