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Ocelaris 04-10-2013 09:52 AM

How to seal around metal duct boxes in attic?
So we just had central air installed in the attic, and I'd like to tidy up a bit the vent boxes which aren't wholly sealed as much as I'd prefer. Basically the box is wrapped with fiberglass, ducts are wrapped, but there is plenty of cold metal in a hot attic which I'd prefer not to have. What's the preferred method, wrap the boxes in a XPS or isocynate foam and spray the areas on top? I just don't think it's a good idea to foam just the top where it touches the flex duct, because then it would be very difficult to remove the flex duct, not to mention it would use a lot of foam. Any suggestions would be appreciated. I intend to foam around the gaps where the metal box splays out on the drywall. I'm trying to do this all before we put more insulation in the attic. Thanks, Bill

joecaption 04-10-2013 11:13 AM

Looks like you have less then 1/2 the required insulation in that attic.

Just add more blow in around them.

Seattle2k 04-10-2013 11:13 AM

Once you insulate the attic, those will be covered anyhow.

Ocelaris 04-10-2013 11:19 AM

Thanks, I'm planning on blowing in cellulose once we've done all the work we need to up in the attic (which is still a few months away), but I'm trying to air seal before it becomes a winter wonderland of cellulose dust etc..., I just didn't know if I should specifically cover the metal with something besides the blown-in insulation. Also trying to figure out where/what I'm supposed to air seal for those duct boxes. I think it should be sufficient to seal around the tabs which make contact with the drywall...


Seattle2k 04-10-2013 11:30 AM

Ok, if you're asking about air sealing...then aluminum-backed tape is what you want. The big box stores carry it.

Ocelaris 04-10-2013 11:33 AM

Are you talking about using the aluminum tape to secure the duct boxes to the floor? it's pretty dusty, not sure the tape would stick well, I've had pretty good success with my foam gun and the high expanding foam. I have some aluminum tape I was using to build boxes around my ceiling speakers, I can try that, just not sure how the dusty drywall will hold up over time...

This is the foam gun I have:
Great Stuff Pro14 Foam Dispensing Gun

Seattle2k 04-10-2013 11:39 AM

clean the dust off. expanding foam should be fine too. And I don't see why, under normal circumstances, that you would need to remove the flex duct down the road.

Windows on Wash 04-11-2013 08:43 AM

Do not just cover them will cellulose.

Spray foam/caulk all the seam in the boot and at any of the connections between the flex lines and the boot.

You can also use mastic or foil faced tape at the duct connections.

Gary in WA 04-11-2013 11:11 PM

Does that line make a 180* turn-around? Bad install, if so... They should have also taped the white f.g. insulation from showing (hence the foil vapor-barrier covering).... As you already have a foam gun... air-seal the drywall joints at the ceiling/wood top plate as picture under the end of the ceiling joist. I'd even use some (or cheap caulking) on the drywall seams like that pictured, 5" from the wall, unless that piece was added to cover a sloppy install of the supply boot. On third look, it appears to be 2 flex supplies off the rectangular trunk line in the background...

Help you find them;

Any holes (wiring/plumbing) from the basement/crawl to the first floor walls to the attic or through the second floor walls can transmit air/moisture from below to deposit moisture on the framing. Exterior openings (light boxes/switches) in outside walls are similar except they are a closer air path to the attic, air-seal the drywall/top plate joint there, especially.

PS. you probably know this already, for other readers;

Ocelaris 04-12-2013 08:37 AM

Thanks for the links, I have read a few of those a couple times! lol... but there are some new ones too. I wasn't sure whether to have the guys loop up the ducts, but for a lot of them it didn't make any sense to have it go up and back down for a short run. So they'll all just eventually be half covered in cellulose. The flex duct has R-8, so I hope that will be enough to keep condensation down.

Just some more pictures for details. There are probably a bunch of places that I can better seal, I'm planning on adding a catwalk with 2x6s and then add another 8-10" of cellulose, but there is a lot of sealing and some wiring to do. Also I have to do metal flashing around the chimney as well.

Gary in WA 04-13-2013 11:48 PM

Is that one supply running over another between chimney/furnace? I'd switch them at the main trunk, if so. Your return air duct is pinched from the narrow overhead support strap. I'm surprised to any ducts they could have shortened to remove the flex compressions, let alone the one 10' too long instead of the direct route next to the WHF. Speaking of which, I'd duct exhaust that out the gable, not into the attic, IMO.

Very important not to restrict flex with a sharp turn, narrow strap;

The R-8 is minimum code, I believe, though considering your heat is produced/delivered from in the attic (which should be the same as outside temps), I would install some lightweight batts/blanket over the tops. R-8 is a 90% reduction in heat loss, R-25 is a 97% reduction, big difference, and the air is delivered warmer to the rooms. Some of them will be covered by the blow-in. Be careful not to overload 1/2" ceiling drywall, if applicable;


Ocelaris 04-14-2013 09:12 AM

Thanks for the advice, so flex duct should be stretched tighter than loose? I think I'll put a few more straps on the hanging ducts, as even if they're not sagging now it doesn't seem like there are very many hangers.

I think you're right about that one duct is crossing over another. I'm going to have to look at why that is...

This system is actually only Air conditioning, we have a boiler and cast iron radiators throughout the house. The attic is too small to build into, so because it's cheaper to do venting in the attic, we went that route. I'll see about increasing the duct insulation, right now it's R-8, but when I blow cellulose in, I don't want any condensation on the exposed portion particularly.

The whole house fan actually did vent through that gable vent, or at least the gable vent opened when the whole house fan kicked on. But there is no ventilation in the attic, so I've temporarily hooked up the motorized gable vent to the power fan to give it a cross ventilation effect. Eventually we'll take down the soffits (which have no vents) and replace them with a continuous soffit, and put the motorized gable vent back on the whole house fan. Ideally I'd get rid of the whole house fan that currently exists, and just do a ducted gable vent so the fan is more remote, but It's probably a 20' run from the whole house fan louvers and the side of the house, I imagine that would be a long run for a 24" flex duct... we had thought about fabricating one, but still not sure how to utilize the whole house fan.

Thanks again

asinsulation 04-15-2013 08:25 AM

Just a recommendation. You have a very complex airseal job to accomplish there. You may be better suited to find a pro to handle it(just the seal up) so assure it is done correctly. Between the buffered wall space, the change in attic height, chimney, ducts, recess lighting, access, whole house fan. There is alot of stuff going on up there, so getting the full effectiveness of your airseal will be a bit difficult if its your first time.

Gary in WA 04-16-2013 04:20 PM

Of course, since we are a DIY site, (and encourage it) we can help you also; RBE-WH-ALC_Install_Certif_Scheme_Handbook_V1.3_DRAFT.pdf (application/pdf Object)


021105092.pdf (application/pdf Object)


bobinphx 04-20-2013 01:01 PM

not to be harsh, but that is one bad installation. repost those pictures on the hvac forum here and they will help you get it fixed up so it will work better and cost you less money... at this point, leaks are the least of your problems... wow is all I can say...

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