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Attic air handler: New system or unvented attic?

My house has 2 heat pumps with the air handlers in the attic.....in southern Florida.

Typical insulation could be improved.....the HVAC systems are 10 years old. Would I be better off putting in newer, more efficient HVAC system (current system is plain single stage, single speed) or changing the attic to foam unvented attic?

Asked by Steve Romero
Posted Jun 26, 2014 10:25 AM ET
Edited Jun 26, 2014 12:57 PM ET


6 Answers

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There are too many site-specific and system-specific variables to say for sure, but foaming the roof deck and sealing the attic will typically take at least a 1-ton off the actual peak load.

Whether that is a bigger reduction in energy use than swapping out your SEER 13 or whatever single speed for a higher SEER 2-stage depends, but by sealing the attic the net humidity in the house should go down, boosting comfort. If the existing system is 2x oversized for the existing load with the vented, and 3x oversized newly reduced load it'll be less of a comfort boost than if it's right-sized for the load as-is, and 1.5x oversized for the load after you foam the roof deck.

It's all pretty squishy unless you take the time to do the real load calculations. When in doubt, lowering the load is almost always better for comfort than increasing the efficiency of the mechanical systems. Unless your existing system is turning into a maintenance nightmare you can probably get another decade out of it, and it would have to be a dramatic uptick in rated performance to be equivalent on energy use to insulating & sealing the attic.

Answered by Dana Dorsett
Posted Jun 26, 2014 2:05 PM ET
Edited Jun 26, 2014 2:11 PM ET.


Before you make your decision, you may want to read this article: Creating a Conditioned Attic.

Answered by Martin Holladay
Posted Jun 26, 2014 2:22 PM ET


Thank you both.

I had a salesman out for sealing the attic. I was a little uncomfortable with the Rvalues he was promoting...small number. Also he only wanted to see me open cell which this site raises some large questions. I asked about spraying 2 inches of closed cell followed by his more favored open cell. He never got back to me. I guess I asked too many questions.

The current ac works ok but the single speed air handlers are not as comfort producing as our last house that had variable speed air handlers. I have to run this house at 73 degrees to get the same summer weather comfort that I got at 78 degrees at the last house. There may be other factors including poor construction infiltration issues (florida quality building) but the difference in the two houses was substantial.

Answered by Steve Romero
Posted Jun 26, 2014 3:20 PM ET


Let me make one more response to Dana about HVAC sizing. In the current summer conditions, the house AC has to run almost all of the time to keep up with demand. If I put the oven on, the kitchen gets pretty warm. Two story house with 5000 square foot with 2 4 ton heat pumps.....air handlers in attic with flex duct. So bringing down the load could make the current system more comfortable. Would love to put in new HVAC with variable speed AH and foam the attic......too expensive.

Thanks for the input.

Answered by Steve Romero
Posted Jun 26, 2014 3:48 PM ET


The oven causing over heating is a zoning issue more than a sizing issue- kitchens are hard to really control for both the minimum & maximum potential heat sources. (Sitting by the window sipping coffee and reading the paper by daylight is puts a lot less heat into the zone than roasting a duck with 1000 watts of lights on.)

In a southern FL climate there are fewer issues with using open-cell-only on the roof deck than in GA or AL (or the FL panhandle.) In your climate it would only be prone to moisture loading on north facing pitches with the most heat rejecting of "cool roof" shingles.

You may find that by sealing the attic your latent load issues that make keeping it at 73F necessary for comfort may go away too. A 2-story house has a lot of stack effect, and air sealing at the attic floor with all of the duct / electrical / plumbing penetration is hard to do even for those who try (let alone those who don't.) Any duct leakage under those leaky attic conditions translates into air-handler-driven air infiltration, and higher latent loads while it's running, and still significant stack effect driven infiltration when it isn't. Moving to a variable speed unit may help, but not as much as taking the infiltration drives down an order of magnitude.

If you can, have the place blower-door tested DURING the foam installation, to be able to fix any of the inevitable leak points while the truck is still there and the crew is still suited up. (Curt Kinder, a FL HVAC contractor who posts here occasionally, usually tests for leaks with a blower door and a theatrical smoke machine while the foam guys are still on site, making it dead-obvious where the leaks are.)

Answered by Dana Dorsett
Posted Jun 26, 2014 4:13 PM ET


I don't know if it is polite to ask but does anyone have recommendation of spray foam company in Sarasota/Bradenton area........maybe with blower door?

Answered by Steve Romero
Posted Jun 26, 2014 4:30 PM ET

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