Hello again,
Was just reading on another site where they've found insulation unnecessary if there's a drywalled ceiling below.  My floor joists are ll 7/8 TJI's with 1/2" sheetrock ceilings applied below. 

Any thoughts?  (SIP house, ICF foundation,  Spray foam on all rim joist and roof deck)

Ben

Radiant barriers stop radiant energy and can act as a moisture barrier in one product. Green Insulation Products.com

Insulation is definitely not "unnecessary". You might be able to heat the upper floor, but you are making the floor work a lot harder than it has to if you do not insulate, and you risk not being able to meet the upper load as well. Also, rim joist insulation is critical to efficiency given the elevated temperatures of joists.

Whether that insulation should be a radiant barrier or not is highly debatable. It definitely provides some benefit at first, but Oak Ridge National Laboratory showed that reflective insulation does lose a LOT of its effectiveness when it gets dusty. So a lot depends on how dusty you think it will get in the joist bay, facing upwards.

Personally, I choose not to gamble, and instead to use a product with a real R-value that is not compromised by dust. R13 between heated floors for joist radiant methods. R19 to R30 over unheated spaces depending on climate and how exposed it is.

Fiberglass blanket insulation can also lose more than half it's R value in humid areas. (A 1-1/2% change in the moisture content of fiberglass insulation will result in a 36% decrease in performance (referenced from HVAC Manual 10.6; McGraw-Hill).
A radiant barrier reflects radiant heat energy instead of trying to absorb it. What does this mean in your home or business? During the winter, 50-75% of heat loss through the ceiling/roofing system and 65-80% of heat loss through walls is radiant. In the summer, up to 93% of heat gain is radiant. If you are depending on R-value (resistance) alone to insulate against heat gain and loss, you are losing half the battle. Remember that traditional forms of insulation are virtually transparent to radiant energy and are affected by changes in humidity (moisture levels).

I'm not a huge fan of fiberglass, though I'm a bit skeptical of wild claims of degradation since most of us out here are not de-rating fiberglass walls by any significant margin and our heating systems aren't coming up short, but I will concede that's not scientific (especially since most systems are vastly oversized anyway), and there is the additional "infiltration" factor of fiberglass for sure. Of course, between heated floors this is of much less concern. In other areas, I am a strong advocate for "supertight" insulation products of many types, but the fact is there is no forever-lasting magic bullet out there.

The idea that insulation is "invisible" to radiation though is an outright lie, however. Radiant heat energy does not radiate through solid or most opaque objects. It radiates until it hits an object. That object must then conduct the heat from one side to the other in order to transmit it to the outside of the envelope, and so if it is resistance to conductive heat transfer (a number handily approximated by its R-value), then it will reduce that transmission loss even if the heat came to the insulation by way of radiation. It may then radiate out into space from the outside shell of the envelope (which will not change unless you change the emissivity of your outer shell), but it doesn't radiate *through* walls and no form of insulation I am aware of is "invisible" to radiant energy. If it were, radiant systems would be the least efficient heating systems in the world as all that energy just "beamed" out of your house.

If you want to discuss this stuff, fine, but please do not make ridiculous marketing claims that are outright lies and just serve to muddy the waters for lay people. It just makes you, and the products you sell which DO have proper applications look bad.

So, explain your statement that "traditional forms of insulation are virtually transparent to radiant energy".

This is completely false. Sliding it in among a couple of statements that might have some more merit does not make this any less false. Care to explain your claim?

This is just like the "65-80% of heat loss through walls is radiant" statement. That might be true, except it's not really THROUGH walls, it's FROM walls. The heat isn't beaming through the wall. Put in a radiant barrier on the inside of the wall, and 65-80% of your heat loss from the wall will STILL be radiant to the outside, because as the outside of the envelope heats up it will radiate heat to the colder outdoor space, period. That's basic heat transfer. Even using that number is preying upon laymen and their lack of understanding of what those numbers mean.

Let me do you one better, the stuff is great because it cuts heat loss by "95%" right? I mean, that sounds really good too... really space age!... you'll see it in reflective advertising all the time. Of course, they don't tell you that an R-16 cuts heat loss about 94%... that's a lot less impressive. Even calling it "mass insulation" is disengenuous. There is mass, and there is insulation, and these two things do not function the same way. You are not charging up a mass battery with rigid foam, or fiberglass. You are slowing the rate of heat transfer, not storing the heat.

This is why the FTC has rules on insulation marketing, and this is why reflective insulation mfgs have been, are being, and will continue to be sued by the FTC until they stop playing these games, trying to confuse people. Hopefully you won't be included in any of those lawsuits.

And in the meantime, if you want to help people learn the "benefits of all types of insulation", then help them learn. I'd be more than happy to learn more myself. But you are not doing much educating; you're spreading bad marketing, and whatever educational tidbits you might have nestled in there are washed out by the cacophany of misleading marketing claims.

I think you'd be a lot more successful if you stuck to the facts. Your manufacturers too. It would certainly save a lot of my time.