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I'm in the process of re-taping the joins in the old duct-work in the very hot attic. They are quite cool to the touch so I'm considering insulating the ducts with Reflectix or even fiberglass batts.

Waste of time & money?

Ideas experiences opinions?

rad
 

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I'm in the process of re-taping the joins in the old duct-work in the very hot attic. They are quite cool to the touch so I'm considering insulating the ducts with Reflectix or even fiberglass batts.

Waste of time & money?

Ideas experiences opinions?
My own experience has been positive.

I had my oversized furnace replaced with a Rheem modulating unit (wonderful). I have close to 50' of trunk with takeoffs.

One section, about 30', has a rather noticeable temperature drop from one end to the other. Once the house temperature stabilizes from setback, the furnace usually runs at a very low speed (can't hear it), with the result that the conductive heat losses were high. I was heating the basement.

The last run at the end of the trunk is quite long and feeds the 2nd story master BR. I estimate the run to be 40'+. Needless to say, the heat loss (trunk and run) was excessive.

So far, I have insulated that section of trunk and the accessible portion of the duct, with the result that the delivered temperature is 4 - 5 degrees higher.

Before insulating, holding my hand above the trunk, got quite hot. Afterwards, there was a slight feeling of heat in places.

Reflectix provides an R value of 3. With a proper airspace, it is about R6. It was not terribly difficult to work with, and got easier and better as I went along. I was even able to 'artfully' cover curved transitions.

Before I started the project, I purchased a package of the tubular fiberglass insulation to see how it would go, as an experiment. I had to cut through it lengthwise to use (didn't want to dismantle the duct. It was difficult to work with, and not as neat.

With regard to the airspaces, I used double strips of Reflectix (about 1.5" and wrapped them around the trunk/duct at 3' intervals. I used a heavy aluminum tape (about 3" wide) available from Home Depot.

When measuring the cut, allow plenty of extra for overlap and for (air space) slack. You should be able to press on it and have good movement, before you contact the duct.

I think that the foil double foil surfaces will work well in your attic.

V
 

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I'm in the process of re-taping the joins in the old duct-work in the very hot attic. They are quite cool to the touch so I'm considering insulating the ducts with Reflectix or even fiberglass batts.

Waste of time & money?

Ideas experiences opinions?

rad
After sealing the joints, definitely insulate with the Reflectix. Stay away from duct tape and use the foil mastic tape. Ductwork in an unconditioned space should always be insulated.
 

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Run your A/C and see how cold the taped areas feels now.
If its cold. You should have sealed the duct first(mastic), insulated and taped second.
 

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This is what I've done so far. The tape was rated well in temps for flexible ducting. It cost about $8 per roll.
That looks like the kind of tape that I started the project with. Is it a mylar based tape? I bought it thinking it was foil, but was shocked after cutting a piece, I noticed that I could see through it.

I found it very hard to use, as the cut end would cling to itself (static). I abbandoned using that and got the high temp foil stuff (around $14 dollars a roll).

V
 

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My own experience has been positive.

I had my oversized furnace replaced with a Rheem modulating unit (wonderful). I have close to 50' of trunk with takeoffs.

One section, about 30', has a rather noticeable temperature drop from one end to the other. Once the house temperature stabilizes from setback, the furnace usually runs at a very low speed (can't hear it), with the result that the conductive heat losses were high. I was heating the basement.

The last run at the end of the trunk is quite long and feeds the 2nd story master BR. I estimate the run to be 40'+. Needless to say, the heat loss (trunk and run) was excessive.

So far, I have insulated that section of trunk and the accessible portion of the duct, with the result that the delivered temperature is 4 - 5 degrees higher.

Before insulating, holding my hand above the trunk, got quite hot. Afterwards, there was a slight feeling of heat in places.

Reflectix provides an R value of 3. With a proper airspace, it is about R6. It was not terribly difficult to work with, and got easier and better as I went along. I was even able to 'artfully' cover curved transitions.

Before I started the project, I purchased a package of the tubular fiberglass insulation to see how it would go, as an experiment. I had to cut through it lengthwise to use (didn't want to dismantle the duct. It was difficult to work with, and not as neat.

With regard to the airspaces, I used double strips of Reflectix (about 1.5" and wrapped them around the trunk/duct at 3' intervals. I used a heavy aluminum tape (about 3" wide) available from Home Depot.

When measuring the cut, allow plenty of extra for overlap and for (air space) slack. You should be able to press on it and have good movement, before you contact the duct.

I think that the foil double foil surfaces will work well in your attic.

V
Hah. I replaced an 80% Armstrong with a Rheem mod basement
heater too. My gas savings for the first heating season after
installation was ZERO. After meticulously sealing and
insulating the basement ductwork with fiberglas I'm now saving
about 20%. My calculations take into account the differences
in outdoor temperature over the last five heating seasons.

It's really too bad they don't tell you about the possible
duct heat losses on multistage furnaces.
 

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Hah. I replaced an 80% Armstrong with a Rheem mod basement
heater too. My gas savings for the first heating season after
installation was ZERO. After meticulously sealing and
insulating the basement ductwork with fiberglas I'm now saving
about 20%. My calculations take into account the differences
in outdoor temperature over the last five heating seasons.

It's really too bad they don't tell you about the possible
duct heat losses on multistage furnaces.
Sorry to hear about your 'loss'. I opted for the furnace for comfort, which has been great. Prior to that, I had to run a space heater in a bedroom, otherwise I'd get a chill. Now, it's just fine; hadn't needed the the space heater this season.

I've been lucky in that, based on 'heating' therms per Heating Degree Days, for this season, vs. the average of the last 3 seasons, my fuel savings have averaged about 18%.

V
 

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After doing that work, I would blow in loose-fill insulation, bury the ducts, and add another foot of insulation to the entire attic. You do it once and it saves you money forever, tax free.
 

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After doing that work, I would blow in loose-fill insulation, bury the ducts, and add another foot of insulation to the entire attic. You do it once and it saves you money forever, tax free.
That can lead to more problems.
The duct can sweat on the outside.
The insulation will contain air. And being covered by insulation the ducts outer temp can drop below dew point.
 

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sweaty
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I believe you're wrong. Sweating comes from cool metal hitting warm, moist air. If the ducts are sealed and insulated, that would not happen. It is the lack of sealing and insulation that causes sweating.
 

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The surface area can be metal, glass, plastic, wood, etc.

Once its temp is below dew point, moisture can and will condense on it.

Blowing insulation on top of insulated duct. Can cause sweating. And does in most areas.
Unless you happen to be in a very low humidity area.

Blown insulation contains a lot of air. That air has moisture in it. And it will condense on a cold surface such as the duct surface(weather metal, or insulation foil face).

If your brave. You can test it.

Take 2 small pieces of foil backed insulation.


Place one in your freezer over night.
Next morning. Pace the one that wasn't in the freezer on top of the one in the freezer and place them out side your front door.

Wait 1 to 2 minutes. Pull off the top one that was on the one that was in the freezer. You'll see moisture was condensing on the foil of the one that was in the freezer.

You can do it by using the refrigerator instead of the freezer. But if the pieces are small, it takes longer for the moisture to condense.
 

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Discussion Starter #13
So what is the solution to this? I live in a fairly high-humidity area (S. Central Texas) so this is of great concern to me.

Seems to me Reflectix would condensate as well.

I've installed a thermometer in the first vent on the main trunk of ducting. When it isn't blowing it pretty much reflects the current temp of the attic. Pointing the infra-red up into the duct confirms this.

I've seen it blow anywhere from 65F on down to 49F depending on time of day and humidity but usually it'll stabilize around 50-52F. This is probably a subject for another thread...
 

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Next time the A/C is running. Check the temp of the ducts insulation in the attic. When the attic is over 100° is best.

Yep, means you have to go up and bear the attic heat a bit.
 

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You can wrap the duct.

But, it must be wrapped tight. So fresh air can't work its way in every time the A/C runs.

Blown in insulation contains fresh air, and allows air to be exchanged through it. Don't use blow in insulation.
 
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