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
What would you suggest in this regard, and why?
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.
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,
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??
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
Regarding your other questions, perhaps it would be best if
I simply ship to you the plans and specs for the
All the best,
What we have presently is very close to what you describe,
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,
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,
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.
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,
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
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.
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