bighammer

I have been wondering what would be the best way to properly install my propervents and insulation in my 8/12 pitch cathedral roof. I have framed it with 2"x8" and have it sheathed,roofing materials applied and "low-omni" roll installed at ridge for my exhaust. I have some options, but wondering what other discussion could be made on this topic, as it can be very important for long term performance.

1) installing 1"x2" tight to sheathing on the underside of the roof deck continuous from plate to ridge and than installing masonite to the 1"x2" to make my proper vent (air chutes). Filling the rest of the cavity with fiberglass

2) installing spray foam in the joist spacing and not thinking twice about it.

3) installing 1"-2" bead board style foam board to the underneath side of the roof deck and than using fiberglass to fill the cavity.


Thoughts and discussion and very welcome. Thank you!!

doubleaction

I would use a rigid foam insulation board cut to fit snugly between the rafter bays to create about a 2" channe for air flowl. then seal with cans of spray foam and put fiberglass on that.

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maceycon

If you can get the R value up to 50 you can avoid ventilation all together. I am not sure you can pull that off with 2 X 8 Rafters. You maybe able to with foam insulation.
Another option a "cold roof" insulate the rafter bays with fiberglass. Install sleepers on top of the roof rafters and sheathing. Install sheathing on top of that and you have a ventilation channel.
I live in the north east and see problems with cathedral or vaulted ceiling all the time. If you insulate the bays and have your ventilation channel above the insulation you will get condensation it the ventilation bay.
My preference is to get the ceiling up to an R 50.

bighammer

than do you use a vapor barrier after installing the fiberglass? the rigid foam could trap moisture? the foam is than just friction fit into place?

bighammer

I have never heard that about the R50, however, if I install "sleepers" over my roof deck , how do I get that to ventilate, do I have to add additional vents from my soffits? Maybe I am not understanding...

KHouse75

I'd go with spray foam.

bighammer

Have you ever done this, does the foam "breath"? I suppose close cell will not cause any condesation..no way for air to travel thru it? Would that not make the roof deck extremely hot in the summer, I was told at one time that code in this area required atleast 1" space between roof deck and insulation.

SLSTech

Closed cell spray foam - Demilic
The system works great, & you don't have to worry about ventilation
The problems with all the other systems, if you mess it up you can run into problems real quick

It is not cheap but in my book worth it. Here is one job I had done with the Demilic Soy Foam which was done by AIT Insulation out of Auburn

If money is an issue I would go with you 1st choice except for masonite I would go with what double action stated

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tomstruble

http://www.hpanels.com/pages/pdfs/Li...ol-Vent-bw.pdf
check this out

maceycon

That is the best solution!!!, but skip the 2" bay for ventilation.