[dorlando]

I am having a heat gain issue in my flex duct. Airflow seems good but I am picking up a lot of heat in the 45 foot run. It was suggested to try a duct fan to move the air faster through the duct so it won't pick up as much heat.

It was suggested I use an axial fan vs booster fan. I can't find much on the axial fans. Any suggestions as to which one I should use? Is it best to put it closer to the register or closer to the supply? It will be a 7 or 8" depending on where I place it because I have a reducer in the line

 

[roughneck]

I would first repair the issue of using flex for a 45’ run of duct.

 

[dorlando]

I don't disagree but I don't know why I have the heat gain. I have talked to two hvac guys and they are not sure either. I have stretched it tight, I don't have any 90's or deep sags, etc. One guy thinks that I needed a bigger duct because he thinks the air is just swirling around giving it more time to heat up, but the airflow is pretty strong out of the register.

I have been told I should not be picking up that much heat gain even in a 45 foot run. It was very difficult to install so I am not looking to do it over or replace it with a larger duct.

Any ideas on why I can be picking up so much heat or is that normal for flex?

 

[Bud9051]

Is this run through a hot attic space?
Flex at best is usually an r-8 and I have little confidence that it even performs that well. R-8 from fiber insulation would be 2" thick. Ducts also have a lot of surface area.

I'm a retired energy auditor and have tested heat loss/gain through ducts and the solution is just MORE insulation.

If they used a foil exterior although it can help it is not as magic as often claimed.

You mentioned a difficult installation, thus I assume adding more insulation would also be difficult, but any areas you can improve will help.

Bud

 

[dorlando]

Hi Bud, thanks for your help. I actually did wrap R 19 around it. It isn't air tight as I just took strips of batts and wrapped it. The entire length is not wrapped but a lot of it is, small sections open here and there. I think the flex was R 6. The R19 helped a little but still getting 6-7 degree heat gain. Yes, it is a hot attic in Florida.

The rest of the house has the R4 ductboard. The run to our bedroom using the duct board is about the same length and I don't pick up as much heat.

I don't mind trying a duct fan if that will speed up the air through it.

 

[Bud9051]

Where are you measuring the heat rise and what are the numbers?

The duct is gaining heat in two ways, convection from the hot air in the attic and radiant energy from the bottom of the roof above.

What measures do you have in place for attic ventilation, every little bit helps.

If you can slide in or wrap a foil faced product to shield that duct from the radiant source it could help. If exposed fiberglass then the radiant energy is just shinning right through. Although I don't believe the claims for foil/bubble/foil wrap it is a workable foil surface. Tough to pull in a roll or aluminum foil.

If new shingles are in your future they offer ones that reflect the infrared energy thus blocking it before it gets inside.

There are also paints that can be applied to the bottom of the roof to reduce its admittance.

Bud

 

[dorlando]

Hi Bud,

I am measuring at the plenum and then the register into the room. I also went into the attic and measured the temp where the reducer is in the duct, I had 3-4 degree heat gain in the first 25 feet of the run. The attic does have soffits and a ridge vent.

The flex duct has the original foil insulation plus the R 19 I wrapped around it.

Do you think a duct booster fan would work to push or pull the air through the duct faster so it doesn't have as much time to heat up? If I can shave two degrees that would probably make a big difference.

 

[Bud9051]

I've been contemplating how velocity would affect the heat gain. As the inside of the duct cools from the air flow the heat source drops and has to be replenished from wherever in the attic. Ultimately it is the replenishment rate that contributes to the heat gain. 2° maybe, but not sure that would make a big difference.

My preference would be to drape where possible a layer of some reflective material. Think of the hamburgers you buy wrapped in foil, they stay hot, in your case cool.

Bud

 

[dorlando]

Even though the flex has the foil around it, add more foil?

 

[dorlando]

Bud, I was doing some reading on what you said. Are you suggesting just a foil radiant guard or a foil with insulation like Reflectix? Also, since I wrapped R 19 around the ducts should that come off first? Lastly, do I need to worry about condensation issues by draping this around the flex duct that already has a vapor barrier?

 

[Bud9051]

No need to remove the r-19. That is why I said draped and underlined it as opposed to wrap. As long as the foil is between the duct and the bottom of the roof it will be blocking much of the radiant heat. Some construction in the deep south uses roof sheathing that has a foil bottom surface.

If you were to start over you might omit the foil layer on the duct, but the duct itself is also a vapor barrier. Ideally a good air barrier would keep the humidity away from the cold surface but we are not in an ideal world.

Drape an easy section to access and compare the temperature under that area to another area without the foil drape obviously with the ac running.

Bud

 

[dorlando]

What type of foil do you recommend? I saw one with just a foil and one with a foil plus some insulation?

 

[Bud9051]

All you need is the foil, but with a little insulation it adds some bulk making it easier to handle. I have used the foil/bubble/foil in a few places ant as a reflector it is fine.

Your choice.

Bud

 

[dorlando]

I found something called Radiantguard, which is just the foil. Can this cause any condensation issues since I already have insulation and a vapor barrier?

 

[Bud9051]

As long as it is draped over the top and not wrapper around the duct I don't see any added risk

Bud

 

[dorlando]

I appreciate your help. I am going to ask this just to be safe and not to be redundant or as an insult. I am not as educated in this as you are. I've already spent a lot of time and some dollars on the R 19 and saw little benefit. I understand you are suggesting the foil to block the radiant heat but in your opinion how much of an impact do you think it will make? I assume I should overalp the edges by an inch. Thanks again.

 

[Bud9051]

Overlap the edges along the length but do not wrap.
This is just a guess but the r-19 was not performing at its potential being exposed to the very hot bottom of the roof. Radiant heat transfer is not only significant it is fast and shines right through the fiberglass. Your total r-value should be doing better and the foil over the top is best I can suggest without starting over, and I know that is not an option.

Bud

 

[dorlando]

Thanks. For my understanding doesn't fiberglass work when it lays on the attic floor to shield heat from the attic into the house?

 

[Bud9051]

"doesn't fiberglass work when it lays on the attic floor to shield heat from the attic into the house?"
Poorly

Insulation likes to be enclosed on all 6 sides like inside wall cavities. To compensate for the poorer performance codes require a lot more insulation in a ceiling. When calculating we still use the r-19 when that is what is installed, but with a wink. There are many places in a home where an auditor has to fudge the numbers to get the actual heat gain/loss to match the calculated.

Bud

 

[dorlando]

Gotcha. I will see what I can find in the foil, try to find something cost effective. I may just go with the foil only since I already have insulation underneath.

Separate question. My house is from the 70's. I had just a few inches of celluose in the attic and I had some more blown in. It was by Certainteed. It wasn't cellouse, maybe some form of fiberglass. They supposedly blew in R 30 but I don't think we ever felt much of a difference. Should I blow in some more and what is the best type to blow in?