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Old 10-16-2010, 08:25 PM   #1
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Attic venting, half heated/half unheated


Hello all from Michigan. I'm trying to alleviate roof snow melting and ice dams, as well as any other roof problems. I have a "ranch style" 30' x 70' wood frame work building with corrugated metal roof (trussed). Half of the building is heated, interior finished with insulation in wall and ceiling (poor), and the other half is unfinished, unheated (no snow/ice problems here). In the "attic", THERE IS NO WALL BETWEEN THE FINISHED AND UNFINISHED ATTIC AREA...ITS OPEN ACROSS ALL THE TRUSSES.

The soffits/roof overhang are not enclosed and the only lower ventilation is the openings where the corrugated roof sits on the wall (.5 sq in. per corrugation). There is NO upper ventilation at all!

Adding a ridge vent or roof "cans" would be difficult according to roofers/contractors I spoke with. That leaves gable vents.

1. How do I handle this finished/heated combined with unfinished/unheated building?

2. What formulas for vent sizing? Do I handle the heated side different than the unheated?

3. Gable vent only in the heated/finished end, or in both ends?

4. Maybe add a gable vent with exhaust fan?

5. Should I wall off the attic between the heated and unheated areas?

6. Could the gable vents create more problems?.....introduce more cold winter air (especially in unheated storage area?).

Thanks for any insight. Please forgive all my questions.


Last edited by Treeplanter; 10-16-2010 at 08:31 PM.
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Old 10-17-2010, 05:07 PM   #2
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Attic venting, half heated/half unheated


your scenario is the prototypical condensation issue waiting to happen. In 5th grade language - condensation is created by temperature differential. When you reach the dew point, condensation is formed - this will create problems for you- unless it can escape.

My 1st response would be to install a ridge vent. If this is not possible - install a few roof louvers.

The other thing to ensure is that the attic is properly insulated. One issue we came across a few years ago was not completely resolved until the attic space's insulation was improved.

Best Luck - condensation issues are ridiculously pesky - try to avoid at all costs.

Remember - eliminate the temperature differential as best as you can.

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Old 10-18-2010, 09:41 PM   #3
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Attic venting, half heated/half unheated


1. How do I handle this finished/heated combined with unfinished/unheated building?----- insulate the heated section, as mentioned. Air seal before insulating, this is what is causing the ice dams: http://oee.nrcan.gc.ca/residential/p...ap4.cfm?attr=4 http://bct.nrc.umass.edu/index.php/p...ting-ice-dams/
http://www.buildingscience.com/docum...d-135-ice-dams

2. What formulas for vent sizing? Do I handle the heated side different than the unheated? ------- Both the same:http://www.airvent.com/homeowner/pro...it-specs.shtml

3. Gable vent only in the heated/finished end, or in both ends? ----- Both to get flow through.

4. Maybe add a gable vent with exhaust fan? ---- No power assist, it will pull conditioned air from the heated side and possibly the furnace pilot light or back-draft: http://www.advancedenergy.org/buildi...%20Studies.pdf

5. Should I wall off the attic between the heated and unheated areas? ---- No, for the flow.

6. Could the gable vents create more problems?.....introduce more cold winter air (especially in unheated storage area?).----- Possibly, if moisture condenses on the stored items. The ventilation's main purpose is to remove heated air coming up from below to stop condensation or frost or ice dams: http://www.energysaversny.com/frost_-_moisture If no temperature difference, no problems: http://www.buildingscience.com/docum...on?full_view=1

On the other hand....... http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf

Gary
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Old 10-28-2010, 09:12 PM   #4
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Attic venting, half heated/half unheated


Gary, thank you for the in depth answer. Excuse my delayed reply as I have been working out of town.

I have not yet digested all of your links, but my initial review throws me into turmoil.

There seems to be simple "common rules" as recommended by companies like Air Vent and Lomanco.....i.e., 1:300 distributed equally top and bottom.

THEN, you reference several articles that seem to dispute the "common rules". In essence, they seem to suggest leaving this stuff to the experts, not the diyer or self taught.

The problem is, local, supposed experts don't seem to exist! Even the professional trades people seem to disagree, as evidenced by some of your links.

What's a person to do? Ie, # 6...achieve no temperature difference? that almost impossible in the spring up here when warm humid air hits building parts that are still cold from winter...metal roof panels.

Thanks!

Last edited by Treeplanter; 10-28-2010 at 09:23 PM.
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