OT - Foam over existing duct insulation...?

[Kenneth]

Howdy,

We have a geothermal water to air heating system with ducts running through an unconditioned attic.

The ducts are not sealed, but they are (very modestly) insulated with a shiny, "quilted" looking product to something like R4.

We are soon to foam over these ducts, but...

There are two different approaches in the bids we have:

One wants to remove the quilted stuff, and then foam over the bare metal of the ducts.

The other wants to foam over the existing insulating material.

What would you suggest in this regard, and why?

Sincere thanks,

[Greg Neill]

Are the ducts to be coated in a layer foam or entirely embedded? If the former, find out how well the foam=20 will adhere to the existing insulation - you don't=20 want it falling off over time.

[Kenneth]

Hi Greg,

Thanks for the response...

The ducts run along the floor and are rectangular in cross section. As a result, the foam would be sprayed up one side, over the top, down the other side, and then down onto the floor for a few inches on both sides.

Any further thoughts?

All the best,

[Greg Neill]

=20

If the foam will adhere to the existing insulation, and that insulation is not going to fall off of the ducts, then I'd leave it in place. It'll add a bit of insulating value. Make sure that the duct ends where the insulation terminates are sealed; an air gap between the exhisting insulation and the duct=20 can form a path for conditioned air to leak out at the terminations if the duct seams are not air tight.

[Kenneth]

Hi Doug,

I appreciate your suggestions...

Yes, the present insulation is firmly attached everywhere, and as I am collecting information about this, it does appear that I will go in the direction of insulating over it, as you have suggested.

All the best,

[whit3rd]

The ducts aren't just insulated, they're muffled by the quilted stuff. You want to keep it.

Because adhesion on the ducts would make (for instance) foam removal destructive to ducting, I'd staple a layer of paper (maybe Tyvek housewrap) over the existing ducts, so the foam wouldn't penetrate and adhere. Tyvek isn't going to chemically interact with the foam.

[Ron Moore]

The "shiny" quilted stuff, probably mylar with a mirror finish, should serve as a radiant barrier and add to the efficiency. Respectfully, Ron Moore

[Kenneth]

Hi Ron,

I want to be certain that I am interpreting your comment correctly...

Am I correct to assume that you are suggesting that its reflective properties would be of benefit, and therefore should be left in place under the foam?

Sincere thanks,

[Ron Moore]

That's my thought on it. We just did a warehouse to office/residence conversion and made extensive use of foil-backed materials. We're very pleased with the efficiency we've achieved. That 10,000' is costing less per month to heat then an older 1500' office that has not been "cocooned" in this manner. Respectfully, Ron Moore

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