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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  • BTW David, if you would like to mention anything to the builder it might be to consider using radiant sheathing in that area - shiny side down of course :) That should help keep the heat up where they want it & hopefully help with any worries you have

  • There are a few things going on here. First, if you want an idea of the effectiveness of the insulation, go to surface temperature. If the bottom surface of the insulation is cool, then the insulation is effective.
    Second, the air gap is a manufacturer's requirement for staple up radiant hydronic tubing. This helps even out the floor temperature especially when there are lots of dividers like joists and cross bracing, etc. I assume the differences you guys are seeing is more based upon the installer, code official and builder trying to interoperate the code correctly. The air gap doesn't hurt the heating system, energy usage, insulation, etc. I would double down on what I would call air barrier, and draft stopping the floor, joists, and anything that goes in or out of the floor assembly.

    Specifically for the timber I beams of this installation, I wouldn't change the installation. I would walk around the house and look for hot spots around the floor and moist cavities. Focus your attention on that.
    Glad someone is peeking in crawl spaces with these radiant floor systems.
    I would love to know more about the Concete radiant system. Was it the manufactured tubing rolled out, push in sub floor, or poured over tubing loops? Boiler, water heater, mixing valve temps, zones, thermostat control or supply temp reset? Sorry. I get too many questions about what works well for these systems.
  • Sounds like you're creating your own building code.  I can understand asking the builder to push up against sheathing since there's no continuous underneath it.  But to ask him to provide more then what code requires is not reasonable.  What if they where 16" deep "I" joist?  Further to tell the homeowners that more insulation is required is also not reasonable. ( I assume the R-30 batts meet code)  You may like to see more, but what does the code require at the houses location?  Since your post doesn't specify just what the purpose of your inspection was, we can only assume it was to verify that the home meets the building codes, and whatever specifications both parties agreed to. 

    • @Bruce, sorry, I just noticed that the original poster did indeed mention adding more insulation to fill the cavity.

      I think David was thinking that additional insulation would help hold the original batt tight to the floor. But he still needs some way to hold the batts from slipping down.The only way another layer would accomplish that would be if they have kraft facings, but that would put a vapor retarder on both sides of the cavity (presumably the original batts have facing on top).

      With radiant flooring on top, it would be a shame not to encapsulate and seal the joist bays, preferably with rigid foam boards attached to bottom of I-joists, in which case the air gap is irrelevant. I can't imagine going to the expense of installing a radiant floor over a vented crawl and then not doing a bang-up job on the floor insulation!

    • Bruce wrote: "What if they where 16" deep "I" joist? Further to tell the homeowners that more insulation is required is also not reasonable."

      I don't think anyone suggested that the joist bays be filled. Insulation can either be in contact with the floor, or at the bottom of the bays if the cavity is closed at the bottom with continuous insulation (boards). See Sean L's reply. The reason the latter was added to the code is recognition of the difficulty of achieving good floor contact with fiberglass insulation.

      • Of course---and fiberglass is garbage in this application and many others

        • Why Charles!   How do you really feel??

  • I too have recently seen this although it sounds like an even worse case. I'd like to stay in the loop as far as specifics to point the homeowner towards as well. In the instance I saw the house has 12" I-joists with 6" r-19 batts sitting on the BOTTOM of the just with metal staves holding them up. So across the whole house there is at least that 6" cavity above the fiberglass batt. There were also instances of areas where the batts had fallen down. Lastly, there seems to be a 2 foot insulation gap around the house's perimeter where the batts were scrunched up and out of the way, exposing the subfloor. I haven't seen this before and didn't know why the installers would have done this?

    • Sounds like another trade was in there after the insulators finished, and scrunched it up.  Typical of mechanical trades, I've yet to find a plumber, electrician or HVAC guy that will replace the insulation they remove, best thing to do is get insulators in there dead last.

      • I could see that point but there didn't seem to be any rhyme or reason to it. No electrical, plumbing, HVAC, cable, or phone line penetrations around any of these spots.

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