Interesting concept - did anyone ever think about the fire hazard rating of
using polystyrene or polyuretane product to fill spaces. This is of concern
and does need careful consideration.
In commercial premises we have thankfully outlawed the use of expanding
polyurethane foam as a sealant for penetrations between compartments.
Duct insulation is required to abide by specific flammability and smoke
generation indices -
Check out what you're doing with your local regulator and make sure that
they endorse it - otherwise you may have just voided your insurance and if a
fire impacts upon any of your neighbours due to such an installation, you'd
be fully liable for any losses that they may incur.
I agree with the comments about not foaming over the ductwork. If you
ever have to do electrical or other work up there, you'll be cursing
Instead, move the ducts into the conditioned living space. Foam the
underside of the roof deck with high density polyurethane. 5+ inches.
Seal the entire attic air tight, just like you would living space. No
roof vents. No soffit vents. Foam and seal.
This has the added advantage of negating the energy loss due to air
infiltration through any penetrations in the main living space
ceiling, through the walls, etc. It a real bear to properly air seal a
home by sealing every seam in the attic, so you negate the issue by
enclosing the attic along the roofline.
consult the following document for the science behind this approach:
The home is a complex system. Make sure you understand the
ramifications. Be especially careful about moisture drive from the
living space into the attic, regardless of what you do. Seal around
bathroom fans and lights; make sure that fans vent properly through
roof vents; if you have a whole home humidifier, make sure you have
hygrometers around the house to ensure your not over-humidifying and
remember, a tight house probably won't need humidification anyway.
There's more to this than insulation. Keep doing your research.
Energy Testing & Consulting
On Sat, 22 Mar 2008 06:09:40 -0700 (PDT), firstname.lastname@example.org
Can you please say more about this aspect of your
When you use the term "up there" I don't know what you mean.
We have under consideration foaming the sides and top of the
ducts that run along the floor of an attic space.
How might that become a hassle if we were to "do electrical
or other work up there" in the future? I ask because the
rest of that attic space would be untouched.
Up there, meaning the attic.
I think the problem that many of have with this idea is we imagine the
entire attic floor foamed, with the ducts encapsulated in the foam.
Reading your note above leads me to think you're just looking at
foaming the ducts themselves but leaving the rest of the attic floor
unfoamed. That's probably far less problematic, as it will just put a
hard shell around the ducts, which should actually protect them in the
As noted in my original response, the far better solution is to seal
up the attic, as that has positive effects for the entire house
envelope. If your air handler is in the attic with the ducts, it also
greatly reduces the wear and tear on the unit because it makes the
attic space much closer to inside temperature rather than the 100+F
that it would be if you use conventional attic venting. Sealing the
attic also helps to negate the effects of the inevitable leaks in the
air handler/ducts, increasing the performance of the system. For
example, in the normal situation, if you have 50 CFM of return air
leaks outside the conditioned envelope of the house, in your climate,
that might mean sucking 0F air into the system, resulting in ~12% of a
4-ton system's capacity wasted. Bring the attic into the conditioned
envelope and now instead of sucking 0F air, you're sucking maybe 60F
air into the system and the loss drops to <5%.
On Sun, 23 Mar 2008 03:30:18 -0700 (PDT), email@example.com
Thanks for clarifying...
As I believe I described, we have no thought about foaming
over the floor to include the ducts. The plan under
consideration was to do only the ducts themselves.
The idea about increasing the envelope of the house to
include the entire attic makes fine sense, but...
The point of payback seems to be extremely far off (and that
is not based upon your comments alone.)
The cost for doing just the ducts is on the order of $3500,
but the cost of doing the entire attic is on the order of
So, based on my calculations, it may be prohibitive.
One of the bids we have suggested removing the glass we how
have on the ducts, and putting it in the roof (instead of
disposing of it) then, foaming the ducts, and adding glass
to the roof where needed. (Also, it would eliminate the
concern about shingles overheating in the sun when foam is
on the other side of the sheathing, though, I don't have any
knowledge of the validity of that concern.)
That might give me (some of) the best of both.
Open-cell allows water to be absorbed and pass through. The closed
cell urathane does not.
As for duct condensation; it does happen quite often here on the very
humid MS Gulf Coast. See it especially during the hot summer months
with the extended run times. Several factors come together to cause
Absolutely, you're a dumb ass that is not listening to some very good
advice you were given, namely, you are in over your head. Just the
fact that you want to foam your ducts proves it. Several people have
told you why not to do it yet you persist. Let me help you here and
give you the answer you are dying to hear; go ahead and foam away!.
On Sat, 22 Mar 2008 19:20:25 -0700, hvac tech wrote:
It is not nearly as simple as you claim...
Though, indeed, some here have suggested that it ought not
be done, others have suggested the opposite.
You seem to believe that those who agree with you are
That may be right, but I have no way to know that at this
I tend not to assume that those who yell the loudest are
All the best,
Down here in the South they will foam the duct but we have very high
humidity and sweating is a problem. The material your
describing( silver quilted stuff) is a new product that when
installed properly works very well. Are you only spraying the ducts?
It would be well worth it to spray the roof and seal off the attic
completely if you stop the heat lose/gain at the source "the roof"
then you are way ahead of the game on insulating and improving the air
quality in your home by stopping moisture. The insulation sprayed
under the roof in your attic will also make your roof(shingles) last
On Mon, 24 Mar 2008 14:36:02 -0700 (PDT),
Thanks for your thoughts on all this...
I was surprised by the very last comment you made, because
that is the opposite of the information I have been
receiving from other folks.
One insulation contractor told me that foaming under the
sheathing could void the shingle warrantee.
I certainly don't disagree, but because I have no first hand
knowledge of any of this I would ask how spraying under the
roof could extend the life of the shingles.
All the best,
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.