I have been in the construction trade for about two years. I love building, but often run into problems. I have recently built an addition and am in the process of insulating the roof rafters. I am venting the rafters but do not know how far out I should run the rafter mate vents? Should they run close to the facia? How much room is to much room or vice versa, and does it defeat the purpose of venting If I insulate the room with a heavy duty plastic/ house wrap before drywalling?
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First off, where are you (climate-wise)?
The purpose of the rafter chutes is to keep the insulation from blocking airflow from the soffit vents into the attic. If you're really "insulating the roof rafters" then the ventillation scheme needs to be carefully integrated into that.
The purpose of the plastic vapor barrier (it's not "insulation" per se) is to keep moisture in the air inside the house from getting into the insulation behind the vapor barrier and condensing in cold weather. In an ceiling/attic situation it works with the soffit/ridge ventillation to prevent condensation in unheated attic spaces.
If you are finishing an attic space it can get tricky. You either need to assure airflow up each rafter bay (with the chutes, etc) or go with a "hot roof" scheme.
Dan,
Thanks for the response. I live in central Maine. The space I am insulating will be to bedrooms. Although there is no facia or soffit in yet, I notice that the plastic air barrier is sucking in and out when the wind blows hard. This will probably be 99% eliminated when the facia etc. is put up. I have thought about taking the rafter vents out and eliminating the whole venting idea, the roof is a shed roof and the pitch is not helping the situation. I am also using the "shingle vent product", but because of the roof pitch the snow accumulates around and on top of it. Currently not heat is being let into the upstairs addition so I figure that is why the snow is not melted around the roof vent. I would appreciate any more advice you have about my questions.
Todd
This is really a more complex topic than can be covered in a few paragraphs. Since you're in snow country you have ice damming to be concerned about as well as moisture problems.This covers ice dams pretty well:
http://www.state.mn.us/mn/externalDocs/Commerce/Ice_Dams_110802041816_IceDams.pdfThis gives a few ideas about ventillation in a situation where you're going up into the rafters:
http://www.ci.minneapolis.mn.us/mdr/docs/attic_vent.pdfGood info in this one too:
http://www.state.mn.us/mn/externalDocs/Commerce/Home_Moisture_110802041415_Moisture.pdfSeveral others listed here:
http://www.state.mn.us/portal/mn/jsp/content.do?action=doc_contentlist&subchannel=-536881511&programid=536885406&id=-536881350&agency=Commerce&sp2=yMaine likely produces some similar publications.
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