Benefit to extra insulation around attic ducts?

[texasmax]

All-

So I burned 3700 kWh cooling my 3,000 sqft house last month. I'm halfway through my radiant barrier job (stapling foil to the rafters myself, but with the heat in Dallas I'm going real slow), added more soffit vents in conjunction with going from 4 to 7 12" whirlybirds, had the AC fully checked out by the co. that installed in 4 years ago (5 ton XL19i), most of the bulbs converted to CFL and airtight baffles on most of the recessed lights, 90% block screens on East and West-facing windows, and in the process of having some of the older (30-year) larger windows replaced with some more efficient ones. Sealed all the supply and return ducts to the drywall. Going to have more insulation blown in the attic as well. Thanks to you all

So, now I'm thinking, air handler/furnace and ductwork all in the attic. Original sheet metal boots, 4-year-old flexduct with the foil-wrapped insulation. I've had the AC company re-inspect ductwork for leaks (visually and by hand, at least). But there's some particularly long runs, especially as the original 2-unit, 2-zone was replaced with a single unit, so there's an 18" duct run, maybe 40', from the handler from one end of the house to the other to meet up with a triangle where the removed unit used to be. And then back out up to 20' to the supply registers in the center of the house.

Are there any gains to be had by throwing some extra insulation wrap around this run (and even some of the shoter trunk line runs)? It's 130 degrees (peak) up there when it's 100 outside. When the RB installation is complete, I'd imagine this would come down a bit.

Thanks!!

[soupcxan]

TM, you sound like me - I'm in DFW, stung by ridiculous electricity bills (0.205/KWH last month). I installed 2000 sq ft of radiant barrier myself (that was brutal), replaced recessed light fixtures with airtight insulation-rated ones, added soffit vents, and just recently blew in some more cellulose insulation. So far, the RB has not drastically reduced the attic air temp (it still gets up to 125 on a bad day). But it may be 5 degrees less than it was before - too many variables and lack of data to make a scientific observation. However, my KWH usage per day dropped since I made those changes (too early to tell on the insulation since that was done recently) in spite of the fact that the average temperature in July was higher than June. So I think I'm seeing some benefit from the RB. I wont know if the extra insulation over the ducts has saved electricity until the end of August. Not sure if dallas gets humid enough for condensation to be a problem with buried ducts but I'm keeping an eye on that as well. How much insulation do you have on the attic floor right now? I was around R-35 and took it up to R-49.

[texasmax]

I've got anywhere from R-11 to R-19 right now.

Most of the ductwork (the larger/longer runs, anyway) is hanging, bottoms 18" or more off the floor, so burying isn't an option unfortunately.

[soupcxan]

At R-11 to R-19, it sounds like you might benefit from adding some more insulation on the attic floor before you tackle the ducts. The department of energy recommends R-30 for the attic in our area.

[texasmax]

Oh yes- I've been getting quotes and plan to do this soon.

But, it still won't help the heat load on the ductwork in the attic...

[beenthere]

No, it won't.

But it will help to knock load off of the house.
Double check for air leaks into the attic.

A check of teh temp difference from teh unit to the triangle will tell you, if you will benefit from wrapping teh duct ork.

[adamk]

How did they check for leaks, by hand ?

With the AC running and feeling for air flow - hopefully.

May need to check for leaks through the ceiling. If you suspect the walls are missing
insulation, check with a temp gun to see if one area is hotter than the rest...

I've found leaks in the AC systems I was walking around in various houses. Lot of
loss there.

Hope you find something.

[Daltex]

Also don't forget to check for return air leaks @ the airhandler. I sprayed perfume on mine and it went all through the house. Mastic fixed it.

[skippedover]

FYI, your greatest savings with insulation is with the first 1". Additional insulation the savings diminishes with each additional inch added. The other hot tip is that insulation does not SEAL anything. It just stops conduction heat. So whether it's a supply leak, return leak or 'chimney effect' with the house leaking, there's plenty of air to be lost. Incidentally, one of the least observed losses of ducted systems is bedrooms where the returns are not MATCHED to the supply air. If there's insufficient return air, then the bedroom will be under pressure when the door is closed. The higher pressure in the room forces air out of the envelope, to either outdoors or to the attic. If there's too much return air, then the room will go negative when the door is closed and you'll be sucking hot, moist air from either outdoor, the attic or whereever it can suck the air from.

[texasmax]

Didn't think about checking the temp difference between the handler and the triangle- will definitely do that

Yes- when the AC folks checked for leaks, it was feeling for air flow with the AC running (so, supply side at least checks out). They didn't check the air handler/furnace assembly itself- roughly half of the seams are foil taped, the rest are bare (it's quite hard to get to)- will check this as well, as well as the perfume test at the unit return...

And on that note, is there something readily available, besides perfume or incense, I could use to check for negative pressure at the return inlet? Something that won't stink the place up? Maybe I'll borrow my 2-year-old's baby powder and puff up a few clouds on it and see what happens

Ive done the infrared thermometer gun all over the house (bought one a month ago- where have you been all my life?)- it jives with what I see are the spotty insulation areas on the attic floor. Again, will have this done in the next few weeks.

Skipped- understand you 100% on the insulation not sealing anything. From the house-side, I've sealed all outlet and switch covers, bathroom and kitchen exhaust fans (I need to ensure the pass through is tight up through the roof), all the supply and return grills (except for a few that are out of reach, and I foamed the heck out of them on the attic side), and airtight can light baffles on all but 8 that are out of reach (and I might foil tape the vent holes on top of the cans- good/bad idea? they're non-IC). I also need to foam up the return boots where they meet the wall conduit returns.

As far as supply/return balance, the only time I've got door-affecting pressure differences is when we've got one zone off at night (return from the whole house, supply just to one side).

[texasmax]

From a temp gun at least, the temps at the supply vents for the longer runs are warmer than the temps for the shorter runs, by 6-7 degrees F. The longest runs go through a 40' 18" run, triangle, 20' smaller diameter, triangle, then smaller individual ducts to the supplies. Looks like I need to inpsect the triangles, check the temps there, but if it's all tight, might wrap some fiberglass around the first 40' run and see what happens from there.

[tpa-fl]

Does anyone have a recommendation for good PAR36 air-tight cans? I love recessed lights, love the PAR36 bulbs even more, but absolutely hate my uninsulated, leaky PAR36 recessed fixtures. Would love to find the 2 & 3-gang PAR rectangles I've seen in higher-end retail stores.

[ampulman]

In response to Texasmax, who wrote in part:


"And on that note, is there something readily available, besides perfume or incense, I could use to check for negative pressure at the return inlet? Something that won't stink the place up? "

This may sound hokey, but it worked just fine for me.

In a previous thread, Dash detailed how, by dangling a thread at the doorway to the garage (if you have one), and cracking it open slightly, one could determine if supply or return leakage is present (return leakage will cause thread to pull out to garage; supply, just the opposite).

Well I did the test and found that I had considerable return leakage. Since my ductwork is in the baement, I took a 4 foot dowel and taped a 1" strip of tissue to the end. I was then able to 'pass' the tissue over every joint/seam, etc., and found a giant (hidden) leak where the installer neglected to join panning to the trunk.

The dowel allows you to 'search' high or low without strain.

AM

[JD Perkins]

Has anyone seen spray foam used on the duct for insulation? We had a high end residential home that had it applied on the duct work. We used small cans to cover the damper handles so they wouldn't get coated. The insulation company did a good job and it really turned out good. It looked good and sealed everything tight plus has a good R value. The fall back is if something was to be changed or added on. But this customer was well off and cost wasn't a major factor. Efficiency was and I think it was achieved.

[texasmax]

That's funny- just last night I was passing a tissue over all my return vents, to see if any pulled stronger or weaker than the others (given that the larger-ducted, larger-registered returns will rpobably have higher suction).

I've got baby powder over half my ductwork, but I like that tissue-on-the-rod idea even better! Thanks!

[beenthere]

A larger return may have less pull then a smaller return.

[texasmax]

That makes sense, they're all pulling from a common handler thus at the same pressure...

[adamk]

When I was looking for return air leaks, I used incense sticks. The smoke quite easily
pulled into any leaking point. It was easy to see as well. Worked well on the supply
sides too.

When all was done the system was definitely quieter (I had a few good leaks.)

Adam