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Old 06-01-2012, 02:26 PM   #1
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To vent, or not to vent...


Greetings all,


I own a small business in North Carolina where our summer day-time temperatures often reach 95+ on a regular basis, and night-time temperatures often remain in the 80s. Summer seems to last from about March through October.


I rent a portion of a strip-mall-type metal building. My portion of the building is 25’ x 70’. The roof is slanted from front to back with about a 0.5:12 pitch, basically flat. It is metal, with rigid foam insulation, laid on metal trusses. There is no venting in the roof. The “attic space” is roughly 48” high at the front, 15” at the back, and there is a firewall between the adjoining spaces. There is an R-30 batt insulation that lies on the ceiling tiles. The AC air handler, supply and return ducts (flex), and drain-lines, all reside in the “attic”.


I would like to increase AC efficiency. I service the AC semi-annually and check the filters monthly and change as needed. I want to determine if venting the roof may reduce heat build-up, and if so, what would be the best placement, type, and quantity of vents for this application.


I am thinking I need intake vents at the back and powered or spinning turbine vents in the front. There is no option to vent from the sides and there are no soffits.


Any and all suggestions are welcome.


Thanks
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Old 06-01-2012, 02:57 PM   #2
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Commercial? Sounds like you need a professional... If they only thing seperating the "attic" from your space is a grid tile ceiling, then introducing unfiltered/untempered outside air to that space may not be a good idea (or a code compliant idea). Sometimes these "attic" spaces are used as return plenums and introducing contaminats will disrupt your systems. I recommend getting a survey of your systems (mechanical and thermal envelope) by a professional if this topic really concerns you.
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Old 06-01-2012, 03:10 PM   #3
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Thanks...

The space is not used as any type of return since there is no air entering or exiting the space from anywhere. The AC (heat pump) is a split system with the air handler being supplied from the inhabited space. The condensing unit is mounted on the roof.
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Old 06-01-2012, 03:14 PM   #4
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That's checks that one off...

But I would check your local codes about circulating untempered air through there...I'm pretty sure that would not be code compliant here in Connecticut.
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Old 06-01-2012, 03:21 PM   #5
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Thanks again...

I always forget about my local code guys/inspectors as a source of information. Although, I do contact them when it comes to my resedential electrical and HVAC.

Hmmm...
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Old 06-02-2012, 07:25 AM   #6
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I have been giving this some thought. My home, and others I have been in, circulate untempered air through the attic. I have blown pink fiberglass insulation resting directly on the drywall ceiling. The roof deck and rafters are fully exposed. Others have unfaced batt directly on the drywall ceiling with the same exposed wood. There are combinations of soffit, gable, ridge, passive, and powered,vents (not all on the same roof). I understand that residential and commercial codes do differ, so I will not move forward without checking with the inspector.

In the event that it is code compliant to do so, my original question still stands:
" I want to determine if venting the roof may reduce heat build-up, and if so, what would be the best placement, type, and quantity of vents for this application?"
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Old 06-02-2012, 08:10 PM   #7
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If the roof deck is insulated with rigid foam, then you may be doing more harm than good in venting the area... There are a variety of factors at play there and a good engineer can survey your conditions and tell you what the pros and cons of your scenario are.
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Old 06-02-2012, 10:44 PM   #8
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http://www.joelstiburek.com/topten/south.htm

Same as fig.4: http://www.buildingscience.com/docum...-roof-systems/

Gary
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Old 06-02-2012, 11:59 PM   #9
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Thanks Gary,

It looks like the answer is not to vent.
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Old 06-25-2012, 01:36 PM   #10
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I laughed at how Joe consistenly blames Yankees for the problems in the South....It's not the Yankee's fault if you copied their plans only to find out that it doesn't work in your climate. Same goes for California, guess it's a Yankee's fault for their houses being flammable in a forest fire zone...hahaha
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Old 06-25-2012, 02:23 PM   #11
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Yet the science foundation is based in Massachusetts. Where real Yankees come from, not the baseball team.

Now I don't know if I need to close all the soffit holes and gable vents and remove the exhaust fan in my home.

If I do, how do I stop the shingles from melting?
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Old 06-25-2012, 02:43 PM   #12
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They won't melt...
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Old 06-25-2012, 04:35 PM   #13
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The real problem is that AC was added to old structures in the South and AC was added to old designs.

I think that comments blaming Yankees for the problems is a, very effective, tongue in cheek method to keep attention focused on the problem.

"Now I don't know if I need to close all the soffit holes and gable vents...."

That is what I plan to do. (I've got no power vent.) Despite the scare tactics by shingle manufacturers, sealing the attic and foaming the roof deck raises the temp of the shingles insignificantly. If you can, install a metal roof. If you are in hurricane zone, it is better for wind too.
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