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Excessive Heat Gain through Attic Ducts ????

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gstein

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Discussion starter · #1 ·
Quick question

We are selling a house here in Houston which was just inspected by a potential buyer's home inspector. The inspector indicated that none of the A/C units is cooling correctly with a measured temp drop from return to supply of about 13 degrees F. Of course, I immediately grabbed my meter, stuck a thermocouple into the ductwork immediately before and after the evaporator and measured a 22 degree drop on each unit, (78 degrees in, 56 degrees out). No problems there. On the other hand, the temperature at the supply registers measured between 58 and 66 degrees F with a return temp of 78 degrees. This made me start thinking about heat (cold) loss from attic ductwork.

The attic was 135 degrees with an exterior temp of about 100 degrees when I checked the system. The ductwork is R6, and the average wall is roughly R15 with R3 windows giving an average R value of about 9. This is fairly usual for a Houston house in summer.

The interior to exterior delta T of an outside wall is about 100-75 = 25 degrees F, whereas the delta T for the duct is 135-56 >= 75 degrees or about 3 times as much. A 10 inch duct has a circumferance of about 32 inches. The higher temperature difference across the duct is the equivalent of increasing the duct circumference to about 96 inches compared with the temperature difference across an exterior wall. With an average R value of 9 for the wall and 6 for the duct, the duct conducts 1.5 times as much heat per unit area, so the duct circumference is effectively increased even more to about 145 inches. Therefore, a 10 inch duct insulated to R6 in the attic has effectively the same heat gain per unit length through conduction as a 12 foot tall exterior wall with 50% of it's surface area covered by windows.

Using this logic and adding up the duct lengths and diameters in the attic, the conduction heat gain from the ducts in the attic is nearly as great as the conduction heat gain through all of the walls and ceilings of the house. Is my logic flawed???

This leads me to two immediate conclusions. 1: Our next house will be built with ductless mini-split systems and 2: the house we are living in here in Houston needs more insulation on its ductwork.

This leads me to a second question. How does one go about adding insulation to the standard flex duct mess that is normally found in the attic of the average house here in Houston?? R16 or more would probably be appropriate.

Thanks and later, George.
 
#2 ·
gstein said:
Quick question



Using this logic and adding up the duct lengths and diameters in the attic, the conduction heat gain from the ducts in the attic is nearly as great as the conduction heat gain through all of the walls and ceilings of the house. Is my logic flawed???

Thanks and later, George.
Click to expand...
Your logic is not flawed. In Arizona only windows have a greater heat gain.

On manual J's we do here the average home has 20% to 30% of its total heat gain coming from its ducting and that is using R6 ( 90% use R4.2).
Windows on average have 35% to 45% of the total gain.

My advice on your next home is to somehow get your ducting into the air conditioned space. In other words move your thermal barrier to the roof line in lieu of the ceiling.:payattention:
 
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#3 ·
hear hear!!
design your next house with ducts in conditioned space.

or foam insulate the roofline (design with lower pitch roof)
and system in attic along with ductwork is in semi conditioned attic.

personally I've seen lots of home with fur downs for ductwork that
most people thought were just architectural features..it can be
done attractively. (I like it myself..too many folks paying
large amounts of $$ to foam 18/12 rooflines..payback is
20+ years..good for foam companies, but for consumer??)

you really can't insulate over flex..problems with two vapor barriers and condensation
in our climate. same with laying ducts on attic floor and blowing insulation over them
creating condensation issues (ok in airzona & ca??)
I've seen some foam companies 'foam seal' ducts. but I don't like it personally.
My testing of duct leakage shows that the seal comes from mastic, foam
is not approved by hvac industry to seal ducts and as it goes on so quickly there
is little control to apply it where needed. the ducts I see with foam on them usually have
voids and gaps..

best of luck
 
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#4 ·
Some of the houses in Sunrise subdivision on Broadway have insulated roofs. What subdivision are you in?? I could probably describe your ductwork. Been in every one here. LOL
 
Discussion starter · #6 ·
Sausage in the deli window

The new Florida retirement house will have tray ceilings and I should be able to have the ductwork run in conditioned space above the sheetrock. Assuming of course it has ducts, those 22+ SEER inverter mini-split systems look pretty interesting, especially if solar panels come down in price a bit.

My concern right now is the Houston house we are currently living in. Our last electric bill was $648.00 at 11 cents/kw. Owwwch. I figure at least $200 of that is the attic ductwork that is hanging up there like sausage in a deli window. I can think of 3 possible options to improve this.

1: Replace it all with rigid ductwork that has plenty of insulation. Probably an expensive proposition that won't pay for itself quickly, but air distribution could be improved which would save some additional energy.
2: Wrap the existing ductwork with R8 or better duct wrap. Probably not an easy job getting the whole thing sealed up good enough to prevent condensation problems, especially since the flex duct is probably much more flexible than the seams would be.
3: Drop the existing R6 insulated ductwork down onto the joists and remove the insulation underneath the ductwork, then install a vapor barrier over the top of the ductwork that is sealed against the sheetrock below. Insulation would be installed over the top of this, probably fiberglass batt, and the area under the vapor barrier would be vented to indoor space.

Does anybody have any other ideas that might work?
 
#7 ·
We've had cellulose blown later over the top of a lot of our ductwork without any condensation problems, however this is in the midwest, so anything near a coast might have too much humidity. Cellulose gets more dense, and allows less air to infiltrate as it settles, so less humidity can get into it. Just make sure and put 6 to 9" over the top of all ductwork, metal and flex. Of course the ductwork must be sealed really well first!
 
#8 ·
Consider a radiant barrier at the roof deck to slow heat gain to the ducts in the attic. This is the largest benefit a radiant barrier can provide...a basic performance boost for your a/c while reducing operating costs.

For your retirement home, build it tight, ventilate right. Read carefully about building in a hot, humid climate like Florida. Lots of things done wrong can lead to lots of heartache for a retirement house. I would not so much as nail up one stud without having a good understanding of making my house survivable and sustainable for the climate it is being constructed in, especially if I planned to retire in it.
 
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#9 ·
jimj said:
Your logic is not flawed. In Arizona only windows have a greater heat gain.
Click to expand...
jimj, I live in Phoenix am planning a blow-in-attic-insulation project in the fall and am concerned about the condensation on flex-ducts if I cover them up...although I must add that some of the smaller flex ducting already seems to be covered up pretty well from the original builder.

Do Valley-of-the-Sunners have the same/similar condensation concerns as those in some of the Eastern/Gulf states?

Obviously, if it is not much of a concern here, then I would like to cover them as much as possible to alleviate the problem addressed by this thread. The ducts are already in the attic and they are there to stay.
 
#10 ·
Some flex manufacturers make R-6 blankets for insulating round duct. (It’s basically flex duct with no duct) You might be able to use this to add an extra layer of insulation to the flex.
 
#11 ·
jonpcar said:
jimj, I live in Phoenix am planning a blow-in-attic-insulation project in the fall and am concerned about the condensation on flex-ducts if I cover them up...although I must add that some of the smaller flex ducting already seems to be covered up pretty well from the original builder.

Do Valley-of-the-Sunners have the same/similar condensation concerns as those in some of the Eastern/Gulf states?

Obviously, if it is not much of a concern here, then I would like to cover them as much as possible to alleviate the problem addressed by this thread. The ducts are already in the attic and they are there to stay.
Click to expand...

In short, for an arid climate like Phoenix I don't think you would, but it's possible. In my own attic the relative humidty really plummets when the temperatures climb.
 
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#12 ·
jonpcar said:
jimj, I live in Phoenix am planning a blow-in-attic-insulation project in the fall and am concerned about the condensation on flex-ducts if I cover them up...although I must add that some of the smaller flex ducting already seems to be covered up pretty well from the original builder.

Do Valley-of-the-Sunners have the same/similar condensation concerns as those in some of the Eastern/Gulf states?

Obviously, if it is not much of a concern here, then I would like to cover them as much as possible to alleviate the problem addressed by this thread. The ducts are already in the attic and they are there to stay.
Click to expand...
It is NOT a concern in Phoenix, in California they give you credit for buried ducts and extra credit for deeply buried ducts ( title 24).

Cover them up and enjoy the savings.
 
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Discussion starter · #13 ·
Flex under insulation not a good idea in humid climates.

Burying flex in insulation is certainly not a good idea in Florida. We bought a 6 year old house about 8 years ago in Jacksonville, FL which had a section of R4.2 flex duct buried in cellulose. When I noticed it and checked it out, I found lots of moisture and wood rot under the duct. Yuck.

After buying the house, I went around the house and sealed all of the supply register boxes to the sheet rock with expando-foam to prevent leakage into the attic. Wound up with a condensation problem around the vents. Turns out some dry air needs to leak out around the vents into the attic in order to prevent condensation on the supply register boxes which are buried under the insulation. A few months later, I replaced a section of mangled 10" flex. I sealed everything up perfectly, but accidentally punched a small hole in the jacket while rummaging around a few months after that. The duct filled up with condensation and tore loose. Splat, wet sheet rock. A leaky connection in the core is apparently necessary to blow cool, dry air through the insulation jacket to prevent moisture buildup in case of an outer jacket leak. Basically, the leakier and cr*ppier a flex duct installation is, the better flex duct works in a hot, humid climate. Flex Duct is a builders best wet dream.
 
#15 ·
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#16 ·
gstein said:
Burying flex in insulation is certainly not a good idea in Florida. We bought a 6 year old house about 8 years ago in Jacksonville, FL which had a section of R4.2 flex duct buried in cellulose. When I noticed it and checked it out, I found lots of moisture and wood rot under the duct. Yuck.

After buying the house, I went around the house and sealed all of the supply register boxes to the sheet rock with expando-foam to prevent leakage into the attic. Wound up with a condensation problem around the vents. Turns out some dry air needs to leak out around the vents into the attic in order to prevent condensation on the supply register boxes which are buried under the insulation. A few months later, I replaced a section of mangled 10" flex. I sealed everything up perfectly, but accidentally punched a small hole in the jacket while rummaging around a few months after that. The duct filled up with condensation and tore loose. Splat, wet sheet rock. A leaky connection in the core is apparently necessary to blow cool, dry air through the insulation jacket to prevent moisture buildup in case of an outer jacket leak. Basically, the leakier and cr*ppier a flex duct installation is, the better flex duct works in a hot, humid climate. Flex Duct is a builders best wet dream.
Click to expand...
"Wet dream" and water logged flex ducts...nice pun.

Flex ducts in attics...just not a good idea period end of sentence. Any section of HVAC equipment out of the conditioned space, save the condensing unit, is not good thinking. Done all the time, as we know...don't know which is harder...to reign in bad thinking or push forward good thinking. Some may compare it to pushing a rope. :D

Your supply registers sweated after you sealed the boots because you were no longer sucking hot attic air into the house (via the venturi effect where the supply air exits the boot) where it mixed with the cold supply air to keep the grill warm enough to not sweat. You sealed up the gap and BINGO. Sweaty grill because it is now colder than the dew point of the room, which may belie other problems with either your HVAC system or your building envelope (or both).

Pertaining to your supply grill, the only way to make it sweat beads of moisture is to make it colder than the dew point temperature of the air surrounding it. By closing off access to attic air you actually reduced a source of moisture, for as you allude above there's plenty of water vapor in your attic where you live. So, that leaves only the moisture in the air of the room.

I used to think a ceiling supply grill with unsealed boot to drywall joints blew supply air into the attic when the a/c ran. I'm rethinking that, based on scenarios like above.
 
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Discussion starter · #17 ·
Sweaty Boxes

The pun was unintended, but I noticed it after the fact. Does that make the pun more funny or less funny?

Actually, the grills weren't sweating, the sheetrock around the grills became slightly damp and discolored when the weather was hot. I figured out that the metal bracket at the bottom of the grille box was conducting cold and sweating in the attic. Previously, I suspect the leaking air was keeping it dry. I fixed it by pulling the insulation back and goobering a bunch of expando foam all over the bottom of the box to cover the metal and bottom edge. This sealed out the moisture. I kept the insulation a little thin over the grille box as well.
 
#18 ·
gstein said:
The pun was unintended, but I noticed it after the fact. Does that make the pun more funny or less funny?

Actually, the grills weren't sweating, the sheetrock around the grills became slightly damp and discolored when the weather was hot. I figured out that the metal bracket at the bottom of the grille box was conducting cold and sweating in the attic. Previously, I suspect the leaking air was keeping it dry. I fixed it by pulling the insulation back and goobering a bunch of expando foam all over the bottom of the box to cover the metal and bottom edge. This sealed out the moisture. I kept the insulation a little thin over the grille box as well.
Click to expand...
According to the Manual J model in Hvac-Calc, the duct system for my house is modeled to be 7-8% of the total (4470 btuh out of 59,104). This is for a house in Fulshear TX which is maybe 40 miles from Pearland. Gstein, I can email you the report if you request -- my address is in the profile.

One of the things I worried about was how well Manual J7 would account for duct losses, after all leakage is most often more than that 8%. The J7 model does not pass the "smells funny" test for a Texas house with ductwork in unconditioned attic. Personally I created a fudge factor for this which was heavily criticised by some on this board. This may illustrate a shortcoming in the J7 model; I am told J8 accounts for it better.

That electric usage is hair raising, at 11 cents/kwh you must be burning close to 6000 kwh for the billing month. I wonder what is the number of days in that billing month and what the typical summer bill is. I'd love to hear there is one or several things you could fix and bring that down. This summer was exceptional for me too, but that means one 2141 kwh bill for a 32 day billing month, vs. a maximum 1757 kwh the previous year.

It would seem worth doing some serious testing to identify the source of high energy consumption. That would mean a thorough energy audit -- most definitely not one from a low bidder like Standard Renewable Energy (SRE) which in my experience was nearly junk. More like someone who has the skills to do a full HERS rating on the house. Hopefully this case study used a good skilled energy rater:
http://www.garyontheair.com/makeover/
However I have not had business experience with Direct Energy's rating service. Just don't trust SRE for quality work.


Best of luck -- Pstu
 
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Discussion starter · #19 ·
Yup, over 6300 kwh. It's a 4500 sf house with 10 tons of AC, single speed air handlers, 13 SEER. I had one of the knowledgeable guys on this board check it out before I bought it and he tactfully insinuated that the ductwork and equipment needs a few upgrades (i.e., its c**p), but then, every house in the area has pretty much the same stuff in the attic so the energy use isn't necessarily a surprise. I have a pretty good idea where the energy is going, just more items in a long list of things the builder left for subsequent owners to deal with. At least the open, unterminated 8" duct the builder left blowing cold air into the attic above the media room has been sealed up. The actual, measured temperature rise of air flowing through attic ducts was a bit of a surprise, however. A 56 degree input temp to 68 degrees output temp with a 21 degree temperature drop across the evaporator is more than a 50% sensible energy loss in the duct. Personally, as a homeowner, I think the Texas code should require R12 or better insulation on exposed ducts in the attic and some sort of insulated enclosure vented to the interior air space for attic equipment. The energy saving would way more than offset the added cost. But then, most homeowners don't have a clue nor do they care and I certainly don't have the same kind of bribe money... er.. political influence that the builders have.
 
#20 ·
gstein said:
Yup, over 6300 kwh. It's a 4500 sf house with 10 tons of AC, single speed air handlers, 13 SEER. I had one of the knowledgeable guys on this board check it out before I bought it and he tactfully insinuated that the ductwork and equipment needs a few upgrades (i.e., its c**p), but then, every house in the area has pretty much the same stuff in the attic so the energy use isn't necessarily a surprise. I have a pretty good idea where the energy is going, just more items in a long list of things the builder left for subsequent owners to deal with. At least the open, unterminated 8" duct the builder left blowing cold air into the attic above the media room has been sealed up. The actual, measured temperature rise of air flowing through attic ducts was a bit of a surprise, however. A 56 degree input temp to 68 degrees output temp with a 21 degree temperature drop across the evaporator is more than a 50% sensible energy loss in the duct. Personally, as a homeowner, I think the Texas code should require R12 or better insulation on exposed ducts in the attic and some sort of insulated enclosure vented to the interior air space for attic equipment. The energy saving would way more than offset the added cost. But then, most homeowners don't have a clue nor do they care and I certainly don't have the same kind of bribe money... er.. political influence that the builders have.
Click to expand...
Do you happen to have discussed this with other homeowners in the area, and do they have similar consumption? You probably received excellent advice from the board pro. Do you intend a project to replace the duct system, or repair it? That 8-inch open supply alone likely dumped 200 CFM of cold air, no doubt that resulted in an extra 200 CFM infiltration of Gulf Coast humid air into the living area.

I am a homeowner not far away, with a 3400 sqft house built in 1989. In 2003 my peak bill was 2700+ KWH for a 32-day month. My 2004 run of Hvac-Calc tells me the total heat gain is something under 60,000 BTUH, that is with sizable single pane windows, attic ductwork and not too much insulation. The house was built with two 3.0 ton AC units, rather a larger sqft/ton measurement than your house. If you were to have the Manual J heat load calculated, you might find the next replacement ACs did not need to be 5 ton. The smaller AC sizes always are a few points more efficient.

Since then I have done a number of small projects to improve energy efficiency, the most productive one in hindsight was testing and sealing the duct system. You cannot buy the R12 insulation you mentioned but you can buy R8 and I hope the best could be used for any new duct sections installed. Personally I would not be surprised if there were a major energy waste in an unforseen place, that is where a competent energy audit would pay off. The testing I imagine useful could cost in the mid-to-hi 3 figures, with your kind of electric bill the payback could be short.

A neighbor had his attic roof lined with foam, and the other insulation removed. He had horrendous electric usage before, but with that one project he put all his ductwork and AC equipment into unconditioned space where duct leakage is practically irrelevant. This would also make ceiling-to-attic air leakage irrelevant and harmless. I am wary of doing this for my own house as the foam is expensive and there continue to be stories circulating about wood damage if it is not done exactly right. Just an idea, but there seems to be concensus among building science people that houses should be built this way in the first place.

Best of luck -- Pstu
 
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