I have seen many failed attempts at doing this method. I know it is not optimal, but many builders are still doing this as an upgrade to ceiling insulation with ducts in the attic, at a lesser cost than spray foam.
Can it effectively be done?
What is needed to make it work right?
I personally do not think you can prevent hot air from the underside of the roof deck bypassing the batt and still having a hot attic.
Looking for any studies or articles on the topic.
Shaking head slowly
1st - no
2nd - ummm, foam like the codes require
Pretty much correct
In all seriousness - it must be vented above the FG, and you need at least an r30 or more perfectly installed (60 would be better). A few tricks to help is radiant sheathing, cool/radiant/polar paint over roofing, or ES (or similar) shingles / metal - you got to remember the Delta T - you are looking at most roof decks reaching 140 to 160 in your area & following that pesky law about heat moving to...
As you are in Tucson - while clay or concrete tile roofs help eliminate some of this as they have that air gap, in many cases it is not enough as they don't allow the air to freely flow
First off, I will say I wholeheartedly agree with not putting batts on the deck. However I was looking for specific information - studies, articles of the issues with this approach's shortcomings.
I do not understand your reference to the codes requiring foam. Code officials allow this assembly with batts alone in an unvented assembly(at least in hot-dry climate zone). As long as the climate is such that the average monthly temperature does not fall below 45 and interior relative humidity below 50%, there is no potential for condensation and the condensing surface does not need to be controlled. (Lstiburek BSD-102) This assembly would be without an air gap above the FG.
This is a retrofit application and I am trying to keep him from moving to batts at the roof deck as recommended to him by an "engineer". I am recommending keeping the insulation at the attic floor(where there is already R30 blown FG) and installing a radiant barrier at the roof deck since his ducts are in the attic and we live in an inferno. I think I got him off this crazy batt idea for now, but I am always looking for good information sources to help the light bulb come on for all these batt crazy builders and general population, when logic isn't working.
Not exactly what you are looking for, but it should fit the bill. There is a lot of solid info for you. http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-in...
I have only seen batts against the attic ceiling a handful of times. I an in Northern California. Not a heavy humidity climate. In the times I have seen these, they had all dropped / sagged down and were no longer against the ceiling. I assume they had absorbed moisture (?) and that was enough weight to bring them down and sag a little below the wires. Left an inch or so gap between the ceiling and the fiberglass.
Vibrations over the years might have contributed to it, not sure. I remember thinking "Well, wont ever do that at my place:. It was summertime in each case and still hotter than blazes in those attics.:
I am no expert on the subject but that is how they looked to me. Maybe it might help build a case for not doing it.
There are a couple of things to consider. First off fiberglass does not work well in an open air environment... that is it needs a 6 sided airthight enclosure to be effective.
Roofs give off most of their heat via radiant heat. Fiberglass is not good at stopping radiant heat.
The performance of the batts would be extremely poor. NREL has come up with alternatives to bring the ductwork inside the envelope. I think the biggest concession would be giving up some floor space for a mechanical closet.
Thanks for all your input guys. In this particular case, common sense prevailed. I guess since there is nowhere that really says don't do it, you just need to provide all the common sense you can and if they can't see the light, let them be in darkness. Good input all around, we seem to have concensus - batts on the roof deck suck.
Batts suck, period. But attic ducts are even a poorer choice. I know, apples and oranges, but my topic is dumb choices.
Preaching to the choir Stan, but hey, what can you do? Probably over 200,000 existing homes in Tucson with ducts in attic space. We can fix them as best we can, but we are not building them new and every home isn't a total remodel. Such is the retro market!