Foam over existing duct insulation...?

[Kenneth]

Howdy,

We have a geo water to air 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.

We are soon to foam over them, 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,

[hvac tech]

Have then foam between your ears while they are at it as the space there is obviously empty.

[Old & Grumpy]

AS long as your origimal insulation is OK I would no see the reason for removing it tony

[Kenneth]

Hi Tony,

The original stuff is in fine condition, and it seemed to me that there was little reason to waste whatever R value it was providing.

Sincere thanks for your comments,

[Stormin Mormon]

Probably less labor to foam over. And the R-value is added.

I'd say leave the old stuff.

[Kenneth]

Hi Christopher,

It certainly seems that I am moving in that direction...

Sincere thanks,

[Noon-Air]

Before you even think of foaming over the old insulation, you need to make sure that the ductwork is correctly sized and each drop has the correct airflow for that space. It would also be a real good idea to make sure that the old insulation is *as new* before covering it with anything. Once its foamed over, your done....and its gonna be a real PITA to correct any and all problems or issues with air distribution.

[Kenneth]

Hi again,

Of course you are correct, but we have no distribution related problems at all.

We have checked it all out quite thoroughly, and all seems designed properly, and sealed and insulated poorly.

All the best,

[Kenneth]

Howdy,

How might the "cat" get into such a sealed system?

But beyond that, based on what I have described of our situation, what would you suggest?

Sincere thanks,

[Kenneth]

Hello again,

What we have presently is very close to what you describe, but...

Given the temperature delta (80 or so in the ducts, and occasionally 0 in the unconditioned surroundings) the glass does a very poor job because of losses to convection.

All the best,

[Stormin Mormon]

I've worked with fiberglass stuff, in rolls. Made for HVAC ducts. And very large tie straps.

[Stormin Mormon]

Fiberglass with vapor barrier?

[Noon-Air]

It doesn't have to be a cat, it could be a squirrel or anything else. As others have said, if you have to have any electrical work done, or run phone lines, or cable, or anything else, spray foam is just not a good idea.

You stated earlier that the ductwork is correctly sized....what method was used to determine proper size?? What type of material is the ductwork?? Was air balancing done when the ductwork was installed?? What size and type of heating/cooling system is it?? What method was used to determine the size of the system?? Where are you located?? When was the house built?? What kind of construction?? How big is the house??

[Noon-Air]

No staples?? no tape?? no mastic?? I guess thats where some of the pics on the "Wall of Shame" come from.

[Kenneth]

Hello again,

Regarding your initial comments, of course you are correct. That is why I would not be foaming anywhere where future modifications might be made. We intend to foam only over the ducts.

Regarding your other questions, perhaps it would be best if I simply ship to you the plans and specs for the house...

All the best,

[Kenneth]

Hi again,

I am not sure what you are asking, but...

It is my understanding (and I am only repeating what I have been told) that if there are very significant temperature differentials, and the fiberglass is not completely boxed in, or otherwise, sealed, its R value will drop significantly due to convection.

Few know less than I about HVAC, but I do know a bit about physics, and that makes sense to me. Certainly, if I were to blow a hair dryer into a foot of fiberglass I could feel the warm air coming though the other side.

I would be happy to understand more,

[KAubert (Air-Care)]

Hi all:

We are seeing more of the foam insulation here on the coast. I have not seen anyone try to foam over the ducts.

I will say this; I have seen where loose fill was applied completely over ducts and made very nice stain tracks on the ceiling. I can see where condensation could be a problem with foam covered ducts; especially if open cell foam were used.

Keith

[Kenneth]

Hi Keith,

Can you say something more about why the (potential) condensation problem would be greater with open cell?

Thanks,

[Kenneth]

Hi Again Keith,

I have continued to think about your comments above, and realize that we have no condensation problem whatever despite having fiberglass simply laid over the ducts.

We use lots of AC for several months of the year, but I have never noticed so much as a drip.

All the best,

[Zyp]

You know, considering I make a living with mechanical failures, it just seems more prudent NOT to cover the ducts with something as permanent as expanding foam.

Vapor barrier: is needed to protect [from the warm side] moisture penetration. Flexible ducts come with a vapor barrier, batted insulation comes with a vapor barrier.

Thinking that, the foam is layered on top, and you're not foaming under the ducts, you'll be leaving an area adjacent to the ceiling without protection. You'll be adding some weight to the duct(s) increasing the crush weight for any insulation on the bottom.

I'm not a big believer in foam as it makes it difficult to repair /change / correct things afterwards.

Good luck...