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Community and Q&A

Attic Ductwork

westjk50 | Posted in Energy Efficiency and Durability on

I live in Nashville, TN climate area 4A

My issue is my attic gets so hot in the summer months and my heat pump runs a lot but does not cool very well. This is a duplex home with each side having around 980 sq foot living space.
I’m not able to move this units to under the house. The attic when you stand up is about 4 feet at the peak of the house. For each side of the duplex I have a 2 ton Rheem unit (one 2 years old the other 1 year old) cooling 5 rooms (2bd, living, kitchen, broom). The overall attic ductwork is 17 years old. I have checked already for any air leaks and used the ductwork mud to seal the best I can. Whatever I do I don’t want to hurt the roof or shingles.

What can I does that would help make my heat pump work more efficient in the summer months?

Should I replace the ductwork does it get too old to work properly?

If I double insulate the ductwork would that help with blowing cooler air? If so how would I do that?

The attic currently has blown insulate that’s not the best and I was thinking about replacing it with the roll kind in between the joints and then criss cross a layer on top in a different directions.

Thanks for any help you can give.

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  1. Expert Member
    Dana Dorsett | | #1

    First, blown insulation generally works BETTER than batts/rolls, because there aren't any gaps. Batts will often have a gap between adjoining batts, or to the wooden framing. But there needs to be ENOUGH of the blown insulation to work well. If you only have 6-8" it's not enough. Current code-minimum under IRC 2012 is R49 for you climate zone, which would be about 13-14" deep for most blown fiberglass or cellulose insulation. If you are going to add insulation it's fine to blow new material over the older insulation, as long as the final total depth (old + new) is over 12" everywhere. A competent insulation contractor would air-seal the ceilings first, and would rake the top of the insulation smooth & level for even coverage at the end, with the same depth everywhere.

    If the ductwork is not insulated, that could be a problem, but fixing it isn't always easy. As long as the ducts are not leaking or collapsed, they still work as well at 17 years of age as they did on the first day. (I have ducts in my house that are 97 years old and still working fine- better now than when I moved in, since I sealed them.)

    If you don't have sufficient air flow the first thing to check is the air filter. if the air flow is too low you can even end up with frost on the cooling coil impeding flow. Running it in heating mode for 10 minutes with the thermostat cranked up would usually be enough to melt the frost, after which it would run fine for at least awhile, unless it frosts up again. If it is frosting up it won't cool well. If that is happening the most common causes are low air flow from clogged filter or obstructions in the ducts, or an improper amount of refrigerant in the cooling system (which would have to be evaluated by a competent technician.)

    With 980' two tons of cooling is usually more than enough, but having the ducts and air handler in the attic adds significant cooling load during the cooling season. But it SHOULD still be enough. During the afternoon hours on the hottest days it should run nearly all the time. If it is so oversized that you can leave it off all day then turn it on in the afternoon and still bring the temp down withing an hour it's too big, since it's duty-cycle will be too low to dehumidify the air very much.

  2. GBA Editor
    Martin Holladay | | #2

    One way to address your problem that Dana didn't mention: You can transform your vented unconditioned attic into an unvented conditioned attic. Here are links to two articles you might want to read:

    Creating a Conditioned Attic

    Keeping Ducts Indoors

  3. westjk50 | | #3

    I have another question about how to help my heat pump work better and my attic. In the winter it blows cool air some before it gets warm and the same thing in the summer hot before it starts blowing cool air. The ductwork does have insulation but I still have the above issue.

    What do you think about building a box around the ductwork using foam board to help with the airflow temp issue? I know this is crazy but I thought may be it would help.

  4. GBA Editor
    Martin Holladay | | #4

    If you have ductwork in your attic, the best thing to do is to move the ductwork to a new location in your house.

    If that is impossible, the next best approach is to convert your attic from a vented unconditioned attic to an unvented conditioned attic.

    If that is impossible, the next best approach is to improve the insulation around your ducts, as your propose.

    Your idea will work. Here are some hints:

    1. First, make sure that you have properly sealed all the duct joints with high quality tape or mastic.

    2. Second, make sure that the existing insulated ductwork is in good condition.

    3. Third, you can manufacture boxes out of rigid foam (I would use foil-faced polyiso if I were you, because it is the easiest foam to tape) and install the boxes around the ductwork. If possible, make these four-sided boxes, not three-sided boxes. If they are three-sided boxes, seal them to the drywall below and make the boxes as airtight as possible

  5. woodreader | | #5

    The ducts also could be encapsulated with at least 1" of close cell spray foam. You would save big time in your labor efforts. You can purchase the materials on line if you want to do it your self. This method will not hurt the roof or shingles.

  6. westjk50 | | #6

    Your suggestion in number 5 - are you saying spray foam directly on the ductwork itself? The stuff you normally spray on the roof rafters? If so, does that stuff flack off or peel once it's been sprayed and sat for a while?

    Also the unit itself that's in the attic what could I use to help it not heat up so much in the summer or is that an issue for the unit and it's performance? Could I place foil-face polyiso board just above the unit to help with that? See attachment for what my unit looks like.

    Thanks for your insight.

  7. woodreader | | #7

    You seldom see insulation on the HVAC unit. You may see some foil tape and mastic on the joints. I never hear of close cell foam peeling or flecking off of the ducts. You could put some radiant barrier above your unit. As long as you can get in to your air filter and not block any air passage, I do not see any reason why you should not spray you unit except for some mad HVAC repair men who made have to go inside and take a panel off.

  8. GBA Editor
    Martin Holladay | | #8

    Do not try to install spray foam against your air handler. You will make a mess and impede access to the unit for servicing.

    To keep the air handler cool, the correct approach is to transform your vented unconditioned attic into an unvented conditioned attic. If you aren't willing to do that, you'll have to live with the performance you're getting.

    It's always possible, of course, that your air conditioner is undersized, defective, or in need of some type of maintenance or repair. You might call an HVAC contractor to have your system assessed.

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