great! radiant heat is "great".
thank you Tim !
R values as defined by ** and as then tested in labs does not allow radiant heat reflectors to shine as well as another test, defined differently, would do.
From wikipedia.org
"...trying to associate R-values with radiant barriers is difficult and inappropriate. The R-value test does not control the amount of heat transfer due to conduction / radiation respectively. There is no standard test designed to measure the reflection of radiated heat energy alone..." This quote describes only a part of a bigger problem, but it's a bigger discussion than we may want to get into here right now.
Your term "airgap" is good; I have always said it needs to be "surrounded by air". I like your term better. I want to congratulate you on something else too, even more important. You have made it clear that radiant barriers need air to work!
Under a basement slab, aluminum coated bubble wrap has to be doubled back onto itself so the foil side is doubled and in contact only with itself and air bubbles -- not in direct contact with solids like concrete or the ground. I have found that hard to explain to people. Or, people have found that hard to understand to the point where they do "the right thing". I'll bet that 90% of all foil faced bubble wrap has been improperly installed.
The same applies to foil faced foam. Where I live, people buy dense foam boards coated with aluminum on both sides, for wall insulation. First of all, there is no gain to be had from two parallel radiant barriers; it is far far better to have the same quantity of metal twice as thick in one layer. Then, people place that foam board so that is in direct contact on one or both sides with other building materials. I have found it very hard to explain why that is not going to help... Or rather, people have not found my explanation easy enough to understand. They install radiant barriers the wrong way, and the manufacturer doesn't print any installation tips.
Enough ranting. Besides, I don't have the kind of job that requires that I "teach" people how to install things the right way. It's only a tiny little frustration to see it being installed improperly everywhere I go.
You probably have seen solar camping cookers that work by reflecting the sun's energy to a central point. When I was a boy scout, these things fascinated me. They work on cloudy days too.
They reflect invisible heat energy. They also reflect some visible energy (i.e. "light" energy). They are mirrors for heat, not light. A mirror on the other hand does not reflect heat, only light.
No one is going to deny that these things exist and that the phenomenon is known as "radiation". Heat radiation. When you put your hand under a broiler heating element, you feel radiation. Heat is radiating downwards onto your skin. (Meaning: Heat does not rise, it goes in all directions equally.)
As you pointed out, it is true that some manufacturers fudge the whole issue by talking about an "equivalent R" factor and then they "recommend the product for situations where there is no air gap, like under a slab." Agh! I'll go see the RPA forum you mention.
Your last line says it's great to have radiant heat. Good to hear it from someone else who's done it.
I think the ASHRAE has published a definition of "comfort" that mentions that the coldest part of a room conditions your sense of comfort, so a cold floor is a No-No if you want to have
a real sense of comfort.
David