Venting heat from a conditioned attic
northeast_kyle
| Posted in Energy Efficiency and Durability on
I have a new construction home in the northeast with a closed-cell spray-foam sealed attic. There’s no insulation between the attic and the 2nd floor bedrooms.
The largest section of the roof is south-facing, which results in lots of free warmth from the sun. This is great in the winter, but is working against me in the summer.
Should I add active ventilation to exhaust the excess heat in the warm summer months? I was thinking of the following:
– Install an exhaust fan + powered damper at a high point
– Install an intake fan + powered damper at a low point (on the northeast side)
– Run both fans when the temp in the attic exceeds 85F, and when the exterior air is 80F or cooler.
– Add a dehumidifier to run in the summer.
This feels like an easy win to save on cooling costs. Am I overlooking anything obvious or subtle?
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Replies
If you're not running air conditioning in the rest of the house this is fine. If you are running air conditioning my gut feeling is it would make the AC work harder than it would without it.
Your biggest problem is going to be sealing the holes where the fans go when they aren't running.
Also -- it takes an enormous amount of air flow to move a significant amount of heat with a low temperature delta. Let's say it's 90F inside, 80F outside, and the solar gain is contributing 20,000 BTU/hr. You'd need about 2000 CFM to remove that much heat. Go look at 2000 CFM fans before proceeding.
That's a good point. Typically in the northeast a summer evening will be cooler - say, low 70s - while the attic is still holding lots of heat from the day. I'm imagining that my ventilation system would be efficient even at 500cfm if it's running after sunset. What do you think?
At present you can’t call it a condition attic. If it was a conditioned the attic would be more or less the same temp as the rest of the house.
Sounds to me you are looking for the free lunch. My guess is they skimped on the R value of the expensive spray foam and you have half the code min R value for your zone installed.
If you really want to scare yourself, check the winter time humidity in the attic.
If you want a conditioned attic you need to spend the money necessary to buy and operate the equipment required to keep the space at the same temp and humidity as the rest of the home.
Note dehumidifiers are effectively heaters and can only make the attic warmer
Walta
How much is the summer temperature delta between the top floor and the attic? Even in an insulated attic there is going to be some delta, maybe 10F greater in the attic. The stack effect still applies.
If that's still too much difference for you (because you are using it for living space or storing temperature sensitive items) then you probably need to cool it with air conditioning.
If none of those apply, then just let it be hotter, and insulate the attic floor to keep the heat out the living space below.
Danan_S,
The problem with splitting the insulation between the roof and attic floor is what happens in the shoulder seasons and winter. Those neither in nor out spaces behave unpredictably and often have moisture issues.
Perhaps then it just needs more/better insulation under the roof, as Walta implied.
Danan_S,
I'll admit I don't know what the best approach is. Intuitively I'd be more comfortable with a passive strategy like more roof insulation
The question is whether the heat is coming through the roof or is due to solar gain through the windows.