vented or unvented attic or radiant barrier....

momwalkinJuly 30, 2008

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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dallasbill

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.

    Bookmark   July 30, 2008 at 5:57PM
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mbarycki

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.

    Bookmark   July 30, 2008 at 9:35PM
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msm859

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.

    Bookmark   August 1, 2008 at 1:03AM
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veesubotee

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

    Bookmark   August 1, 2008 at 7:18AM
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dallasbill

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.

    Bookmark   August 1, 2008 at 11:37AM
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energy_rater_la

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

    Bookmark   August 6, 2008 at 9:27PM
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msm859

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.

    Bookmark   August 9, 2008 at 10:15PM
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