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It’s summer, the attic is hot, our HVAC system is off, but hot air seems to be slowly coming in through the supply vents, making the upstairs bedrooms hot.

It’s certainly normal for some warm air to come in from the vents when first turning the system on because the air in the ducts has heated up during the day. However, I just noticed that the vents leak hot air even before turning the system on, defeating the purpose of my attic insulation.

I think the connections to the ducts are good (though I certainly could be wrong), but could there be another reason for this? It certainly doesn’t seem right. Are we missing dampers of some sort to prevent this from happening?

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    Where are you on this planet? It sounds like your attic insulation is rather..misplaced (most people insulate the attic floor and vent the underside of the roof deck, but that's a bad plan in hot climates for reasons that have to do with what you're experiencing) – ThreePhaseEel Jul 4 '20 at 0:18
  • Southern California. I have R38 on the attic floor and I even have some blown in cellulose on top of that. The duct work is insulated flexible duct which hangs above the insulation on the attic floor. Are you suggesting that the duct work should be below the floor insulation somehow? – Matthew Levine Jul 4 '20 at 2:36
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A tale of misplaced insulation, leaky ductopi, and lazy builders

The problems you are seeing here are due to a combination of three problems:

  • Vented attics in hot climates get scorchingly hot
  • Your ducts are in the attic (and are a lazily-installed "ductopus" of flex, at that)
  • Your ducts, sadly, leak (somewhere)

Fixing the leak will stop the hot air from coming into the house, but it won't cure anything else that's wrong with this setup, as the unstretched flex hampers airflow through the ducts and the R8 or so duct insulation is nowhere near enough to keep attic heat from warming the cold air in the ductwork, either. Considering that there's no ice dams in SoCal, hence no reason to vent the roof deck otherwise, I would go to an unvented conditioned attic, or "hot roof", configuration.

You can either have a couple thick layers of rigid insulation board laid down with the gaps staggered and an overdeck laid over that next time you replace your shingles, spray foam applied to the underside of the existing roof deck if you want to do the retrofit now, or a mix of the two, provided you achieve the R-value you're after. If you are doing spray foam on the underside of the existing roof deck, you'll also want to put some 1/2" drywall on the underside of the rafters to provide a thermal barrier between the foam and any sort of flare-up that might happen in the attic (some folks use a sort of paint-on coating instead, but the drywall is easier to get right and maintain I reckon). Of course, the attic venting will have to be removed and replaced with solid materials instead, and supply and return grilles added to the attic so that the air conditioner can cool it.

While you're at it, you'll want to have that "ductopus" of flex replaced with a rigid trunk-and-branch duct system, reducing the amount of work your air handler has to do to push conditioned air through the system. Between these two upgrades, you'll be able to take a good 10-25% off of the energy usage of your air conditioner, and may even be able to downsize the A/C next time you are shopping for one, since your house no longer wastes effort trying to cool outside air that's just going to get vented anyway, and your air handler no longer has to push as hard to get cold air to you, either.

(As a sidebar: it is possible to "bury" the ducts in attic insulation, but that's trickier than cathedralizing the attic due to condensation issues; it requires careful spray foaming of the ducts in order to keep the dewpoint on the outside of the vapor-impermeable section of the system high enough that humidity in the outside air doesn't go off and condense on the outside of the ducts, creating a moisture problem.)

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  • Having spent several fortunes on this already, I’ll try to find a cool day when I can go up to the attic and locate the leak. I imagine you’re on the money with that since the air upstairs sometimes smells attic-y. I’m nearly the end of life on my current roof, when replacing, I’ll consider the idea of a conditioned attic. Much appreciated, once I check the ductopi I’ll report back – Matthew Levine Jul 4 '20 at 18:54

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