# Best Way to Vent a 12" Fiberglass Insulated Vaulted Ceiling



## zootjeff (Jul 11, 2007)

Hi,

I'm building an addition and I'm using 2x12s 24 on center to make a vaulted ceiling. I'm going to be venting it with a ridge vent and the slope is 4x12.

Looks like there are lots of way to create the 1" air gap above the high density R-38.. What do most people do (that are happy and proud of the results)?

One idea I had was to use: http://www.shop.insulationmaterials.com/Radiant-Panel-Pro-Plus-24-x-48-Bundle-of-25-RP3-24bdl.htm

But this relies on the panel "wedged" into place and will likely compress the insulation and create more than a 1" gap, which might be good..

Do people generally think 1" is the right amount?

Owens corning makes Raft-R-Mate which is a foam baffle, but costs 200/case and seems like you loose a lot of the 1" space for mounting/ribbing.. 

Then people seem to make their own on site, foam strips, cardboard, etc.. 

If you are going to have R38, is using a radiant shield worth it? What about just painting the back of the sheeting white?

Anyway.. Anyone proud of the way they vented a 12" vaulted cathedral ceiling, please drop me a line and send me a link to the materials you used..

Thanks!

Jeff


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## joecaption (Nov 30, 2011)

Have it spray foam or build the roof with Sip roofing panels and no venting would be needed.
I'd use the foam baffles if it was mine and I was going to vent.
Painting it white would be useless. 
A reflective barrier would just cook the shingles.


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## zootjeff (Jul 11, 2007)

joecaption said:


> Have it spray foam or build the roof with Sip roofing panels and no venting would be needed.
> I'd use the foam baffles if it was mine and I was going to vent.
> Painting it white would be useless.
> A reflective barrier would just cook the shingles.



If you spray foam do you use open or closed cell? 
With Spray foam would you still leave a air gap? 
What kind of R-Value do you get out of a 11 1/4" space?
What is the price difference between spray foam and fiberglass?

Thanks!

-Jeff


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## Windows on Wash (Aug 30, 2011)

How big is the space (i.e. area of roof)?

I really like rigid foam here with foil facing on it and a 1.5" minimum air space.


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## BarryP (Jun 14, 2015)

Just dense pack the ceiling to 3.5 psi with cellulose or fiberglass and don't worry about the ventilation. Treat it like a wall.


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## BarryP (Jun 14, 2015)

BarryP said:


> Just dense pack the ceiling to 3.5 psi with cellulose or fiberglass and don't worry about the ventilation. Treat it like a wall.


I didn't know how to edit the above post. Here is an article I just wrote on the subject: http://www.insulationmachines.net/t...-ceilings-dense-pack-with-insulation-machine/

This is an approach that cellulose manufacturers recommend. (I train insulation companies throughout the US and hang with these guys).


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## Windows on Wash (Aug 30, 2011)

BarryP said:


> Just dense pack the ceiling to 3.5 psi with cellulose or fiberglass and don't worry about the ventilation. Treat it like a wall.


Joe would disagree with you on that one. 

http://www.buildingscience.com/documents/insights/bsi-043-dont-be-dense


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## BarryP (Jun 14, 2015)

Sure. Good people have differing opinions. To be fair, the picture was fiberglass batt, unventilated. It was not dense packing.

Dense packing is only as good as the installation. Even bad spray foamers leave gaps.

The article is dated 2010... there is new information about this since 2014. I will see if I can find it. I am not sure it is published yet.

Thanks for the article!!


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## Windows on Wash (Aug 30, 2011)

I don't disagree with you on the differing opinions and next to SPF, I think dense packed cellulose is the best insulation/air barrier combo out there. 

Not sure I trust it just yet given the feedback. We have dense packed the slopes on Capes but we still have some ventilation on the top side of it and the sloped sections aren't getting as bombarded with heat and moisture. 

My point from before is that if you can use a vented application and batts, why not do it. To create and underdeck vent space just requires a bit of forethought and material selections and they you get the best of both worlds.


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## Gary in WA (Mar 11, 2009)

BarryP, does this post #9 sound familiar; http://www.greenbuildingadvisor.com...joe-lstiburek-says-dont-dense-pack-unvented-c the other points are also interesting...

Gary


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## BarryP (Jun 14, 2015)

Gary in WA said:


> BarryP, does this post #9 sound familiar; http://www.greenbuildingadvisor.com...joe-lstiburek-says-dont-dense-pack-unvented-c the other points are also interesting...
> 
> Gary


I read that yesterday. National Fiber had some very valid points. 

Another cellulose manufacturer, who will go unnamed for the moment, was telling me about his studies and he came to the conclusion, after years of thinking that ventilation is the thing to do, that dense packing is fine and now a preferred treatment.

Keeping the cavity free of water is key. So, why let moisture-laden air flow through vents that are most likely crushed and plugged anyways?

I am skeptical about that 2010 article because it has some non sequiturs.

Now... I will say this... attempting to pack 12" at 3.5Psi just won't happen.

Disclaimer: I grew up as a fiberglass-friendly insulator. I have no need to promote a particular product. I don't defend cellulose manufacturers. They can be full of hype.


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