It's really not good to have the HP or the ductwork exposed those temps.
I would take whatever measures you can to insulate and air seal around them.

Unfortunately, I can't encapsulate all of the ductwork. I can install a closet for the unit, and replace the 20 year old ductwork with new R-8 rated materials, but it would cost too much to bring all of the second story ductwork within the envelope of the building.

Who will you get to replace a compressor if the attic is at 150*? A compressor changeout takes the better part of a day. Build a conditioned room, with enough room to work, your equipment will have a longer life span and your repair guy will be happy!! Make sure the ducts are air sealed, insulated and then have them buried in blown in insulation.

Bergy

I'd get some more data before you go to a major expense.

Here's some data for residence in a hot Dallas climate (79 100°+ days last year) .  This may help to frame how big the problem may or may not be.  Structure is a 3400 sf, single story residence.  With it being single story, attic expanse is significant.

Insulation is blown-in fiberglass, at R-38 amount.  Nothing else special about the attic - under eave vents and three wind powered exhaust fans in the roof.

Here's a chart showing the hourly avg attic temp for almost 4 years (measured outside of the cabinet of 1 of 2 geo units).  Key things to note are: (1) peak temp is about 135° F,  (2) for only a few days each summer is the peak above 130°, and (3) it's about a one and a half month period where peaks are constantly above 125°.  I'm getting these conclusions from the 2009 data section of the chart because in Jan. of 2010 I installed a lot of solar panels on the South facing roof and indeed you can see about a 10° reduction in the summer peak temps.

OK, that's the extreme.  Here's another chart showing the daily avg attic temp.  Key things to note are: (1) the highest avg daily temp stays at or below 104°, and (2) for only a few days each summer is the avg daily temp higher than 100°.  In fact, it's about a one month period where avg daily attic temp is above 95°.

And here's the broad look.  Here's another chart showing the monthly avg attic temp.  Key things to note are: (1) Only one year does the avg monthly temp approach 100° (2011, the year we had the record heat wave); and (2) otherwise the avg monthly attic temps stays at about 95° or lower.  (Parenthetically, note that the solar panels affect much more instantaneous peak temps, and not monthly avg temps.)

So the above is the environment to consider.  Your actual temps will be a little higher with a shorter roof height, and a little lower with a higher roof height.  I don't know how the attic floor area sf affects the numbers - the attic floor illustrated here is 3400 sf.

Now let's look at what's happening inside the cabinet where the compressor and electrical parts are located.

Here's a chart showing the hourly avg inside cabinet temps.  Key things to note are: (1) peak temp is about 120°, 15° less than the ambient peak; (2) for only a few days each summer is the peak above 115°, and (3) it's about a one and a half month period where peaks are consistently above 105°.   In this case I'm able to use the entire chart as the solar panels are not showing any affect on the inside cabinet temps.

Now let's look at the daily avg inside cabinet temps.  Key things to note are: (1) the highest avg daily temps over 100° are limited to a few days each season (and for 2 of the 4 summers shown the avg daily temp never got to 100°); and (2) it looks like there's about a month's worth of days where the avg daily temp inside the cabinet is above 95°.

And here's the broad look for inside the cabinet temps.  Here's a chart showing the monthly avg inside cabinet temps.  Key things to note are: (1) the highest montly avg temp ranges from 94 to 98° on a season by season basis; and (2) inside the cabinets are generally at or below 94° every summer on a monthly avg except for occasional years where you may get a couple of months 1 or 2° higher.

I don't know or remember where you're located.  In case you want to interpret what you may expect at your location, here's a chart that show the hourly avg attic to outside temp difference.  Here you can see the peak difference is about 35° for a couple of days in the 2009 summer.  And that it's about a 30° peak difference for about 4 weeks of hot days.  (The effect of solar panel installation is most pronounced on this chart).

Lastly, on this summary page, if you scroll down just one page, you can get an idea of the roof slope to compare to what yours may be.

Conclusion?

Based on the data that I've got, if I were you, I would do 4 things:

(1) Proceed with putting the unit in the attic;

(2) Carefully make sure all duct work is well sealed, particularly on the return side;

(3) Put R8 wrap on steel duct and/or use R8 flex duct (see example here).

(4) Not proceed with the major expense of building a closet around your unit in the attic, or the major expense of foaming the underside of the roof. 

Short summary?  Get on with putting in the unit in the attic, make sure all good installation procedures are followed, and don't bother with the expense of a closet or foam.

Hope this all helps.

Best regards,

Bill

Fortunately for me and my installers, it violates our (Florida) code to install an air handler in an attic. I visited three attic installations (predating this code) in the past several weeks. All 3 airhandlers were sweating profusely, rusting their drain pans, sprouting mold, and, in one case, producing water stains on ceiling. Inspecting all 3 was a miserable experience owing to attic conditions. The last thing I want to do, or ask staff to do, is work on such systems.

Note that application of sprayfoam eliminates the attic, at least in the eyes of the code. The attic ceases to exist, code renames it "indirectly conditioned space", and air handlers are good to go. In fact, that space above the ceiling becomes my favorite place to locate an airhandler since I can generally place it advantageously for ductwork and the install is generally easier than a vertical unit in a tiny closet whose upper reaches defy efforts to reach and properly seal. It runs quieter, and the client keeps all their closets.

I understand the high cost of sprayfoam (I paid right at $10k for mine in 2007) but, done right, it substantially reduces the size of the system needed along with a host of other benefits.

Interesting.

In the Dallas area, it's a rare exception for air handlers / furnaces to be located anywhere except the attic (residential).  Thus, HVAC crews have to accept the occupational challenge of hot attics to work in.  And customers have to accept that it may take multiple visits to accomplish something that may otherwise be doable in one day (because of excessive attic temp being a health risk).

The unfortunate side to the subject here is that there are so many installers looking for business, that for every one installer that declines a job that requires working in a very hot attic, there are 4 others willing to do so for the job.  Further unfortunate, technical expertise is often a secondary concern amongst the leadership of many HVAC companies (residential) here.

Best regards,

Bill

It stands to reason that the quality of work done in such adverse conditions is piss poor, serving no one well.Fortunately my better customers understand that.

I have several attic duct projects identified this past summer back pocketed for November-December. They include duct sealing and plenum rebuilds that will turn out far better absent insane working conditions.

I have been mulling a trailer mounted 4 ton package unit for use at sites where we are forced to deal with hostile attic environments.

Posted By engineer on 11 Sep 2012 12:35 AM
It stands to reason that the quality of work done in such adverse conditions is piss poor, serving no one well.Fortunately my better customers understand that.

I have several attic duct projects identified this past summer back pocketed for November-December. They include duct sealing and plenum rebuilds that will turn out far better absent insane working conditions.

I have been mulling a trailer mounted 4 ton package unit for use at sites where we are forced to deal with hostile attic environments.

Agreed.

Wow!  A trailer mounted package set up would be a first in the Dallas area for residential stuff.  You'd have customers lined up in no time, in spite of the intense competitive market!  Quality work with the time to do it, and at least giving to the customer one room that's cool enough to sleep in.  I think this is an awesome idea!

Best regards,

Bill

The trailer unit would be temporarily ducted into the attic for staff. It wouldn't do much for the house.

I have loaner window and roll-around portables to keep a room or two comfy while we work

Ha! Ha!  Here I thought the trailer unit was for the customer!  It's your crew you're worried about!  Awesome!

The fact that you have loaner window units for the customer and a trailer unit for your crew up in the attic certainly sets you apart from almost all competition in the Dallas market!

Best regards,

Bill

Thank you for all the input from the community. I understand fully at how brutal it can be in the attic since I've been doing all of the work myself over the past two months on this install. From running the electrical circuit, to building a stronger deck for the unit to sit on, I don' think that it has been below 85* during all of my time up there. It sounds like there are two modes of thoughts for the installation and not a clear "right" path. I do plan on installing all new ductwork and ensuring that everything is completely sealed using mastic (not foil tape). I still may entertain the idea of building a closet up in the attic, but I will base that off my observations during the next few months of operation. Thanks again for all the extremely thorough information and input from the community and for entertaining my bantering of questions.

Use foil tape (UL 181 rated) and then fiber reinforced mastic.

They work together, not mutually exclusive.

If you insist on installing a system in an unconditioned attic, look hard at R8 flex, ductboard and wrap.

If you get really good at it and want to move to Jax, FL, let me know, I can always use a good duct guy.

Engineer, are you saying to apply foil tape on top of the mastic? My plan was to use mastic on the majority of the duct work seams and foil tape for the vapor barrier on the insulation. Does this sound about the best solution? Thanks again for all your help. Also, can you wrap the duct work with fiberglass insulation prior to Tlthe 48 hour cure time for the mastic?

I had the ducts delivered today and see a very busy weekend on my future.

Foil tape doesn't stick well to mastic. We tape, then mastic.

Be sure to use a squeegee to both ensure the tape is in full contact and heat-set the insulation (via friction from firm use of squeegee)

It would probably be best to let the mastic fully dry before wrapping it