Insulating ductwork

MaxMercy

Hi guys, my ductwork in my unheated basement is uninsulated, and think it's high time I do this. I've been looking at foiled fiberglass, foiled bubble wrap, and solid foam board cut and fit.

Opinions? Thanks.

Jamie Hall

Fiberglass -- foiled or not -- is by far the easiest to do. Couple of inches thick. But it doesn't look particularly classy. Foam board -- again, a couple of inches -- can be done really elegantly, but one does need to take time to get it cut to really fit properly.

Bubble wrap... is wonderful stuff for shipping the fragile object to somewhere. It won't do much more insulation.. I wouldn't bother with it.

EBEBRATT-Ed

@MaxMercy

2" foil faced duct wrap. Comes in a roll 4' wide x 50' (if I remember right) 200 square feet.

A lot of people just put it on with foil tape but you are supposed to staple it on with a special stapler which the supply houses have. Then use the foil tape to patch any rips or tears.


The first thing if the duct isn't sealed well you should get a bucket of duct sealer (water based type) and a paint brush and paint every seam and joint. You may have to clean the dust/dirt off the duct so the sealer will stick.

Sealing may be more important than the insulation

HVACNUT

If you plan on doing it in place, then brushing on duct mastic as @EBEBRATT-Ed mentioned should be done first. And even though the bubble wrap doesn't have the R value that fiberglass does, it will be a heck of a lot easier. Foil tape on the seams.
If your going to remove and reinstall, then I'd go fiberglass. Still mask all the joints. It should be stapled, then scrim tape the seams, and staple again. Scrim tape has threads running through it, making it more durable than foil tape.
You'll be able to wrap presents like a champ when you're done.

Or you could spray foam the foundation walls. That would make it conditioned space and the ductwork doesn't need to be insulated at all.

EdTheHeaterMan

HVACNUT said:

Or you could spray foam the foundation walls. That would make it conditioned space and the ductwork doesn't need to be insulated at all.

I like that idea the best. Ductwork in the conditioned space is always better!

MaxMercy

HVACNUT said:

Or you could spray foam the foundation walls. That would make it conditioned space and the ductwork doesn't need to be insulated at all.

I've heard that before, and I have to admit it seems counterintuitive given the thickness of the foundation. But it seems the foundation should indeed be insulated.

Question: at least half of my foundation is below ground level - mostly the rear elevation is partially or full above ground. Should the entire foundation be insulated or only the parts above grade?

HVACNUT

I don't know the exact requirements for foundation air sealing. I believe outside should be sealed to below the frost line with masking and foam boards. For inside, instead of spray foam, there are batts available that can be hung. The area above the foundation in the joists still needs to be air sealed.

JakeCK

Foam board insulation can be used on the interior of the foundation all the way to the cove joint and sealed at the top, bottom, and seams. Then spray foam between the joists, or foam board again but use can foam to seal gaps. As long as there is proper drainage so you dont have damage from hydrostatic pressure or flowing water you're good.

I know there are people here who are going to blast me. But before you do please cruise on over to BSC and greenbuildingadvisor. There is plently of science to back me up. And admittedly there is still some back and forth between how best to insulate below grade. The most current thinking is that the foundation is going to get wet no matter what. You can seal it with the best of materials and practices but eventually it will fail. It is also been shown the insulation board on the outside is often subjected to damage even below grade. It also quickly becomes an insect motel. Ants in particular have an affinity for foam in wet places. 

MaxMercy

Thanks everyone. Any preferences for the duct mastic/paint? There's a bewildering array of products on the market. I took a look at my ductwork, and while the tin knocker did a pretty good job there are several areas that are clearly leaking, and this can be seen by eye. I can only imagine what's leaking between the seams.

Larry Weingarten

Hi @MaxMercy , The Red Devil goop gets the best reviews: https://www.bing.com/shop?q=red+devil+0841dw+f-seal+181&FORM=SHOPPA&originIGUID=747626996B4A4239A4930E13D958F71A Watever you use, put on some disposable rubber gloves and let your hands smooth it on just so. If there are any large gaps, you can use some of the mesh tape the dry-wallers use for strengthening joints. It will help hold the mastic in place.

Yours, Larry

GW

You’ll hate life if you try rolling 2” fiberglass. Bubble wrap is easier to work with. Pick your poison as they say 

MaxMercy

GW said:

You’ll hate life if you try rolling 2” fiberglass. Bubble wrap is easier to work with. Pick your poison as they say 

Some supplemental reading has shown bubble wrap to be virtually useless.

I was considering bonding thin foam board to all flat surfaces and then over-wrap with a thin fiberglass - after air sealing with mastic of course.

John