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Old 05-18-2010, 09:29 AM   #1
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Installing Soffit vents w/finished attic


I recently bought a Cape-style house which included an attic that was already finished. The stairs go up in the middle of the house, and there is a small dormer with a bathroom right in front of the stairs. To the sides of the stairs are bedrooms with ~4 foot knee walls and ceiling joists installed between the rafters to make a flat top of the ceiling. The house is old and we are renovating the attic ourselves, so we tore out all the old drywall and now have full access to the eves space and the small space above where the ceiling was installed.

The house has gable vents at either end as well as a ridge vent that was installed when the roof was redone last year. Unfortunately, no soffit vents were put it, so the ridge vent is pretty useless.

Most of the stuff that's out there about properly ventilating your attic is in reference to un-finished attics. I assume that putting soffit vents would be a good idea to prevent ice damming in winter and keep it cooler in the summer. Do I need soffit vents on both sides of the eves? All the way down? How do I figure out how many soffits I need? Does the finished space still get included in the attic floor space?

Also, when installing insulation between the rafters, do I need to put baffles in every cavity? Just in the same cavities that have the soffit vents?


Last edited by Nathan0115; 05-18-2010 at 09:31 AM.
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Old 05-18-2010, 12:33 PM   #2
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Installing Soffit vents w/finished attic


If you are using a ridge vent, you should have a soffit in every bay and a chute in every bay as well, all the way up to the ridge vent. The easiest way to do that is to rip a 3/4" thigh sheet of ridgid foam, like XPS or foil faced polyurathane, into 16" wide strips on the table saw, then reduce your blade depth and make a slot 5/8" deep, 3/4 of the way in from both edges. Fold these blocks over so you make a long chute that is now 14.5" wide with two folded over lips that give you a 3/4" air gap. The air gap side faces the roof deck. Wedge it in between the bays and use canned or gun foam to seal the edges. Refill the remaining rafter space with insulation. Make sure your chute ends in the open air and not over your outside wall - you want to channel any leaks outside your house, not into the top of a wall.

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Old 05-18-2010, 02:09 PM   #3
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Installing Soffit vents w/finished attic


air chute just past ceiling joist
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Old 05-18-2010, 05:59 PM   #4
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Installing Soffit vents w/finished attic


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Originally Posted by xxPaulCPxx View Post
The easiest way to do that is to rip a 3/4" thigh sheet of ridgid foam, like XPS or foil faced polyurathane, into 16" wide strips on the table saw, then reduce your blade depth and make a slot 5/8" deep, 3/4 of the way in from both edges. Fold these blocks over so you make a long chute that is now 14.5" wide with two folded over lips that give you a 3/4" air gap. The air gap side faces the roof deck. Wedge it in between the bays and use canned or gun foam to seal the edges. Refill the remaining rafter space with insulation.
What is the advantage of this method over the preformed vent spacers they sell? Is it more cost effective?
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Old 05-18-2010, 11:15 PM   #5
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Installing Soffit vents w/finished attic


You would have to price it out yourself - I've never seen a vent chute around here, so it's a regionally stocked item. Lowes has them in their circular, but I can't get it at their store!

Using rigid foam board will also add R value to your job, similar to the amount of regular insulation it is displacing while still giving you the air channel for your ridge vent.
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Old 06-04-2010, 08:39 PM   #6
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Installing Soffit vents w/finished attic


It sounds like it would be stronger than what they sell. Probably worth the time and effort.
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Old 06-04-2010, 09:05 PM   #7
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Installing Soffit vents w/finished attic


Check locally. Lowe's has the foam ones, which tear apart on the roof's exposed nail points. H.D. carries the plastic ones with a wind-blocker built in for the price of the other including the insulation wind-blocker. Leave a 1" space at ends to let condensation out (as per instructions) with both. Vent every bay- 9sq.in.NFVA per foot to match your ridge vent- 18sq.in/2=9".

http://www.adoproducts.com/duro.html http://www.adoproducts.com/wind.html http://www.bergerbuildingproducts.co...sAccuvent.html

http://www.fureyco.com/content/image...ng_The_Air.pdf

The math: http://www.airvent.com/homeowner/pro...it-specs.shtml

Close up the gable vents afterward: http://books.google.com/books?id=Z8a...lation&f=false

Insulation tight to drywall: http://oikos.com/library/insulating_...lls/index.html

Seal the joist space at kneewall/attic: http://www.homeenergy.org/archive/he...95/950309.html

Sloped ceilings: ww.buildingscience.com/documents/digests/bsd-115-wood-pitched-roof-construction?full_view=1

House wrap the attic side kneewall to prevent wind-washing the insulation. Air seal for leaks: http://www.rd.com/how-to-seal-attic-...icle18158.html

Be safe, Gary

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