first some questions..
what type of attic venting do you have?
soffit venting with ridge vent
gable end venting
or turbines? (please don't say pav's!)
what is the approx roof pitch?

are you going to abandon the ducts and go with mini splits?
are you going to install a white metal roof??.when??
will you have solid roof decking under the new metal roof?

"In my climate, New Orleans, the common modern advice is to seal the attic and insulate the roof.

This is usually done with foam insulation, which provides
the insulation and air sealing...use open cell which allows
moisture to exit, rather than closed cell which will trap water against roof decking.

I would like to do this, but one flat roof stymies me. One is in communication with the gabled roofs meaning that it is vented at the edges and opens into the gabled part. I think that the other flat roof is not connected, but vented on its own so I can ignore it for now.

You would need to verify this.

What do I do about the one that is in communication?

Seal the openings between the rafters where it is in
communication with the rest of the attic. If you choose the
open cell foam route the foam company can do this.

Do I seal the vents and try to blow in insulation?

No, the insulation value you have here..it just what you have. It would be difficult to blow insulationin these flat roof areas. At some points within the joist bays the insulation would be in contact with the roof decking..which will cause condensation issues and rot the roof decking.

I guess there is about 6 inches of space above existing insulation. I dont think that makes much sense because it will compress and leave a space between the flat roof and the attic space.

you need this space with conventional insulation.

you also have a lot of variables! although I'm sure you
knew that already.
there are some good energy raters in the NO area, go to
LaDNR for a complete list.

best of luck.

Thanks, energy, very much. You have given me some good information to chew on.

Sometimes I dream of raising the pitched roof over the flat ones to make this easier. I think it would be way too expensive. My existing garage roof is in shot (separate building) and the house gabled roofs are not that much better. I will have to fix both of them before I install solar panels (PV and water) on them. The house is a sort-of cottage style built in 1940.

Before I answer your questions, let me know if some pictures or an aerial view would make it easier on you. I can send some information.

Attic venting:

On the (pitched-roof) original structure there are soffit vents and gable end vents. On one gable end, at the rearmost flat roof, there is a big old belt-driven fan. It does not appear to operate and its screen is decayed and difficult to get to. (Under the fan what I thought was metal sheet flashing is really the only thing between the attic space and the outside. I think it pretty unconventional and disturbing.)

The flat roof in communication with the main attic has soffit vents. I did not pay that much attention to the other, rearmost flat roof venting. I think that I can deal with that later provided it does not communicate with the main attic. IIRC, the only venting are some minimal stack-type vents penetrating it.

Roof pitch: I wish I knew. All I can say is that it is in line with other houses in the neighborhood so nothing radical. It is steep enough so I can't really rest anything on it because it will slide/roll down the shingles. Want to see those pics?

I am seriously thinking of going the mini-split route. I have a hard time, however, deciding what rooms (of 7 in 2000 sq feet) won't need an indoor unit. We bought the house, last November, because it has all its original doors and walls intact and we like to close the doors. Seven units will cost a lot of money. If the ducts are in good shape, it may cost a lot less to replace the current split AC/gas furnace. Doing a duct test as it sits might be difficult because the company that diagnosed an evaporator leak before we bought the house, apparently, did not seal up the air handler the way it should have been.

are you going to install a white metal roof??.when??

I think so, on the pitched part at least. Since the LA subsidy for solar is not secure past the end of the year, I guess I better do it soon and get the solar installed.

will you have solid roof decking under the new metal roof?

Wow, yet another complication. I did not know that I had the option of no decking under the new roof. I guess that retaining it would have the advantages of sound deadening, and lower expense since the old decking does not have to be torn off and hauled away. What I would need to find out is what it means to wind resistance. I wonder if removing the shingles to replace with lighter metal will be negative due to the reduced resistance to uplift.

I am speculating that you question the utility of bothering with attic sealing if mini-splits and white roof are installed. It must reduce the marginal improvement of both of those modifications and I wonder how much.

pictures would be nice.

you have a lot of options...radiant barrier on roofline,
foam on roofline,,seal ducts, abandon ducts. new central
vs mini-splits. solar/new roof..

I truly think that the way to get the best bang for
your buck is to first fix the house..before hvac install..
because stopping air leakage will result in a smaller
better performing hvac system or even with solar.
folks install solar to reduce utility bills.
in a perfect world..and definately another state..net meteting would be easily done rather than be an uphill battle with utility providers. but entergy..cleco..it just
isn't easily done.

just say you had unlimited dollars for a solar install.
wouldn't it make sense to make the house energy efficient FIRST..then install solar to handle part or full load of the house? The house would be tight,, the solar install
smaller because load of house has been reduced.
what I see happening with solar here in La is that the homeowner pays for a solar install but the solar company's
interest is selling all the solar they can. When common sense tells you that if your house is sucking air from attic
and walls that it will still be expensive to condition.
The same holds true with many hvac companies, Rule of thumb
sizing, no duct sealing resulting in a 2000 sq ft house with 5 tons of hvac. Every day. Short cycling, poor humidity removal, higher utility costs.

When I first rented my place my rent and utilities were the same..$300 per month each. Now I own the same place,,and my utility bill this month was $35. ($2.00 more than last year) My Kwh usage is low and I pay more per kwh than my neighbors.

The bulk of the savings came from air sealing. cases of caulk, because everything leaked..floor moldings, ceiling moldings.door and window trims. I am in full sun, so I installed a radiant barrier on the roof rafters and on the
gable end wall that got west sun.
duct sealing materials are cheap, and its not rocket science. A bucket of paint on mastic and you can make a huge
difference. Prolly sealing the supply boxes to the attic floor is the most difficult. (but you can do it from inside the house with mastic tape)
stop comminication between the attic and the living space.
This is the humidity and temp gain that you feel every afternoon.

mini splits are set up to handle multiple air handling units. the condensing unit is located outside and the ahu's
are placed in different rooms inside. do a search for minisplits ..daikin is one of the best for mulitple ahu's.

I asked about the roof decking because if you spray foam against the decking it is a good thing,,but I think companies would hesitate to spray a residential home
with a metal roof. Not all of them..but there may be questions you would want to ask them first.

why don't you get a couple of bids for hvac..
ask that they evaluate the duct system, and bid a unit
that will meet federal tax credits.
variable speed units help out with oversizing and provide
good humidity control in most cases.
for more hvac info..but no diy..visit www.hvac-talk.com
very educated folks there.
there are even some building science folks there.
also visit www.buildingscience.com and read the hot/humid climate info. www.fsec.org is also a good source for info
for our climate.

I feel that I've given you homework..but it is worthwhile.
after all its your home.

best of luck.

Thanks again, Wow, you have an enviable record with the utility bill reduction. I am not sure what your complaint about net-metering in LA is about. Could it be that if I produce more power than I use, I only get wholesale price and then only after I die? I have not looked into how it is done in other parts of the country. Maybe it is done better elsewhere. So far, I have not been pressured to buy more photovoltaic than I need. I am dealing with the big dogs in the region and a smaller NOLA-based company. The latter seems to be flexible while the former has a sales staff that seems to have "packages". The problem is that I have to decide what I need and that will not be easy.

I understand that fixing the house first would be a better idea in a perfect world. There are some things that make that difficult. First, I dont know how long the generous Fed and state programs are going to last. Second. My AC is FUBAR. There are two, 2-ton, compressors, one rather old, feeding a split evaporator. One circuit in the evaporator is leaking. I have an estimate from the previous family that says it will cost $2K to replace. I guess it is a custom evaporator. We also have an unusual tax situation that makes it better to spread the costs out over a few years, though we could do a loan.

One thing that I forgot to mention is that when the AC ducts were installed, the central hallway ceiling was dropped and, later, the ceiling in a closet and part of a bath were dropped all to accommodate ducts. Unfortunately, the areas appear to have been boxed off open to the attic. That is right, the attic "floor" is actually boxed "down". Part of this is under the furnace and air handler. I am having a difficult time figuring out what is going on because I can not see all of it. Sealing the supply ducts/boots to the attic floor will be a piece of cake compared to that mess. That is why I was relieved to know that sealing the attic was a great idea. Then I had to think about the flat roofs.

Speaking of which, if I close off the flat roof from the gable, and leave the flat roof vented for now, I will need to add vents to let continue the air flow that enters through the soffit vents, right?

Believe me, I am counting on variable speed equipment to adjust to my changes in the house in the next few years. For us, oversized, but adjustable will be a big boon. We are both gone all day so having a fast-cooling system that can throttle and control humidity will be nice whether ducted or mini-split. I have not looked at daikin. I have seen the Sanyo, Mitsubishi and LG "multi-minis" though. I will check them out.

I have been to the HVAC-talk site, but not for a while. I learned a lot about AC and air quality there. I am a tech-minded guy, but I had a lot to learn about AC and humid climates since I moved here from drier places. I think I have read most of the stuff about hot-humid climates at the building science site, but I should check it again. Will you check the www.fsec.org link. I dont get a rational response.

As a historical note. This home was probably one of the early adopters of central AC. There was originally a water-cooled condenser and the house still has three-phase power. There are some interesting-looking motor controls left in the attic. It is hard to say when the AC was first installed wrt the flat-roof additions. I suspect the first one was there because it is fed from the duct in the dropped-ceiling hallway. The later flat-roofed room in the back, with a lower ceiling, has AC supply that runs under the house rather than through the attic.

Again, thanks from the bottom of my heart. You are very helpful.

http://www.fsec.ucf.edu/en/

sorry about that florida solar energy center has
great info..not biased..not selling products.
use the search feature to search for specific info.
like radiant barriers.

You are very welcome and I hope to help! I replied to your email so check your mail!

best of luck.

Thanks