So, I'm ready to start investigating where to seal the duct work before insulating. Just for reference, this house is only six years old. One obvious place to address is the joints between sections. But, are the long seams where the rectangular trunks or round branches are assembled suspect as well?

This is the end of the duct in my attic -- http://i1218.photobucket.com/albums/...EndBoxDuct.jpg. I'll be pulling up the batts around it to check the joints. I haven't yet tried to lift it, but should I try to put some foam board or something under it? There is only the floor, some fg batts and lots of air space in those joists (garage ceiling below).

The other end of that trunk, where it comes up from the basement, is uninsulated for about 6-8 feet. Once I get that all sealed, good enough to just rewrap it in the fg?

 

Hi Yukon,
I'll attach some more reading for you below. Think wind chill and how it transfers heat more rapidly than just static air. Inside those ducts you have a considerable flow of air, so any shortage of insulation will result in increased heat loss/gain depending upon the season. IMO, R-40 if you can fit it in, R-20 minimum.

Now, before you bury everything, air seal the ducts and any areas of the house over conditioned living spaces. Not part of your post, but those electrical wires need to be installed per code. Easier now rather than later. Also, mark the location from above of any electrical boxes you bury.

As for what to seal on the ducts, all seams, everything. Any leakage to unconditioned spaces usually results in double the loss as replacement air has to be pulled into the system from another location. Be sure to continue your sealing throughout the duct network.

As a safety measure, anytime you change the air flow or air pressure anywhere within your home, you should have the combustion zone tested for proper air pressures, referred to as a Combustion Air Zone pressure test, or CAZ test.

Home Energy Magazine Online

Luck
Bud

 

Wow u got some issues! First batt insulation should not be used for duct. Duct wrap should be used. 2nd seal the joints between the duct long seams should be air tight from shop! When your at it I would replace the flex for a better grade

 

Now that I've seen a little better how these thing go together, there never really was an question about sealing the long seams. These are a bit tighter than the section joints, but still I see how they may leak significantly.

Now, if the radiant barrier part of duct wraps work like they are purported to, is additional insulation then necessary? What I have seen comes with about an inch under foil. Not much for R-factor, so I assume it really is just there to 1) damp some noise, 2) eliminate direct contact and conduction, and 3) protect the foil from sharp corners. Without the function of the r.b. the need is obvious for more R. My thought about the foam board under was just about adding real R by not compressing fg. I can remove some of the floor to make room for it.

 

Code in most areas is 3 inch.

 

So. while I am sealing up this trunk, would it be worth the effort to pull up the whole vertical portion where it comes up from the basement to seal all that also? I think I have enough access. Or is is enough just to seal off the chase it comes through?

Whether batts or duct wrap, the weight of the duct will compress the bottom portion of the fiberglass to pretty much useless. Would that have enough impact to make it worth doing something different underneath -- like double the glass thickness or use foam board?

I'm getting foil barrier for the general attic anyway. Any reason I could not just rewrap the duct with the batts and wrap with my own foil shell and tape it up well?

Thanks for all your comments.

 

I got the sealing done some time ago, but never finished the insulation. Spoke to the inspector today who said R19 top and sides but that the floor was assumed to be insulated and so did not necessarily need anything directly under the duct. That last bit doesn't make much sense to me.

The duct wraps I've seen are all less than R10 with maybe 1" glass plus foil. I guess they are meant to be a convenience to install, but don't seem to address the requirement. That suggests to me additional batts.

Does a vapor barrier enter into this at all to minimize condensation during cooling season? Or would it just trap inevitable moisture inside?