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Heating & Air Conditioning

vented or unvented attic or radiant barrier....

15 years ago

We live in a hot humid climate. When we built our house 9 years ago, it was standard to build vented attics and to locate your hvac system in the attic.

I am now reading on different web sites that it is a really dumb idea to locate hvac units in a vented attic in this type climate! Makes sense to me now that I think about it.

Our house is constructed with ICF's and our energy bills are astronomical. We are consuming 120+ kwh/day.

We have a complicated roof system consisting of 10/12, 12/12, 10/12 pitch, resulting in an enormous attic space.

Our energy monitor shows our consumption at night is about the same as during the day until about 4 a.m.

Our theory is that the large attic space is retaining heat, which results in our hvac system working harder...

Should we consider converting our vented attic to an unvented attic? Is it fiscally feasible to attempt such a feat?

We have ridge and soffit vents. Should we ventilate better and add insulation to duct work? Or maybe consider some type of radiant barrier?

Any thoughts or feedback would be appreciated.

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Comments (7)

dallasbill
15 years ago
How big is your home and how many tons is the system? Is it a variable speed air handler? Was you system properly sized? Is it short-cycling -- on 10 min/off 15/on 10/etc.? Were duct and load designs (manual D & J) done?

We have a 3100 sq ft 1.5 story ICF home and our monthly consumption is about 1000kwh (33/day) total for 2 separate 3-ton systems. We have spray foam attic insulation and a vented attic, with one system in the attic. But then, we had a complete energy-audit of our plans/D & J design done before we started.

The pros need way more info.
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mbarycki
15 years ago
You think you are storing heat in your attic... try sealing it up and see what happens... it will be twice as bad. You probably need more ventilation. 50% soffit vents and 50% roof vents square inches wise is needed. Your large attic space will ventilate well if there is enough. Don't depend on what your builder put in to be adequate, they usually only do the minimum required to meet code. The height of the roof will cause a good draft due to the heat rising. I've read that ridge vents aren't the best method for the roof becuase they require the hot air to turn back on itself to release. Do you have insulation covering your soffit vents. Sounds like that may be a problem. Easy to clean off with a blower from the outside.
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I agree with Joe. if your ductwork & equipment are in the attic, use foam to create an unvented semi conditioned attic space. 130 degree attic with R-8 ducts surrounded by this extreme temp air & all the humidity we have... condensating equipment, plenum & ducts. I see it everyday. with mid to high efficiency equipment the air is much colder entering the plenum (vs air handler....two stage units). so in a hot unvented attic the equipment condensates, and the first 4' of the plenum and any ducts taken off in that first 4' condensate. I'd never foam the walls & not the attic..well I'd never foam the walls period. foam sheathing on exterior of walls yes...but not foam in wall stud bays. big waste of money. our heat gain is via the roof...where the sun shines all day...not the walls that get sun at different parts of the day for certain periods of time. that said...I have a radiant barrier. But no ducts no equipment in attic. my air barrier @ ceiling is near perfect. all ducts are in fur downs inside living space & equipment is in interior closet. this was planned for a long time...not just a last minute choice. invest in the foam in the roofline. educate yourself about foam sheathing on exterior of walls, conventional insulation in wall stud bays, and air tight drywall approach for sheetrock to interior. and understand that you still have to reach R-value requirements. 2-3" of foam doesn't meet code requirements. inches of foam X R-value of foam = code requirement. don't buy into the whole 2-3" performs as R-30 it is bull...and will not pass code inspection. open or closed cell? we use open cell 7" in roof rafters & covering faces of rafters, here in La. as Renovator * pointed out...radiant barrier & foam... RB has to have minimum of 3/4" air space between it and anyother product. waste of $$ to foam over tech shield. best of luck.
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msm859
15 years ago
i have an encapsulated attic, radiant barrier no venting. basically my attic never gets hotter than it is outside. live in nor cal were we get 100+ in the summer. 1 hvac system is in the attic.
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veesubotee
15 years ago
Mbarycki;

Your statement:

"I've read that ridge vents aren't the best method for the roof becuase they require the hot air to turn back on itself to release."

Could you clarify, please? Not sure what you mean, but I'm very interested since I have a soffit/ridge vent system.

Thanks.

V
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dallasbill
15 years ago
msm859... what is an "encapsulated" attic? And, an RB needs at least 1 inch of air space between it and the attic below it in order to work, so how is your attic roof "encapsulated?"

If the OP's home is 9 yrs old, and the furnace up there is original, then it's very likely not high-efficiency with outside-air-source venting. So, the attic cannot be sealed because the furnace requires combustion air.
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energy_rater_la
15 years ago
thought you would find this of interest!
study on spray on Radiant Barriers...
note that the OP used a company for his install
and not a diy.
it really makes a difference!
his RB achieves the same temp difference as my
install of single sided foil RB that I used button
cap nails to install.

http://www.hvac-talk.com/vbb/showthread.php?t=177709
0
msm859
15 years ago
dallasbill.... my attic is part of the insulated envelope. i have sprayed in foam insulation under my roof. i then have a radiant barrier that is double foil bubble wrap. i do not have any ventilation in my attic. my attic basically never gets hotter than whatever the outside temperature is. i have a 2 story and one of my ac air handlers is in the attic.
0

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