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-   -   Are rafter vents always needed? (http://www.diychatroom.com/f103/rafter-vents-always-needed-112327/)

SonOfPirate 07-29-2011 07:44 AM

Are rafter vents always needed?
 
I am interested in "finishing" my attic to provide additional storage space. Nothing fancy, really, all I want to do is put down a plywood floor and close-up the rafters to better control the dust, etc. in the space.

I currently have two layers of fiberglass insulation - the first is suspended between the floor joists (the ceiling for the rest of the house) and the second is laid perpendicular on top.

I would like to remove the top layer of fiberglass and attach it to the rafters (I may go with new once I take a closer look at the condition of what's there). I know that airflow is critically important and have a ridge vent, soffit vents as well as a few other roof vents already in place.

From what I understand, rafter vents are a good way to ensure that there is proper airflow from the soffit to the ridge but I'm wondering if there are really necessary?

My rafters are 2"x8" so attaching 4" fiberglass insulation leaves a gap. Is this enough or are there other factors I'm not aware of?

It's not that I'm cheap but anywhere I can save a few bucks is always helpful.

Thx!

Gary in WA 08-06-2011 11:11 PM

http://docs.google.com/viewer?a=v&q=...1txlzU01V4JQIg

Gary

SonOfPirate 08-08-2011 06:44 AM

While I appreciate the link, I'm not sure I see the relevance.

parts 08-08-2011 11:31 AM

Well the insulation will not work as effectively installed in that manner but that was not the question so here you go. When I began installing insulation 25 plus years ago vents where almost never used and R30C 8.25 inch insulation was originally sold to go into a 2x10 without one. That being said insulation puffs up a lot in the heat but I have seen 3.5 in 2x8s that has been there for years and had not cut off the air flow. I personally always use them just in case though. Now let the multitude of posts saying I am wrong and quoting the code to begin.:whistling2:

Gary in WA 08-08-2011 12:22 PM

Sorry about that, right before bedtime.... The site shows there are requirements for using an attic for storage space. I see the site also removed the main ideas I was showing.......Anyway, make sure the floor joists are strong enough for the future loads as most were sized to only hold up the ceiling material and insulation.
Air sealing any holes from the conditioned rooms below is more important than insulation. Putting plywood or OSB sheets on the ceiling joists will limit the amount of insulation and possibly lead to mold/mildew by trapping the moisture from below (because of its perm rating) if you don't have plastic on the ceiling already. Try it for a heating season then remove and inspect the bottom face of the sheets. If you are in a zone above 4, it would be unwise to limit your insulation there. What is your location?
The insulation in the rafters would not be as effective as on the joists unless you do it properly; http://www.buildingscience.com/docum...d-roof-systems
That would be if the attic room was conditioned and air-tight from the attic (outside) air.
http://www.jlconline.com/cgi-local/v...170a32100a05c7

Gary

AGWhitehouse 08-08-2011 02:14 PM

Quote:

Originally Posted by GBR in WA (Post 702911)
Putting plywood or OSB sheets on the ceiling joists will...possibly lead to mold/mildew by trapping the moisture from below (because of its perm rating)

I agree that the numbers do say that 1/2" plywood or OSB should be considered a class II vapor retarder, but why then do we consistently sheath exterior walls with it creating a "double barrier" system and don't have the mold issues?

My take is that the open joints between sheets allows for the moisture to migrate through. So if you are going to sheath the attic floor, don't use toungue and groove plywood.

AGWhitehouse 08-08-2011 02:23 PM

Quote:

Originally Posted by SonOfPirate (Post 696112)
From what I understand, rafter vents are a good way to ensure that there is proper airflow from the soffit to the ridge but I'm wondering if there are really necessary?

My rafters are 2"x8" so attaching 4" fiberglass insulation leaves a gap. Is this enough or are there other factors I'm not aware of?

"Hot Roofs", or un-vented roofs, do exist, though I don't personally recommend them.

Just putting in the 4" batt into an 8" bay won't gain you much of anything. The air flow within the remaining joist space from the soffit vents to the ridge vents will negate most of the insulative value. Batt type insulation gets its insulative value from trapping air, so with the air moving through it, it loses its ability to trap air, and in turn loses its effective R-value. I would recommend putting in the foam baffles and putting the batt tight to the baffles to eliminate the exposed air gap behind the batts.

SprayFoamer 08-08-2011 06:20 PM

a hot roof system with sprayfoam (2Lb medium density) is effective and in some circumstances required when you run into situation where venting is unobtanable due to roof line design and lack of cross strapping

regardless vent where ever possible.
air flow is just as important as insulation in most cases (i.e. attic, insufficent airflow can lead to premature shingle faliure, mold, dryrot and if you live in an area with snow ice damming will happen)


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