Foam over existing duct insulation...?

[New Directions In Building Ser]

[tedinoue]

Kenneth,

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 yourself. 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.

-tdi Energy Testing & Consulting

[Kenneth]

Howdy,

Can you please say more about this aspect of your interesting comments?

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.

Sincere thanks,

[KAubert (Air-Care)]

Hi there;

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 it.

bye now

Keith

[hvac tech]

How long are you all going to continue playing this trolls game?

[Zyp]

Best advice yet.

[hvac tech]

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!. Dumbass.

[tedinoue]

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 future.

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

[Kenneth]

Hi again,

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 "correct."

That may be right, but I have no way to know that at this point.

I tend not to assume that those who yell the loudest are always correct.

All the best,

[Kenneth]

Hello again,

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 $12000.

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.

Thanks again,

[Kenneth]

And to those among you who have been most generous, once again, I offer my sincere thanks,

[csstouffer]

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 longer.

[Kenneth]

Howdy,

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,

[Zyp]

I'd think that sealing under the shingles prevents the moisture from escaping. But I'm not a roofer and really don't know.