New Roof

[Chris Padilla]

As some of you may know from the Skylight thread several lines below, I am preparing for a new roof to be installed on my house.

It is a 3:12 pitch, currently cedar shakes on skip sheathing. 5 sections of my roof are vaulted ceilings and therefore have rafters as opposed to trusses (which is an attic space, of course).

My roofer will rip off the cedar shakes but leave the 1x8 (or whatever size they are) skip sheathing in place and put OSB (foil on one side) 4x8 panels down on top of it. Then I'll have some nice asphalt shingles (CertainTeed brand) layed down to complete the reroofing.

My question to the collective here is about insulation in the rafter bays for the vaulted ceiling sections:

It seems to me this is the TIME to upgrade/improve the insulation. I could arrange to have cellulose or fiberglass blown into the rafter bays between the skip sheathing or perhaps some skip sheathing could be removed and batts stuffed in the bays.

Now I understand about my roof needing air space for air flow (breathe) especially since the asphalt roof breathes a lot less than my cedar shingled roof right now. I figured if the OSB panels were being nailed to the skip sheathing, that would provide a decent enough air gap per bay.

My roofer claims it would seal things up too much and not allow enough air flow and doesn't recommend I do anything to my rafter bays. I've seen those "trays" that can be placed in the bays that keeps an air-channel open so I figured those could be installed as well.

Any thoughts about this? I'm all ears!!

[Jim Becker]

Outside of using closed cell spray foam, you must have the air gap for ventilation so you can't add insulation to the bays if it takes away that air flow. Those "trays" are called baffles, and would normally be used to keep the insulation away from the roof sheathing. Spray foam can be used right on the deck, although some contractors still prefer to maintain an air gap. (Mine did, but we had 12" rafters to work with...and lucky me, the insulation contractor still filled them which was much more than the R38 we contracted for... )

[Chris Padilla]

Yeah, my rafters are 2x6s so I don't have a lot of space but geez, I'd sure like to improve the insulation.

The owner of the roofing company will come out and go over my roof with me step by step so we'll see what he has to say about it.

[Jim Becker]

Chris, with a 2x6 structure, about the only way to really do some "good" retro-fit insulation is to have spray foam done while it's open to the sky. That may also mean they need to rip off the skip sheathing to do it effectively. But you could get yourself to R30 or better that way with closed cell foam in a 2x6 wall.

[Chris Padilla]

Agree...but it'll cost extra to rip off all the skip sheathing...was hoping to keep that to a minimum if possible.

Thinking, thinking, thinking....

[Chris Padilla]

http://www.daviddarling.info/encyclo...l_ceiling.html

Interesting link.

In the first paragraph titled, "Cathedral celiling ventilation", they have a quote from a lab saying, "Should cathedral ceilings be ventilated? Not if that space is tightly packed with insulation."

Then the paragraph right below that titled, "Insulation techniques for cathedral ceilings", the first sentence says you need an air space! LOL!

So if I spend the extra money to have the skip sheathing removed and that expanding foam insulation sprayed in the rafter bays, it appears that I don't have to ventilate the cathedral ceiling. They can just lay the foil-covered OSB right on the rafters and go to town. Hmmmm...........

http://www.nuwool.com/products/technical/cathedral.html More fodder for reading although it appears to be largely the exact same text in some cases. Interesting, interesting....

[Jim Becker]

Yes, that's correct Chris. At this point, it's an acceptable technique to use spray foam without an air gap. It will also provide a significant increase in structural rigidity...something a 2x6 rafter ceiling will benefit from, IMHO. Do note that you want closed cell foam, not open cell foam. The latter has half the R value and less of the structural and air/moisture infiltration properties that closed cell foam has.

[Chris Padilla]

So I had the skylights installed and was able to see a little bit of my roof from below and above. The skip sheathing is 1x6 spaced 6" apart. My skylight guy said my tar paper has reached the consistency of a potato chip!

Anyway, with this spray foam insulatin investigation I have going, I'm figuring that maybe every other row of skip sheathing could be removed and that would allow reasonable access to the rafter bays to fill 'em full of gunk.

Oh, I should add that we've discoverd that I have the additional headache of batts AND loose-fill insulation in my rafter bays! That will make for messy/time consuming (and probably more costly) removal for me should I go the spray foam route.

[Lee Schierer]

Just to add to what Jim has said. For the "ventilation" to work you have to have a way for air to get in and for the hot air to get out. Normally this is done with soffet vents and a ridge vent. If the space you are referring to is closed off, or doesn't have both an inlet and outlet, adding insulation may not help the life of the roof much.

[David G Baker]

I don't know if this will apply to your situation because I don't have cathedral ceilings and I have an attic. The attic is not very accessible.
I had a new roof put on. I decided to have the old roofing material removed. During the removal process it was discovered that my soffits were completely enclosed by wood. If there were vents, they would have been useless for ventilation because Fiberglass insulation was crammed into the space between joists and trusses. The old roof was approximately 10 years old and was asphalt shingles over 1/2" OSB and the original 1"X8" boards. The old roofer did not lay down roofing felt, the shingles were on bare OSB. Due to the lack of ventilation the OSB was black with mold and crumbling and the majority of the 1"X8" were rotted. There was no place for the moisture in the attic to ventilate so it stayed in the attic and destroyed the roofing material. There were several of the metal vents in the roof but there was no air source to allow the attic to vent. $14,000 later I had a new roof.

[Chris Padilla]

David,

I do have an attic but only 2 of 7 sections of my roof have such a thing...everything else is a cathedral ceiling. My attic is vented at the gables so no problems there...the "floor" of the attic is the place to insulated well.

[Chris Padilla]

Just to add to what Jim has said. For the "ventilation" to work you have to have a way for air to get in and for the hot air to get out. Normally this is done with soffet vents and a ridge vent. If the space you are referring to is closed off, or doesn't have both an inlet and outlet, adding insulation may not help the life of the roof much.

Lee, et. al. Google with this phrase:

oak ridge laboratory moisture control handbook cathedral ceilings

You'll find some interesting stuff in regards to venting roofs. From what I can discern, if air penetration can be 100% stopped, there is no reason to ventiliate (as ventilating is a form of air flow that can bring moisture in as well as remove it).

You'll also find that the heat gain of a roof is most strongly dependent upon the color of the roof and not the venting methods.

[Rob Russell]

Chris,

I have a personal bias against using OSB for anything structural. I belive in using plywood, even if it is more expensive.

We don't have enough historical data to know how long the glues are really going to hold up. As woodworkers, we all know that the glue will fail. With the manufactured products like OSB, once the glue goes - there is nothing in the panel to provide any structural integrity. At least with plywood, the sheets of ply will provide support even if the glue between the plys fails.

Just my 2 cents.

FWIW, when we did our addition, we used plywood for everything - no OSB.

Rob

[Chris Padilla]

Hi Rob,

I asked my roofer about that but he really prefers the OSB and said he's had success with it for over 20 years so who am I to argue. He has seen the plywood fail/delaminate. I, too, wanted to use plywood but you make a good argument for sure. Hmmmm.......

[David G Baker]

Rob,
I think I may have to agree with you. When my house was re-roofed there was an area that was covered with plywood that was an addition to the old farm house's original structure. The plywood was not as thick as it should have been and it was spongy to walk on but it was still all wood. The OSB, if it did not have the 1"X8" underneath it, when walking on it a person would have fallen through because it was as you described. The OSB sheets were originally 1/2 inch thick but when removed they were closer to 3/4 inch thick due to swelling.
Had the old roof been done correctly I believe that the OSB would have been fine.