Attic Ventilation Questions


Old 08-25-06, 07:46 AM
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Question Attic Ventilation Questions

I want to finish and insulate an old attic that already has stairs, dormers and windows but no ventilation (well it leaks alot!) and no insulation. I am also getting the roof replaced soon and have the opportunity to install ridge and soffit vents but...

I want to drywall much of it right against the sloped roof with insulation behind. I can put in plastic spacers to keep the insulation off the roof but there are so many barriers that I can't create a continuous path from soffit to ridge on more than about 2 out of about 60 spaces-between-the-joists. For example along one long wall there is a large diagonal joist cutting through the normal joists. I could cut small one inch holes in the center and put tubes through it to allow air to flow past this barrier I suppose. Also there are hips. When a space-between-the-joists gets up to a hip there is nowhere for the venting to go unless I drill side holes through the middle of the joists near the top to let the air get to the next space-between-the-joists and so on until we get to a space above the to-be-created finished ceiling. Do they do hip ridge vents? I doubt it! It would probably leak water!

This is Massachusetts by the way - we get lots of 20 degree weather. And plenty of 90 degree weather.

So - should I just forget about venting? If I insulate well will moisture go through the insulation and get trapped under the roof to cause warping and mold? Would tyvec just under the drywall be worth the trouble? Or should I put in soffit vents but skip the ridge vent since the air from the soffit can typically only get up to the hip in most cases? Or should I put tons of holes in my rafters to allow an air passage and get both ridge and soffit vents (holes in rafters will not allow much air through and I'll need some kind of tube to keep insulation from blocking them)?

What about heat - will the roof survive for at least 15 years with the extra heat caused by insulation and no venting just under the roof boards? That's the least of my worry actually but I'd love to hear people's thoughts.

I forgot to mention that the new roof will be light colored aphalt shingles on 30 lb paper on old horizontal boards with gaps.

Thanks for your thoughts!
Old 08-25-06, 09:57 AM
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Actually, they do install ridge vents on some low pitch hips. I'd talk to a good roofer about it. Whatever you do, don't put a million holes in your framing - for one thing it won't be an effective means of ventilation. Hip roofs are just not conducive to the natural soffit to ridge airflow - see if mechanical fans are an option. No ventilation could be trouble and will probably void the shingle warranty. So... Sit down with the guy who will be doing your roof and see what he has to say. Good luck and keep us posted

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