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-   -   Ventilated Cathedral Ceiling "Open or Closed Cell" ? (http://www.diychatroom.com/f103/ventilated-cathedral-ceiling-open-closed-cell-150503/)

brulin 07-16-2012 06:59 PM

Ventilated Cathedral Ceiling "Open or Closed Cell" ?
 
I had new cellulose insulation blown into my attic about 4 yrs, ago; the area above the cathedral ceiling was "dense packed". However, not being able to see the area I am not sure if it is really "dense" packed. Anyways, I am now in need of a new roof and I would like to create a ventilated attic. I am willing to remove the necessary decking to provide access to the cathedral portion of the attic.

I have contacted a foam insulation contractor who recommends removing the cellulose in the cathedral portion of the attic and replacing with "open cell" foam after applying a vapor barrier on top of the sheetrock. Is "open cell" the way to go or should "closed cell" be used? I know the closed cell has a higher R value and the rafters are only 2 X 6.

To compensate for the limited R value I propose to add 2 or 3 inches of polyiso under the cathedral ceiling. Will there be a problem with vapor barriers as I don't know if polyiso is considered to be a vapor barrier? The polyiso would be covered with tongue and groove boards.

Also, in the front of the house (where the cathedral ceiling is located) I only have sufficient soffitt area for venting across 1/2 of the area. Should I have soffitt venting on the 1/2 and drip edge venting on the other 1/2 or should I run drip edge venting for the entire area?

I am located in the Albany, NY area.

Thanks for any suggestions!!

jklingel 07-18-2012 12:02 AM

i'm a little confused as to what you have. it sounds like a cathedral ceiling in part, and a truss part. yes? cellulose is likely not the best answer for a cathedral, as it needs ventilation over it, like most insulations. if you spray open cell, it needs vapor retarder paint. you'll have to dig into the reading to see how thick you can apply closed or open cell; don't trust the contractor. buildingscience.com and greedbuildingadvisor.com have info on this. cellulose is great on the rafter/truss side. i would not install a vapor barrier, esp if you have foam sprayed to the sheathing; vapor retarder paint on the open cell, if used. no vb on the sheet rock. if you have an open attic area, no vb there, either. vapor retarder and air seal.

Windows on Wash 07-18-2012 09:57 AM

Can you post a picture of the exterior of the home at the vaulted sections?

There are a bunch of ways to skin this cat and spray foam, while great, is the most expensive.

brulin 07-18-2012 04:33 PM

6 Attachment(s)
Thanks for taking your time to help me out. Please see the attached photos of the exterior and interior. Let me know if you need any additional information.

Windows on Wash 07-18-2012 06:24 PM

Polyiso with T&G over it and a vapor barrier inside the rafter bays should be avoided.

If he wants a vapor retarder level, he could just use closed cell SPF and kill the R-Value and vapor issue bird with one stone.

The issue is that still does not address the thermal bridging of the rafter without thermally breaking it from the outside.

If you are pulling off the deck already, would you consider making your own sealed SIP roof? The reason I ask is because putting iso to the interior side is a bit unconventional and will drop you ceiling height a bit and somewhat defeat the purpose of the vaulted ceiling.

The issue with this approach is it will throw off the intersecting roof lines unless you raise the roof line on all phases.

Easiest thing to do would be to skip the vapor retarder paint/layer and the open cell SPF. Pull the roof deck, lay down 5.5" of close cell SPF and be done with it. The rafters will still be cold spots but you get and R-38'ish out of the insulation section of the roof and zero moisture movement through the roof. The rafter will still be cold but the only want to fix that is the interior iso or exterior foam and seal up the whole thing.

That is my only concern is the R-5.5 in the rafters in that case and the cold spots that they will create. If this area is over a kitchen, you can get rafter sweating in that case. That can only be fixed by exterior or interior foam to break the bridging of the rafter.

framer52 07-18-2012 06:41 PM

You do not need to have ventilation in this area.

it is called a hot roof and is accepted as good now.

Read at building science.com

brulin 07-18-2012 08:58 PM

If closed cell foam is used I will still have cellulose in the remainder of the attic (non cathedral). Would it then be alright to have an unventilated attic and close off all soffitt vents, gable vents and the roof ridge vent?

Windows on Wash 07-18-2012 11:49 PM

You are only going to a hot/unvented attic on the cathedral portions.

The rest stays vented.

brulin 07-19-2012 12:03 AM

I'm not sure I understand. The cathedral portion of the attic is not physically (structurally?) separated from other portions of the attic.

Windows on Wash 07-19-2012 01:34 AM

I understand.

The only section that would be a hot roof is that square footage of vaulted ceiling and the rest would be vented. It doesn't appear there are any soffits on that section either so the intake air would still come from the front of the home to feed the venting on the other sections.

You wouldn't close up the exhaust venting by doing the vaulted sections as it appears they don't run to the top of the ridge line.

Gary in WA 07-20-2012 11:08 PM

WoW mentioned the reason you can see the rafter outlines in the ceiling/attic wall. Winter-time warm moist conditioned air is rising and condensing on the cold rafters leaving water to collect dust and outline them. You can even see the mid-span full-depth rafter blocking. Your punched vinyl soffits are probably inadequate, a triple 4' center vent only has 4.8 NFVA per foot deep, your soffits appear 12" deep?: http://bluelinxco.com/Portals/0/docs...20Brochure.pdf You'd need 2' deep to get minimum for 1/300. What are yours, per link?

One problem with removing the sheathing and SPF from above is no air sealing at SPF/sheathing. Any air infiltrating (from ends or through ply joints) could still condense on the framing. A few benefits of SPF is the air-sealing and racking resistance of the roof as a unit (need the bonding wet SPF provides).

Adding foam---- fig.#1: http://www.buildingscience.com/docum...+in+flat+attic

Gary


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